Extended Red Emission and the evolution of carbonaceaous nanograins in NGC 7023

0.72 MB·English

by  O. Berne

Astronomy&Astrophysicsmanuscriptno.OB9158 (cid:13)c ESO2008
February2,2008
LE
Extended Red Emission and the evolution of carbonaceous
⋆
nanograins in NGC 7023.
O.Berne´1,C.Joblin1,M.Rapacioli2,J.Thomas3,J.-C.Cuillandre4,andY.Deville3
8 1 Centred’EtudeSpatialedesRayonnements,Universite´ PaulSabatierToulouse3etCNRS,ObservatoireMidi-Pyre´ne´es,9Av.du
0 ColonelRoche,31028Toulousecedex04,France
0 2 LaboratoiredeChimieetPhysiqueQuantique,IRSAMC,Universite´ PaulSabatierToulouse3etCNRS,118RoutedeNarbonne,
2 31062ToulouseCedex,France
3 Laboratoired’AstrophysiquedeToulouse-Tarbes,Universite´ PaulSabatierToulouse3etCNRS,ObservatoireMidi-Pyre´ne´es,14
n Av.EdouardBelin,31400Toulouse,France
a 4 Canada-France-HawaiiTelescopeCorporation65-1238MamalahoaHighwayKamuela,Hawaii96743,USA
J

2 Received?;accepted?
2
ABSTRACT

]
h Context.Extended Red Emission (ERE) was recently attributed to the photo-luminescence of either doubly ionized Polycyclic
p AromaticHydrocarbons(PAH++),orchargedPAHdimers([PAH ]+).
2
- Aims.Weanalysedthevisibleandmid-infrared(mid-IR)dustemissionintheNorth-WestandSouthphoto-dissociationregionsof
o
thereflectionnebulaNGC7023.
r
t Methods.Using a blind signal separation method, we extracted the map of ERE from images obtained with the Hubble Space
s Telescope,andattheCanadaFranceHawaiiTelescope.WecomparedtheextractedEREimagetothedistributionmapsofthemid-IR
a
emissionofVerySmallGrains(VSGs),neutralandionizedPAHs(PAH0andPAH+)obtainedwiththeSpitzerSpaceTelescopeand
[
theInfraredSpaceObservatory.

1 Results.ERE is dominant in transition regions where VSGs are being photo-evaporated to form free PAH molecules, and is not
v observedinregionsdominatedbyPAH+.Itscarriermakesaminorcontributiontothemid-IRemissionspectrum.
0 Conclusions.TheseresultssuggestthattheEREcarrierisatransitionspeciesformedduringthedestructionofVSGs.[PAH2]+appear
0 asgoodcandidatesbutPAH++moleculesseemtobeexcluded.
4
Keywords.astrochemistry—ISM:dust—ISM:linesandbands—reflectionnebulae—infrared:ISM—methods:numerical—
3
methods:observational
.
1
0
8 1. Introduction nmandwithapeakwavelengthlongwardof600nmtobeyond
0
800 nm (Smith&Witt 2002). ERE is likely due to the photo-
: Unveiling the composition, structure and charge state of the
v luminescence of carbonaceous macromolecules or nanograins
i smallest interstellar dust particles remains one of today’s chal- exposed to UV photons (see Witt et al. 2006 and references
X lenges in astrochemistry. Progress in this field requires a de- therein). The ERE is observed in many environments exposed
r tailed analysis of the spectral signatures of these dust popula- toUVphotons(Photo-dissociationRegions;PDR)foundforin-
a
tions. Amongst these signatures is the ExtendedRed Emission stanceinthediffuseinterstellarmedium,reflectionnebulaeand
(ERE),abroademissionfeaturerangingfrom540tobeyond900 planetarynebulae.Ithasbeenobservedingalaxies,NGC3034
(Perrinetal.1995)andNGC4826by(Pierinietal.2002).
Sendoffprintrequeststo:
O.Berne,e-mail:[email protected]
ThepossiblelinkwiththecarriersoftheAromaticInfrared
⋆ This work is based on observations made with the Spitzer
Bands(AIBs;alsocalledunidentifiedinfraredbandsat3.3,6.2,
SpaceTelescope, whichisoperated bytheJetPropulsion Laboratory,
CaliforniaInstituteofTechnologyunderacontractwithNASA.Based 7.7, 8.6 and 11.3 µm) has been discussed by several authors.
on observations made with the NASA/ESA Hubble Space Telescope, Both types of emission features, ERE and AIBs, were found
obtained from the Data Archive at the Space Telescope Science toberelativelycospatialalthoughnotmatching(Furton&Witt
Institute, which is operated by the Association of Universities for 1990),pointingtodifferentbutrelatedmaterialsfortheircarri-
Research in Astronomy, Inc., under NASA contract NAS 5-26555. ers(Furton&Witt1992).Recently,adetailedstudyofthespa-
These observations are associated with program 9471. Based on ob- tial distribution of the ERE in the northern PDR of NGC7023
servations obtained at the Canada-France-Hawaii Telescope (CFHT)
hasbeenperformedbyWittetal.(2006)whoconcludedthatthe
which is operated by the National Research Council of Canada, the
ERE mechanism is a two-step process involving the formation
InstitutNationaldesSciencesdel’UniversoftheCentreNationaldela
of the carrier and then the excitation of the luminescence, and
RechercheScientifiqueofFrance,andtheUniversityofHawaii.Based
proposed doubly-ionized Polycyclic Aromatic Hydrocarbons
onobservationswithISO,anESAprojectwithinstrumentsfundedby
ESAMemberStates(especiallythePIcountries:France,Germany,the (PAHs) as plausible carriers. On the other hand, recent quan-
Netherlands and the United Kingdom) and with the participation of tum chemistry calculations point to PAH dimers ([PAH2]+) as
ISASandNASA. thecarrierofERE(Rheeetal.2007).
2 O.Berne´etal.:ExtendedRedEmissionandtheevolutionofcarbonaceousnanograinsinNGC7023.
Fig.1. On the left: HST images of the NGC 7023 North-West
PDR in threeSDSS wide-bandfilters(cf. Witt etal. 2006).On
theright:scatteredlightandEREimagesextractedwithFastICA
fromtheobservations
In this letter, we provide an efficient and non biased way
to extract the ERE map in NGC 7023 from the Hubble Space
Telescope(HST)dataandnewCanadaFranceHawaiiTelescope
(CFHT)imagesusingablindsignalseparationmethod.Wethen Fig.2.Ontheleft:CFHTimagesoftheNGC7023North-West
compare the extracted ERE map to the maps of the different PDRinthreeBVRfilters.Ontheright:scatteredlightandERE
mid-IR emission carriers, namely Very Small Grains (VSGs), imagesextractedwithFastICAfromtheobservations
neutral and ionized PAHs (PAH0 and PAH+), as extracted by
Rapaciolietal.(2005)andBerne´etal.(2007)fromtheInfrared
SpaceObservatory(ISO)andSpitzerSpaceTelescope(Spitzer)
the detrending and stacking process, ensuring no alteration of
observations.
theintrinsicbrightnessfeaturesofthenebula.
2. Observations
3. ExtractionoftheEREmap
2.1.Infraredobservations
OneofthedifficultiesinproperlyextractingtheEREmapisto
The analysis of the mid-IR emission of the North-West (NW) remove the contribution of the scattered light from the central
and South PDRs was first performedby Rapaciolietal. (2005) HerbigBe star,HD 200775.To performthisanalysisin an un-
using ISOCAM-CVF data from ISO. More recent data on the biasedway,weappliedaBlindSignalSeparation(BSS)method
NW PDR obtained with the Infrared Spectrograph (IRS) on- successivelytotheACSandCFHTimagesobtainedinthethree
board Spitzer in mapping mode was analysed by Berne´etal. wide-bandfilters (F475W, F625W, F850LP,for ACS and B, V,
(2007). The achievedangularresolutionis 3.6”for IRS and 5” RforCFHT:seeFigs.1-2).Forawide-bandfiltercenteredatλ,
forISOCAM. itisassumedthattheobservedimageIm canbewrittenas:
λ
Im =a ERE+b Scattered. (1)
2.2.Visibleobservations λ λ λ
ERE was observed in NGC 7023 by Witt&Boroson (1990) Inthisequation,itisassumedthatthespatialpatterns,ERE
and Wittetal. (2006). The NW PDR was observed with the and Scattered are unknown but do not depend on wavelength.
HST by Wittetal. (2006), using the Advanced Camera for The ”mixing” coefficients a and b are unknown and vary
λ λ
Surveys(ACS)andtheNearInfraredCameraandMulti-Object with the filter central wavelength λ. ERE arises from the UV-
Spectrometer (NICMOS). We retrieved from the archive the illuminatedlayersofthecloudandthereforedoesnotsufferfrom
calibrated, geometrically corrected, dither-combined ACS im- significant extinction in the studied reflection nebula. It is thus
ages (Fig. 1) in three wide-band Sloan Digital Sky Survey fil- reasonable to assume that its spatial pattern (ERE in Eq. 1) is
ters(Smithetal. 2002):g,r, andz(respectivelycalledF475W, independent of wavelength. In the case of scattered light, this
F625WandF850LPonHST).ThewholeNGC7023nebulawas mightnotbetrueifstrongradiativetransferispresentasshown
observedin August2002 in the B,V and R filters at the CFHT forinstancebyNuzillard&Bijaoui(2000)inthecaseofgalax-
usingtheCFH12KCCDmosaic(Cuillandreetal.2001)aspart ies. However, in the present object, the scattered light is sub-
oftheCFHToutreachprogram.Asetof5ditheredexposuresof ject to much less extinction than in galaxies, we therefore as-
60secondseachwereobtainedineachfilterinordertoremove sumethatitsspatialpattern(Scattered inEq.1)is independent
the mosaic gapsand otherphysicalblemishes. No sky subtrac- of wavelength.The BSS method we used here, called FastICA
tion,norconvolutionofanysortwasappliedtothedataduring (Hyvarinen 1999), enables us to recover the spatial patterns of
O.Berne´etal.:ExtendedRedEmissionandtheevolutionofcarbonaceousnanograinsinNGC7023. 3
Fig.3. Distribution maps of the three populations of mid-IR
emitters in NGC 7023 NW from Spitzer-IRS observations:
VSGs in red, PAH0 in green and PAH+ in blue (cf. Berne´ et
al2007).OverlayedincontoursistheemissionmapofEREex-
tractedfromHSTimages(Fig.1).
Fig.4. Distribution maps of the three populations of mid-IR
emitters in NGC 7023 from ISOCAM observations: VSGs in
red,PAH0ingreenandPAH+inbluefirstpresentedbyRapacioli
ERE and scattered light using the statistical information avail- etal.(2005)andreanalysedinthiswork.Overlayedincontours
ableinthethreeimages.FastICAusestheassumptionsofinde- istheemissionmapofEREextractedfromCFHTimages(Fig.
pendenceandnon-Gaussianityoftheoriginalsources(herethe 2).
spatialpatternsofEREandscatteredlight)andthusbelongsto
the class of methods called Independent Component Analysis
(ICA). The FastICA algorithm maximizes the non-Gaussianity
of the output signals (i.e. estimations of the sources) by using
afixed-pointalgorithm.TheextractedimagesofEREandscat-
tered light from ACS and CFHT images are presented on the
rightofFigs.1and2respectively.Wecanqualitativelycompare 4.2.IstheEREcarrieranimportantcontributortothemid-IR
these extracted ERE images to the one obtained by Wittetal. emissionspectrum?
(2006).Theimagesareverysimilarbutwefinda smallcontri-
butionofEREinthecavity.TheCFHTandHSTERE patterns A simple energy budget including ERE and the mid-IR emis-
appeartobeconsistentwitheachother. sioncanbemade.First,wecanconsiderthattheEREcarrieris
excitedbyphotonsof atleast7eV energy.Thiscorrespondsto
themeanenergyabsorbedbyVSGsaccordingtoRapaciolietal.
(2005).If we assume thatoneERE photonis emittedat anen-
4. LinkingEREandmid-IRemissioncarriers ergyofabout1.7eV (correspondingtoacentralwavelengthof
the ERE band at 680 nm detected by Witt&Boroson 1990),
4.1.Comparingthespatialdistributionofmid-IRemitting then the rest of the absorbed energy (5.3 eV) will be emitted
populationsandERE in the IR. Thus, about 25 % of the absorbed light is converted
into ERE. Table 1 summarizes the values of the mid-IR and
Rapaciolietal.(2005) andBerne´etal.(2007) haveusedsignal ERE fluxes for the regions of NGC 7023 where ERE is de-
processing methods to analyse the mid-IR spectral cubes form tectedaswellasfortheprototypicalcaseoftheRedRectangle.
ISO and Spitzer and extract the spectra and associated spatial The gross ratio between ERE and mid-IR flux for NGC 7023
distributionsforthedifferentemittingpopulations,VSGs,PAH0 NWPDRisbelow2%(0.4%fortheSouthPDR).Considering
and PAH+ as shown in Figs. 3-4. In Fig. 3, the HST ERE map the above approximationfor the ratio of energy emitted in the
was overlayed on the Spitzer maps showing that ERE arises IR vs visible for one particle, this yields a proportion of less
fromtheregionwherethepopulationidentifiedasPAH0 domi- than about 3 × 2% = 6% of the mid-IR emission due to the
natesthemid-IRemission.InFig.4,weoverlayedtheEREex- ERE carrier in the NW PDR (1.2% for the South PDR ). As
tractedfromtheCFHTobservationsonthemapsextractedfrom a comparison, in the Red Rectangle protoplanetarynebula, the
ISOCAMdata.ThesamecorrelationisfoundfortheNWPDR strongestknownsourceofERE,wefindthatthisratioisaround
as with Spitzer/HST, though the level of detail reached here is 3×0.8%=2.4%.ThisimpliesthattheEREcarriercontributesto
lower due to the lower spatial resolution of both ISOCAM vs onlyafewpercentofthemid-IRemission,andthusitssignature
IRSandCFHT vsHST. However,Fig. 4 providesthe informa- willbedifficulttoidentifyinthisspectralregion.Followingour
tionontheSouthPDR,whereagain,theEREfilamentisfound assignment, PAH0 are excluded as possible candidates for the
in a region dominated by PAH0, and close to the frontier with ERE, though they were initially proposed as potential carriers
VSGs. byd’Hendecourtetal.(1986).
4 O.Berne´etal.:ExtendedRedEmissionandtheevolutionofcarbonaceousnanograinsinNGC7023.
Table 1. Mid-IR and ERE fluxes in NGC 7023 and the Red and, perhaps more important, to have an efficient reformation
Rectangle. path.
Mid-IR∗ ERE∗∗ I /I 6. Conclusion
ERE mid−IR
PDR (Wm−2sr−1) (Wm−2sr−1)
InthisletterwehavecomparedinNGC7023thespatialdistri-
NGC7023-S 4.1110−5 1.4410−7 0.0035 bution of ERE and that of the mid-IR emitters : VSGs, PAH0,
NGC7023-N 1.4010−4 2.3010−6 0.0164 PAH+.WefindstrongevidencethattheEREcarrierisproduced
Red-Rect 9.0410−4 7.1010−6 0.008 intheregionofdestructionofVSGsandshowthatithasanegli-
∗IntegratedSpitzer-IRS(5-14µm)spectrumatthepositionsat giblecontributiontothemid-IRemission.WeshowthatPAH++
whichEREwasobservedbyWittandBoroson(1990),∗∗from areunlikelytobethecarrierofEREandconcludethat[PAH ]+
WittandBoroson(1990),integratedbetween550and850nm 2
are attractive candidates, on the basis of qualitative arguments
on the balance between photodissociation and reformation of
these clusters. Further investigations are needed, following the
strategy presented in this letter, but in other PDRs. Spectro-
5. PossiblecarriersofEREinNGC7023
imagery of such regions in the mid-IR with a high spatial res-
5.1.ThecaseofPAH++ olution is needed in order to be able to trace the thin frontier
between PAHs and VSGs where small clusters are likely to be
UsingtheACSobservations,Wittetal.(2006)showedthatERE present.Finally,laboratoryspectroscopicstudieson[PAH ]+are
2
islikelyatwostepprocessinvolvingtheformationofthecarrier
needed to progress in this identification. In particular, closed-
and then the excitation of the luminescence. The first step re-
shell cation dimers as proposed by Rheeetal. (2007) deserve
quiresfar-UVphotons(E>10.5eV)andsupportstheideathat
additionalstudies.
thecarrierofEREisproducedbythephoto-dissociation/photo-
ionizationofaprecursor.Fromthisresult,theyproposedPAH++ Acknowledgements. Weacknowledgetheanonymousrefereeforhiscomments
asthe carrierofERE, invokingthatthesespecieshavean ioni- onthemanuscript.
sationpotentialabove10.5eVandhavestrongabsorptionbands
in theopticalandnear-UVregions.In theprevioussection, we
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24 12 13
ergiesofaround10eV.Thisisinagreementwiththethreshold
of10.5eVsetbyWittetal.(2006)fortheproductionoftheERE
carrier.Then,forEREtobeobserved,thecarriershouldsurvive
long enough in the PDR i.e. be reformed as efficiently as it is
destroyed.[PAH ]+ appearasgoodcandidatesbecause(1)their
2
stability is expected to be increased relative to neutral dimers
because of charge delocalization effects (Bouvieretal. 2002),
(2) their abundanceis favored because they constitute the final
stageinthephotodissociationcascadestartingfromlargerclus-
ters;(3)theycanbereformedefficientlybycollisionofaneutral
andionizedPAHasthisprocessisfavoredbythelongrangeion
↔induceddipoleinteraction(see discussioninRapaciolietal.
2006). PAH+ are not abundant species in the ERE region but
are present(∼ 10% of total mid-IR emission). This fraction of
PAH+wouldbeenoughtoleadtothereformationof[PAH ]+in
2
regionswherePAH0 areabundant.Thus,[PAH ]+ arefavorable
2
candidates since they are expected to be relatively more stable