AnewschemeofradiationtransferinHIIregionsincludingtransientheatingofgrains
S.K.Ghosh∗&R.P.Verma
TataInstituteofFundamentalResearch,HomiBhabhaRoad,Bombay400005Received2000April15;accepted2000May11
Abstract.Anewschemeofradiationtransferforunderstand-inginfraredspectraofHIIregions,hasbeendeveloped.Thisschemeconsidersnon-equilibriumprocesses(e.g.transientheat-ingoftheverysmallgrains,VSG;andthepolycyclicaromatichydrocarbon,PAH)also,inadditiontotheequilibriumthermalemissionfromnormaldustgrains(BG).Thesphericallysym-metricinterstellardustcloudissegmentedintoalargenumberof“onionskin”shellsinordertoimplementthenon-equilibriumprocesses.TheschemeattemptstofittheobservedSEDorigi-natingfromthedustcomponent,byexploringthefollowingpa-rameters:(i)geometricaldetailsofthedustcloud,(ii)PAHsizeandabundance,(iii)compositionofnormalgrains(BG),(iv)radialdistributionofalldust(BG,VSG&PAH).
TheschemehasbeenappliedtoasetoffivecompactHIIre-gions(IRAS18116–1646,18162–2048,19442+2427,22308+5812,&18434–0242)whosespectraareavailablewithadequatespec-tralresolution.Thebestfitmodelsandinferencesabouttheparametersforthesesourcesarepresented.
Keywords:HIIregions–radiativetransfer–PAH–VSG
1.Introduction
Tillrecently,themidtofarinfraredspectralenergydistribution(SED)ofGalacticstarformingregionsingeneralwasavailableonlyinthefourIRASbands(12,25,60&100µm).Insomerelativelyrarecases,spectroscopyinthe10µmbandthroughtheatmosphericwindow,wasalsoavailable.However,thesituationhaschangeddrasticallyrecently,duetotheadventoftheInfraredSpaceObservatory(ISO).TheISOPHOTphotometeralong
62S.K.Ghosh&R.P.Verma
withISO-SWSandISO-LWSspectrometerstogetherhasrevolutionizedtheavailabilityofinformationaboutSEDoftheastrophysicalsourcesingeneral.
Intheliterature,severalradiationtransferschemeshavebeenusedforinterstellardustcloudswithembeddedYSOsinspherical(e.g.Scoville&Kwan1976,Leung1976,Churchwell,Wolfire,&Wood1990)aswellascylin-drical(Ghosh&Tandon1985,Dent1988,Karnik&Ghosh1999)geometries.Alloftheseconsideredthedustgrainstobeinthermalequilibrium.Theroleofnon-equilibriumprocesses(resultingintransientheating/excitationofgrains,particularlyinthevicinityofasourceofUVradiation)hasbecomeevidentfromsignificantnear&midinfraredcontinuumemissiondetectedinGalacticstarformingregions(Sellgren1984,Puget,Leger,&Boulanger1985,Boulanger,Baud&vanAlbada1985)aswellasspectralfeatures(Leger&Puget1984,Allamandola,Tielens,&Barker,1985,Puget&Leger1989).Theimportanceoftheseprocesseshasalsobeendemonstratedinex-tragalacticnuclei/starformingregions(Moorwoodetal.,1996,Metcalfeetal.,1996,Ghosh,Drapatz,&Peppel,1986).Acomparisonoftheobservedmid-IRspectralfeatureswithlaboratorydata,hasledtotheidentificationofanewconstituentoftheinterstellarmedium-polycyclicaromatichydro-carbons(PAH).Theenhancedcontinuumemissioninthenear&midIRhasbeenmainlyattributedtotheverysmallgrains(VSG)ofradii10-100˚A.Hence,itisobviouslyimportanttoincludethenon-equilibriumprocessesinattemptingtomodeltheobservedSEDofstarformingregionsingeneral.Basically,grainsofverysmallsizeoralargeorganicmolecule,witheffectiveheatcapacitycomparabletotheenergyofasingleUVphotongetexcited(forashorttime)toanenergystatewellaboveitsthermalequilibriumstatecorrespondingtothelocalradiationfield.Thephotonsemittedduringthede-excitationprocesscontributetothenear/midIRpartoftheSED,whichshowscontinuumexcessandemissionfeatureswhichareunexplainedbyradiativetransfermodelsconsideringtheemissionfromlargegrainsinthermalequilibriumalone.Recently,Siebenmorgen&Krugel(1992)haveattemptedtoquantifythepropertiesofthedustcomponentsrelevantfornon-equilibriumprocesses(VSG&PAH),fromtheinfrareddataofsourcesindifferentastronomicalenvironmentsinourGalaxy.TheroleofVSGontheinfraredemissionfromexternallyheateddustcloudshasbeenstudiedbyLis&Leung(1991).Krugel&Siebenmorgen(1994)havepresentedamethodtomodelthetransferofradiationindustygalacticnuclei,whichincludesthepresenceofVSGandPAH.
Herewepresentaschemeofradiativetransferdevelopedbyuswhichisapplicableinsphericalgeometry.Thisincludes,inadditiontothedustgrainsinthermalequilibrium(ofnormalsize,hereafterbiggrainsorBG),thetransientheatingofverysmallgrains(VSG)aswellasthePAHmolecules.AnattempthasbeenmadetomodelfivecompactHIIregions:IRAS18116–1646,18162–2048,19442+2427,22308+5812&18434–0242usingtheabovescheme.
RadiationtransferinHIIregions63
Insection2,theradiativetransfermodellingschemeisbrieflydescribed.TheresultsofmodellingthefivecompactHIIregionsarepresentedinsection3.Thelastsection(4)consistsofdiscussion.
2.
2.1
TheModellingScheme
Dustcomponentsandtheirproperties
Thenormalgrains(BG)consistoftwocomponents:astronomicalsilicateandgraphite.TheirsizedistributionistakenasperMathis,Rumpl&Nordsieck(1977)tobeapowerlaw,n(a)∝aγ,with–3.5astheexponent.Thelowerandupperlimitsofthegrainradiiaretakentobe0.01µmand0.25µmrespectivelyasrecommendedbyMathis,Mezger&Panagia(1983),forbothastronomicalsilicateaswellasgraphitegrains.Thescatteringandabsorptioncoefficients,andanisotropicscatteringfactorshavebeentakenfromDraine&Lee(1984)andLaor&Draine(1993).
TheVSGcomponentistakentobegraphitegrainsofasinglesize:either10˚Aor50˚Ainradius.TheiropticalpropertieshavealsobeentakenfromDraine&Lee(1984).AbundanceofVSGisconnectedtothatofthenormalgrainsthroughascalingfactorYVSG,whichgivesthefractionofdustmassinVSGformtothenormalBGform.ThevalueofYVSGwastakenfromDesert,Boulanger,&Puget(1990),whichisneededtoaccountforthe2200˚AbumpintheaverageinterstellarmediumintheGalaxy,andithasbeenheldfixedforallmodelsconsideredhere.
ThePAHcomponentisassumedtobeeitherasinglemoleculewithabout15–30atoms,oralargecomplexconsistingof10–20ofthesemoleculesasusedbySiebenmorgen(1993).Theiropticalproperties,featurecentres,fea-tureshapeandwidthshavebeentakenfromLeger&d’Hendecourt(1987).TheabundanceofPAHcomponentisalsoconnectedthroughascalingfac-torYPAHtothenormalgrains(BG).TherearetwoadditionalparametersexploredinthemodellingoftheobservedPAHspectralfeatures:(i)theradiusofthePAHmolecule/complex,aPAH;and(ii)thede-hydrogenationfactor,fde−H.Thevalueoffde−Hliesbetween0and1(fde−H=0referstocompletelyhydrogenatedPAH).WhereasaPAHhasimplicationsofheatcapacityandhencetheefficiencyoftransientheatingforagivenradiationfield,thefde−HaffectstheratiosofPAHfeaturesresultingfromtheC-HversusC=Cstretchmodes.2.2
Geometry
Thestarformingregionisconsideredasasphericaldustcloudimmersedinanisotropicinterstellarradiationfield,withanembeddedsourceofenergy(e.g.aZAMSstar)atitscentre.Acentralcavityinthiscloudrepresents
64S.K.Ghosh&R.P.Verma
Figure1.Schematicdiagramoftheshellstructureofthecloud.
sublimation/destructionofgrainsintheintenseradiationfieldofthecentralsource.AschematicofthedustcloudispresentedinFigure1.
Thissphericallysymmetricdustcloud,isdividedintoalargenum-berofconcentriccontiguoussphericalshells(saySh1,Sh2,...,ShN)like“onionskins”.Eachshell,Shi,isidentifiedbyitsinnerandouterradiimin&Rmax;seeFigure1).Theseshellscanbeofdifferentselectable(Rii
thicknesses,dependingontheopticaldepthattheshortestrelevantwave-length.Inordertoincorporatethepresenceofboth–normalgrains(BG,responsibleforemissionatthermalequilibrium),aswellasthegrainsrespon-siblefornon-equilibriumemission(VSGandPAH),eachshellissubdivided
Pcorrespondingtothesetwocom-&ShVintoapairofsub-shells,ShBGii
ponentsrespectively.WhereastheformerconsistsofonlyBG,thelatterconsistsofonlytheVSGandPAH.
Thefulldetailedradiativetransfercalculationsassumingthenormalgrainstobeinthermalequilibrium,areperformedineachofthesub-shells
VPShBGi,fori=1,2,...,N.ThesubshellsShi,gothroughastatisticalme-chanicaltreatmentdescribingthenon-equilibriumemissionprocessesforthe
PVSGsandthePAHs.Forsimplicityofcomputations,thesub-shellsShVi
areconsideredtobeverythincomparedtothetotalthicknessoftheshellShi,andthissub-shellisassumedtobeplacedattheinneredgeoftheshell
RadiationtransferinHIIregions65
Shi(seeFigure1).Thefinalresultsareexpectedtobeinsensitivetotheabovesimplificationsinceindividualshellsareopticallythin.
Radiativetransportateachofthetwosub-shellsiscarriedoutasatwopointboundaryvalueproblem,thetwoboundaryconditionsbeingtheincidentradiationfieldsatthetwosurfaces.Thecalculationsbeginwiththegivenspectrumemittedbytheembeddedenergysource(ingeneral,anInitialMassFunctionweightedsyntheticstellarspectrumensemble)incidentattheinnerboundaryofthefirstshellSh1.Theoutersurfaceofthelast(outermost)shell,ShN,hastheinterstellarradiationfield(ISRF)incidentonitfromtheoutside.Startingfromthe“core”sideofthefirstshell,the
Pfirstandtheemergentradiationistransportedthroughthesub-shellShV1
processedspectrumisconsideredtobeincidentontheothersub-shellShBG1.TheemergentspectrumfromthelatteristheprocessedoutputoftheentireshellSh1andisusedasinputboundaryconditionforthenextshellSh2.Inthismanner,theradiationfieldistransportedoutwardfromshellSh1toSh2...tillthelastshell,viz.,ShNisreached.Thisentireprocessingfromshell1,2,...toN,constitutesoneiteration.Severalsuchiterations(typically5–10)arecarriedoutuntilasetofpredeterminedconvergencecriteriaaresatisfied.Theemergingspectrumfromthelastshell,ShN,isthedesiredoutputofthefullmodel.Thenumberofshellsusedforaspecificsourceisdeterminedbythecriterionthattheshellisopticallythinintheshortestrelevantwavelength.2.3
ProcessingoftransientheatingoftheVSGandthePAH
Asdescribedabove,thedustcomponentsinashellforwhichthenon-equilibriumemissionprocessesareimportant,aresegregatedintoasepa-Pratesub-shell(ShVk)consistingonlyoftheVSGandthePAH.Thein-teractionofthetotalincidentradiationfieldfromboththesurfacesofthis
ν),withtheVSGcomponent,isconsideredusingacodede-sub-shell,(Iin
velopedbyusbasedonthestatisticalmechanicaltreatmentprescribedbyDesert,Boulanger&Shore(1986).Theincidentradiation,partlyextin-ννν×e−τVSG),isconsideredincidentontheguishedbytheVSGs(IV=IinP
PAHcomponentandasimilarcomputationisrepeated.Thefinalemergingspectrumconsistsofthreecomponents:(i)theoriginallyincidentradiation
νννν×e−(τVSG+τPAH)),extinguishedbybothVSGaswellasPAH,(Iout=Iin
(ii)theemissionfromtheVSGcomponent,and(iii)theemissionfromthePAHcomponent.Whereasthefirstcomponentisdirectionsensitive(thetwosurfacesgetdifferentcontributionsdependingontheoriginalspectrumincidentattheothersurfaces),thelattertwocontributeequallytothetwosurfaces.
TheVSGandPAHcomponentsofgrainshavefluctuatingtemperature,mainlybecausetheirenthalpy(internalenergy)iscomparabletotheenergyofUVorvisualphotons.Thismeans,themultiphotonabsorptionprocesses
66S.K.Ghosh&R.P.Verma
canbecomeimportant(dependingontheexactradiationfieldandthede-tailsofthermal&opticalpropertiesofthesegrains)astheycanleadtoamodifiedtemperaturedistribution.AniterativemethodhasbeenusedheretoconsiderthesemultiphotonprocessesforVSGsandPAHsseparately.Themethodassumesasinglegraininanisotropicradiationfield,andfollowstheevolutionofthegraintemperaturebysolvingtherelevantstochasticdiffer-entialequation.
Aschemeofbetween100to400levelsofinternalenergy(covering0.5eVto200eV)forconsideringdiscreteheating/coolingprocesses;and400energylevels(forenergies1.25×10−3eVto0.5eV)forconsideringthecontinuumprocesses,hasbeenincorporated.Atotalof97frequencygridpointscovering0.0944µmto5000µmhavebeenused.SeveralgridpointsaredenselypackedaroundthefivePAHfeaturesat3.3,6.2,7.7,8.6and11.3µm.2.4
Radiationtransportthroughnormalgrains(BG)
Eachofthesub-shellsconsistingofthenormalgrains,(ShBGk,k=1,2,...N),separatelyundergoesfullradiativetransportcalculationusingthecodeCS-DUST3developedbyEgan,Leung&Spagna(1988)(seealsoLeung1975).InCSDUST3,themomentequationofradiationtransportandtheequationofenergybalancearesolvedsimultaneouslyasatwo-pointboundaryvalueproblem.Theeffectsofmultiplescattering,absorptionandre-emissionofphotonsonthetemperatureofdustgrainsandtheinternalradiationfieldhavebeenconsideredself-consistently.Inaddition,multigraincomponents,radiationfieldanisotropyandlinearanisotropicscatteringarealsoincorpo-rated.
Samefrequencygridof97points,asusedforVSGandPAH,hasbeenusedhere.Inordertoavoidnon-convergenceproblemsduetosharpchangesinopticaldepthatanyofthefrequencygrids,logarithmicallyincreasingradialgridspacingshavebeenusedattheinnershellboundary.Similarlyasmoothlydecreasinggridspacingshavebeenusedneartheoutershellboundary.2.5
TheModellingScheme
TheschemeaimstoconstructamodelconstrainedbytheobservedSEDcoveringtheentireinfraredandthesub-mm/mmregion.Basedoncom-parisonsofthemodelpredictedSEDswiththeobservedSED,variousmodelparametersarefinetunedtillthebestfitmodelisidentified.Thefollowingmodelparametersareexplored:(i)thetotalradialopticaldepth(repre-sentedatafiducialwavelengthof100µm);(ii)exponentofthedustdensitydistributionpowerlaw;(iii)theratioofgraphitecomponenttotheastro-nomicalsilicatecomponentforBGs;(iv)sizeofVSGs,aVSG:either50˚Aor
RadiationtransferinHIIregions
Table1.InputparametersofthecompactHIIregions
67
IRASSource
(L⊙)1.6×105
18162–204822308+5812
5.4×1041.0×106
T∗
(kpc)4.4
30,900
2.3
37,500
7.4
θdia
(pc)1.1263
0.5295
0.69
10˚A;(v)sizeofPAHmolecule/cluster,aPAH:4.6˚Aor8˚Aor13.6˚A;(vi)relativeabundanceofPAHcomparedtoBGs,YPAH(aconstantorvaryingwiththeradialdistance);and(v)thede-hydrogenationfactor,fde−H.
Theinnerradius(Rin),foreachmodelofthesphericalcloud,hasbeendeterminedusingtheconstraintthatthetemperatureoftheBGisequalto1500K,thesublimationtemperatureofthenormalbiggrains(graphiteandastronomicalsilicate).Theradialdustdensitydistributionlawhasbeenassumedtobeapowerlawnd(r)∝r−α,andthevaluesofαthathavebeenexploredare0,1and2.
3.Applicationofthemodellingscheme
Inordertodemonstratetheusefulnessoftheschemedescribedabove,anattempthasbeenmadetoapplythesametoafewHIIregions.Thequestionis:inspiteofrathersimplistictreatment,canwegetanyinsightintophysicaldetailsofthesesources?
3.1ThesampleofcompactHIIregions
WiththeadventofInfraredSpaceObservatory(ISO),ithasbecomenowpossibletohaveprecisespectroscopicinformationintheentireinfraredbandencompassingneartofarinfraredregion.ThespectroscopicresultsforasampleofsixGalacticcompactHIIregions,coveringfourofthefivemajorPAHfeatureshavebeenpublishedbyRoelfsemaetal.(1996).Wehavechosenfiveoutoftheirsixsourcesforourdetailedstudy.ThesixthsourceIRAS21190+5140,identifiedwithM1-78andvariouslyconsideredasHIIregionandplanetarynebula(Pucheetal.1988,Ackeretal.1992),hasnotbeenconsideredhere.AlthoughthepublishedspectralresultsfromISOisratherlimited(6-12µm),iftheIRASPointSourceCatalog(IRASPSC)measurements(at12,25,60&100µm),IRASLowResolutionSpectra(IRASLRS;between8–22µm)andgroundbasedspectroscopyaround
68S.K.Ghosh&R.P.Verma
Table2.DustparametersvalidfortheentiresampleofcompactHIIregions
Dustcomponent
amin(µm)amax(µm)γaVSG(˚A)YVSGaPAH(˚A)YPAHfde−H
Value
RadiationtransferinHIIregions69
Fig. 2 (1_a)
27
’18116-1646_MODEL’’18116-1646_SWS’’18116-1646_OTHERS’
25.5
Fig. 2 (1_b)
’18116-1646_MODEL’’18116-1646_SWS’’18116-1646_OTHERS’
26
25
log-SED (erg/sec/Hz) log-SED (erg/sec/Hz) 2524.5
24
24
23.5
23
23
22
22.5
21
00.511.5
log-lamda(micron)
22.53
22
0.50.60.7
0.80.9 log-lamda(micron)
11.11.2
Figure2.SpectralenergydistributionofthecompactHIIregionIRAS18116-1646.Theordinateisthelogofthefluxdensitymultipliedbythesurfaceareaoftherespectivecloud.SolidlinesshowtheISO-SWSspectrafromRoelfsemaetal.(1996);dottedlinesshowourbestfitmodelspectra;diamondsshowotherobservations.Otherobservationsinclude—3µmobservationsfromdeMuizon,d’Hendecourt&Geballe(1990),IRASLRSspectrafromOlnon&Raimond(1986)orfromVolk&Cohen(1989),IRASPSCfluxdensities,sub-mmobservationsfromMcCutcheonetal.(1995),Jenness,Scott,&Padman(1995)orBarsony(1989),and1.3mmobservationofChini,Krugel,&Kreysa(1986).InordertoshowthePAHfeaturesclearly,midIRregionoftheSEDsareshownseparately(ontheright).
tioni.e.n(r)∝r0(asopposedton(r)∝r−1orr−2)gavemuchbetterfitstotheSEDs.TheVSGswithaVSG=50˚AandthePAHswithinter-mediatesize(i.e.aPAH=8˚A)givebetterfitstotherespectivespectra.Thede-hydrogenationfactor,fde−H,needstobezero(correspondingto√NH=
70S.K.Ghosh&R.P.Verma
Fig. 2 (2_a)
26
’18162-2048_MODEL’’18162-2048_SWS’’18162-2048_OTHERS’
25
Fig. 2 (2_b)
’18162-2048_MODEL’’18162-2048_SWS’’18162-2048_OTHERS’
25
24
24 log-SED (erg/sec/Hz) log-SED (erg/sec/Hz) 23
2322
2221
2120
200
0.5
1
1.5
log-lamda(micron)
2
2.5
3
19
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
log-lamda(micron)
Figure2.Continued...(forIRAS18162-2048)
Fig. 2 (3_a)
26
’19442+2427_MODEL’’19442+2427_SWS’’19442+2427_OTHERS’
Fig. 2 (3_b)
’19442+2427_MODEL’’19442+2427_SWS’’19442+2427_OTHERS’
25
25
24 log-SED (erg/sec/Hz) log-SED (erg/sec/Hz) 24
2323
22
22
21
21
200
0.5
1
1.5
log-lamda(micron)
2
2.5
3
20
0.5
0.6
0.7
0.80.9 log-lamda(micron)
1
1.1
1.2
Figure2.Continued...(forIRAS19442+2427)
ducethePAHfeatures.Thesizeofthisregionisquantifiedbyaparameter
PAH−R)/(RηPAH,whichisdefinedas:ηPAH=((Routinout−Rin)).This
parameterhadtobevariedforeachsource,tillagoodfittothespectrumwasobtained.Inaddition,theabundanceofPAHrelativetoBGs,YPAH,neededtobeincreasedbyafactor10relativetothenormalvalueobtainedbyDesert,Boulanger&Puget(1990).Howeverthisdoesnotleadtoany
RadiationtransferinHIIregions71
Fig. 2 (4_a)
’22308+5812_MODEL’’22308+5812_SWS’’22308+5812_OTHERS’
25.5
Fig. 2 (4_b)
’22308+5812_MODEL’’22308+5812_SWS’’22308+5812_OTHERS’
26
25
25 log-SED (erg/sec/Hz) log-SED (erg/sec/Hz) 0
0.5
1
1.5
log-lamda(micron)
2
2.5
3
24.5
24
24
23.5
23
23
2222.5
22
21
21.5
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
log-lamda(micron)
Figure2.Continued...(forIRAS22308+5812)
Fig. 2 (5_a)
27
’18434-0242_MODEL’’18434-0242_SWS’’18434-0242_OTHERS’
27
Fig. 2 (5_b)
’18434-0242_MODEL’’18434-0242_SWS’’18434-0242_OTHERS’
26
26
log-SED (erg/sec/Hz) 24
log-SED (erg/sec/Hz) 0
0.5
1
1.5
log-lamda(micron)
2
2.5
3
2525
24
23
23
22
22
21
2021
0.50.60.70.80.911.11.2
log-lamda(micron)
Figure2.Continued...(forIRAS18434-0242)
conflictwiththeavailablecarbon,sinceηPAH<<1.Thevaluesofthebestfitparametersspecifictoeachsource,arepresentedinTable3.
72S.K.Ghosh&R.P.Verma
Table3.BestfitparametersofthecompactHIIregionsasdeterminedbymod-elling
IRASSourceName
RoutMTotGraphite:Silicate
(pc)
(cm−3)18116–16461.121.9×1035.3×10−4
1.32×1050.1419442+2427
7.7×1021.5
×10−3
0.52
1.38×
104
0.068
18434–0242
0.69
1.8×103
4.Discussion
Thefollowinginferencescanbedrawnaboutthesourcesmodelledhere,providedthebasicassumptions(e.g.sphericalsymmetry;sourcesofenergylocatedonlyatthecentreofthecloud;etc)arenotatgreatvariancefromthereality.
Themostfavouredradialdustdensitydistributionlaw,forallfivesources,turnsouttobeofuniformdensity.Thiscanperhapsbeunderstoodintermsofthefarinfraredconstraints(IRAS-PSC60&100µmdata).Ifthedustdensityisfallingwithradialdistance,theninordertofittheFIRpartoftheSED,sohighadustdensityisrequiredatthevicinityoftheembeddedZAMSstar,thatthemidinfraredemissionbecomesinvisible.Thisprob-lemcanperhapsbeavoidedinanon-sphericallysymmetricgeometry.WehaveexploredtheeffectsofrelaxingtheassumptionthatRin=R0,whereR0istheradialdistanceatwhich(BG)graintemperaturebecomesequaltothesublimationtemperature(1500K).ThemostimportanteffectofmakingRin>R0,istodrasticallymodifythenearandmidinfraredcontinuumlevelofthepredictedspectrum.Inaddition,theroleofnon-equilibriumprocessesvis-a-visthermalequilibriumemissionofthePAHfeatures,aswellasthecontinuumduetoVSG,changessignificantly.
AquickperusalofFigure2andTables2and3bringsoutthefollowingfacts:
1)AllthecompactHIIregionsconsideredhere,aredeeplyembeddedstars;totalopticaldepthat100µmintherangeof0.056–0.14.ThisisnecessarytoexplainthefarIRspectraobservedbyIRAS.
2)PAHisconfinedonlytoathincentralshell;thethicknessofthisshellbeingjustafewpercent(1.3–5.5%)ofthetotalthicknessofthedustcloud.AsthesesourcesareopticallythickatmidIR,ifthePAHisdistributedthroughoutthecloud,itsemissionwhichoccursintheinnerhot
75:252.7×10−295:51.3×10−2
95:5
RadiationtransferinHIIregions73
regionwherehighenergyphotonsresponsiblefornon-equilibriumprocessesarepresent,willbeabsorbedbytheoutercoolershells,andPAHfeatureswillnotbedetectable.
3)TheBGsaredominatedbygraphites,withsilicatescontributinglessthan25%.Thelatterhasbeentieddownratherpreciselybythe10µmsilicatefeature.
4)ISO-SWSfluxesaregenerallymuchsmallerthanIRASfluxesatsimilarwavelengths,indicatingthatSWSisnotsamplingfullemissionatmidIRandthesourcesizesatthesewavelengthsaremuchlargerthantheSWSbeamsize(14′′×20′′).Withthisinmind,wehavenottriedtofittheabsolutefluxesoftheSWSbutonlyuseditsshapeasindicativeoftheimportanceofPAHmolecules.
Followingcommentscanbemadeabouttheindividualsources:•IRAS18116–1646:Ithasrelativelyloweropticaldepth.ThefittotheIRdataisquitereasonable,exceptat100µmwhereIRASfluxishigher;nosub-mmobservationexistsforthissource.•IRAS18162–2048:Thissource(GGD27)wasoriginallythoughttobeaHHobject.Howevernowithasbeenestablishedasastarformingregionwithreflectionnebulosityaswellasoutflow(seeforexampleStecklumetal.1997).TheregionhasseveralnearIRandmidIRsources;thesourceofenergybeingclosetoIRS2.Thesizeofthissourceatsub-mmwavelengthsis∼1′(McCutcheonetal.1995),consistentwiththesizeforthebestfitmodel.ThemassoftheenvelopeestimatedbyourmodelisnotfarfromtheestimateofYamashitaetal.(1987),viz.,200M⊙.Theyhaveproposedadiskgeometryforthissource.Thissourcehasveryhighopticaldepth.ThefittotheIRdataisquitereasonablebutatthesub-mmwavelengthsthecalculatedfluxdensitiesarelowerthantheobservedones.•IRAS19442+2427:ThissourceliesintheHIIregionS87.Thesizeofthissourceatsub-mmwavelengthsis∼1′(Jenness,ScottandPadman,1995),consistentwiththesizeforthebestfitmodel.Thissourcehasmediumopticaldepth.Thefittoalltheobservationsfrom3µmto850µmisquitereasonable.•IRAS22308+5812:Ithasrelativelyloweropticaldepth.ThefittotheIRdataisquitereasonable;nosub-mmobservationexistsforthissource.•IRAS18434–0242:Thissourceisthemostluminoussourcewithhighopticaldepth.TherearenoIRobservationsforthissourceotherthanthosefromIRASandISO.IRASPSC100µmaswellas1.3mmobservationsarehigherthancalculated.
74S.K.Ghosh&R.P.Verma
Fromtheabove,weconcludethatournewschemeofradiativetransferwhichincludesnon-equlibriumprocesses(transientheatingofthegrains/PAH/VSG)inadditiontotheemissioninthermalequilibrium,cangiveimportantphysicalinsightintoGalacticstarformingregions.Ifthesimplify-inggeometricalassumptionsofourschemearevalid,thenseveralimportantinferencescanbemadeaboutthefivecompactHIIregionsconsideredformodellinghere.
AcknowledgementsItisapleasuretothankBhaswatiMookerjeaforherhelpinpreparingsomeinputparameters.
RadiationtransferinHIIregions75
References
Acker,A.,Marcout,J.,Ochsenbein,F.,Stenholm,B.,Tylenda,R.1992,
“Strasbourg-ESOCatalogueofGalacticPlanetaryNebulae”(Munchen,ESO).
Allamandola,L.J.,Tielens,A.G.G.M.,Barker,J.R.1985,Astrophys.J.Letter,
290,L25.
Allamandola,L.J.,Tielens,A.G.G.M.,Barker,J.R.1989,Astrophys.J.(Suppl.),
71,733.
Barsony,M.1989,Astrophys.J.,345,268.
Boulanger,F.,Baud,B.,vanAlbada,G.D.1985,Astr.Astrophys.,144,L9.Chini,R.,Krugel,E.,Kreysa,E.1986,Astr.Astrophys.,167,315.
Churchwell,E.,Wolfire,M.G.,Wood,D.O.S.1990,Astrophys.J.,354,247.
deMuizon,M.J.,d’Hendecourt,L.B.,Geballe,T.R.1990,Astr.Astrophys.,227,
526.
Dent,W.R.F.1988,Astrophys.J.,325,252.
Desert,F.X.,Boulanger,F.,Puget,J.L.1990,Astr.Astrophys.,237,215.Desert,F.X.,Boulanger,F.,Shore,S.N.1986,Astr.Astrophys.,160,295.Draine,B.T.,Lee,H.M.1984,Astrophys.J.,285,89.
Egan,M.P.,Leung,C.M.,Spagna,G.F.1988,ComputerPhysicsCommunications,
48,271.
Ghosh,S.K.,Drapatz,S.,Peppel,U.C.1986,Astr.Astrophys.,167,341.Ghosh,S.K.,Tandon,S.N.1985,Mon.Not.R.Astr.Soc.,215,315.
Jenness,T.,Scott,P.F.,Padman,R.1995,Mon.Not.R.Astr.Soc.,276,1024.Karnik,A.D.,Ghosh,S.K.1999,J.Astrophys.Astr.,20,23.Krugel,E.,Siebenmorgen,R.1994,Astr.Astrophys.,282,407.Laor,A.,Draine,B.T.1993,Astrophys.J.,402,441.
Leger,A,d’Hendecourt,L.1987,“PAHandAstrophysics”(eds.A.Leger,L.
d’Hendecourt&N.Boccara),p223.
Leger,A.,Puget,J.L.1984,Astr.Astrophys.,137,L5.Leung,C.M.1975,Astrophys.J.,199,340.Leung,C.M.1976,Astrophys.J.,209,75.Lis,D.C.,Leung,C.M.1991,Icarus,91,7.
Mathis,J.S.,Mezger,P.G.,Panagia,N.1983,Astr.Astrophys.,128,212.Mathis,J.S.,Rumpl,W.,Nordsieck,K.H.1977,Astrophys.J.,217,425.
McCutcheon,W.H.,Sato,T.,Purton,C.R.,Matthews,H.E.,Dewdney,P.E.1995,
Astr.J.,110,1762.
Metcalfe,L.,Steel,S.J.,Barr,P.,etal.1996,Astr.Astrophys.,315,L105.Mookerjea,B.,Ghosh,S.K.1999,Bull.Astr.Soc.India,27,567.
Moorwood,A.F.M.,Lutz,D.,Oliva,E.,etal.1996,Astr.Astrophys.,315,L109.OlnonF.M.,Raimond,E.(IRASScienceTeam)1986,Astr.Astrophys.(Suppl.),
65,607.
Puche,D.,Zijlstra,A.A.,Boettcher,C.,etal.1988,Astr.Astrophys.,206,89.Puget,J.L.,Leger,A.1989,Annu.Rev.Astron.Astrophys.,27,161.Puget,J.L.,Leger,A.,Boulanger,F.1985,Astr.Astrophys.,142,L19.
Roelfsema,P.R.,Cox,P.,Tielens,A.G.G.M.,etal.1996,Astr.Astrophys.,315,
L289.
Scoville,N.Z.,Kwan,J.1976,Astrophys.J.,206,718.
76S.K.Ghosh&R.P.Verma
Sellgren,K.1984,Astrophys.J.,277,623.
Siebenmorgen,R.1993,Astrophys.J.,408,218.
Siebenmorgen,R.,Krugel,E.1992,Astr.Astrophys.,259,614.
Stecklum,B.,Feldt,M.,Richichi,A.,Calamai,G.,Lagage,P.O.1997,Astrophys.
J.,479,339.
Thompson,R.I.1984,Astrophys.J.,283,165.Volk,K.,Cohen,M.1989,Astr.J.,98,931.
Yamashita,T.,Sato,S.,Nagata,T.,Suzuki,H.,Hough,J.H.,McLean,I.S.,Garden,
R.,Gatley,I.1987,Astr.Astrophys.,177,258.
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