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Astronomy&Astrophysicsmanuscriptno.18544 c ESO2013 (cid:13) January10,2013 δ The X-ray spectrum of Orionis observed by LETGS aboard Chandra A.J.J.Raassen1,2 andA.M.T.Pollock3 1 SRONNetherlandsInstituteforSpaceResearch,Sorbonnelaan2,3584CAUtrecht,TheNetherlands 2 AstronomicalInstitute”AntonPannekoek”,SciencePark904,1098XHAmsterdam,UniversityofAmsterdam,TheNetherlands 3 EuropeanSpaceAgencyXMM-NewtonScienceOperationsCentre,EuropeanSpaceAstronomyCentre,Apartado78,Villanueva delaCanãda,28691Madrid,Spain 26November2012 3 1 ABSTRACT 0 2 Aims.Weanalyzethehigh-resolutionX-rayspectrumofthesupergiantO-starδOrionis(O9.5II)withlineratiosofHe-likeionsand n athermalplasmamodel,andweexamineitsvariability. a Methods.TheO-supergiantδOriwasobservedinthewavelengthrange5–175ÅbytheX-raydetectorHRC-Sincombinationwith J thegratingLETGaboardChandra.WestudiedtheHe-likeionsincombinationwiththeUV-radiationfieldtodeterminelocalplasma 9 temperaturesandtoestablishthedistanceoftheX-rayemittingionstothestellarsurface.Wemeasuredindividuallinesbymeansof Gaussianprofiles,foldedthroughtheresponsematrix,toobtainwavelengths,linefluxes,halfwidthsathalfmaximum(HWHM)and ] lineshiftstocharacterizetheplasma.Weconsidermultitemperaturemodelsincollisionalionizationequilibrium(CIE)todetermine R temperatures,emissionmeasures,andabundances. S Results. AnalysisoftheHe-liketripletsextendedtoNviandCvimpliesionizationstratificationwiththehottestplasmatobefound withinafewstellarradii3R (Mgxi)andthecoolestfartherout,farbeyondtheaccelerationzone,upto49R (Nvi)and75R (Cv). . h Theobservedtemperaturesc∗overarangefromabout0.1to0.7keV,i.e.,1–8MK.TheX-rayluminosity(L )∗is 1.5 1032e∗rg/sin p therangefrom0.07to3keVcoveredbyLETGS.Velocitywidthsofabout1040kms 1havebeendeterminexd. ∼ × − - o Keywords.techniques:spectroscopic–stars:individual:δOri–stars:stellarwindstars:early-type–missions:Chandra r t s a 1. Introduction shouldprovidenewconstraintsontheory,it hasbeenproposed [ torecordthespectrumofδOri,thesoftestoftheX-raybrightest 1 Since the launch of the first X-ray observatories, it has been O-starsobservedwith ROSAT.ThispaperdiscussestheLETGS v knownthatO-typestarsandearlyB-typestarsareX-raysources. dataobtainedinthecontextofindividuallinediagnosticstoob- 7 Harndenetal.(1979),Sewardetal.(1979),andCassinellietal. tainplasmacharacteristics,aswellasoverallthermalmodels. 4 (1981)describedthefirstobservationswithEinstein.Theinves- TheO-star δ Ori(HD 36486)is the western starin the belt 8 tigationswerecontinuedwithASCA(Corcoranetal.1994)and of Orion, named Mintaka, with a parallax of 4.71(0.58) mas, 1 ROSAT(Haberl&White1993,Bergho¨feretal.1996).Although . i.e., at a distance of 212 pc (SIMBAD: Van Leeuwen 2007) 1 the origin of the emission was initially a matter of debate, it 1. The position in J2000 coordinates is RA=053200.400 and 0 is now generallybelievedthatinstabilities and clumpingin the Dec= 001756.74.ItisabrightX-raysourceandsufferslower 3 strongline-drivenstellarwindisthemechanismthatheatssmall − interstellarabsorptionthanζOri.Itispartofatriplestarsystem, 1 parts of the wind (e.g., Lucy & Solomon 1970;Lucy & White containingofδOriA,δOriB,andδOriC.Herewefocusonthe : v 1980;Lucy1982;Owockietal.1988;Dessart&Owocki2005). “single star” δ OriA, which in itself is also a triple star system i ThelaunchesofChandraandXMM-Newtonin1999withhigh- (Aa1,Aa2,andAb).Thetertiarystar(Ab)istoofarawaytoplay X resolution instruments aboard have offered the possibility to aroleinourobservation.Theprimary(Aa1),asupergiant,isof r study the X-ray spectra in more detail, resulting in, e.g., esti- a spectral type O9.5II, with a temperature of 33000K. The sec- mates of line ratios, line profiles, temperatures, emission mea- ondary(Aa2)is of spectraltype B0.5III,with a temperature of sures,andabundances(Waldron&Cassinelli2001,2007;Cohen 27000K(Voelsetal.1989andTarasovetal.1995).Theyforma et al. 2006; Kahn et al. 2001; Raassen et al. 2008). Generally, binarysystemwithaperiodof5.7325days(Harveyetal.1987). theoverallspectrahavebeendescribedbymeansofcollisional Different stellar mass values are given in the literature. Harvin ionization equilibrium (CIE) models with several temperature etal.(2002)giveM =10.3M asaveragewithR =11R and p p components,oftencomplementedbyanalysisofindividualline forthesecondarystar M = 5.2⊙M andR = 4R ,whileM⊙ayer s s profiles,linefluxes,andlinefluxratiosofHe-likeions.Basedon etal.(2010)suggestvaluesofM ⊙=25M withR⊙ =16 17R . p s RGSobservationsof ζ Ori,Pollock(2007)suggestsan alterna- Theprimaryandsecondarystarsaresepa⊙ratedby33R ,−equiva⊙- tivemechanismduetothenon-equilibriumion-ioninteractions lentto3R .ThepropertiesofδOri,whichwehaveap⊙plied,are p faroutinthewind.Sinceextendingthehigh-resolutioncoverage giveninTable1. to the longer wavelengths available with the Chandra LETGS The spectrum of δ Ori was studied in X-rays with earlier satellitesbyCassinelli&Swank(1983),Haberl&White(1993), Sendoffprintrequeststo:A.J.J.Raassen, e-mail:[email protected] 1 http://simbad.u-strasbg.fr/simbad/sim-fid 1 A.J.J.Raassen&A.M.T.Pollock:TheLETGSX-rayspectrumofδOrionis Table1.AdoptedpropertiesofδOri. Property δOri DistancefromEarth(pc) 212a Spectraltype O9.5IIb Radius(R ) 11b Mass(M ⊙) 10.3b T (K)⊙ 33000c eff Terminalwindvelocity(kms 1) 2000d − M˙(M yr 1) 1.07 10 6d − − ⊙ × Notes: a:VanLeeuwen(2007) b:Milleretal.(2002),takenfromHarvinetal.(2002). c:Milleretal.(2002),takenfromVoelsetal.(1989). d:Milleretal.(2002),takenfromLamers&Leitherer(1993). andCorcoranetal.(1994)andmostrecentlyathighresolution Fig.1. The zeroth-order lightcurve of δ Ori, observed with byWaldron&Cassinelli(2007)andMilleretal.(2002)withthe LETGS,andrebinnedtotimebinsof15700seconds.Thecurve ChandraHETGS.Theyfocusedonlineprofilesandlinewidths, indicatesa minimumat the beginningof the observationand a togetherwith ratios of forbiddenand intercombinationlines in flatteningoftheincreaseattheendoftheobservation. He-likeions. The set up of this paper is as follows. The emphasis of the paper is on determining the location of the X-ray emitting countsinthiszeroth-orderimageis26350counts.Noperiodicity plasmaanditstemperatureinrelationtothestellarsurfaceand couldbeestablishedinthelightcurve,asthisobservationistoo theUV-radiationfield.Thereforeweneedindividuallinefluxes. short to cover several stellar revolutions. However, after rebin- In Sect. 2 the observationand the lightcurveare discussed. We ningtotimebinsofabout16ksthelightcurveshowssomevari- showindividuallinefluxes,linewavelengths,andlinebroaden- ation in count rate from 0.264(0.004)to 0.278(0.004)counts/s ings in Sect. 3. Based on the individual line fluxes of He-like during this observation. This is a change of about 5%. During ions, the location relative to the stellar surface, as well as the the observation the phase of the binary system was 0.83-0.02, temperatureoftheplasma,isderived.TheyaregiveninSect.4. derivedfromHarvinetal.(2002),or0.87-0.07,asderivedfrom We discuss a multitemperature model fit to the total spectrum Mayeret al. (2010).This implies thatthe secondaryis in front with a CIE-model in Sect. 5. As suggested by the light curve, oftheprimaryduringtheobservation.Inlatersectionsweinves- wealsomakeatimesplit,dividingthetotalobservationof98ks tigateapossiblerelationbetweenthebehaviorofthelightcurve intothefirstpartof49ksandthesecondpartof49ks. andtheobservedspectrumbycuttingtheobservationintoafirst partfrom0to49ksandasecondpartfrom49to98ks. 2. Observationandlightcurve 3. Individuallinefluxes 2.1.Dataandpipelining TheindividuallinefluxesoftheLETGSspectrumofδOriwere The X-ray spectrum of δ Ori was obtained by means of HRC- measured.ForeachobservedlineaGaussianprofilewasfolded S in combination with the Low-Energy-Transmission-Grating through the response matrix and fit to the observed line pro- (LETG) on board Chandra on November9, 2007 during97 ks file, establishing the line positions, line fluxes, as well as the (JD=54413.92705-54415.05066).The log of the observationis linewidths. A powerlaw was added to describe the continuum showninTable2. near the line features. The line profiles are symmetric. This is The data were pipelined with CIAO3.4 with CALDB ver- showninFig.2fortheOVIIIandCVIlines.Thisagreeswith sion 3.4.1and preparedforSPEX by the programs extract and observations of δ Ori by Miller et al. (2002), when applying creledevelopedatSRON.Thespectrumderivedfromthisobser- HETGS. Seeming asymmetriesin the data are due to asymme- vationisthesumofthe+1and-1order.Thebackground,which triesintheinstrumentallinespreadfunction(LSF).Italsoinflu- isquitedominantinthehigherwavelengthareaof LETGspec- encestheobservedwidthsofthelines.Thesituationinζ Oriis tra,hasbeensubtracted.Thespectrumcontains31500countsin different.Inthatstarthelinesareasymmetricandbroadenedby thespectralrangefrom5to175Å. themselvesasshownbyPollock(2007). The measured wavelengths (Å) and line fluxes (in units 10 4 photons cm 2s 1 and in units 10 13 ergs cm 2s 1) are 2.2.Zeroth-orderlightcurve − − − − − − collected in Table 3, together with the theoretical wavelengths Inotherwavelengthranges(opticalandIUE)fluctuationsdueto from laboratory measurements collected by Kelly (1987) and eclipsing have been observed in the lightcurve of δ Ori. These the NIST4 database 2. The stronger lines have a low value for changesareabout.12mfortheprimaryeclipseand.07mforthe the(1σ)uncertainty.TheiruncertaintiesareafewmÅ. secondaryeclipse(Harvinetal.2002).Thiscorrespondstoade- There are very few line features above 50 Å, and they creaseofabout11%duringtheprimaryeclipseandofabout6% are often dueto higher-orderlines, especially third-orderlines, during the secondary eclipse. Based on the zeroth-orderimage which have their strong first-order componentbetween 10 and a lightcurve has been constructed. Figure 1 shows the zeroth- orderlightcurveofthisobservationofδOri.Thetotalnumberof 2 http://www.nist.gov/pml/data/asd.cfm 2 A.J.J.Raassen&A.M.T.Pollock:TheLETGSX-rayspectrumofδOrionis Table2.ObservationlogofthedataofδOri. ObsID Instrument Grating StartDate EndDate Duration(ks) Wavelengthrangea 7416 HRC-S LETG 2007-11-09T10:14:57 2007-11-10T13:12:57 97.08 5-175Å JD54413.92705 JD54415.05066 Notes: a:Rangealsousedduringthefittingprocedure. iconandiron,andtheiroptimalionizationtemperatureisaround 0.1keV. Some stronger lines are collected in Table 5 and shown in Fig.3.Thistableshowsthelineshiftsandthelinebroadenings. Thelineshiftsshowvelocitiesbetween-160and+160kms 1. − The earth’s orbitalvelocity in the direction of δ Ori duringthe observation and the velocity of δ Ori itself (Wilson 1963) are both about 16 km s 1, but they have opposite signs and are − canceling. The obtained velocities are comparable with those givenbyMilleretal.(2002),basedonobservationsbyHETGS. The HWHMs of the strong lines result in a velocity of about 840 km s 1, below the terminal velocity of 2000 km s 1. Our − − value is higher than the values given by Miller et al. (2002), based on HETGS. This might be due to an inconsistency between the instrumentalLSFs of LETGSandHETGS. However, our value agrees with Wojdowski & Schulz (2005), who also usedHETGS.Theultimatevelocityattheendofthewing,cor- rected for the instrumental width are also given. These values are about1850km s 1 with an uncertaintyof 200km s 1. The − − linesin the separatedspectraobtainedfromthe two time inter- vals (0-49 ks and 49-98 ks) were measured, and no significant differences between the line measurements of the two time in- tervalshavebeenestablished. 4. He-likelineratios TheindividuallinefluxesofthelinetripletsintheHe-likeions, consisting of a resonance line (r), an intercombinationline (i), and a forbidden line (f), are tools for density and temperature diagnostics(GabrielandJordan 1969),becausethe upperlevel oftheforbiddenline (1s2s3S ) isdepopulatedbycollisionsin 1 favoroftheupperleveloftheintercombinationline(1s2p3P ). 1 Ontheotherhand,thesameeffectofdepopulationcanbecaused byastrongUVradiationfield(Blumenthaletal.1972).Thelat- ter gives the size of the UV field flux and therewith an indica- Fig.2.LineshapesoftheOviiilineat18.97ÅandtheCviline tionofthedistancetothestellarsurfacewheretheX-rayemit- at 33.74 Å in the spectrum of δ Ori. Data are in red/gray, the tingionsareformed.Whileforlatetypestarsthecollisionsand fittedGaussianmodellinefoldedthroughtheresponsematrixis therewith the electron density are the dominant processes, for giveninblack.Therestwavelengthisalsoshown.Keepinmind hot, early type stars the strong UV radiation field (the location thatthewidthisalsoinfluencedbytheinstrumentallinespread relativetothestellarsurface)ismostimportant.Thishasbecome function. awell-provendiagnostictoolforestablishingtheradiallocation of the X-ray source in the stellar wind of hot stars (Miller et al.2002;Leuteneggeretal.2006;Waldron&Cassinelli2007). The appropriatefeaturesin the He-like ions of Cv, Nvi, Ovii, 35 Å. The second-order lines of Fexvii at 16.777, 17.054, Neix,Mgxi,andSixiiilieintherangefrom5to45Å,whichis and 17.097 Å contribute a bit to the wings of the Cvi line at coveredbyLETGS. 33.736 Å. The third-order Nex resonance line at 12.134 Å is Thelinefluxesofthesespecialcaseshavebeenmeasureda blendedbytheSxiitransitionat36.500Å,andthesecond-order slightlydifferentwayfromtheotherlinesgiveninTable3.After OviiiresonancelinecontributestothewingoftheSxiandSixiii the final overall multitemperature CIE-model fit (see Sect. 5), featureat37.787Å.Somefreeunblendedhigher-orderlines are limitedwavelengthrangeswereselectedjustaroundtherelevant giveninTable4.Thehigherorderlinesareimplementedinthe line features of the He-like ions. Then the lines from the ions model fits of Sects. 5.1 and 5.2. First-order lines in the wave- under investigation were ignored from the model. In this way lengthrangeabove50Åbelongtolowerionizationstagesofsil- thecontinuumisbasedonthemultitemperaturefitandpossible 3 A.J.J.Raassen&A.M.T.Pollock:TheLETGSX-rayspectrumofδOrionis Table3.LinepositionsandlinefluxesinδOri. ion λa λb fluxc fluxd 0 obs [Å] [Å] ion λa λb fluxc fluxd Cvi 33.736 33.728(.008) 4.88(.39) 2.88 [Å0] [Åob]s Caxi 35.212 35.175(.078) 0.53(.18) 0.30 Sxv 5.038+.0664.977(.103)f 0.27(.15) 1.07 Caxi 35.576 35.580(.054) 0.62(.18) 0.35 Sixiv 6.182 6.168(.029) 0.05(.03) 0.14 Sxii 36.563+.398 36.500(.036) 0.74(.20) 0.40 Sixiii 6.648 6.635(.007) 0.14(.03) 0.43 Sixiii+Sxi 37.786+.773 37.787(.040) 0.83(.23) 0.43 Sixiii 6.688+.704 6.698(.011) 0.08(.03) 0.24 Sxi 39.240+.323 39.234(.036) 1.43(.29) 0.72 Sixiii 6.740 6.751(—) 0.01(.01) 0.03 Cv 40.268 40.240(.020) 2.74(.74) 1.35 Mgxii 8.422 8.423(.009) 0.09(.03) 0.21 Cv 40.729 40.730e 1.96(.53) 0.97 Mgxi 9.168 9.165(.008) 0.18(.04) 0.39 Cv 41.472 41.474(.024) 0.66(.32) 0.32 Mgxi 9.231 9.229(.010) 0.12(.04) 0.25 Mgx+Sixii 44.050+.020 44.031(.022) 0.85(.13) 0.39 Mgxi 9.314 9.315(.011) 0.08(.03) 0.17 Siix+xii 44.249+.165 44.265(.017) 0.92(.13) 0.41 Nex 9.710 9.716(.019) 0.08(.04) 0.17 Sixii 45.520+.680 45.533(.033) 0.20(.09) 0.09 Nex 10.24010.238(.012) 0.08(.03) 0.15 Sixi 46.300+.410 46.308(.062) 0.42(.11) 0.18 Fexix 10.64010.647(.008) 0.11(.04) 0.21 Mgx+Six 47.280+.500 47.410(.050) 0.58(.12) 0.24 Fexvii 11.13011.136(.019) 0.05(.03) 0.09 Arix+Sixi 49.180+.220 49.165(.025) 1.10(.16) 0.44 Fexvii 11.25111.274(.016) 0.09(.04) 0.16 Six 50.254+.305 50.284(.015) 1.07(.21) 0.42 Fexviii 11.53011.552(.009) 0.16(.04) 0.28 Six 50.524+.691 50.545(.031) 1.21(.20) 0.48 Nex 12.13412.131(.003) 1.00(.08) 1.66 Six 52.1784+.248 52.232(.038) 0.72(.22) 0.28 Fexxi 12.28612.271(.008) 0.28(.05) 0.45 Fexv+Siix 52.911+.834 52.838(.044) 1.04(.24) 0.39 Nixix 12.43012.437(.015) 0.08(.04) 0.13 Fexvi 54.710 54.700(.045) 0.72(.19) 0.26 Nixix 12.65412.662(.022) 0.07(.04) 0.11 Siix 55.356+.272 55.340(.028) 1.50(.22) 0.54 Fexx 12.818+.84712.845(.009) 0.25(.05) 0.39 Fexv 59.404 59.350(.103) 0.38(.15) 0.13 Fexx 12.946+.97812.973(.014) 0.12(.04) 0.18 ??? — 60.003(.078) 0.40(.15) 0.13 Siix+xiii 61.050-.8461.416(.087)f 1.90(.14) 0.61 ??? —13.257(.016) 0.10(.04) 0.16 Neix 13.44713.450(.006) 1.45(.15) 2.15 Siix+xii 68.148+.548 68.328(.081) 0.70(.22) 0.20 Neix 13.55313.542(.007) 1.10(.14) 1.61 Siviii 69.825 69.812(.055) 0.81(.21) 0.23 Neix 13.70013.723(.011) 0.27(.05) 0.39 Notes: Fexvii 13.82313.824(.009) 0.42(.06) 0.60 a:λ arethetheoreticalwavelengthvaluesfromKelly(1987). Nixix 14.040+.08114.052(.008) 0.31(.05) 0.43 0 b:λ istheobservedwavelengthwiththestatistical1σerrorin Fexviii 14.202+.25514.209(.005) 0.76(.11) 1.06 obs parentheses. Fexviii 14.344+.36114.364(.008) 0.34(.05) 0.48 c:inunits10 4photonscm 2s 1 Fexvii 15.01515.011(.002) 2.54(.11) 3.36 − − − d:inunits10 13ergscm 2s 1 Oviii 15.17615.175(.012) 0.50(.10) 0.64 − − − e:wavelengthfixedattheoreticalvalues Fexvii 15.26015.264(.008) 1.03(.11) 1.34 f:broadline ??? —15.844(.013) 0.20(.12) 0.24 Oviii 16.00716.002(.006) 0.89(.11) 1.11 Fexvii 16.07316.092(.014) 0.34(.09) 0.42 Fexvii 16.229 16.220(—) <0.07 0.08 Table4.Higher-orderlinesandtheirlinefluxesinthespectrum Fexix 16.28416.296(.020) 0.∼06(.03) 0.07 ofδOri. Fexvii 16.77716.775(.003) 1.39(.09) 1.62 Fexvii 17.054 17.054e 2.48(.18) 2.89 ion λa λb λ /order fluxc Fexvii 17.097 17.097e 1.02(.16) 1.18 0 obs obs Ovii 17.76817.763(.012) 0.16(.05) 0.18 [Å] [Å] [Å] Ovii 18.62718.628(.008) 0.58(.07) 0.62 Fexvii 15.015 30.020(.012) 15.010(.006) 0.27(.09) Oviii 18.96918.968(.002) 6.60(.18) 6.91 Neix 13.447+.553 40.545(—-) 13.515(—-) 1.12(—-) Nvii 20.910 20.894(—) 0.06(—) 0.05 Fexvii 15.015 45.004(.026) 15.001(.009) 0.47(.10) Ovii 21.60221.614(.003) 6.29(.24) 5.81 Fexvii17.054+.097 51.204(.067) 17.068(.022) 0.50(.17) Ovii 21.80421.793(.003) 5.57(.23) 5.07 Oviii 18.969 56.936(.030) 18.978(.010) 1.79(.24) Ovii 22.101 22.101e 0.25(.09) 0.22 Ovii 21.602+.804 65.171(.165) 21.724(.055) 1.86(.52) Caxiv+Sxiv24.133+.28524.167(.056) 0.26(.11) 0.21 Cvi 33.736101.222(.348) 33.741(.116) 2.02(.88) Nvii 24.78124.788(.011) 1.38(.15) 1.11 Notes: Arvii 25.68425.705(.030) 0.46(.12) 0.36 a:λ arethetheoreticalwavelengthvaluesfromKelly(1987). Cvi 26.357 26.356(—) 0.26(—) 0.20 0 b:λ higher-orderobservedwavelengthwiththestatistical1σerror Cvi 26.99026.990(.047) 0.30(.13) 0.23 obs inparentheses. Cvi 28.46628.462(.017) 0.57(.15) 0.40 c:inunits10 4photonscm 2s 1. Nvi 28.78728.763(.028) 1.27(.20) 0.88 − − − Nvi 29.08429.065(.037) 0.86(.18) 0.58 Nvi 29.53429.475(.060) 0.36(.14) 0.24 blendsaretakenintoaccount.Fromthatpointonthesamepro- cedure was used as in Sect. 4: a Gaussian was folded through ThelinefluxratiosarecomparedwiththosegivenbyMilleretal. theresponsematrixandfittedtothelineprofilesofeachionline (2002)usingHETGS. Theirvaluesandoursarein goodagree- individually. The line flux ratios (f/i) obtained in this way are ment.TheratioshavebeenappliedtoaPlanckUV-radiationfield collectedinTable6.Theymightdifferfromratiosderivedfrom of33000Katthewavelengthsofthe1s2s3S–1s2p3Ptransitions Table 3, as in that table total feature fluxes, polluted by blends ofthecorrespondingions,incombinationwithscaledformulas andsatellite linesare givenwith theirdominantidentifications. given by Blumenthal et al (1972). This UV radiation field de- 4 A.J.J.Raassen&A.M.T.Pollock:TheLETGSX-rayspectrumofδOrionis Table6.He-likeandH-likeoverHe-likelinefluxratiosandcor- The influence of the UV radiation field on the population respondingdistancesinR andtemperatures.Valuesobtainedby ofthe 1s2s3S and1s2p 3P levelsis reflectedinthe f/iratio, 1 1 Milleretal(2002)areals∗ogiven. hence in the distance to the stellar surface as shown in Fig. 4. Theshapesofthecurvesare thesameasthosegivenbyMiller thiswork Milleretal.2002 etal.(2002)forMgxi,Neix,andOvii. Thecurvesof Nviand ion f/i distance(R ) f/i Cvarenewandestablishedasaresultofourinvestigation.The Cv 0.31(.10) 75(14) ∗ —- boldintervalsonthetrajectoryshowtheaveragedistancefrom Nvi 0.38(.17) 49(13) —- the stellar surface (includingthe 1σ statistical error)where the Ovii 0.052(.016) 9.0(1.5) <0.2 ion is formed. From this figure it is clear that the hotter ions Neix 0.10(.06) 3.4(1.1) <∼0.1 (MgxiandNeix)areformedclosertothestellarsurfacethanthe Mgxi 0.73(.48) 3.2(1.4) 0∼.5(.4) coolerones(CvandNvi).Intheshortwavelengthsourresults Sixiii <0.6 — 2.2(1.5) are in goodagreementwith Fig. 4 in the paperby Miller et al. ion ∼(f +i)/r T(MK) f +i/r (2002) and with the values of Mgxi and Ovii by Waldron & Cv 0.96(.34) 1.0 —- Cassinelli(2007),basedonHETGSobservations.Atthelonger Nvi 0.86(.23) 1.6 —- wavelengths,NviandCv(outsidetheHETGSrange)showthat Ovii 0.93(.06) 1.7 0.78(0.35) theX-raysareformedmuchfartheroutinthewindthanthewind Neix 0.94(.14) 2.3 1.09(0.44) accelerationzone,whichstopsatabout10R .TheNviislocated Mgxi 1.08(.37) 2.1 0.54(0.28) Mgxia 0.81(.23) 4.4 —- at 49(13)R and Cv at 75(14)R with temp∗eratures of 1.6 MK Sixiii 0.69(.27) 8.0 0.84(0.34) and1.0MK∗ respectively(seeTa∗ble6). Sixiiia 0.77(.22) 6.4 —- Only an upper value has been determined for Sixiii. This ion rb/(r+i+ f)T(MK) feature is better imaged by HETGS, and therefore the distance H Cvi/v 0.91(.18) 1.18(0.07) ofitsplasmatothestellarsurfaceisexcludedfromTable6and Nvii/vi 0.55(.09) 1.64(0.08) Fig. 4. Also included in Table 6 are the temperature indicators Oviii/vii 0.55(.02) 2.44(0.02) G = (f + i)/r of the He-like ions, as well as the ratios of the Nex/ix 0.35(.04) 3.8(0.1) H-likeresonancelinesandtheHe-likelinetriplets. Theyresult Mgxii/xi 0.24(.09) 6.0(0.5) intemperaturesfortheX-rayemittingplasma,relatedtothelo- Sixiv/xiii0.22(.14) 9.4(1.3) cationofthespecificionsintheplasma. Notes: a:AverageofourvalueandtheHETGSvalueobtainedbyMilleretal. (2002) 5. Multitemperaturefitting b:r isthelinefluxfortheH-likeresonancelinedoublet H Along with the individual line analysis we investigated the LETG-spectrum of δ Ori using a multithermal model for opti- callythinplasmainCIEasimplementedinSPEX(Kaastraetal. creaseswith distancefromthe stellarsurfacebya dilution fac- 1996a)incombinationwithMEKAL(Meweetal.1985,1995; tor: Kaastraetal. 1996b)3. Itprovidesa calculationwiththousands oflinesandamodelelectroncontinuum.Theionizationequilib- 1 R 2 1/2 rium is based on calculations by Arnaud & Rothenflug (1985) W(r)= 1 1 ∗ . (1) 2  − −(cid:18) r (cid:19) !  aAnpdplAicrantaioundo&f aRanyomnoenqdui(li1b9r9iu2m), itohneizlaatttieornemspoedceialldlyidfonrotiriomn-. As a result of the radial dependence of the radiation field, the provethedescriptionofthespectrum. observed f/iratiocanbeusedtoderivetheradiallocationofthe The fits were applied using the C-statistics approach. For He-likeionsthatareproducingtheobserved firlines. the transition probabilities of the model line features, an uncertainty of 15% was assumed, a value that accords with the NISTdatabase4.Thefirstcalculationwasfittedtothedata,tak- ingtheerrorsonthedatacountrateintoaccount.Inthe follow up of the fitting procedure the errors on the model count rate were applied to avoid over-influences of incidental low count ratebins. 5.1.A3-Tmodel We started the fitting procedure with more fixed temperature components, out of which only three components were significant. From that point on a three temperature (3-T) fit was applied for the CIE-model. The values of the fit are collected in Table7.ThistableshowstheinterstellarcolumndensityN ,the H obtainedtemperatures,the emission measures,the luminosities ofthedifferenttemperaturebins,absorptioncolumndensitiesfor thethreetemperaturecomponentsinthewind,abundances,line shifts(z),andlinebroadening(vmic). Fig.4.The f/iratioversusthedistancetothestellarsurfacein 3 http://www.sron.nl/SPEX stellarradii(R*). 4 http://www.nist.gov/pml/data/asd.cfm 5 A.J.J.Raassen&A.M.T.Pollock:TheLETGSX-rayspectrumofδOrionis Table 7.The3-T fitto the LETGSspectrumof δ Ori, applying aCIE-modeltothetotal(time)spectrum.Thedistanceisputat 212pc(VanLeeuwen2007). Parameters model logN [cm 2] 20.18(.03) H − z(Redshift) -2.3(.5)e-4 v 1039(21) mic T [keV] 0.098(.005) 1 T [keV] 0.228(.008) 2 T [keV] 0.647(.010) 3 EM [1054cm 3] 2.05(.20) 1 − EM [1054cm 3] 2.27(.09) 2 − EM [1054cm 3] 0.88(.04) 3 − L [1031erg/s]a 7.10 1 L [1031erg/s] 5.14 2 L3[1031erg/s] 2.59 Fig.5.ThespectrumofδOri(red/gray),togetherwiththemodel LTot[1031erg/s] 14.83 (black) derived from the multitemperature fit applying the pa- Nh [1020cm 2] 0.21(.16) 1 − rametersgiveninTable7. Nh [1020cm 2] 1.52(.37) 2 − Nh [1020cm 2] 26.7(4.0) 3 − C 1.080(.016) TheInterStellarMedium(ISM)N was fittedandfoundto H N 1.046(.020) be1.5 1020cm 2. Threeadditionalwindabsorptionparameters O 0.859(.009) × − wereappliedusingthehotmoduleinSPEX.Itdescribesawarm Ne 1.243(.015) plasma,astheCIE-model,whilecalculatingthecontinuumab- Mg 1.650(.022) Si 0.913(.012) sorption,aswellasthelineabsorption.Emissionlineswiththe Fe 1.000(fix) highest oscillator strength are mostly influenced.The tempera- C stat/d.o.f. 2836/1609 turesofthishotplasmawerecoupledtothetemperaturesofthe − C stat 1.76 components in the CIE-model. The hottest component, which − red is formed deeper in the wind (see Sect. 4), results in the high- a:Intherangefrom0.07to3keV.(LETGSband). estabsorptioncolumndensity.Noabsorptionforthecoolwind (33000 K) could be determined. This implies that the overall coolstellarwindabsorptionisnottakenintoaccountandmight Threestatisticallywell-determinedtemperaturecomponents havebeentakenoverpartlybythefittedISM-parameterNH. at 0.10, 0.23, and 0.65 keV have been established. The two A slight shift in λ ( -2.3e-4) was applied to optimize the ≈ coolestcomponentscorrespondtothelinesoftheHe-likeions, fitted wavelengths. A line broadening parameter vmic of 1039 discussedinSect.4.Accordingtoconclusionsfromthatsection, km/s was added and fitted to the overall spectrum to take into the hottestcomponent(0.65keV), thatis responsibleforSixiv accountthebroadeningofthelinefeatures.TheHWHMofaline andS xvisformeddeeperinthewind. isrelatedtothevmic bythefollowingformula(seealsoSect.3): Theresultsofthetotal3-Tfittothespectrumareshownin Fig.5.Thelinesandcontinuafromthethreeindividualtempera- HWHM/λ = √ln2vmic. (5) turecomponentsareshowninthethreetoppanelsofFig.6.The obs c bottom panel shows the data with the calculated continuum as sumofthethreecontinuaofthesecomponents. The X-ray luminosities of the three individual temperature Thetabulatedemissionmeasureisdefinedas componentsaregivenovertheLETGenergyrangefrom0.07to 3.0keV.Fortheluminositiesnoindividualstatisticalerrorshave EM =nenHV, (2) been determined. Their uncertainties are supposed to have the samerelativevaluesasthosefortheemissionmeasuresbecause inwhichn =0.85 n . H e × the luminosity is mostly determined by the emission measures Knowledge of the electron density offers the possibility to in this temperature range, and therefore L EM. The total calculatetheX-rayemittingvolume.Theelectrondensityforhot X ∝ luminosities (summed over the three temperature components) starswitha sphericallysymmetricstellarwindcanbeobtained in other energy bands (0.5-4 keV, 0.1-2.4 keV, and 0.4-4 keV) viathefollowingcontinuityequation: for Einstein, ROSAT, ASCA given in the papers by Cassinelli M˙ =4πR2ρ(R)V(R). (3) & Swank (1983), Haberl & White (1993), and Corcoran et al. (1994),respectively,do agreewith ourvalueswhentaking dif- Inthisformula M˙ isthemasslossrate(1.07 10−6M /year ferencesininterstellarabsorptionordistanceintoaccount. (Lamers&Leitherer1993))forδOri,Risthedis×tancefr⊙omthe The abundances are relative to solar photospheric values stellarsurface,ρ(R)thedensityatdistanceR,andV(R)thewind (Loddersetal.2009).Theabsolutevaluesoftheabundancesare velocityatdistanceR,relatedtotheterminalvelocity(V )by stronglyinfluencedbytheobtainedvaluesoftheemissionmea- ∞ V(R)=V (1 R /R)β, (4) sures. The productofabundancesandemission measures(A × ∞× − ∗ EM),however,istherobustquantityforthelinespectrum,there- inwhichβisacoefficientbetween0.85forsubgiantsand1.1for fore,abundance-ratiosareoftengivenrelativetoanabundantel- supergiants.Weuseavalueof1.0forβ. ementinthestellaratmosphere.Heretheabundanceofironwas 6 A.J.J.Raassen&A.M.T.Pollock:TheLETGSX-rayspectrumofδOrionis fixedatitssolarphotosphericvalue.Theabundancevaluesagree to 10 R , respectively. This is in accordance with observations withthevalueof 0.9thatSimoń-D´ıaz(2010)findsforoxygen by Wald∗ron & Cassinelli (2007) and Miller et al. (2002)using ≈ andsiliconabundancesintheOrionstar-formingregion. HETGS. Using LETGS that extendsto longerwavelengths, the A fit to the spectrum taken by HETGS applying the same coolerions Nvi and Cv have been observedas part of this in- setofparametersneedsalowervalueforv (seealsoSect.3). vestigation.TheX-raysfromtheseionsareformedonaverageat mic ThismightbeduetoinconsistencybetweentheLSFsoftheused 50R and75R ,respectively,muchfartheroutinthewindthan instruments.AlsothedescriptionoftheSixiiiandSixivlinefea- thea∗cceleration∗zone,wherethefinalvelocityhasbeenreached. turesisnotsatisfactory.Thismightbebecausethesefeaturesare Here the wind is expected to be steady. However, small devia- neartheendofthecalibratedareaforbothinstruments. tionsinvelocity(weakshocks)willbeabletoproducethecooler Toinvestigatethetimedependenceofthespectrum,assug- ions. gestedbythelightcurve(seeFig1)wesplittheobservationinto ThespectrumisdescribedwellbyaCIEmodel.Threewell- twotimeintervals,0-49ksand49-98ks.Theemissionmeasure distinguished temperature components have been determined: ofthehottestcomponent(formedclosesttothestellarsurface)is 0.10, 0.23, and 0.65 keV. The first two are in good agreement about10%lowerinthefirsttimeintervalthaninthelast49ks. with the two temperatures found by Haberl & White (1993). Thismightagreewithsomeeclipsingbythecompanionstar. Theydidnotdetermineathirdtemperaturecomponent.Thetwo lowertemperaturecomponents(0.098and0.228keV)describe the line-forming regions of the cooler ions, Nvi and Cv, and 5.2.DEMmodeling themoderatelyionizedspeciesMgxi,Neix,Ovii,andOviii,re- To show the smooth connection between the separated tem- spectively,while thehottestcomponentis supposedto produce perature bins, we performed a continuous DEM-modeling for evenhotterionsdeeperinthewindandahotcontinuum. the spectrum of δ Ori by applying the regularization method Threeemission measuresare established, related to the ob- (Kaastraetal.1996b)intheSPEX-code.Thismethodusesdirect tained temperatures. Knowledge of the electron density offers matrix inversionwith the additionalconstraintthat the second- thepossibilitytocalculatetheX-rayemittingvolume.Applying orderderivativeof the solution with respectto the temperature thedistances(seeTable6andFig.4)andthewindvelocityand isassmoothaspossible.InthisDEMmodelingtheabundances densityformulas4and5,giveninSect.5.1,wehavecalculated giveninTable7havebeenused.TheresultsareshowninFig.7. theelectrondensitiesfortheregionswherethespecificionspro- Theresultsofthefits,collectedinTable7andFig.7,arein duceX-rayradiation. good agreement. The peak of our DEM agrees with the peaks BecausethedistancesgiveninTable6andFig.4areaverage foundbyMilleretal. (2002)andWojdowski&Schulz(2005). valueswiththestatisticalerroronthisaverageandnotthe lim- However, our DEM extends more to lower temperatures and its of the X-ray emitting distance, it is difficultto calculate the their values extend further to higher temperatures as a conse- total wind volume in which the conditions are optimal for the quenceofusingtwodifferentinstruments.Thepeakvaluegiven corresponding He-like ion emission. By assuming a wind vol- byWojdowski&Schulz(2005)ishigher. umefromhalftotwicetheobtainedaveragedistancewegetthe percentageofthewindthatcontributestothation.Theemitting As forthe CIE-fit we also performeda DEM-modelingfor the partial spectra of 0-49 ks and 49-98 ks, again applying the volumesareabout1%ofthewindvolumeforthecoolerplasma regularizationmethod.ThesetwoDEM-modelingsareshownin andlessforthehotterions. Fig. 8. Although the hottest component is again a bit stronger TheNH oftheISMismainlybasedonthelongwavelength for the last 49 ks, no significant difference can be established partofthespectrum.Itsvalueof1.51(.06) 1020cm−2 isingood betweenfitstothepartialspectra. agreementwiththevalueof1.56 1020cm−×2foundbyJenkinset × al.(1999)basedon57IUEobservations,andslightlylowerthan thevaluefoundbyHaberl&White(1993)fortheirT=0.1keV 6. Discussionandconclusions component. Additional wind absorption column densities have been determined(see Table 7). They are related to the temper- The X-ray spectrum of the O-star δ Ori obtained with LETGS ature components of 0.10, 0.23, and 0.65 keV, with distances isrichinlinesbelongingtoH-likeandHe-likeionsofC,N,O, to the stellarsurfaceof about60R , 6R , and2R , respectively, Ne, Mg, and Si. A weak He-like (Sxv) triplet is also present. wheretheplasmaisformedonaver∗age.I∗ncombin∗ationwithfor- Otherpossibleweakfeaturespresentbelow5Åfalloutsidethe mula 4 we calculated that about7% of the wind contributesto calibratedbandwidthoftheLETGS.IronL-shelllinesaredomi- thehotwindabsorption.Noabsorptionoftheoverallcoolstel- nant.TheNe-liketransitions,betweenelectronconfigurationsof lar wind could be determined. This means that this absorption aninertnoblegas,arefavoredforiron(FeXVII)aswellasfor componentisnotexplicitlytakenintoaccountduringthefitting calcium(CaXI),andnickel(NiXIX). procedure. However, the effect might have been taken over by Above 50 Å a few new line features were identified, some theotherabsorptionparameters. of whichare third-orderlinesassociated with strongfirst-order The X-ray luminosities of the three temperature compo- featureslocatedbetween15and35Å.Nolinesofcoolionswere nentsinthetotalLETGSband(0.07-3keV)are7.10,5.14,and detectedatwavelengthshigherthan70Å.Themainreasonsfor 2.59 1031erg/s. Two partial spectra were derived: one from the theabsenceoftheselinefeaturesistheabsenceofanoticeable first49ksandanotherfromthelast49ksoftheobservation.This volumeofcool(0.05keV)plasmaandtheinterstellarabsorption. divisionwassuggestedbythelightcurve(seeFig1).Nosignif- Basedontheforbiddenlineoverintercombinationlineratio icant differences between the spectra, their temperature struc- inHe-likeionstheaveragedistanceoftheX-rayemittingplasma tures,theirlineshapes,orlinefluxeshavebeenestablished. tothestellarsurfacehasbeendetermined.Dependingontheion The obtained abundancesare relative to solar photospheric (temperature),theplasmaemittingregionisbetween2and80R values (Lodders et al. 2009). They were fit by fixing the iron forionsfromMgxitoCv.Thecoolerplasma(lowerionization∗ abundanceatitssolarphotosphericvalueandleavingthecarbon, stages)islocatedfartheroutinthewind.Forthehotions,Mgxi, nitrogen,oxygen,neon,magnesium,andsiliconabundancesfree Neix, and Ovii, the average X-ray emitting plasma is from 2 to vary.Fixingtheironabundancewasdonebecausethe abun- 7 A.J.J.Raassen&A.M.T.Pollock:TheLETGSX-rayspectrumofδOrionis dancesandemissionmeasuresarestronglyanticorrelated,espe- Leutenegger,M.A.,Paerels,F.B.S.,Kahn,S.M.,&Cohen,D.H.2006,ApJ,650, ciallyinspectrawithaweakcontinuumtoestablishtheemission 1096 measure.Theobtainedvaluesforoxygenandsiliconareingood Lodders, K., Palme, H., & Gail, H.-P., 2009, Landolt-Bornstein, New Series, Astronomy and Astrophysics, Springer Verlag, Berlin (eprint agreementwithvaluesfoundbySimoń-D´ıaz(2010)intheOrion arXiv:0901.1149) star-formingregion. Lucy,L.B.1982,ApJ,255,286 The lines are symmetric and their HWHMs are about Lucy,L.B.,&White,R.1980,ApJ,241,300 840 km s 1, far below the terminal velocity of 2000 km s 1, Lucy,L.B.,&Solomon,P.M.1970,ApJ,159,879 − − Mayer,P.,Harmanec,P.,Wolf,M.,Bozˇic,H.,&Sˇlechta,M.2010,A&A,520, but higher than the values found by Miller et al. (2002) using A89 HETGS.Thismightbeduetoinconsistenciesbetweentheinstru- Mewe,R.,Gronenschild,E.H.B.M.,&vandenOord,G.H.J.1985,A&AS,62, mental line spread functions in the calibration files of HETGS 1(mekal) andLETGS.Theultimatevelocities,measuredattheendofthe Mewe,R.,Kaastra,J.S.,&Liedahl,D.A.1995,Legacy6,16 wings,areabout1850kms 1withanuncertaintyof200kms 1. Miller,N.A.,Cassinelli, J.P.,Waldron,W.L.,MacFarlane, J.J.,&Cohen,D.H. − − 2002,ApJ,577,951 The measuredline shiftsgivenin Table 5, as well as the z- Oskinova,L.M.,Feldmeier,A.,&Hamann,W.-R.2006,MNRAS,372,313 valuegiveninTable7,indicatethatthevelocityoftheradiative Owocki,S.P.,&Cohen,D.H.2006,ApJ,648,565 plasmainthedirectionfromortotheobserverislow.Thisiswell Owocki,S.P.,Castor,J.I.,&Rybicki,G.B.1988,ApJ,335,914 known for OB stars, especially among giants and supergiants Pollock,A.M.T.,2007,A&A,463,1111 Raassen, A.J.J., van der Hucht, K.A., Miller, N.A., & Cassinelli, J.P., 2008, (Waldron&Cassinelli2007). A&A,478,513 The conclusion should be that the radiation we observe is Seward,F.D.,Forman,W.R.,Giacconi,R.,etal.1979,ApJ,234,L55 producedin plasma that has a very low velocity or movesper- Simoń-D´ıaz,S.2010,A&A,510,A22 pendicularto thelineof sight.Theremightbeseveralexplana- Tarasov,A.E.,Harmanec,P.,Horn,J.,etal.1995,A&AS,110,59 tions.Theclumps,whichcollidewiththewindandproducethe VanLeeuwen,F.2007,A&A,474,653 Voels, S.A.,Bohannan, B., Abbott, D.C.,& Hummer, D.G.. 1989, ApJ, 340, X-rayradiation,havelowvelocities.Inthatcasetherelativeve- 1073 locity of the clumps to the wind is highest far out in the wind Waldron,W.L.,&Cassinelli,J.P.2001,ApJ,548,L45 at terminalwind velocity. As a consequencethe higheststages Waldron,W.L.,&Cassinelli,J.P.2007,ApJ,668,456 ofionizationareproducedfaroutinthewind,whichcontradicts Wilson, R.E. 1963, General Catalogue Of Stellar Radial Velocities, Carnegie InstitutionofWashingtonpublication601 ourobservationbasedontheforbidden-to-intercombinationline Wojdowski,P.S.,&Schulz,N.S.2005,ApJ,627,953 ratiosinHe-likeions(seeFig.4).Additionally,afterthe accel- eration phase one does not expect velocity differences that are too high. The secondary star and its wind play importantroles in the explanation of the δ Ori spectrum. However, this is dis- cussedbyMilleretal.(2002)andrejected.Athirdoptionisthat the clumps screen their own X-ray radiation when they are in between the star and the observer. Only when the wind-clump collision is almost perpendicular to the line of sight is the radiation observable and spread over a narrow velocity range as derivedfromtheHWHM.Theeffectsofclumpingandporosity onX-Rayemission-lineprofilesfromhot-starwindshavebeen describedextensivelybyOwocki&Cohen(2006)andOskinova etal.(2006). Acknowledgements. TheSRON National Institute forSpace Research is sup- portedfinanciallybyNWO. 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Watanabe (eds.), UV and X-ray Spectroscopy of Astrophysical and LaboratoryPlasmas(Tokyo:UniversalAcademyPress,Inc.),p.411(spex) Kaastra,J.S.,Mewe,R.,Liedahl,D.A.,etal.1996b,A&A,314,547 Kahn,S.M.,Leutenegger,M.A.,Cottam,J.,etal.2001,A&A,365,L312 Kelly,R.L.1987,J.Phys.Chem.Ref.Data,16,Suppl.1 Lamers,H.J.G.L.M.&Leitherer,C.1993,ApJ,412,771 8 A.J.J.Raassen&A.M.T.Pollock:TheLETGSX-rayspectrumofδOrionis Fig.7. DEM-modeling of δ Ori, based on the regularization method (Kaastra et al. 1996b) (see text). The EM is given in unitsof1054cm 3. − Fig.8. DEM-modeling of δ Ori, based on the regularization method.ThetwoDEMsshowamodelingofthespectrumover thetimeinterval0-49ks(red/gray)and49-98ks(black). Fig.6.ThespectrumofδOri(red/gray),togetherwiththethree individualtemperaturecontributionstothemodel(black),apply- ingtheparametersgiveninTable7.Theyimplythelineandcon- tinuum radiation from the individual components. The bottom panelshowsthemodelelectroncontinuum(black).Thisimplies thesumofthethreecontinua. 9 A.J.J.Raassen&A.M.T.Pollock:TheLETGSX-rayspectrumofδOrionis Fig.3.PartofthespectrumofδOri.Themostdominantlinesareindicated.Dataareinred(gray),theCIE-modelinblack) Table5.Wavelengthshiftsandhalfwidthathalfmaximum(HWHM)valuesforsomestrongisolatedlinesinthespectrumofδOri ThevalueshavebeencomparedwiththosefromMilleretal.2002. ion λa λ ∆λ shift shift(Miller) HWHM HWHM(Miller)bluewing redwing 0 obs [Å] [Å] [Å] inkms 1 inkms 1 inkms 1 inkms 1 inkms 1 inkms 1 − − − − − − Nex 12.134 12.1305(.0030) -0.0035(.0030) -87(74) -150(100) 653(123) 420(170) -1168 1246 Fexvii15.015 15.0100(.0023) -0.0050(.0023) -100(46) -50(100) 840(80) 510(220) -1734 1915 Fexvii16.777 16.7745(.0034) -0.0025(.0034) -45(57) 100(150) 853(89) 420(250) -1804 1783 Oviii 18.969 18.9684(.0015) -0.0006(.0015) -9(24) 60(100) 854(30) 700(100) -2009 2067 Ovii 21.602 21.6148(.0038) 0.0128(.0038) 178(53) ——- 792(47) ——– -1808 1826 Ovii 21.804 21.7930(.0041) -0.0110(.0041) -151(56) ——- 812(61) ——– -1915 2066 Nvii 24.781 24.7866(.0100) 0.0056(.0100) 68(121) 120(400) 750(169) 420(870) -1704 1669 Cvi 33.736 33.7252(.0050) -0.0108(.0050) -96(44) ——- 835(55) ——– -1834 1751 Notes: a:λ arethetheoreticalwavelengthvaluesfromKelly(1987). 0 10