Table Of Content

Astronomy&Astrophysicsmanuscriptno.sanchez (cid:13)c ESO2008 February3,2008 PPAK Integral Field Spectroscopy survey of the Orion Nebula Data release S.F.Sa´nchez⋆1,N.Cardiel1,2,M.A.W.Verheijen3,D.Mart´ın-Gordo´n4,J.M.Vilchez4,andJ.Alves1 1 CentroAstrono´micoHispanoAlema´n,CalarAlto,CSIC-MPG,C/Jesu´sDurba´nRemo´n2-2,E-04004Almeria,Spain. 2 DepartamentodeAstrof´ısica,FacultaddeF´ısicas,UniversidadComplutensedeMadrid,28040Madrid,Spain. 7 3 KapteynAstronomicalInstitute,UniversityofGroningen,POBox800,9700AVGroningen,TheNetherlands. 0 4 InstitutodeAstrof´ısicadeAndalucia,CaminoBajodeHuetor,S/N,Spain. 0 ; 2 n ABSTRACT a J Aims.Wepresentalow-resolutionspectroscopicsurveyoftheOrionnebula.Thedataarereleasedforpublicuse.Weshowthepos- 8 sibleapplicationsofthisdatasetanalyzingsomeofthemainpropertiesofthenebula. Methods.Weperformanintegralfieldspectroscopy mosaicofanareaof∼5′×6′ centeredontheTrapeziumregionofthenebula, 3 includingtheionizationfronttothesouth-east.Analysisofthelinefluxesandlineratiosofboththeindividualandintegratedspectra v allowedustodeterminethemaincharacteristicsoftheionizationthroughtoutthenebula. 3 Results. Thefinaldatasetcomprises8182individualspectra,sampledinacircularareaof∼2.7′′ diameter.Thedatacanbedown- 6 loadedasasinglerow-stackedspectrafitsfileplusapositiontableorasaninterpolateddatacubewithafinalsamplingof1.5′′/pixel. 3 Theintegratedspectrumacrossthefield-of-viewwasusedtoobtainthemainintegratedpropertiesofthenebula,includingtheelec- 1 trondensityandtemperature,thedustextinction,theHαintegratedflux(aftercorrectingfordustreddening),andthemaindiagnostic 1 lineratios.Theindividual spectrawereusedtoobtainlineintensitymapsofthedifferentdetectedlines.Thesemapswereusedto 6 studythedistributionoftheionizedhydrogen,thedustextinction,theelectrondensityandtemperature,andtheheliumandoxygen 0 abundance. Allofthemshowaconsiderabledegreeofstructureasalreadyshowninpreviousstudies.Inparticular,thereisatight / relationbetweentheheliumandoxygenabundancesandtheionizationstructurethatcannotbeexplainedbycaseBrecombination h theory.Simpleargumentslikepartialionizationanddustmixedwiththeemittinggasmayexplaintheserelations.Howeveramore p detailedmodelingisrequired,forwhichweprovidethedataset. - o Keywords. ISM:individualobjects:M42–(ISM:)HIIregions–(ISM:)dust,extinction–ISM:abundances r t s a : 1. Introduction ratio). Such properties are frequently compared with those of v distant H II regionsin our Galaxy and extragalacticones (e.g., i TheOrionnebulaisthebrightestandbeststudiedHIIregionin X Pageletal.1992; Ferland2001). Due to their distance most thesky.Ithasbeenusedfordecadesasafundamentallaboratory of these regions are poorly resolved. However their integrated r in the study of the star formation regions, ionization processes a propertiesare comparedwith those ofparticularresolvedareas andheliumandheavyelementenrichment.However,despitethe intheOrionnebula. large number of studies on this target much remains unknown aboutit,evenfromopticalstudies.Themoreweknow,themore complexitseemstobe. Very little effort has been made to study the distribution of Several spectroscopic surveys have tried to characterize physical propertiesacross the nebula, except for some particu- its spectroscopic properties, taking spectra at different “repre- lar areas (e.g.Baldwinetal.1991), or their integrated values in sentative” locations (e.g., Kaler1976; Baldwinetal.1991; largeapertureareas.Poggeetal.(1992)performedFabry-Perot Estebanetal.1998; Estebanetal.2004). In particular, (FP)imagingspectrosphotometryonanareaof∼6′×6′centered Osterbrocketal. (1990)andOsterbrocketal. (1992)presented intheTrapezium.TheirFPdatacoveredthebrightestemission a compilation of high and low dispersion deep spectra of the lines in the nebula: Hβ, [OIII]λ5007, Hα, [NII] λ6548,6583, centralbrightregionin the optical-NIRregion(3000-11000Å). [SII]λ6716,6731and HeIλ6678. They derived some of the av- Theymeasured225emission lines, 88 atthe wavelengthrange eragespectroscopicparametersofthecoreofthenebula,includ- ofourIFSdata.Usingtherelativeintensitiesoftheselinesthey ing the integrated Hα flux, the average extinction, the average derived the relative abundances of several elements, the elec- ionization line ratios ([OIII]/Hβ, [NII]/Hα and [SII]/Hα), the tron temperature (T=9000 K) and density (N =4×103 cm−3), He+/H+ ionicabundanceratio,andtheaverageelectrondensity. e and the extinction (A ∼1.08 mag, derived from the Hα/Hβ Theyderivedthefirstrealiablemapsofthedistributionofthese V propertiesacrossthenebula,whichshowaconsiderabledegree ofstructure.Theyfoundsignificantdifferenceswithsomeofthe Sendoffprintrequeststo:S.F.Sa´nchez,[email protected] ⋆ BasedonobservationscollectedattheCentroAstronm´icoHispano previouslyreportedvaluesthatcorrespondtoparticularregions Alema´n (CAHA) at Calar Alto, operated jointly by the Max-Planck inthenebula(e.g.,Osterbrocketal.1992).However,duetothe Institut fu¨r Astronomie and the Instituto de Astrof´ısica de Andaluc´ıa reducedspectroscopiccoverageoftheirdatasetitwasnotpossi- (CSIC). bletofullycharacterizetheionizationconditionsinthenebula. 2 S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula Fig.1.IntegratedspectrumoftheOrionNebulaobtainedcoaddingthe8281individualspectraofourIFSdatasetoverthefullfield- of-view,plottedonalogarithmicscale.Theidentificationsofthedetectedemissionlines,aslistedinTable1,areindicatedattheir correspondingwavelength. Wepresentherealow-resolutionIntegralFieldSpectroscopy hexagon.Thesky-fibersaredistributedamongthescienceones (IFS) survey of a similar field-of-view of the FP observations inthepseudo-slit,inordertohaveagoodcharacterizationofthe presented by Pogge et al. (1992), but covering the entire opti- sky. The remaining15 fibers are used for calibrationpurposes, cal wavelength range (∼3700-7100 Å). The data are obtained, asdescribedbelow. reduced and released for the community.In this article we de- Thedataweretakenasabackupprogramofthe“DiskMass scribe the observationsand data reduction,indicating the main Project”(Verheijenetal.2004)underbadtransparencyandnon characteristicsofthedataset,andweshowafewexamplesofthe photometricconditions.A mosaic of 27 single exposuresof 2s applicationsofthisdataset.Toourknowledgethisisthelargest each was taken. The exposuretime was selected to avoid satu- areaeversampledwithanIFSunit,showingthecapabilitiesof ration of the brightest emission lines, like Hα and [OIII]. The new tecniques like 3D mosaicing to overcome the inherent re- initialexposurewascenteredintheTrapeziumareaoftheOrion ducedfield-of-viewoftheseinstruments. Nebula.Consecutivepointingsfollowedahexagonalpattern,ad- The layout of this article is as follow: In Section 2 we de- justing the mosaic pointings to the shape of the PPAK science scribetheobservationsanddatareduction.Section3summarizes bundle.Eachpointingcenterliesat60”fromthe previousone. thebasicpropertiesofthedataset.InSection4welistexamples DuetotheshapeofthePPAKbundleandbyconstructionofthe oftheapplicationsofthisdataset,includingtheintegratedprop- mosaic,11spectraofeachpointing,correspondingtooneedge ertiesofthenebula,thedescriptionofthelineintensitymapsof of the hexagon,overlapwith the same numberof spectra from some of the brightestemission lines, a distribution of the elec- thepreviouspointing.Thispatternwasselectedtomaximizethe tron density and temperature, a study of the dust extinction, a coveredarea,allowingenoughoverlappingto match thediffer- descriptionoftheionizationstructure,andastudyofthehelium entexposurestakenundervariableatmosphericconditions.The andheavyelementabundances.Section 5summarizesthe con- offsetprocedurewascarriedoutuntilclosingduetoriskofrain. clusionsofthearticle. Thecompletemosaicwascompletedin1.5hoursintotal,mostly duetothereadouttimeofthePMASCCDcamera(∼1.5minin thesetupused). 2. Observationsanddatareduction Data reduction was performed using R3D (Sa´nchez2006), in combination with IRAF packages (Tody1993)1 and E3D Observations were carried out on November 21st, 2004 at the (Sa´nchez2004). The reduction consists of the standard steps 3.5mtelescopeoftheCalar AltoobservatorywiththePotsdam forfiber-basedintegral-fieldspectroscopy.A masterbiasframe MultiAperture Spectrograph, PMAS, (Rothetal.2005) in the was created by averaging all the bias frames observed during PPAKmode(Verheijenetal.2004;Kelzetal.2006).TheV300 the nightand subtracted from the science frames. The location grating was used, covering a wavelength range between 3700- of the spectra in the CCD was determined using a continuum 7100 Å with a resolution of ∼10 Å FWHM, corresponding to illuminated exposure taken before the science exposures. Each ∼600kms−1,muchlargerthantheinternalvelocitieswithinthe spectrumwasextractedfromthescienceframesbycoaddingthe nebula.ThePPAKfiberbundleconsistsof382fibersof2.7”di- flux within an aperture of 5 pixels around this location along ametereach(seeFig.5inKelzetal.2006).Ofthem,331fibers the cross-dispersion axis for each pixel in the dispersion axis (thesciencefibers)areconcentratedin asinglehexagonalbun- dlecoveringafield-of-viewof72”×64”,withafillingfactorof 1 IRAF is distributed by the National Optical Astronomy ∼65%. The sky is sampled by 36 additional fibers, distributed Observatories, which are operated by the Association of Universities in 6 bundles of 6 fibers each, located following a circular dis- forResearchinAstronomy,Inc.,undercooperativeagreementwiththe tribution at ∼90” of the center and at the edges of the central NationalScienceFoundation. S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula 3 Table1.Listofdetectedemissionlines,uncorrectedforextinction. Wavelength Wavelength Flux1 LineId. Wavelength Wavelength Flux LineId. Lab.(Å) Obs.(Å) Lab.(Å) Obs.(Å) 3726.03∗ 3724.94 35219.70 [OII] 5006.84∗ 5006.99 143002.31 [OIII] 3728.82∗ 3730.08 40670.09 [OII] 5056.02 5055.80 35.09 SiII 3750.15 3751.09 676.63 H12 5056.35 5056.35 39.79 SiII 3770.63 3771.25 1574.32 H11 5158.81 5157.92 52.29 [FeII] 3797.90 3798.18 2550.31 H10 5197.90 5195.79 25.86 [NI] 3819.64 3819.54 876.94 HeI3819 5199.00 5199.49 76.77 SKY NI 3835.39 3835.15 3391.71 H9 5200.26 5201.02 142.51 [NI] 3857.53 3853.55 561.27 HeI 5270.40 5270.64 171.06 [FeIII] 3868.75 3868.59 8266.59 [NeIII] 5360.00 5359.94 6.06 SKY? 3888.65 3887.97 4416.27 HeI 5461.00 5461.07 215.29 SKY HgI 3889.05 3890.22 3957.53 H8 5517.71 5517.71 284.96 [ClIII] 3964.73 3967.69 3634.16 HeI 5537.88 5537.89 281.24 [ClIII] 3967.46 3968.09 642.14 [NeIII] 5577.00 5576.40 789.97 SKY OI 3970.07 3969.20 5478.41 Hǫ 5577.31 5578.10 791.73 [OI] 4009.27 4007.41 318.24 HeI 5685.00 5684.97 118.17 SKY NaI 4026.21 4026.00 722.02 HeI 5754.64∗ 5754.66 400.01 [NII] 4046.00 4046.02 32.09 SKY HgI 5770.00 5769.91 15.66 SKY? 4068.60 4068.39 458.02 [SII] 5790.00 5790.04 1.16 SKY? 4076.35 4074.88 159.72 [SII] 5875.62 5875.56 7538.90 HeI 4101.74 4101.56 12021.26 Hδ 5893.00 5892.61 348.91 SKY NaID 4120.86 4121.23 53.23 HeI 5930.00 5927.95 123.02 SKY? 4143.76 4148.23 23.02 HeI 5957.61 5957.11 41.97 SiII 4267.15 4265.12 85.48 CII 5958.58 5957.49 12.78 OI 4340.47 4340.32 23860.65 Hγ 5978.97 5978.80 81.35 SiII 4358.00 4364.15 113.00 SKY HgI 6000.00 6000.26 12.91 SKY? 4363.21∗ 4366.76 831.27 [OIII] 6046.40 6046.57 41.92 OI 4387.93 4379.43 125.13 HeI 6240.00 6239.95 60.97 SKY? 4414.91 4471.37 1937.47 OII 6265.00 6264.91 30.67 SKY? 4416.98 4418.47 62.93 OII 6300.00 6299.33 297.01 SKY OI 4471.50 4412.41 97.99 HeI 6300.30 6301.39 362.98 [OI] 4658.10 4658.22 379.59 [FeIII] 6312.10 6312.27 950.13 [SIII] 4701.62 4701.26 123.43 [FeIII] 6330.00 6329.99 46.28 SKY? 4713.20 4713.36 279.38 HeI 6347.09 6347.30 141.11 SiII 4733.93 4735.80 40.15 [FeIII] 6363.78 6363.72 208.24 [OI] 4754.83 4755.52 62.28 [FeIII] 6370.36 6370.53 107.90 SiII 4769.60 4775.10 62.48 [FeIII] 6548.03∗ 6547.63 10743.11 [NII] 4777.88 4783.04 29.99 [FeIII] 6562.82∗ 6562.87 171573.70 Hα 4815.55 4815.82 82.38 [FeII] 6583.41∗ 6583.52 32494.66 [NII] 4861.33∗ 4861.41 50768.69 Hβ 6678.15∗ 6678.37 1907.79 HeI6678 4921.93 4922.20 201.94 HeI 6716.39 6716.47∗ 2354.08 [SII] 4958.91∗ 4959.02 47337.07 [OIII] 6730.85∗ 6731.27 3447.13 [SII] 110−11Ergcm−2s−1 ∗Emissionlineusedinouranalysisofthedata. and stored in a row-staked-spectrum file RSS (Sa´nchez2004). mission. Then, we used multi-band photometric data available Wavelength calibration was performed using a He lamp ex- intheliteratureforthebrighteststarintheTrapezium,Θ1OriC posure obtained from archive data, and corrected for distor- (Lee1968; Vitrichenko2000) to perform an absolute flux cali- tionsusingThArexposuresobtainedsimultaneouslytothesci- brationbycomparingwiththeun-calibratedspectrumofthisob- enceexposuresthroughthecalibrationfibers(indicatedabove). jectextractedfromthecentralpointingofthemosaic.Anaddi- Differences in the fiber-to-fiber transmission throughput were tionalcorrectionwasperformedtotakeintoaccountfluxlosesin correctedbycomparingthe wavelength-calibratedRSS science theun-calibratedspectrumoftheobjectduetodifferentialatmo- frameswiththecorrespondingcontinuumilluminatedones.The sphericrefraction.Thiscorrectionwasdeterminedbycomparing natureofthe datasetpreventsusfromperforminganyskysub- theemissionlineintensitieswiththosederivedbyBaldwinetal. traction. However,due to the strength of the detected emission (1991) in the spatial region (O’Dell, private communications). lines and the short exposure time, sky contamination is not an Thisprocedureensuresafluxcalibrationandskyextinctioncor- importantissueformostoftheforeseeablesciencecases. rectionofthiscentralpointing. Sincethedataweretakenunderbadweatherconditions,no After reducing each individual pointing we built a single attempt was made to performa flux calibration on the basis of RSS file for the mosaic following an iterative procedure. The the standard stars observed during the night. On the contrary, spectraofeachpointingwasscaledtothoseofthepreviousone fluxcalibrationwasperformedfollowingatwo-stepprocedure. by the average ratio of the emission of Hβ in the overlapping First,wecarriedoutafluxcalibrationusingastandardstar ob- spectra. To address the possible variable extinction during the servedwithasimilarinstrumentalsetuponapreviousnight.This observations,theaverageratiobetweenthesescaledspectraand procedure corrects, to a first order, for the instrumental trans- the previous ones was determined. We fitted this wavelength- 4 S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula dependentratiotoaloworderpolynomialfunctionanddivided Table1liststhedetectedemissionlines,including,foreachline, all the spectra in the new pointing by the resulting wavelength thelaboratorywavelength(Osterbrocketal.1992),theobserved dependentscale.Inmostofthecasesthissecondcorrectionhas wavelength, the integrated measured flux and the line identi- littleeffectonthedata,indicatingthattheskyextinctiondidnot fication. The final effective resolution of our integrated spec- change too strongly during the observations, prehaps because trumwasFWHM∼10Å,measuredinthebrightestdetectedlines. they comprise only a limited amountof time (1.5h).The over- Therefore,anyemissionlineatdistanceslessthan∼3Åfromits lappingspectra were then replaced by the averagebetween the adjacentneighborsisblended,andthelistedintegratedfluxmust previous pointing and the new rescaled spectra. The resulting betakenwithcare. spectra were includedin the final RSS file, updatingthe corre- spondingpositiontable.Theprocessstartsin thecentralpoint- There is a considerable amount of blue continuum emis- ing, that is flux calibrated and sky extinction corrected as ex- sion in the integrated spectrum shown in Fig. 1, whose inten- plained before, ensuring an homogeneous flux calibration and sitylevelrangesfrom∼10−8 ergcom−2 s−1 Å−1 intheultravio- skyextinctioncorrectionforallthedataset.Theprocedurewas let to a few 10−9 erg com−2 s−1 Å−1 in the reddest wavelength repeateduntilthelastpointingwasincluded,endingwithafinal ranges.Thiscontinuumis partiallydueto the starssampledby setof8182individualspectraanditscorrespondingpositionta- ourIFSdata,inparticularthebrightmembersoftheTrapezium. ble.Inaddition,wecreatedadatacubewith1.5”/pixelsampling However,itiswellknown(e.g.,Poggeetal.1992)thattheOrion byinterpolatingthisRSSframeusingE3D(Sa´nchez2004). nebula presents a diffuse continuum emission, whose origin is associated with the light from the stars within the ionized gas 3. Results thatisscatteredbydust.Unfortunally,themeasuredcontinuum is strongly affected by the fact that we could not subtract the The final data set consists of 8182 flux calibrated spectra sam- skyemission.Thereforethisissuecannotbeaddressedpropertly plingacircularareaof∼2.7′′diametereachandcoveringacon- withthecurrentIFSdata. tiguous field-of-view of ∼5′×6′ centered approximately on the Trapeziumarea of the Orion nebula. Each spectrumcoversthe wavelength range between ∼3700-7100 Å, which includes the most prominent recombination and collisionally excited emis- 4.1.2. Averagedustextinction sionlines,from[OII]λ3727,3729to[SII]λ6716,6731(seeTable 1). Near the lower wavelength range, the [OII] flux was mea- The H I Balmer series are normally used to determine the suredwithoutprobleminall fibers.Thefaintestdetectedemis- extinction. One must compare the observed line ratios with sionlines,upto5σ,intheindividualspectrahavesurfacebright- ness of the order of 10−14 erg s−1 cm−2 arcsec−2. It is out of the expected theoretical values. These lines are known to be emitted as a consequence of recombination, and the the- thescopeofthisarticletoextractalltheinformationpotentially ory for them has been worked out in considerable detail available in this huge dataset. We consider here a few relevant (Hummer&Storey1987; Osterbrock1989). Case B (optically propertiesof the Orion nebula using these data. Howevermwe thickinalltheLymanlines)isthebestsimpleapproximationto arereleasingthefulldatasetinordertomakeiffreelyavailable tothecommunityontheprojectwebpage2. describetherealphysicalconditionsintheionizationofthegas in the Orion nebula (Osterbrock1988; Osterbrocketal.1992; Ferland2001). The main deviation of the real nebula from the 4. Applications theoryisprobablytheabsorptionofLyman-linephotonsbydust mixedwiththeemittinggas,whoseobservableeffectsareatthe 4.1.IntegratedpropertiesoftheNebula levelofafewpercentintheHα/Hβratio(Cota&Ferland1988; Baldwinetal.1991).Table2liststheobservedandexpectedra- In this section we describe some of the more interesting spec- tiosbetweentheHIBalmerserieslineslistedinTable1.They troscopic properties of the nebula derived by analyzing the in- show a goodagreementwithin the errors,particularlythe ratio tegratedspectrumoverthefullfield-of-viewofourIFSdataset. forthe5brightestemissionlines(whoseerrorsareestimatedto Contrary to previous studies that attempt to describe the aver- be lower than ∼10%), indicating the presence of little dust ex- age ionizationconditionsin the nebulabyanalyzingindividual spectratakenatdifferentlocations,we could,forthe firsttime, tinctiononaverage. describetherealaveragespectroscopicparameters. An average extinction of A =0.75 mag is found as- V suming an intrinsic value of Hα/Hβ=2.86, corresponding to 4.1.1. IntegratedspectrumoftheNebula case B recombinationfor T=9000 K (Hummer&Storey1987; Figure 1 shows the integrated spectrum of the Orion Nebula Poggeetal.1992; Osterbrocketal.1992), and using the Orion obtained by coadding the 8281 individual spectra of our IFS extinctionlawofCardelli,Clayton&Mathis(1989),witharatio dataset, plotted on a logarithmic scale. 82 emission lines are of total to selective extinction of RV =5.5 (Mathisetal.1981). detected in the integrated spectrum. They were identified us- This average extinction is smaller than the value reported by ing the list of 88 emission lines detected by Osterbrock et al. Poggeetal.(1992).However,adetailedcomparisonofthefield- (1992)using deep spectroscopy in the same wavelengthrange, of-view covered by their FP data and our IFS ones shows that incombinationwiththelistofknownskyemissionlinesatCalar their data are approximately centered ∼60′′ more towards the Alto (Sa´nchez, in prep.). The observed wavelength and inte- east, covering a larger area of the Orion “Dark Bay”, an area grated flux of each emission line were measured using FIT3D withdustextinctionsuptoAV∼2mag.Therearebetterlineratios (Sa´nchezetal.2006),removingthebackgroundcontinuumand to estimate the extinction(Osterbrocketal.1992), since, as we deblendingdifferentemissionlineswhenneeded(andpossible). notedbefore,theintrinsicHα/Hβratiointherealnebulaisafew percentsmallerthanthecaseBvalue(e.g,Baldwinetal.1991). 2 http://www.caha.es/sanchez/orion/ Wewilldiscussitseffectinfurthersections. S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula 5 Fig.2.Left:observedHαlinefluxmapinunitsof10−12 ergs−1 cm2 arcsec−2 obtainedbyfittingasingleGaussianfunctiontothe Hαemissionlineforeachsinglespectruminthedataset.Theintensitymapisthenreconstructedbyinterpolatingtherecoveredflux atthelocationofeachspectrum,asdescribedinthetext.Right:observedHα/Hβlineratiomap.Nocorrectionfordustextinction wasappliedtoanymaps. Table2.ObservedBalmerlineratios,uncorrectedforextinction, of N =2150 cm−3 assuming an average temperature of 9000 e comparedwiththetheoreticalvaluesobtainedfromOsterbrock K(Baldwinetal.1991).OurIFSspectroscopicdatadocoverat etal.(1992) leasttwooftheclassicalindicatorswithenoughsignal-to-noise, the[OIII]ratio: Lineratio Obs.Value Exp.Value I(λ4959)+I(λ5007) Hα/Hβ 3.380 2.86 =228.97 I(λ4363) Hγ/Hβ 0.470 0.486 Hδ/Hβ 0.237 0.259 andthe[NII]ratio: Hǫ/Hβ∗ 0.108 0.159 H8/Hβ∗∗ 0.078 0.105 H9/Hβ 0.067 0.0731 I(λ6548)+I(λ6583) =108.09 H10/Hβ 0.050 0.0530 I(λ5755) H11/Hβ 0.031 0.0398 H12/Hβ 0.013 0.0307 Bothratiosareremarkablysimilartothevaluesreportedby ∗Blendedwith[NeIII] Osterbrocketal.(1992),forthecenteroftheNebula,despitethe ∗∗BlendedwithHeI evidentvariableconditionsoftheionizationacrossthenebula. Theconversionbetweenbothratiosandthetemperature,T, dependsonN /T1/2(Osterbrock1989).Therefore,itispossible 4.1.3. Averageelectrondensityandtemperature e toderivethetemperatureusingbothratiosandtheratiobetween The electron density, N , in the S+ zone of the nebula can the[SII]doublet.Oncethetemperatureisobtained,itispossible e be estimated using the observed [SII]λ6716,6731 doublet ra- toderivetheelectrondensitytoo.Wefoundanelectronicdensity tio(Osterbrock1989).Theestimated[SII]λ6716/[SII]λ6731ra- ofNe =2104cm−3(2019cm−3)andatemperatureofT=9682K tio based on the integrated spectrum is 0.683, that can be (T=8922 K) for the [OIII] ([NII]) ratio. To derive the electron converted to an electron density, N . Although mild, this density from the [SII] lines ratio and the temperature derived e conversion shows a dependency with the electronic temper- fromthe[OIII]linesratios(T3),wetransformedthistemperature ature (∼ Ne/T1/2, Osterbrock1989). Many previous spectro- to T2 using the relation between both quantities presented by scopic studies sampling the Orion nebula at different loca- Pilyugin et al. (2006). Since the [OIII]λ4363 may be affected tions reported a range of temperatures between 8000-10000K byanunsubtractednightskyHgIemissionlineatλ=4358Åwe (e.g., Osterbrocketal.1992). Therefore, the measured ratio consider the estimation based on the [NII] ratio more reliable. could correspond to electron densities in a range between Althoughthelineswerenotcorrectedbyextinction,itseffecton 2000-4000 cm−3. Pogge et al. (1992) reported an average thederivedtemperatureisnegligible(evenforA >2mag).The V [SII]λ6716/[SII]λ6731 ratio of 0.680, similar to our measure- average oxygen abundance, 12+log(O/H) ∼ 8.6, derived from ment.However,sincetheirFPdatadidnotcoveranyoftheusual the fluxesof theintegratedspectrumquotedinTable 1 is close temperature indicators, they convertedit to an electron density tothesolarvalue. 6 S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula Fig.3.Left:Electrondensitymap,N incm−3,derivedfromthe[SII]λ6716/[SII]λ6731lineratio,representedinalogarithmscale. e Right:Electrontemperaturemapinunitsof103K,derivedfromthe([NII]λ6548+[NII]λ6583)/[NII]λ5755lineratio. Table3.IntegratedSpectroscopicpropertiesofthenebula. individual spectra as independentprobes of these properties at differentlocations. Parameter Value A 0.75(mag) 4.2.1. Lineintensitymaps V N 2019(cm−3) e The ionized gas in the Orion nebula exhibits a rather com- T∗ 8922(K) [O2III]λ5007/Hβ 2.820 plexstructure.Duetothat,ithasbeentargetedbynarrow-band [NII]λ6583/Hα 0.190 imaging using the HST and groundbased telescopes in order [SII]λ6716+31/Hα 0.034 to disentangle the structure of its ionized gas (e.g., Wen and ∗Electrontemperaturederivedfromdoubleionizedions,likeN++, O’Dell 1995; O’Dell&Yusef-Zadeh2000; O’Delletal.2003; usingtheformulaedescribedinOsterbrock(1989). Rubinetal.2003 and references therein). In many cases these narrow-bandimages catch more than one single line (e.g.,Hα, the [NII]λ6548,6583 doublet or the [SII]λ6716,6731 doublet), 4.1.4. Averageionizationconditions reducing its use to study the basic parameters of the ionized gas (as listed in Table 3). As we quoted before, Pogge et al. It is widely accepted that the ionization of the Orion (1992) presented FP observations that allowed them to disen- nebula is dominated by photoionization by the star Θ1 tangle some of the main diagnostic emission lines. Following Ori C. The derived values for the classical diagnostic themwehaveobtainedlineemissionmapsforthesameemission line ratios, [OIII]λ5007/Hβ=2.82, [NII]λ6583/Hα=0.19 and lines.Wehavealsoderivedlineemissionmapsfor[NII]λ5755, [SII]λ6716+31/Hα=0.034,are consistent with this picture, be- [OIII]λ4363 and [OII]λ3727,29, used as temperature, oxygen ing the typical values for H II regions ionized by OB stars abundance and ionization indicators in combination with the (Osterbrock1989). As already noticed by Pogge et al. (1992), lines quoted before. All the analyzed lines are listed in Table the integratedHα luminosity,7.6×1037 erg s−1 (extinctioncor- 1,labeledwithanasterisk. rected), and the electron density indicate too that Orion is a Toderivetheemissionlinemapseachspectruminthedataset ratherdense,low-luminosityHIIregion(Kennicuttetal.1989). wasfittedwithasingleGaussianfunctionperemissionlineplus Indeed, this Hα luminosity agrees with the nebula ioniza- aloworderpolynomialfunctiontodescribethecontinuumemis- tion dominated by a single O 5 star (Poggeetal.1992; sionagainusingFIT3D(Sa´nchezetal.2006).Insteadoffitting Hummer&Storey1987).Asummaryoftheaverageproperties theentirewavelengthrangeinarow,weextractedshorterwave- of the nebula derived from our integrated spectrum is listed in length ranges for each spectrum that sampled one or a few of Table3. theanalyzedemissionlines,inordertocharacterizethecontin- uumwiththemostsimplepolynomialfunction,andtosimplify thefittingprocedure.Inafew caseswe hadtoincludesomeof 4.2.SpatialdistributionofthepropertiesoftheNebula these lines: e.g., [OIII]λ4363 is blended with a night sky HgI While in previous sections we focused on the integrated prop- line that must also be included in the fit. When more than one erties of the nebula, in this section we describe the spatial dis- emission was fitted simultaneously, their FWHMs were forced tributionofthese propertiesalongthenebula.We use the 8182 tobeequal(sincetheFWHM isdominatedbythespectralres- S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula 7 mostoftheextinctionisassociatedwiththenebula.Poggeetal. (1992)proposedthatitmustariseintheneutralgasbetweenus andtheionizedzonesofthenebula.Therefore,itispossibleto assume a simple screen geometry for the dust extinction. This assumptionimpliesthatmostofthedeviationsoftheHα/Hβra- tio fromthe expectedones are due to dust. Poggeet al. (1992) estimated an average intrinsic ratio of Hα/Hβ=2.86throughout thenebula,assumingcaseBrecombinationforaelectronictem- perature of T=9000 K and a density of N ∼103 cm−3, to de- e rivetheirextinctionmap.Insteadofusingthissimplification,we areabletodetermineboththeelectrondensityandtemperature maps,andderivetheexpectedHα/HβlineratiomapforcaseB recombination. 4.2.2. Electrondensityandtemperature Asalreadyindicated,itispossibletodeterminetheelectronden- sityandtemperatureusingthe[SII]λ6716,6731doubletratioand the [NII]λ6548+83/[NII]λ5755ratio. Figure 3 shows the elec- tron density map, N (left panel) and the electron temperature e map (right panel) derived from the indicated line ratio maps, following the formulae by Osterbrock (1989), as indicated in Section4.1.Theestimateddensityrepresentsonlyalowerlimit Fig.5.Hαintensitymapcorrectedfordustextinctioninunitsof totherealoneinthehydrogenionizedzone,sincethe[SII]emis- 10−12 erg s−1 cm2 arcsec−2 from the uncorrected Hα intensity sion is also presentin partiallyionizedregions,especially near map(Fig.2),andcorrectingbythederiveddustextinctionmap theionizationfronts,wherethedensitycanchange. (Fig.4). To our knowledge this is the first time that both the elec- trondensityandtemperaturemapsareestimatedsimultaneously. Pogge et al. (1992) derived an electron density map using the olution),inordertodecreasethenumberoffreeparametersand [SII]λ6716,6731doubletratio but assumed an average temper- increasetheaccuracyofthedeblendingprocess(whenrequired). ature of T=9000 K. This approximation is valid, if we take A final interpolated map with a 1′′/pixel scale was derived for into account the reduced effect of the temperature in the den- eachemissionlinebyinterpolatingthediscretesampleofinten- sityestimationandthenarrowrangesofestimatedtemperatures sitiesobtainedforeachspectrum(locatedatacertainpositionin throughout the nebula (Fig. 3). This explain why, despite the thesky).Theinterpolationwasperformedusingtheroutinesin- differences in the procedure to derive both maps, a consider- cludedinE3D(Sa´nchez2004).Thedataatthelocationofbright able agreement in the shape of the substructures and the cov- starsinthefieldweremaskedpriortoanyinterpolation,inorder ered range of electron densities is found between both studies todecreasetheeffectsoftheircontamination. (N ∼100-15000 cm−3). This agreement is expected since the e Figure 2, left panel, shows the derived Hα flux map. The derivation of the electron density does not depend strongly on well known structures of this region of the Orion nebula are thetemperature.However,wefindaricherlevelofdetailinthe easily identified: the strong Hα emission area at the west of substructuresinourmapthanthatofPoggeetal.(1992).Since the Trapezium, the “Dark Bay”, a strong decrease in the Hα thefinalspatialresolutionsofbothdatasetsaresimilar,wemay intensity, at the east of the Trapezium, classically explained assumethatthesesubstructuresarereal,mostprobablyinduced by dust absorption, and the Orion “Bar”, a filamentary area of bythemildeffectofthetemperatureinthedensityderivation. strong Hα emission at the south-east of the Trapezium. Other The highest densities are found in two regions: (1) two fainter structures, like the small arc of M42-HH2 at the north- clumps nearby and towards the west of the Trapezium and (2) westoftheTrapeziumcanalsobediscerned.Thedifferentsub- asetofclumpsat∼60′′south-southwestoftheTrapezium,both structures seen in the reconstructed Hα intensity map can be regionswithanelectrondensityoftheorderofN ∼3×104cm−3. e compared individually with those in the published HST and ThefirstregionwasnotnoticedbyPoggeetal.(1992),sinceit ground-basedobservations,whichreinforcethereliabilityofour wastoo nearto theirmaskedareasdueto thecontaminationof adopted procedure and results. In particular, this figure can be thebrightstarsoftheTrapezium.Thesecondoneiscoincident compared with the one derived from FP observations (Fig. 1e, withtheobservedpeakintheradiocontinuumemission(Yusef- Poggeetal.1992), since it does notinclude any contamination Zadeh 1990), and the [CII] λ 158 µm IR structure detected by from [NII]. Despite the fact that there is an offset between the Stacey et al. (1992),as already noticed by Pogge et al. (1992). sampled areas of ∼1′ they agree perfectly well, indicating that Ourdensitymapisricherinsubstructures,andmostofthemare ourfinalresolutionmustbesimilartotheoneobtainedbythem coincidentwiththehigherresolutionstructuresinthisregion.It (i.e.,∼2′′). seems that this is a dense regionon the backside of the nebula Figure 2, right panel, shows the observed Hα/Hβ line ratio (Poggeetal.1992). map. As stated before, this ratio can be used to trace the dust AnenhancementofthedensityoccursintheOrionbarion- extinctionwhencomparedwiththeintrinsicone.Thereddening izationfront,from∼2000cm−3to∼4000cm−3,andthenitdrops ofstarsintheforegroundoftheOrionnebulaisknowntobelow againacrossthefrontfromnorth-westtosouth-east.Theregions (A ∼0.2mag;Breger,Gehrz&Hackwell1981),whilethered- ofhigherdensityandthedetectedsubstructuresarealsocoinci- V deningtowardsthe Trapeziumis much higher(A ∼ 1.0−1.4 dentwiththeareasofhigherradiocontinuumemissioninthebar V mag;Osterbrocketal.1992;Bohlin&Savage1981).Therefore (Yusef-Zadeh1990).Thisisconsistentwiththeclassicalpicture 8 S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula Fig.4. Left: Hα/Hβ line ratio map calculatedfor case B recombinationusing the electron density, N , and temperaturemapsde- e scribedabove(Fig.3).Right:Dustextinctionmap(A ),derivedcomparingthemeasuredHα/Hβlineratiomapwiththecalculated V forcaseBrecombination(leftpanel),andassumingtheCardelli,Clayton&Mathis(1989)extinctionlaw. Fig.6.Classicaldiagnosticlineratiomaps.Left:[NII]λ6583/Hαlineratiomap.Right:[OIII]λ5007/Hβlineratiomap. of ionization fronts described by Hester (1991). In addition to Estebanetal.1998;Estebanetal.2004).Mostofthemhavede- these large structures, there are a number of isolated substruc- terminedtheelectrondensityandtemperatureattheselocations turesthatcanbeeasilyassociatedwithknownHHandHH-like using the same line ratios (usually S++ and N++) and formulae objectsinthenebula(Poggeetal.1992),althoughitisoutofthe usedhere.Baldwinetal.(1991)obtainedlow-resolutionoptical scopeofthecurrentstudytodescribeoranalyzethemindetail. slit spectroscopy with the slit located at ∼30′′ west of Θ1 Ori C, in an east-west orientation, reaching ∼5′ west. In the range As we indicated before, several previous spectroscopic coveredbyourIFSdatatheyfoundadecayoftheelectronden- studies have attempted to determine the ionization conditions sity from ∼6310 cm−3 to ∼158 cm−3 and of the electron tem- in the Orion nebula by obtaining slit spectroscopy at differ- peraturefrom∼11000K to ∼8000K. At the same location we ent locations (e.g., Baldwinetal.1991; Osterbrocketal.1992; S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula 9 found similar values, with the electron density dropping from imumnearbytheTrapezium,towardsthesouth-east,andasec- ∼6166 cm−3 to ∼166 cm−3, and the temperature from ∼10000 ond one, less prominent,at ∼40′′ south-west. Most of the sub- K to ∼ 7000 K. Esteban et al. (1998) obtained high resolution structuresareclearlyassociatedwithfluctuationsintheelectron echellespectroscopyattwolocationsofthenebulea,oneat45′′ densityand/ortemperature(seeFig.3). northandanotherat20′′ southand10′′ westofΘ1 OriC.New Figure4,rightpanel,showstheextinctionmap(A )derived deeperdata ata similar resolutionwereobtainedbyEstebanet V usingtheobservedHα/HβlineratiomappresentedinFig.2and al.(2004)inthesecondlocation.Theyfoundanelectrondensity assumingtheestimatedintrinsicvalueofthislineratioforeach of4020cm−3and6410cm−3,andaelectrontemperatureof9850 positiondescribedabove(Fig.4,leftpanel).WeusedtheOrion Kand10710Kateachoftheselocations.UsingourIFSdatawe extinctionlawofCardelli,Clayton&Mathis(1989),witharatio foundan electrondensityof ∼3716cm−3 and∼7141cm−3 and of total to selective extinction of R =5.5 (Mathisetal.1981), atemperatureof∼9743Kand∼10217Katthesamelocations. V as in previoussections. Insome locationsthe observedline ra- Osterbrocketal.(1992)obtainedopticalandnear-infraredlow- tioislowerthantheestimatedone.Asweindicatedpreviously, resolution spectroscopy at ∼45′′ north of the Trapezium. They case B recombination is the best simple approximation to de- estimatedtheelectrondensityandtemperaturetobe∼4000cm−3 scribethephysicalconditionsinthisnebula,butitdoesnotcom- and∼9000K,similartothevaluesderivedusingourIFSdataat pletely describe the ionizationconditions.Indeed,the presence asimilarlocation. of dustmixedwith the emitting gasdecreasesthe realvalueof O’Dell et al. (2003)presenteda highresolutionmap of the theHα/Hβratiobyafewpercent.Thisdeviationismostproba- electron temperature based on the [OIII] line ratio described blythecauseoftheobserveddiscrepancybetweentheestimated abovederivedfromnarrow-bandimagingusingtheWFPCatthe intrinsic line ratio and the observedone in areas with low dust HST. Their narrow-bandimages have the characteristic WFPC extinction. field-of-view of ∼160′′×160′′, located to the south-west of Θ1 Ori C. Despite their very fine resolution, the presented image ThereforethedustextinctionmapshowninFig.4hastobe hasanotmuchbettersignal-to-noisethantheonederivedusing consideredasalowerlimittotherealone,sincetheintrinsicline ourIFSdata.Thereasonforthatcomesfromthestepsrequired ratiomaybeoverestimated.Thedescribedeffectismostproba- to derive the electron density from narrow-band imaging, and blynothomogeneous,whichpreventsusfromapplyingasingle howtheyaffectthesignal-to-noiseofthefinalderivation.They correctiontothefulldataset.Adetailedmodelingoftheioniza- used a differention to derive the electron temperature(Fig. 3). tionconditionswouldberequiredtoperformabetterestimation Howeverwe can comparethem since there is a simple relation ofthespatialdistributionofthedustextinction.Suchamodeling betweenbothtemperatures(e.g.Pilyuginetal. 2006).Theval- isoutofthescopeofthecurrentstudy. uespresentedbyO’Delletal.(2003)intheirFig.2atdifferent Asexpected,the“Dark-Bay”structureisassociatedwithan selected locations and the average temperature throughout the increase in the foregrounddust extinction,with two extensions field-of-view roughly agree with those obtained using the pre- towardsthenorthandthesouthoftheTrapezium.Anotherpeak sented IFS data at similar locations. The temperature distribu- of the extinction is located 2 arcmin south-west of Θ1 Ori C. tionisalsocoincident,howevertheirlowersignal-to-noiseratio Similar dust extinctiondistributionswere reportedby Poggeet preventusfromperformingamoredetailedcomparison.Theau- al.(1992)andO’Delletal.(2000).ThePoggeetal.(1992)ex- thorsclaimthatalthoughtheyfoundrealvariationsofthetem- tinction map was affected by ring-like reduction artifacts, typ- peratureacrossthe nebula,basedon astatistical analysis,there ical in Fabry-Perotdata, absent in the currently presented dust is no obvious relation to the position in the nebula. Based on extinction map. O’Dell et al. (2000) derived the extinction co- the temperature distribution shown in Fig. 3 we cannot agree efficient (C ) using two different methods: (i) by comparing withtheirinterpretationofthedata,sincewe foundanobvious Hβ high resolution radio continuum emission maps obtained with rich structure in the temperature distribution, which rises near the VLA and WFPC/HST Hβ narrow-band images and (ii) by theTrapeziumanddropsintheouterregionsofthefield-of-view deriving the Hα/Hβ line ratio using WFPC/HST narrow-band ofourdata. images. In both cases it was assumed that the intrinsic Hα/Hβ Thus, the electrondensitiesandtemperaturesderivedusing line ratio was constant across the nebula.Despite this different thepresentedIFSdataagreewiththosefoundusingclassicalslit approachtheirhighresolutionextinctionmapsaresimilartothe spectroscopyandnarrow-bandimagingatdifferentlocationsin onepresentedhere,althoughtheestimatedvaluesfortheextinc- thenebula. tionseemtobeslightlylowerfortheIFSdata. Figure 5 shows the extinction-corrected Hα flux map, af- 4.2.3. Dustextinction ter the dust extinction map shown in Fig. 4 has been ap- Figure 4, left panel, shows the expected distribution of the plied. As already noticed by Pogge et al. (1992), the appear- Hα/HβlineratiomapcalculatedforcaseBrecombinationusing ance of the nebula is not significantly modified by the struc- theelectrondensity,N ,andtemperaturemapsdescribedabove ture in the foreground dust extinction. The most remarkable e (Fig.3). We have employed the line ratios listed in Osterbrock structure unaffected by the dust extinction correction is the (1989)for a discrete set of electron densities and temperatures Orion bar. The decrease of the Hα emission south-east of this in order to create a grid whose interpolation allowed us to es- structure is not due to dust extinction, but rather a flatten- timate the line ratio for any possible set of these values. This ing of the surface of the nebula (Wen&O’Dell1995). The estimation is just an approximation and a more accurate one “Dark Bay” is less prominent after correcting for dust extinc- would require a complete modeling of the nebula. However, tion, but it is still present, indicating that it is not simply a it is clearly a much better approximation than any one used region of high foreground extinction but a region with an ab- in previous studies, where a single intrinsic Hα/Hβ line ratio sence of emission. The fact that this structure is also seen wasestimatedfortheentirenebula(e.g.,Osterbrocketal.1992; in radio continuum and CO maps (e.g., Peimbertetal.1988; Poggeetal.1992). Theestimated intrinsic line ratio rangesbe- vanderWerf&Goss1989;Thronsonetal.1986)stronglysup- tween∼2.8and∼3.3,showingarichstructure,withaclearmin- portsthisconclusion. 10 S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula Fig.7.Left:MapoftheionicabundanceofHe+/H+derivedfromtheHλ6678/Hαlineratiomapcorrectedfortheeffectsofextinction andcollisionalexcitation.Right:Oxygenabundancemapderiveddirectlyusingthemeasuredtemperaturevaluesandintensitiesof the[OII]and[OIII]lines. 4.2.4. Ionizationstructure (e.g.,Cantoetal.1980;Meaburn1986).Thus,itisnotpossible to decouple them from the resolution of our dataset, requiring Theionizationstructureofthenebulacanbeinprincipleinves- highspatialandspectralresolutionspectroscopy. tigated by exploring the line ratio maps for the usual diagnos- tic lines(e.g.,Baldwinetal.1981; Osterbrock1989).Figure 6 showsthelineratiomapsfor[NII]/Hα,leftpanel,and[OIII]/Hβ, 4.2.5. He+ Ionicabundanceandmetalicity rightpanel.As alreadynoticedby Poggeetal. (1992),the first oneistherichestinstructure3.TheOrionbaristhemostpromi- The Orion nebula has been used as an anchor point nentsubstructure,clearlyseenasadiagonalregionsouth-eastof for HII regions to determine the primordial helium abun- the Trapezium. Another two structures are also seen as an en- dance (Y ) and the slope of the relationship between the hance of the [NII]/Hα ratio, one a less prominent north-south p helium and heavy-element abundance enrichment (dY/dZ), bartotheeastoftheTrapezium,attheedgeofthe“DarkBay”, (e.g, Peimbert&Torres-Peimbert1976; Baldwinetal.1991). andanotheroneatthenorth-eastendofthefield-of-viewofour Howeverlittleefforthasbeenmadetoexplorepossibleinternal IFSdata.The[NII]emissionisoriginatedinthe singleionized effectsonbothquantities.Poggeetal.(1992)presentedastudy regions between the ionized and partially ionized zones in the ofthesystematicbehavioroftheheliumemissionwithrespectto nebula,tracingtherapidchangesinlocalionization,while[OIII] thehydrogentounderstandtheeffectsthatinfluencethederived isoriginatedinthefullyionizedzones(e.g.,Osterbrock1989), heliumabundance.Theypresentedthedistributionofthisabun- tracing the level of ionization in these zones. The [NII]/Hα as dance(He+/H+map)incomparisontotheionizationstructurein well as [OII]/Hβ increases in areas where the [OIII]/Hβdrops, thenebula. thusthemapsshowtheeffectivereciprocitybetweenbothratios. Poggeet al. (1992)noticedthatitis notpossibleto use the FollowingPoggeetal.(1992)wedeterminedtheionizedhe- classical diagnostic diagrams (e.g., [OIII]/Hβ vs. [NII]/Hα) to liumabundancebasedontheHeIλ6678/Hαlineratio.Inionized distinguishbetweenregionsdominatedbyphotoionization(HII- nebulaeitisexpectedthattheHe+ zoneisspatiallyincludedin regions like) and regions dominated by shocks. We have re- theH+ one,andthereforetherelativestrengthofHeItoHIwill peatedtheexperimentusingourowndata.Theyconfirmthatthe provideadirectestimateoftheionicabundanceHe+/H+(y+).To line ratios at any position in the nebula are located in the gen- performsucha conversionweusedtheeffectiverecombination eral locus of HII-regions.No major deviationsare found, even ratesofHummer&Store(1987)forHIandofSmits(1991)for in the location of well known HH objects, like M42-HH2 and HeI,recentlyupdatedbyBenjaminetal.(1999,2002).Figure7, M42-HH1,where shocksmay play an importantrole.The rea- leftpanel,showsthederivedHe+/H+ionicabundancemap,after sonisthattheseregionsareembeddedinahighsurfacebright- applyinga4′′×4′′medianfiltertoincreasethesignal-to-noisera- nessphotoionizationnebula.ItisknownthatmanyHH objects tio.AsalreadynoticedbyPoggeetal.(1992),thereisaconsider- differby ∼ 60 km s−1 fromthe systemic velocityof the nebula abledegreeofstructureintheheliumionicabundance,strongly correlatedwith the ionizationstructurein the nebula.In partic- 3 The[OIII]/HβmappresentedbyPoggeetal.(1992)hasanerrorof ular,the partially ionizedzonescoincidewith areaswith lower afactor10inthedisplayedscale,easytocross-checkwhencomparing He+/H+ abundance.Intheseregions,thefractionofneutralhe- withthevaluesreportedintheirTable2. liumcanbeimportant,makingitnecessarytocorrectthederive

ebook img

PPAK Integral Field Spectroscopy survey of the Orion Nebula: Data Release PDF

0.88 MB·English
by  S. F. Sanchez
#additional_collections #journals #arxiv
Save to my drive
Quick download
Download
Upgrade Premium
Most books are stored in the elastic cloud where traffic is expensive. For this reason, we have a limit on daily download.

Preview PPAK Integral Field Spectroscopy survey of the Orion Nebula: Data Release

Astronomy&Astrophysicsmanuscriptno.sanchez (cid:13)c ESO2008 February3,2008 PPAK Integral Field Spectroscopy survey of the Orion Nebula Data release S.F.Sa´nchez⋆1,N.Cardiel1,2,M.A.W.Verheijen3,D.Mart´ın-Gordo´n4,J.M.Vilchez4,andJ.Alves1 1 CentroAstrono´micoHispanoAlema´n,CalarAlto,CSIC-MPG,C/Jesu´sDurba´nRemo´n2-2,E-04004Almeria,Spain. 2 DepartamentodeAstrof´ısica,FacultaddeF´ısicas,UniversidadComplutensedeMadrid,28040Madrid,Spain. 7 3 KapteynAstronomicalInstitute,UniversityofGroningen,POBox800,9700AVGroningen,TheNetherlands. 0 4 InstitutodeAstrof´ısicadeAndalucia,CaminoBajodeHuetor,S/N,Spain. 0 ; 2 n ABSTRACT a J Aims.Wepresentalow-resolutionspectroscopicsurveyoftheOrionnebula.Thedataarereleasedforpublicuse.Weshowthepos- 8 sibleapplicationsofthisdatasetanalyzingsomeofthemainpropertiesofthenebula. Methods.Weperformanintegralfieldspectroscopy mosaicofanareaof∼5′×6′ centeredontheTrapeziumregionofthenebula, 3 includingtheionizationfronttothesouth-east.Analysisofthelinefluxesandlineratiosofboththeindividualandintegratedspectra v allowedustodeterminethemaincharacteristicsoftheionizationthroughtoutthenebula. 3 Results. Thefinaldatasetcomprises8182individualspectra,sampledinacircularareaof∼2.7′′ diameter.Thedatacanbedown- 6 loadedasasinglerow-stackedspectrafitsfileplusapositiontableorasaninterpolateddatacubewithafinalsamplingof1.5′′/pixel. 3 Theintegratedspectrumacrossthefield-of-viewwasusedtoobtainthemainintegratedpropertiesofthenebula,includingtheelec- 1 trondensityandtemperature,thedustextinction,theHαintegratedflux(aftercorrectingfordustreddening),andthemaindiagnostic 1 lineratios.Theindividual spectrawereusedtoobtainlineintensitymapsofthedifferentdetectedlines.Thesemapswereusedto 6 studythedistributionoftheionizedhydrogen,thedustextinction,theelectrondensityandtemperature,andtheheliumandoxygen 0 abundance. Allofthemshowaconsiderabledegreeofstructureasalreadyshowninpreviousstudies.Inparticular,thereisatight / relationbetweentheheliumandoxygenabundancesandtheionizationstructurethatcannotbeexplainedbycaseBrecombination h theory.Simpleargumentslikepartialionizationanddustmixedwiththeemittinggasmayexplaintheserelations.Howeveramore p detailedmodelingisrequired,forwhichweprovidethedataset. - o Keywords. ISM:individualobjects:M42–(ISM:)HIIregions–(ISM:)dust,extinction–ISM:abundances r t s a : 1. Introduction ratio). Such properties are frequently compared with those of v distant H II regionsin our Galaxy and extragalacticones (e.g., i TheOrionnebulaisthebrightestandbeststudiedHIIregionin X Pageletal.1992; Ferland2001). Due to their distance most thesky.Ithasbeenusedfordecadesasafundamentallaboratory of these regions are poorly resolved. However their integrated r in the study of the star formation regions, ionization processes a propertiesare comparedwith those ofparticularresolvedareas andheliumandheavyelementenrichment.However,despitethe intheOrionnebula. large number of studies on this target much remains unknown aboutit,evenfromopticalstudies.Themoreweknow,themore complexitseemstobe. Very little effort has been made to study the distribution of Several spectroscopic surveys have tried to characterize physical propertiesacross the nebula, except for some particu- its spectroscopic properties, taking spectra at different “repre- lar areas (e.g.Baldwinetal.1991), or their integrated values in sentative” locations (e.g., Kaler1976; Baldwinetal.1991; largeapertureareas.Poggeetal.(1992)performedFabry-Perot Estebanetal.1998; Estebanetal.2004). In particular, (FP)imagingspectrosphotometryonanareaof∼6′×6′centered Osterbrocketal. (1990)andOsterbrocketal. (1992)presented intheTrapezium.TheirFPdatacoveredthebrightestemission a compilation of high and low dispersion deep spectra of the lines in the nebula: Hβ, [OIII]λ5007, Hα, [NII] λ6548,6583, centralbrightregionin the optical-NIRregion(3000-11000Å). [SII]λ6716,6731and HeIλ6678. They derived some of the av- Theymeasured225emission lines, 88 atthe wavelengthrange eragespectroscopicparametersofthecoreofthenebula,includ- ofourIFSdata.Usingtherelativeintensitiesoftheselinesthey ing the integrated Hα flux, the average extinction, the average derived the relative abundances of several elements, the elec- ionization line ratios ([OIII]/Hβ, [NII]/Hα and [SII]/Hα), the tron temperature (T=9000 K) and density (N =4×103 cm−3), He+/H+ ionicabundanceratio,andtheaverageelectrondensity. e and the extinction (A ∼1.08 mag, derived from the Hα/Hβ Theyderivedthefirstrealiablemapsofthedistributionofthese V propertiesacrossthenebula,whichshowaconsiderabledegree ofstructure.Theyfoundsignificantdifferenceswithsomeofthe Sendoffprintrequeststo:S.F.Sa´nchez,[email protected] ⋆ BasedonobservationscollectedattheCentroAstronm´icoHispano previouslyreportedvaluesthatcorrespondtoparticularregions Alema´n (CAHA) at Calar Alto, operated jointly by the Max-Planck inthenebula(e.g.,Osterbrocketal.1992).However,duetothe Institut fu¨r Astronomie and the Instituto de Astrof´ısica de Andaluc´ıa reducedspectroscopiccoverageoftheirdatasetitwasnotpossi- (CSIC). bletofullycharacterizetheionizationconditionsinthenebula. 2 S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula Fig.1.IntegratedspectrumoftheOrionNebulaobtainedcoaddingthe8281individualspectraofourIFSdatasetoverthefullfield- of-view,plottedonalogarithmicscale.Theidentificationsofthedetectedemissionlines,aslistedinTable1,areindicatedattheir correspondingwavelength. Wepresentherealow-resolutionIntegralFieldSpectroscopy hexagon.Thesky-fibersaredistributedamongthescienceones (IFS) survey of a similar field-of-view of the FP observations inthepseudo-slit,inordertohaveagoodcharacterizationofthe presented by Pogge et al. (1992), but covering the entire opti- sky. The remaining15 fibers are used for calibrationpurposes, cal wavelength range (∼3700-7100 Å). The data are obtained, asdescribedbelow. reduced and released for the community.In this article we de- Thedataweretakenasabackupprogramofthe“DiskMass scribe the observationsand data reduction,indicating the main Project”(Verheijenetal.2004)underbadtransparencyandnon characteristicsofthedataset,andweshowafewexamplesofthe photometricconditions.A mosaic of 27 single exposuresof 2s applicationsofthisdataset.Toourknowledgethisisthelargest each was taken. The exposuretime was selected to avoid satu- areaeversampledwithanIFSunit,showingthecapabilitiesof ration of the brightest emission lines, like Hα and [OIII]. The new tecniques like 3D mosaicing to overcome the inherent re- initialexposurewascenteredintheTrapeziumareaoftheOrion ducedfield-of-viewoftheseinstruments. Nebula.Consecutivepointingsfollowedahexagonalpattern,ad- The layout of this article is as follow: In Section 2 we de- justing the mosaic pointings to the shape of the PPAK science scribetheobservationsanddatareduction.Section3summarizes bundle.Eachpointingcenterliesat60”fromthe previousone. thebasicpropertiesofthedataset.InSection4welistexamples DuetotheshapeofthePPAKbundleandbyconstructionofthe oftheapplicationsofthisdataset,includingtheintegratedprop- mosaic,11spectraofeachpointing,correspondingtooneedge ertiesofthenebula,thedescriptionofthelineintensitymapsof of the hexagon,overlapwith the same numberof spectra from some of the brightestemission lines, a distribution of the elec- thepreviouspointing.Thispatternwasselectedtomaximizethe tron density and temperature, a study of the dust extinction, a coveredarea,allowingenoughoverlappingto match thediffer- descriptionoftheionizationstructure,andastudyofthehelium entexposurestakenundervariableatmosphericconditions.The andheavyelementabundances.Section 5summarizesthe con- offsetprocedurewascarriedoutuntilclosingduetoriskofrain. clusionsofthearticle. Thecompletemosaicwascompletedin1.5hoursintotal,mostly duetothereadouttimeofthePMASCCDcamera(∼1.5minin thesetupused). 2. Observationsanddatareduction Data reduction was performed using R3D (Sa´nchez2006), in combination with IRAF packages (Tody1993)1 and E3D Observations were carried out on November 21st, 2004 at the (Sa´nchez2004). The reduction consists of the standard steps 3.5mtelescopeoftheCalar AltoobservatorywiththePotsdam forfiber-basedintegral-fieldspectroscopy.A masterbiasframe MultiAperture Spectrograph, PMAS, (Rothetal.2005) in the was created by averaging all the bias frames observed during PPAKmode(Verheijenetal.2004;Kelzetal.2006).TheV300 the nightand subtracted from the science frames. The location grating was used, covering a wavelength range between 3700- of the spectra in the CCD was determined using a continuum 7100 Å with a resolution of ∼10 Å FWHM, corresponding to illuminated exposure taken before the science exposures. Each ∼600kms−1,muchlargerthantheinternalvelocitieswithinthe spectrumwasextractedfromthescienceframesbycoaddingthe nebula.ThePPAKfiberbundleconsistsof382fibersof2.7”di- flux within an aperture of 5 pixels around this location along ametereach(seeFig.5inKelzetal.2006).Ofthem,331fibers the cross-dispersion axis for each pixel in the dispersion axis (thesciencefibers)areconcentratedin asinglehexagonalbun- dlecoveringafield-of-viewof72”×64”,withafillingfactorof 1 IRAF is distributed by the National Optical Astronomy ∼65%. The sky is sampled by 36 additional fibers, distributed Observatories, which are operated by the Association of Universities in 6 bundles of 6 fibers each, located following a circular dis- forResearchinAstronomy,Inc.,undercooperativeagreementwiththe tribution at ∼90” of the center and at the edges of the central NationalScienceFoundation. S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula 3 Table1.Listofdetectedemissionlines,uncorrectedforextinction. Wavelength Wavelength Flux1 LineId. Wavelength Wavelength Flux LineId. Lab.(Å) Obs.(Å) Lab.(Å) Obs.(Å) 3726.03∗ 3724.94 35219.70 [OII] 5006.84∗ 5006.99 143002.31 [OIII] 3728.82∗ 3730.08 40670.09 [OII] 5056.02 5055.80 35.09 SiII 3750.15 3751.09 676.63 H12 5056.35 5056.35 39.79 SiII 3770.63 3771.25 1574.32 H11 5158.81 5157.92 52.29 [FeII] 3797.90 3798.18 2550.31 H10 5197.90 5195.79 25.86 [NI] 3819.64 3819.54 876.94 HeI3819 5199.00 5199.49 76.77 SKY NI 3835.39 3835.15 3391.71 H9 5200.26 5201.02 142.51 [NI] 3857.53 3853.55 561.27 HeI 5270.40 5270.64 171.06 [FeIII] 3868.75 3868.59 8266.59 [NeIII] 5360.00 5359.94 6.06 SKY? 3888.65 3887.97 4416.27 HeI 5461.00 5461.07 215.29 SKY HgI 3889.05 3890.22 3957.53 H8 5517.71 5517.71 284.96 [ClIII] 3964.73 3967.69 3634.16 HeI 5537.88 5537.89 281.24 [ClIII] 3967.46 3968.09 642.14 [NeIII] 5577.00 5576.40 789.97 SKY OI 3970.07 3969.20 5478.41 Hǫ 5577.31 5578.10 791.73 [OI] 4009.27 4007.41 318.24 HeI 5685.00 5684.97 118.17 SKY NaI 4026.21 4026.00 722.02 HeI 5754.64∗ 5754.66 400.01 [NII] 4046.00 4046.02 32.09 SKY HgI 5770.00 5769.91 15.66 SKY? 4068.60 4068.39 458.02 [SII] 5790.00 5790.04 1.16 SKY? 4076.35 4074.88 159.72 [SII] 5875.62 5875.56 7538.90 HeI 4101.74 4101.56 12021.26 Hδ 5893.00 5892.61 348.91 SKY NaID 4120.86 4121.23 53.23 HeI 5930.00 5927.95 123.02 SKY? 4143.76 4148.23 23.02 HeI 5957.61 5957.11 41.97 SiII 4267.15 4265.12 85.48 CII 5958.58 5957.49 12.78 OI 4340.47 4340.32 23860.65 Hγ 5978.97 5978.80 81.35 SiII 4358.00 4364.15 113.00 SKY HgI 6000.00 6000.26 12.91 SKY? 4363.21∗ 4366.76 831.27 [OIII] 6046.40 6046.57 41.92 OI 4387.93 4379.43 125.13 HeI 6240.00 6239.95 60.97 SKY? 4414.91 4471.37 1937.47 OII 6265.00 6264.91 30.67 SKY? 4416.98 4418.47 62.93 OII 6300.00 6299.33 297.01 SKY OI 4471.50 4412.41 97.99 HeI 6300.30 6301.39 362.98 [OI] 4658.10 4658.22 379.59 [FeIII] 6312.10 6312.27 950.13 [SIII] 4701.62 4701.26 123.43 [FeIII] 6330.00 6329.99 46.28 SKY? 4713.20 4713.36 279.38 HeI 6347.09 6347.30 141.11 SiII 4733.93 4735.80 40.15 [FeIII] 6363.78 6363.72 208.24 [OI] 4754.83 4755.52 62.28 [FeIII] 6370.36 6370.53 107.90 SiII 4769.60 4775.10 62.48 [FeIII] 6548.03∗ 6547.63 10743.11 [NII] 4777.88 4783.04 29.99 [FeIII] 6562.82∗ 6562.87 171573.70 Hα 4815.55 4815.82 82.38 [FeII] 6583.41∗ 6583.52 32494.66 [NII] 4861.33∗ 4861.41 50768.69 Hβ 6678.15∗ 6678.37 1907.79 HeI6678 4921.93 4922.20 201.94 HeI 6716.39 6716.47∗ 2354.08 [SII] 4958.91∗ 4959.02 47337.07 [OIII] 6730.85∗ 6731.27 3447.13 [SII] 110−11Ergcm−2s−1 ∗Emissionlineusedinouranalysisofthedata. and stored in a row-staked-spectrum file RSS (Sa´nchez2004). mission. Then, we used multi-band photometric data available Wavelength calibration was performed using a He lamp ex- intheliteratureforthebrighteststarintheTrapezium,Θ1OriC posure obtained from archive data, and corrected for distor- (Lee1968; Vitrichenko2000) to perform an absolute flux cali- tionsusingThArexposuresobtainedsimultaneouslytothesci- brationbycomparingwiththeun-calibratedspectrumofthisob- enceexposuresthroughthecalibrationfibers(indicatedabove). jectextractedfromthecentralpointingofthemosaic.Anaddi- Differences in the fiber-to-fiber transmission throughput were tionalcorrectionwasperformedtotakeintoaccountfluxlosesin correctedbycomparingthe wavelength-calibratedRSS science theun-calibratedspectrumoftheobjectduetodifferentialatmo- frameswiththecorrespondingcontinuumilluminatedones.The sphericrefraction.Thiscorrectionwasdeterminedbycomparing natureofthe datasetpreventsusfromperforminganyskysub- theemissionlineintensitieswiththosederivedbyBaldwinetal. traction. However,due to the strength of the detected emission (1991) in the spatial region (O’Dell, private communications). lines and the short exposure time, sky contamination is not an Thisprocedureensuresafluxcalibrationandskyextinctioncor- importantissueformostoftheforeseeablesciencecases. rectionofthiscentralpointing. Sincethedataweretakenunderbadweatherconditions,no After reducing each individual pointing we built a single attempt was made to performa flux calibration on the basis of RSS file for the mosaic following an iterative procedure. The the standard stars observed during the night. On the contrary, spectraofeachpointingwasscaledtothoseofthepreviousone fluxcalibrationwasperformedfollowingatwo-stepprocedure. by the average ratio of the emission of Hβ in the overlapping First,wecarriedoutafluxcalibrationusingastandardstar ob- spectra. To address the possible variable extinction during the servedwithasimilarinstrumentalsetuponapreviousnight.This observations,theaverageratiobetweenthesescaledspectraand procedure corrects, to a first order, for the instrumental trans- the previous ones was determined. We fitted this wavelength- 4 S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula dependentratiotoaloworderpolynomialfunctionanddivided Table1liststhedetectedemissionlines,including,foreachline, all the spectra in the new pointing by the resulting wavelength thelaboratorywavelength(Osterbrocketal.1992),theobserved dependentscale.Inmostofthecasesthissecondcorrectionhas wavelength, the integrated measured flux and the line identi- littleeffectonthedata,indicatingthattheskyextinctiondidnot fication. The final effective resolution of our integrated spec- change too strongly during the observations, prehaps because trumwasFWHM∼10Å,measuredinthebrightestdetectedlines. they comprise only a limited amountof time (1.5h).The over- Therefore,anyemissionlineatdistanceslessthan∼3Åfromits lappingspectra were then replaced by the averagebetween the adjacentneighborsisblended,andthelistedintegratedfluxmust previous pointing and the new rescaled spectra. The resulting betakenwithcare. spectra were includedin the final RSS file, updatingthe corre- spondingpositiontable.Theprocessstartsin thecentralpoint- There is a considerable amount of blue continuum emis- ing, that is flux calibrated and sky extinction corrected as ex- sion in the integrated spectrum shown in Fig. 1, whose inten- plained before, ensuring an homogeneous flux calibration and sitylevelrangesfrom∼10−8 ergcom−2 s−1 Å−1 intheultravio- skyextinctioncorrectionforallthedataset.Theprocedurewas let to a few 10−9 erg com−2 s−1 Å−1 in the reddest wavelength repeateduntilthelastpointingwasincluded,endingwithafinal ranges.Thiscontinuumis partiallydueto the starssampledby setof8182individualspectraanditscorrespondingpositionta- ourIFSdata,inparticularthebrightmembersoftheTrapezium. ble.Inaddition,wecreatedadatacubewith1.5”/pixelsampling However,itiswellknown(e.g.,Poggeetal.1992)thattheOrion byinterpolatingthisRSSframeusingE3D(Sa´nchez2004). nebula presents a diffuse continuum emission, whose origin is associated with the light from the stars within the ionized gas 3. Results thatisscatteredbydust.Unfortunally,themeasuredcontinuum is strongly affected by the fact that we could not subtract the The final data set consists of 8182 flux calibrated spectra sam- skyemission.Thereforethisissuecannotbeaddressedpropertly plingacircularareaof∼2.7′′diametereachandcoveringacon- withthecurrentIFSdata. tiguous field-of-view of ∼5′×6′ centered approximately on the Trapeziumarea of the Orion nebula. Each spectrumcoversthe wavelength range between ∼3700-7100 Å, which includes the most prominent recombination and collisionally excited emis- 4.1.2. Averagedustextinction sionlines,from[OII]λ3727,3729to[SII]λ6716,6731(seeTable 1). Near the lower wavelength range, the [OII] flux was mea- The H I Balmer series are normally used to determine the suredwithoutprobleminall fibers.Thefaintestdetectedemis- extinction. One must compare the observed line ratios with sionlines,upto5σ,intheindividualspectrahavesurfacebright- ness of the order of 10−14 erg s−1 cm−2 arcsec−2. It is out of the expected theoretical values. These lines are known to be emitted as a consequence of recombination, and the the- thescopeofthisarticletoextractalltheinformationpotentially ory for them has been worked out in considerable detail available in this huge dataset. We consider here a few relevant (Hummer&Storey1987; Osterbrock1989). Case B (optically propertiesof the Orion nebula using these data. Howevermwe thickinalltheLymanlines)isthebestsimpleapproximationto arereleasingthefulldatasetinordertomakeiffreelyavailable tothecommunityontheprojectwebpage2. describetherealphysicalconditionsintheionizationofthegas in the Orion nebula (Osterbrock1988; Osterbrocketal.1992; Ferland2001). The main deviation of the real nebula from the 4. Applications theoryisprobablytheabsorptionofLyman-linephotonsbydust mixedwiththeemittinggas,whoseobservableeffectsareatthe 4.1.IntegratedpropertiesoftheNebula levelofafewpercentintheHα/Hβratio(Cota&Ferland1988; Baldwinetal.1991).Table2liststheobservedandexpectedra- In this section we describe some of the more interesting spec- tiosbetweentheHIBalmerserieslineslistedinTable1.They troscopic properties of the nebula derived by analyzing the in- show a goodagreementwithin the errors,particularlythe ratio tegratedspectrumoverthefullfield-of-viewofourIFSdataset. forthe5brightestemissionlines(whoseerrorsareestimatedto Contrary to previous studies that attempt to describe the aver- be lower than ∼10%), indicating the presence of little dust ex- age ionizationconditionsin the nebulabyanalyzingindividual spectratakenatdifferentlocations,we could,forthe firsttime, tinctiononaverage. describetherealaveragespectroscopicparameters. An average extinction of A =0.75 mag is found as- V suming an intrinsic value of Hα/Hβ=2.86, corresponding to 4.1.1. IntegratedspectrumoftheNebula case B recombinationfor T=9000 K (Hummer&Storey1987; Figure 1 shows the integrated spectrum of the Orion Nebula Poggeetal.1992; Osterbrocketal.1992), and using the Orion obtained by coadding the 8281 individual spectra of our IFS extinctionlawofCardelli,Clayton&Mathis(1989),witharatio dataset, plotted on a logarithmic scale. 82 emission lines are of total to selective extinction of RV =5.5 (Mathisetal.1981). detected in the integrated spectrum. They were identified us- This average extinction is smaller than the value reported by ing the list of 88 emission lines detected by Osterbrock et al. Poggeetal.(1992).However,adetailedcomparisonofthefield- (1992)using deep spectroscopy in the same wavelengthrange, of-view covered by their FP data and our IFS ones shows that incombinationwiththelistofknownskyemissionlinesatCalar their data are approximately centered ∼60′′ more towards the Alto (Sa´nchez, in prep.). The observed wavelength and inte- east, covering a larger area of the Orion “Dark Bay”, an area grated flux of each emission line were measured using FIT3D withdustextinctionsuptoAV∼2mag.Therearebetterlineratios (Sa´nchezetal.2006),removingthebackgroundcontinuumand to estimate the extinction(Osterbrocketal.1992), since, as we deblendingdifferentemissionlineswhenneeded(andpossible). notedbefore,theintrinsicHα/Hβratiointherealnebulaisafew percentsmallerthanthecaseBvalue(e.g,Baldwinetal.1991). 2 http://www.caha.es/sanchez/orion/ Wewilldiscussitseffectinfurthersections. S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula 5 Fig.2.Left:observedHαlinefluxmapinunitsof10−12 ergs−1 cm2 arcsec−2 obtainedbyfittingasingleGaussianfunctiontothe Hαemissionlineforeachsinglespectruminthedataset.Theintensitymapisthenreconstructedbyinterpolatingtherecoveredflux atthelocationofeachspectrum,asdescribedinthetext.Right:observedHα/Hβlineratiomap.Nocorrectionfordustextinction wasappliedtoanymaps. Table2.ObservedBalmerlineratios,uncorrectedforextinction, of N =2150 cm−3 assuming an average temperature of 9000 e comparedwiththetheoreticalvaluesobtainedfromOsterbrock K(Baldwinetal.1991).OurIFSspectroscopicdatadocoverat etal.(1992) leasttwooftheclassicalindicatorswithenoughsignal-to-noise, the[OIII]ratio: Lineratio Obs.Value Exp.Value I(λ4959)+I(λ5007) Hα/Hβ 3.380 2.86 =228.97 I(λ4363) Hγ/Hβ 0.470 0.486 Hδ/Hβ 0.237 0.259 andthe[NII]ratio: Hǫ/Hβ∗ 0.108 0.159 H8/Hβ∗∗ 0.078 0.105 H9/Hβ 0.067 0.0731 I(λ6548)+I(λ6583) =108.09 H10/Hβ 0.050 0.0530 I(λ5755) H11/Hβ 0.031 0.0398 H12/Hβ 0.013 0.0307 Bothratiosareremarkablysimilartothevaluesreportedby ∗Blendedwith[NeIII] Osterbrocketal.(1992),forthecenteroftheNebula,despitethe ∗∗BlendedwithHeI evidentvariableconditionsoftheionizationacrossthenebula. Theconversionbetweenbothratiosandthetemperature,T, dependsonN /T1/2(Osterbrock1989).Therefore,itispossible 4.1.3. Averageelectrondensityandtemperature e toderivethetemperatureusingbothratiosandtheratiobetween The electron density, N , in the S+ zone of the nebula can the[SII]doublet.Oncethetemperatureisobtained,itispossible e be estimated using the observed [SII]λ6716,6731 doublet ra- toderivetheelectrondensitytoo.Wefoundanelectronicdensity tio(Osterbrock1989).Theestimated[SII]λ6716/[SII]λ6731ra- ofNe =2104cm−3(2019cm−3)andatemperatureofT=9682K tio based on the integrated spectrum is 0.683, that can be (T=8922 K) for the [OIII] ([NII]) ratio. To derive the electron converted to an electron density, N . Although mild, this density from the [SII] lines ratio and the temperature derived e conversion shows a dependency with the electronic temper- fromthe[OIII]linesratios(T3),wetransformedthistemperature ature (∼ Ne/T1/2, Osterbrock1989). Many previous spectro- to T2 using the relation between both quantities presented by scopic studies sampling the Orion nebula at different loca- Pilyugin et al. (2006). Since the [OIII]λ4363 may be affected tions reported a range of temperatures between 8000-10000K byanunsubtractednightskyHgIemissionlineatλ=4358Åwe (e.g., Osterbrocketal.1992). Therefore, the measured ratio consider the estimation based on the [NII] ratio more reliable. could correspond to electron densities in a range between Althoughthelineswerenotcorrectedbyextinction,itseffecton 2000-4000 cm−3. Pogge et al. (1992) reported an average thederivedtemperatureisnegligible(evenforA >2mag).The V [SII]λ6716/[SII]λ6731 ratio of 0.680, similar to our measure- average oxygen abundance, 12+log(O/H) ∼ 8.6, derived from ment.However,sincetheirFPdatadidnotcoveranyoftheusual the fluxesof theintegratedspectrumquotedinTable 1 is close temperature indicators, they convertedit to an electron density tothesolarvalue. 6 S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula Fig.3.Left:Electrondensitymap,N incm−3,derivedfromthe[SII]λ6716/[SII]λ6731lineratio,representedinalogarithmscale. e Right:Electrontemperaturemapinunitsof103K,derivedfromthe([NII]λ6548+[NII]λ6583)/[NII]λ5755lineratio. Table3.IntegratedSpectroscopicpropertiesofthenebula. individual spectra as independentprobes of these properties at differentlocations. Parameter Value A 0.75(mag) 4.2.1. Lineintensitymaps V N 2019(cm−3) e The ionized gas in the Orion nebula exhibits a rather com- T∗ 8922(K) [O2III]λ5007/Hβ 2.820 plexstructure.Duetothat,ithasbeentargetedbynarrow-band [NII]λ6583/Hα 0.190 imaging using the HST and groundbased telescopes in order [SII]λ6716+31/Hα 0.034 to disentangle the structure of its ionized gas (e.g., Wen and ∗Electrontemperaturederivedfromdoubleionizedions,likeN++, O’Dell 1995; O’Dell&Yusef-Zadeh2000; O’Delletal.2003; usingtheformulaedescribedinOsterbrock(1989). Rubinetal.2003 and references therein). In many cases these narrow-bandimages catch more than one single line (e.g.,Hα, the [NII]λ6548,6583 doublet or the [SII]λ6716,6731 doublet), 4.1.4. Averageionizationconditions reducing its use to study the basic parameters of the ionized gas (as listed in Table 3). As we quoted before, Pogge et al. It is widely accepted that the ionization of the Orion (1992) presented FP observations that allowed them to disen- nebula is dominated by photoionization by the star Θ1 tangle some of the main diagnostic emission lines. Following Ori C. The derived values for the classical diagnostic themwehaveobtainedlineemissionmapsforthesameemission line ratios, [OIII]λ5007/Hβ=2.82, [NII]λ6583/Hα=0.19 and lines.Wehavealsoderivedlineemissionmapsfor[NII]λ5755, [SII]λ6716+31/Hα=0.034,are consistent with this picture, be- [OIII]λ4363 and [OII]λ3727,29, used as temperature, oxygen ing the typical values for H II regions ionized by OB stars abundance and ionization indicators in combination with the (Osterbrock1989). As already noticed by Pogge et al. (1992), lines quoted before. All the analyzed lines are listed in Table the integratedHα luminosity,7.6×1037 erg s−1 (extinctioncor- 1,labeledwithanasterisk. rected), and the electron density indicate too that Orion is a Toderivetheemissionlinemapseachspectruminthedataset ratherdense,low-luminosityHIIregion(Kennicuttetal.1989). wasfittedwithasingleGaussianfunctionperemissionlineplus Indeed, this Hα luminosity agrees with the nebula ioniza- aloworderpolynomialfunctiontodescribethecontinuumemis- tion dominated by a single O 5 star (Poggeetal.1992; sionagainusingFIT3D(Sa´nchezetal.2006).Insteadoffitting Hummer&Storey1987).Asummaryoftheaverageproperties theentirewavelengthrangeinarow,weextractedshorterwave- of the nebula derived from our integrated spectrum is listed in length ranges for each spectrum that sampled one or a few of Table3. theanalyzedemissionlines,inordertocharacterizethecontin- uumwiththemostsimplepolynomialfunction,andtosimplify thefittingprocedure.Inafew caseswe hadtoincludesomeof 4.2.SpatialdistributionofthepropertiesoftheNebula these lines: e.g., [OIII]λ4363 is blended with a night sky HgI While in previous sections we focused on the integrated prop- line that must also be included in the fit. When more than one erties of the nebula, in this section we describe the spatial dis- emission was fitted simultaneously, their FWHMs were forced tributionofthese propertiesalongthenebula.We use the 8182 tobeequal(sincetheFWHM isdominatedbythespectralres- S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula 7 mostoftheextinctionisassociatedwiththenebula.Poggeetal. (1992)proposedthatitmustariseintheneutralgasbetweenus andtheionizedzonesofthenebula.Therefore,itispossibleto assume a simple screen geometry for the dust extinction. This assumptionimpliesthatmostofthedeviationsoftheHα/Hβra- tio fromthe expectedones are due to dust. Poggeet al. (1992) estimated an average intrinsic ratio of Hα/Hβ=2.86throughout thenebula,assumingcaseBrecombinationforaelectronictem- perature of T=9000 K and a density of N ∼103 cm−3, to de- e rivetheirextinctionmap.Insteadofusingthissimplification,we areabletodetermineboththeelectrondensityandtemperature maps,andderivetheexpectedHα/HβlineratiomapforcaseB recombination. 4.2.2. Electrondensityandtemperature Asalreadyindicated,itispossibletodeterminetheelectronden- sityandtemperatureusingthe[SII]λ6716,6731doubletratioand the [NII]λ6548+83/[NII]λ5755ratio. Figure 3 shows the elec- tron density map, N (left panel) and the electron temperature e map (right panel) derived from the indicated line ratio maps, following the formulae by Osterbrock (1989), as indicated in Section4.1.Theestimateddensityrepresentsonlyalowerlimit Fig.5.Hαintensitymapcorrectedfordustextinctioninunitsof totherealoneinthehydrogenionizedzone,sincethe[SII]emis- 10−12 erg s−1 cm2 arcsec−2 from the uncorrected Hα intensity sion is also presentin partiallyionizedregions,especially near map(Fig.2),andcorrectingbythederiveddustextinctionmap theionizationfronts,wherethedensitycanchange. (Fig.4). To our knowledge this is the first time that both the elec- trondensityandtemperaturemapsareestimatedsimultaneously. Pogge et al. (1992) derived an electron density map using the olution),inordertodecreasethenumberoffreeparametersand [SII]λ6716,6731doubletratio but assumed an average temper- increasetheaccuracyofthedeblendingprocess(whenrequired). ature of T=9000 K. This approximation is valid, if we take A final interpolated map with a 1′′/pixel scale was derived for into account the reduced effect of the temperature in the den- eachemissionlinebyinterpolatingthediscretesampleofinten- sityestimationandthenarrowrangesofestimatedtemperatures sitiesobtainedforeachspectrum(locatedatacertainpositionin throughout the nebula (Fig. 3). This explain why, despite the thesky).Theinterpolationwasperformedusingtheroutinesin- differences in the procedure to derive both maps, a consider- cludedinE3D(Sa´nchez2004).Thedataatthelocationofbright able agreement in the shape of the substructures and the cov- starsinthefieldweremaskedpriortoanyinterpolation,inorder ered range of electron densities is found between both studies todecreasetheeffectsoftheircontamination. (N ∼100-15000 cm−3). This agreement is expected since the e Figure 2, left panel, shows the derived Hα flux map. The derivation of the electron density does not depend strongly on well known structures of this region of the Orion nebula are thetemperature.However,wefindaricherlevelofdetailinthe easily identified: the strong Hα emission area at the west of substructuresinourmapthanthatofPoggeetal.(1992).Since the Trapezium, the “Dark Bay”, a strong decrease in the Hα thefinalspatialresolutionsofbothdatasetsaresimilar,wemay intensity, at the east of the Trapezium, classically explained assumethatthesesubstructuresarereal,mostprobablyinduced by dust absorption, and the Orion “Bar”, a filamentary area of bythemildeffectofthetemperatureinthedensityderivation. strong Hα emission at the south-east of the Trapezium. Other The highest densities are found in two regions: (1) two fainter structures, like the small arc of M42-HH2 at the north- clumps nearby and towards the west of the Trapezium and (2) westoftheTrapeziumcanalsobediscerned.Thedifferentsub- asetofclumpsat∼60′′south-southwestoftheTrapezium,both structures seen in the reconstructed Hα intensity map can be regionswithanelectrondensityoftheorderofN ∼3×104cm−3. e compared individually with those in the published HST and ThefirstregionwasnotnoticedbyPoggeetal.(1992),sinceit ground-basedobservations,whichreinforcethereliabilityofour wastoo nearto theirmaskedareasdueto thecontaminationof adopted procedure and results. In particular, this figure can be thebrightstarsoftheTrapezium.Thesecondoneiscoincident compared with the one derived from FP observations (Fig. 1e, withtheobservedpeakintheradiocontinuumemission(Yusef- Poggeetal.1992), since it does notinclude any contamination Zadeh 1990), and the [CII] λ 158 µm IR structure detected by from [NII]. Despite the fact that there is an offset between the Stacey et al. (1992),as already noticed by Pogge et al. (1992). sampled areas of ∼1′ they agree perfectly well, indicating that Ourdensitymapisricherinsubstructures,andmostofthemare ourfinalresolutionmustbesimilartotheoneobtainedbythem coincidentwiththehigherresolutionstructuresinthisregion.It (i.e.,∼2′′). seems that this is a dense regionon the backside of the nebula Figure 2, right panel, shows the observed Hα/Hβ line ratio (Poggeetal.1992). map. As stated before, this ratio can be used to trace the dust AnenhancementofthedensityoccursintheOrionbarion- extinctionwhencomparedwiththeintrinsicone.Thereddening izationfront,from∼2000cm−3to∼4000cm−3,andthenitdrops ofstarsintheforegroundoftheOrionnebulaisknowntobelow againacrossthefrontfromnorth-westtosouth-east.Theregions (A ∼0.2mag;Breger,Gehrz&Hackwell1981),whilethered- ofhigherdensityandthedetectedsubstructuresarealsocoinci- V deningtowardsthe Trapeziumis much higher(A ∼ 1.0−1.4 dentwiththeareasofhigherradiocontinuumemissioninthebar V mag;Osterbrocketal.1992;Bohlin&Savage1981).Therefore (Yusef-Zadeh1990).Thisisconsistentwiththeclassicalpicture 8 S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula Fig.4. Left: Hα/Hβ line ratio map calculatedfor case B recombinationusing the electron density, N , and temperaturemapsde- e scribedabove(Fig.3).Right:Dustextinctionmap(A ),derivedcomparingthemeasuredHα/Hβlineratiomapwiththecalculated V forcaseBrecombination(leftpanel),andassumingtheCardelli,Clayton&Mathis(1989)extinctionlaw. Fig.6.Classicaldiagnosticlineratiomaps.Left:[NII]λ6583/Hαlineratiomap.Right:[OIII]λ5007/Hβlineratiomap. of ionization fronts described by Hester (1991). In addition to Estebanetal.1998;Estebanetal.2004).Mostofthemhavede- these large structures, there are a number of isolated substruc- terminedtheelectrondensityandtemperatureattheselocations turesthatcanbeeasilyassociatedwithknownHHandHH-like using the same line ratios (usually S++ and N++) and formulae objectsinthenebula(Poggeetal.1992),althoughitisoutofthe usedhere.Baldwinetal.(1991)obtainedlow-resolutionoptical scopeofthecurrentstudytodescribeoranalyzethemindetail. slit spectroscopy with the slit located at ∼30′′ west of Θ1 Ori C, in an east-west orientation, reaching ∼5′ west. In the range As we indicated before, several previous spectroscopic coveredbyourIFSdatatheyfoundadecayoftheelectronden- studies have attempted to determine the ionization conditions sity from ∼6310 cm−3 to ∼158 cm−3 and of the electron tem- in the Orion nebula by obtaining slit spectroscopy at differ- peraturefrom∼11000K to ∼8000K. At the same location we ent locations (e.g., Baldwinetal.1991; Osterbrocketal.1992; S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula 9 found similar values, with the electron density dropping from imumnearbytheTrapezium,towardsthesouth-east,andasec- ∼6166 cm−3 to ∼166 cm−3, and the temperature from ∼10000 ond one, less prominent,at ∼40′′ south-west. Most of the sub- K to ∼ 7000 K. Esteban et al. (1998) obtained high resolution structuresareclearlyassociatedwithfluctuationsintheelectron echellespectroscopyattwolocationsofthenebulea,oneat45′′ densityand/ortemperature(seeFig.3). northandanotherat20′′ southand10′′ westofΘ1 OriC.New Figure4,rightpanel,showstheextinctionmap(A )derived deeperdata ata similar resolutionwereobtainedbyEstebanet V usingtheobservedHα/HβlineratiomappresentedinFig.2and al.(2004)inthesecondlocation.Theyfoundanelectrondensity assumingtheestimatedintrinsicvalueofthislineratioforeach of4020cm−3and6410cm−3,andaelectrontemperatureof9850 positiondescribedabove(Fig.4,leftpanel).WeusedtheOrion Kand10710Kateachoftheselocations.UsingourIFSdatawe extinctionlawofCardelli,Clayton&Mathis(1989),witharatio foundan electrondensityof ∼3716cm−3 and∼7141cm−3 and of total to selective extinction of R =5.5 (Mathisetal.1981), atemperatureof∼9743Kand∼10217Katthesamelocations. V as in previoussections. Insome locationsthe observedline ra- Osterbrocketal.(1992)obtainedopticalandnear-infraredlow- tioislowerthantheestimatedone.Asweindicatedpreviously, resolution spectroscopy at ∼45′′ north of the Trapezium. They case B recombination is the best simple approximation to de- estimatedtheelectrondensityandtemperaturetobe∼4000cm−3 scribethephysicalconditionsinthisnebula,butitdoesnotcom- and∼9000K,similartothevaluesderivedusingourIFSdataat pletely describe the ionizationconditions.Indeed,the presence asimilarlocation. of dustmixedwith the emitting gasdecreasesthe realvalueof O’Dell et al. (2003)presenteda highresolutionmap of the theHα/Hβratiobyafewpercent.Thisdeviationismostproba- electron temperature based on the [OIII] line ratio described blythecauseoftheobserveddiscrepancybetweentheestimated abovederivedfromnarrow-bandimagingusingtheWFPCatthe intrinsic line ratio and the observedone in areas with low dust HST. Their narrow-bandimages have the characteristic WFPC extinction. field-of-view of ∼160′′×160′′, located to the south-west of Θ1 Ori C. Despite their very fine resolution, the presented image ThereforethedustextinctionmapshowninFig.4hastobe hasanotmuchbettersignal-to-noisethantheonederivedusing consideredasalowerlimittotherealone,sincetheintrinsicline ourIFSdata.Thereasonforthatcomesfromthestepsrequired ratiomaybeoverestimated.Thedescribedeffectismostproba- to derive the electron density from narrow-band imaging, and blynothomogeneous,whichpreventsusfromapplyingasingle howtheyaffectthesignal-to-noiseofthefinalderivation.They correctiontothefulldataset.Adetailedmodelingoftheioniza- used a differention to derive the electron temperature(Fig. 3). tionconditionswouldberequiredtoperformabetterestimation Howeverwe can comparethem since there is a simple relation ofthespatialdistributionofthedustextinction.Suchamodeling betweenbothtemperatures(e.g.Pilyuginetal. 2006).Theval- isoutofthescopeofthecurrentstudy. uespresentedbyO’Delletal.(2003)intheirFig.2atdifferent Asexpected,the“Dark-Bay”structureisassociatedwithan selected locations and the average temperature throughout the increase in the foregrounddust extinction,with two extensions field-of-view roughly agree with those obtained using the pre- towardsthenorthandthesouthoftheTrapezium.Anotherpeak sented IFS data at similar locations. The temperature distribu- of the extinction is located 2 arcmin south-west of Θ1 Ori C. tionisalsocoincident,howevertheirlowersignal-to-noiseratio Similar dust extinctiondistributionswere reportedby Poggeet preventusfromperformingamoredetailedcomparison.Theau- al.(1992)andO’Delletal.(2000).ThePoggeetal.(1992)ex- thorsclaimthatalthoughtheyfoundrealvariationsofthetem- tinction map was affected by ring-like reduction artifacts, typ- peratureacrossthe nebula,basedon astatistical analysis,there ical in Fabry-Perotdata, absent in the currently presented dust is no obvious relation to the position in the nebula. Based on extinction map. O’Dell et al. (2000) derived the extinction co- the temperature distribution shown in Fig. 3 we cannot agree efficient (C ) using two different methods: (i) by comparing withtheirinterpretationofthedata,sincewe foundanobvious Hβ high resolution radio continuum emission maps obtained with rich structure in the temperature distribution, which rises near the VLA and WFPC/HST Hβ narrow-band images and (ii) by theTrapeziumanddropsintheouterregionsofthefield-of-view deriving the Hα/Hβ line ratio using WFPC/HST narrow-band ofourdata. images. In both cases it was assumed that the intrinsic Hα/Hβ Thus, the electrondensitiesandtemperaturesderivedusing line ratio was constant across the nebula.Despite this different thepresentedIFSdataagreewiththosefoundusingclassicalslit approachtheirhighresolutionextinctionmapsaresimilartothe spectroscopyandnarrow-bandimagingatdifferentlocationsin onepresentedhere,althoughtheestimatedvaluesfortheextinc- thenebula. tionseemtobeslightlylowerfortheIFSdata. Figure 5 shows the extinction-corrected Hα flux map, af- 4.2.3. Dustextinction ter the dust extinction map shown in Fig. 4 has been ap- Figure 4, left panel, shows the expected distribution of the plied. As already noticed by Pogge et al. (1992), the appear- Hα/HβlineratiomapcalculatedforcaseBrecombinationusing ance of the nebula is not significantly modified by the struc- theelectrondensity,N ,andtemperaturemapsdescribedabove ture in the foreground dust extinction. The most remarkable e (Fig.3). We have employed the line ratios listed in Osterbrock structure unaffected by the dust extinction correction is the (1989)for a discrete set of electron densities and temperatures Orion bar. The decrease of the Hα emission south-east of this in order to create a grid whose interpolation allowed us to es- structure is not due to dust extinction, but rather a flatten- timate the line ratio for any possible set of these values. This ing of the surface of the nebula (Wen&O’Dell1995). The estimation is just an approximation and a more accurate one “Dark Bay” is less prominent after correcting for dust extinc- would require a complete modeling of the nebula. However, tion, but it is still present, indicating that it is not simply a it is clearly a much better approximation than any one used region of high foreground extinction but a region with an ab- in previous studies, where a single intrinsic Hα/Hβ line ratio sence of emission. The fact that this structure is also seen wasestimatedfortheentirenebula(e.g.,Osterbrocketal.1992; in radio continuum and CO maps (e.g., Peimbertetal.1988; Poggeetal.1992). Theestimated intrinsic line ratio rangesbe- vanderWerf&Goss1989;Thronsonetal.1986)stronglysup- tween∼2.8and∼3.3,showingarichstructure,withaclearmin- portsthisconclusion. 10 S.F.Sa´nchezetal.:PPAKIntegralFieldSpectroscopysurveyoftheOrionNebula Fig.7.Left:MapoftheionicabundanceofHe+/H+derivedfromtheHλ6678/Hαlineratiomapcorrectedfortheeffectsofextinction andcollisionalexcitation.Right:Oxygenabundancemapderiveddirectlyusingthemeasuredtemperaturevaluesandintensitiesof the[OII]and[OIII]lines. 4.2.4. Ionizationstructure (e.g.,Cantoetal.1980;Meaburn1986).Thus,itisnotpossible to decouple them from the resolution of our dataset, requiring Theionizationstructureofthenebulacanbeinprincipleinves- highspatialandspectralresolutionspectroscopy. tigated by exploring the line ratio maps for the usual diagnos- tic lines(e.g.,Baldwinetal.1981; Osterbrock1989).Figure 6 showsthelineratiomapsfor[NII]/Hα,leftpanel,and[OIII]/Hβ, 4.2.5. He+ Ionicabundanceandmetalicity rightpanel.As alreadynoticedby Poggeetal. (1992),the first oneistherichestinstructure3.TheOrionbaristhemostpromi- The Orion nebula has been used as an anchor point nentsubstructure,clearlyseenasadiagonalregionsouth-eastof for HII regions to determine the primordial helium abun- the Trapezium. Another two structures are also seen as an en- dance (Y ) and the slope of the relationship between the hance of the [NII]/Hα ratio, one a less prominent north-south p helium and heavy-element abundance enrichment (dY/dZ), bartotheeastoftheTrapezium,attheedgeofthe“DarkBay”, (e.g, Peimbert&Torres-Peimbert1976; Baldwinetal.1991). andanotheroneatthenorth-eastendofthefield-of-viewofour Howeverlittleefforthasbeenmadetoexplorepossibleinternal IFSdata.The[NII]emissionisoriginatedinthe singleionized effectsonbothquantities.Poggeetal.(1992)presentedastudy regions between the ionized and partially ionized zones in the ofthesystematicbehavioroftheheliumemissionwithrespectto nebula,tracingtherapidchangesinlocalionization,while[OIII] thehydrogentounderstandtheeffectsthatinfluencethederived isoriginatedinthefullyionizedzones(e.g.,Osterbrock1989), heliumabundance.Theypresentedthedistributionofthisabun- tracing the level of ionization in these zones. The [NII]/Hα as dance(He+/H+map)incomparisontotheionizationstructurein well as [OII]/Hβ increases in areas where the [OIII]/Hβdrops, thenebula. thusthemapsshowtheeffectivereciprocitybetweenbothratios. Poggeet al. (1992)noticedthatitis notpossibleto use the FollowingPoggeetal.(1992)wedeterminedtheionizedhe- classical diagnostic diagrams (e.g., [OIII]/Hβ vs. [NII]/Hα) to liumabundancebasedontheHeIλ6678/Hαlineratio.Inionized distinguishbetweenregionsdominatedbyphotoionization(HII- nebulaeitisexpectedthattheHe+ zoneisspatiallyincludedin regions like) and regions dominated by shocks. We have re- theH+ one,andthereforetherelativestrengthofHeItoHIwill peatedtheexperimentusingourowndata.Theyconfirmthatthe provideadirectestimateoftheionicabundanceHe+/H+(y+).To line ratios at any position in the nebula are located in the gen- performsucha conversionweusedtheeffectiverecombination eral locus of HII-regions.No major deviationsare found, even ratesofHummer&Store(1987)forHIandofSmits(1991)for in the location of well known HH objects, like M42-HH2 and HeI,recentlyupdatedbyBenjaminetal.(1999,2002).Figure7, M42-HH1,where shocksmay play an importantrole.The rea- leftpanel,showsthederivedHe+/H+ionicabundancemap,after sonisthattheseregionsareembeddedinahighsurfacebright- applyinga4′′×4′′medianfiltertoincreasethesignal-to-noisera- nessphotoionizationnebula.ItisknownthatmanyHH objects tio.AsalreadynoticedbyPoggeetal.(1992),thereisaconsider- differby ∼ 60 km s−1 fromthe systemic velocityof the nebula abledegreeofstructureintheheliumionicabundance,strongly correlatedwith the ionizationstructurein the nebula.In partic- 3 The[OIII]/HβmappresentedbyPoggeetal.(1992)hasanerrorof ular,the partially ionizedzonescoincidewith areaswith lower afactor10inthedisplayedscale,easytocross-checkwhencomparing He+/H+ abundance.Intheseregions,thefractionofneutralhe- withthevaluesreportedintheirTable2. liumcanbeimportant,makingitnecessarytocorrectthederive

See more

The list of books you might like

Upgrade Premium
Most books are stored in the elastic cloud where traffic is expensive. For this reason, we have a limit on daily download.
DMCA & Copyright: Dear all, most of the website is community built, users are uploading hundred of books everyday, which makes really hard for us to identify copyrighted material, please contact us if you want any material removed.
Copyright @ PDFdrive.to, 2022 | We love our users