1995ApJS ... 99. .565S tic nuclei(Planesas,Scoville,&Myers1991;Rotaciucetal. (Genzel, Hollenbach,&Townes1994),andstarburstgalaxy berg &Dalgarno1989).PDRsareubiquitousintheinterstellar etary nebulae(Grahametal.1993;Latter,Walker,&Maloney heijde, Jansen&vanDishoeck;derWerfetal.1995),plan- al. 1993;vanderWerfetZhouHoger- from spaceusingmid-andfar-infrared spectrometersonboard nuclei (Staceyetal.1991;Henkel,Baan,&Mauersberger Howe etal.1991;Meixner1992),theGalacticcenter star-forming regions(Stutzkietal.1988;Lane1990; Dishoeck, &Black1994),insideclumpymolecularcloudsin Dalgarno, &Boland1980;TielensHollenbach1985;Stern- diation fields(6-13.6eV)fromexternalsources(deJong, drogen cloudswhichareexposedtofar-ultraviolet(FUV)ra- medium andarepresentattheedgesofHnregions(Staceyet cade. Amajordevelopment will betheobservationofPDRs techniques ofground-basedandair-borneinfraredimaging observational studieswillcontinue throughoutthenextde- al. 1993;Sargent&Welch Tielensetal.1993).These Poglitsch etal.1991;Genzel 1992;PhillipsKrabbeet Howe 1989;Genzel,Harris,&StutzkiHollenbach1990; millimeter waveinterferometry(Harrisetal.1987;Jaffe & spectroscopy, heterodynesubmillimeter and individual PDRsingreatdetailusingthemultiwavelength line emission.Inrecentyearsithasbecomepossibletostudy ionization andphotodissociation.Thegastemperatures in and drivethecloudchemistryinsequencesinitiatedbyphoto- trolled bytheincidentFUVphotonswhichbothheat gas able sourcesofinfraredandmillimeteratomicmolecular PDRs rangefrom—20to^1000K,andtheyareoftendetect- 1994). 1991; Jacksonetal.1993;Blietz1994;Tacconi 1991; Krabbe,Sternberg,&Genzel1994;Sternbergetal. 1994) ,andprobablyinthenarrow-lineregionsofactivegalac- 1993) ,andreflectionnebulae(Fuenteetal.1993;Jansen,van The AstrophysicalJournalSupplementSeries,99:565-607,1995August © 1995.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. Photon-dominated regions(PDRs)forminneutralhy- The physicalandchemicalpropertiesofPDRsarecon- Subject headings:galaxies:ISM—ISM:moleculesmolecularprocesses molecular diagnosticsofphoton-dominatedchemistryindenseclouds. production ofatomicandmolecularspeciesintheseveraldistinctzonesthatoccurcloud.Weidentify exposed toanintensefar-ultravioletradiationfield.Weuseitillustratethegas-phaseprocessesthatlead We presentadetailedchemicalmodelofphoton-dominatedregionproducedindensemolecularcloud © American Astronomical Society • Provided by theNASA Astrophysics Data System 1. INTRODUCTION Harvard-Smithsonian CenterforAstrophysics,60GardenStreet,Cambridge,MA02138 School ofPhysicsandAstronomy,TelAvivUniversity,Ramat69978,Israel CHEMISTRY INDENSEPHOTON-DOMINATEDREGIONS Received 1994March17;acceptedAugust10 A. Sternberg A. Dalgarno ABSTRACT AND 565 and thedumpinessofclouds. Allofthesequantitiesvary erties ofthedustgrains,gas-phase elementalabundances, dent FUVradiationfield,the absorptionandscatteringprop- (hydrogen) gasdensityandpressure, theintensityofinci- regions dependonseveral cloud parametersincludingthe processes thatleadtotheefficientproductionofatomic and posed toanintensefar-ultravioletradiationfield.Weuse our ton-dominated regionproducedinadensemolecularcloud ex- molecular speciesindensePDRs. calculations toillustratethevariousgas-phasephotochemical line emissionsproducedintheclouds,andpresentedlimited discussions ofthecloudchemistry. such clouds,andontheatomicfine-structureHCO lens 1993;Kösteretal.1994;Goldshmidt&Sternberg1995). These authorsfocusedontheFUVabsorptionpropertiesof lens, &Hollenbach1990;Hollenbach,Takahashi,Tielens lens 1993;Burton,Hollenbach,&Tielens1990;Wolfire,Tie- incident FUVradiation.ManymodelsofopticallythickPDRs gate moleculeformationindensecloudswhichfullyabsorbthe & Dalgarno1989;McKeeWolfire,Hollenbach,Tie- have beenpresentedintheliterature(Tielens&Hollenbach hoeck &Black(1986,1989).However,theydidnotinvesti- difluse andtranslucentcloudshavebeenpresentedbyvanDis- variety ofmultiwavelengthGalacticandextragalacticemis- the InfraredSpaceObservatory{ISO)(Encrenaz&Kessler sion-hne data.Comprehensivechemicalmodelsoflow-density al. 1994)andhavebeenusedsuccessfullytointerpretawide Tielens &Hollenbach1985;vanDishoeckBlack1988; constructed anddevelopedbyseveralresearchgroups(e.g., Sternberg &Dalgarno1989;LeBourlotetal.1993;Köster 2 1991; Elmegreen1993;LeBourlotetal.Meixner&Tie- 1985; Black&vanDishoeck1987;Sternberg1988; 1992). The physicalandchemicalpropertiesofphoton-dominated In thispaperwepresentadetailedchemicalmodelofpho- Theoretical modelsofphoton-dominatedregionshavebeen 2. MODEL 1995ApJS ... 99. .565S -6 + 3_1 + -1 -45 -4 + + + + + + + + 6-3 5 -17 tion fieldandgastemperature vary.Wedescribethesepro- cesses indetail§3. molecular speciesvarywiththe clouddepthasthelocalradia- reaction pathwaysthatleadto thesynthesisofatomicand van Dishoeck&Black1986;1988;Millaret al. Figures 1-6(Dalgamo&Black1976;PrasadHuntress1979; carbon, sulfur,nitrogen,andsiliconchemistryareshown in data weusedinourcalculations. the cosmic-rayhydrogenionizationrate.Inourmodel we depth z(cm),andthecosmic-raydestructionratesdepend on vary withclouddepth. con. Weassumedthatthetotalelementalabundancesdonot depleted abundanceof1X10fortherefractoryelementsili- the carbonintermediatesCS,CS,HCS,HCSCN,HCN, adopted aninterstellarcosmic-rayHionizationratef= 5 X pend ontheincidentFUVradiationfieldandcloud et al.1991;Millar1991).Thephotodestructionrates de- between speciesjand/thatleadtotheproductionofz, dependent) ratecoefficients(cms)forchemicalreactions dances oftheheavyelements.Forvolatileelements,car- ties ofthehydrogenfamilyH,H,HJ,H3;oxygen (s )ofspeciesjwithproductsz(Gredeletal.1989;Roberge density ofz),andwhere/c(T)arethe(temperature- where x*=rii/«istheabundanceofatomormoleculei(where 6 X10,and1relativetohydrogen.Weadopteda solar abundancesequalto,respectively,3X10,1 bon, nitrogen,oxygen,andsulfur,weadoptedapproximately CN, HCNHandOCS. NH, NHJ,NandH;thesüiconfamilySi,Si and HS;thenitrogenfamilyN,N,NH,NH CH, CHJ,andCH5;thesulfurfamilyS,SSH,HSH carbon family,C,C,CH,CHCHJ, interstellar FUVfieldmultipliedbyanintensityscalingfactor equilibrium chemicalequationsoftheform SO, S0,SONO,NOSiO,Si0SiOandSiOH; SiH, andSiH;theoxygenintermediatesCO,CO,HCO family O,O,OH,H0,OH0andOJ;the incident FUVfieldequaltoDraine’s(1978)fittheaverage density n=10cmthroughoutthecloud,andadoptedan Tij(z) andarethephotoncosmic-raydestructionrates n isthetotalvolumedensityofhydrogennuclei,andrii X =2X10. In ourcalculationsweassumedaconstanthydrogenparticle cloud whichisexposed,ononeside,toanisotropicFUVfield. ments. considerably inthewiderangeofpossibleinterstellarenviron- 2 /Cy/(T)xx,+[r(z)tijlXj 566 1991). Thedominantformation anddestructionreactions 10 s.IntheAppendixwediscussbasicphotochemical 2 2 Z7/ 2 23 23 32 234 jl j 2 2 23 T jy The chemicalnetworksthatgovernthehydrogen,oxygen, The chemicalstructuredependsonthetotalgas-phaseabun- We computedtheatomicandmoleculardensitiesbysolving We usedourmodeltocomputethedepth-dependentdensi- Our modelconsistsofastatic,plane-parallel,semi-infinite © American Astronomical Society • Provided by theNASA Astrophysics Data System x 2 kjiii+[IMz)&;]■, j' i•> STERNBERG &DALGARNO (1) -3 1 we adoptedinourcalculations. the heatingandcoolingprocessesthatweincludedinour cal- ature equalto20K. heating ratesarelesscertainweadoptedauniformgastemper- culations intheAppendix.Atlargeclouddepthswhere the molecular rotationalandvibrationallineemission.Wediscuss is cooledbyatomicandionicfine-structurelineemission, and pumped hydrogenmolecules(Tielens&Hollenbach1985; Lepp &Dalgamo1988;Sternberg1989).The gas combination ofgrainphotoelectricemission,photoionization incident FUVradiationfielddominatesthegasheatingby a of ‘Targemolecules,”andcollisionaldeexcitationFUV- puted thegastemperatureinoutercloudlayerswhere the cooling rates(ergscm),respectively.Weexplicitlycom- where G(T)andL(T)arethetotallocalvolumeheating Grain photoelectricthreshold through thecloud(Sternberg&Dalgamo1989). to theconstrainingequation Turbulent Dopplerwidth Nitrogen abundance subject totheadditionalconstrainingequation H1-0 vibrationalcriticaldensity PAH abundance Silicon abundance Carbon abundance Oxygen abundance hydrogen particledensityn=+remainsconstant Sulfur abundance FUV intensity Hydrogen particledensity magnetic fieldwhichisfrozenintothegas,sothattotal our modelweassumethatthegaspressureisdominatedbya and ejectionofmoleculesfromgrainsurfaces(Breakers1991; mass density,andbisthe“turbulent”velocityparameter.In ular hydrogengasdensities,Bisthemagneticfield,p Shalabiea &Greenberg1994). and turbulentpressures,««haretheatomicmolec- where thetotalpressurepissumofthermal,magnetic, 2 THU2 H2 1 In Table1wesummarizethe inputmodelparametersthat In thermalequilibriumequation(1)mustalsobesolved In pressureequilibriumequation(1)mustbesolvedsubject With theexceptionofHweneglectpossibleformation Notethatn=-\-2nand n= +«hsothat±. 2 uH2 Tn2 Parameter 2 p =(n+n)kT—-pb(2) iiH2 G(T)-L(T) =0(3) Model Parameters TABLE 1 d2 1 -1 c3 3 £o(eV) b(km s) n cm [PAH] x n(cm ) [Si] [N] [S] [C] [O] H T Symbol 4 5 4 -6 7 4 5 6 XIO“ 2X 10 3 XKT 1 X10 1 XKT 1 XlO" 1 X10‘ Vol. 99 6 Value 10 ? n c/} LO s No. 2, 1995 CHEMISTRY IN DENSE PHOTON-DOMINATED REGIONS 567

Fig . 1.—Hydrogen network. Endothermic reactions involving atomic or molecular hydrogen are indicated by the symbols H and H2, respec- tively. Reactions involving FUV-pumped hydrogen molecules are indi- cated by the symbol H f.

3. RESULTS The results of our computations are shown in Figures 7-22. Figure 7 shows the thermal profile of our model cloud, and Figures 8-22 show the abundances, xz (z), of the various atomic and molecular species as functions of the cloud depth, and the integrated atomic and molecular column densities

Ni{z) = n f Xi(z)dzcttT2 (4) Jo

also as functions of the cloud depth. In all of these figures the cloud depth is parameterized by the total visual extinction, Á y, measured from the cloud surface. Figures 8-22 display the depth, and many species show distinct density peaks at several complex chemical structure of photon-dominated regions. The locations through the cloud. atomic and molecular abundances are sensitive to the cloud The behavior illustrated in Figures 8-22 may be understood by noting that several distinct zones exist, at various depths in the cloud, in which the chemical structure is governed by spe- cific sets of chemical reactions. These zones may be identified as an H i and H/H2 transition layer, followed by C n, S n, Si n, and S i zones, and a dark core. We define these zones below, and we discuss the detailed atomic and molecular gas-phase production and destruction processes that occur in each of them.

3.1. Hot H i Zone and H/H2 Transition Layer Close to the cloud surface rapid photodissociation of molec- ular hydrogen2

H2 + v —H + H (Rl)

produces an H i zone in which most of the hydrogen is present in atomic form. Hydrogen molecules form by the association of hydrogen atoms on grain surfaces

H + H + gr. H2 -h gr., (R2)

and the H2 density increases with increasing cloud depth as the rate of reaction (Rl ) is reduced by the combined effects of H2 2 The identification number of each reaction is listed with the letter R when it is displayed for the first time in the text.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1995ApJS ... 99. .565S the hydrogenbecomesmoleculardependsonratiox/^r cloud surface(seeFig.8).WediscussourtreatmentofHself- hydrogen densitiesbecomeequalatadepthy4=0.7fromthe (Sternberg 1988),andinourmodeltheatomicmolecular shielding andgrain-surfaceformationintheAppendix. “self-shielding” anddustopacity.Theclouddepthatwhich 2 F 568 © American Astronomical Society • Provided by theNASA Astrophysics Data System Fig. 5.—Nitrogennetwork STERNBERG &DALGARNO Fig. 4.—Sulfurnetwork 3 part oftheH/Htransitionlayerisdrivenbyrapidphotoion- but coolsrapidlyasthehydrogenbecomesfullymolecular. mains hotthroughtheouterpartofH/Htransitionlayer, comes efficientwhileemission-linecoolingisquenched temperatures exceeding10K.TheHizonesindensePDRs (Sternberg &Dalgamo1989;Burtonetal.1990).Thegasre- become hotbecausegasheating(bygrainphotoelectricemis- sion, andcollisionaldeexcitationofFUV-pumpedH)be- 2 2 2 The molecularchemistryinthehotHIzoneand As isshowninFigure7thegasHIzonehot,with Vol. 99 1995ApJS ... 99. .565S 10- -71 3 + the endothermicabstractionreaction oxygen inatomicformthroughoutthehotHizoneand H/ the removalofmolecularions. where inreaction(R4)H*represents FUV-pumpedhydrogen the photodissociationratesareequalto~10~xsand and dissociativerecombinationarecompetitiveprocesses for Thus, intheHizoneofourmodelcloudphotodissociation dissociative recombinationratesareequalto~10«s. the outerlayersofPDRswhen0.1cmsince,typically, and by H transitionlayer,andtheoxygenchemistryisinitiated by dissociative recombinationofthemolecularions. of atomiccarbon,andthechemistryisalsomoderatedbyrapid sociation andphotoionization.Alargeelectrondensity,n~ population of(superthermal)vibrationallyexcitedhydrogen No. 2,1995 hot gaschemistryismoderatedbyrapidmolecularphotodis- ated intheHIzoneandH/Htransitionlayer, drogen abstractionreactions(seeAppendix). zone, andtheinternalenergiesofthesemoleculesmayalsobe used toovercometheendothermicitiesofmediatinghy- molecules ismaintainedbyrapidFUV-pumpingintheHI toionization andphotodissociation.Thesereactionsbecome sufficiently warmforthereactionstoproceedefficiently.A H particledensitybecomeslargebutwherethegasisstill particularly effectiveintheH/Htransitionregionwhere tween thehydrogenmoleculesandabundantatoms ions (C,N,O,andS)whichareproducedbytherapidpho- The chemistryisinitiatedbyseveralendothermicreactionsbe- ization andphotodissociationbyFUVheatingofthegas. i? 2 10"V, ismaintainedintheHizonebyphotoionization e 2 2 2 Rapid molecularphotodestructionmaintainsmostofthe Photodissociation ofmolecularionsbecomesimportantin The incidentFUVradiationfieldisnotsignificantlyattenu- © American Astronomical Society • Provided by theNASA Astrophysics Data System 3.1.1. OxygenFamily O +H|-►OHH, (R4) O +H-►OH 2 CHEMISTRY INDENSEPHOTON-DOMINATEDREGIONS (R3) 6 in theHizonewhereatomichydrogendensityislarge. comes ineffectivewhiletheOHphotodissociationrateremains and thereverseofreaction(R7) comes fullymolecular,andthegascools,reaction(R3)be- layer is throughout theHizone. this partofthecloud. of theH/Htransitionregion.However,ashydrogenbe- formation reaction(R3)becomesveryrapidintheouterpart The OHdensityincreasesastheHincreases,and and thereverseofreaction(R3) by reactions(R3)and(R4)arerapidlyremovedphotodis- tons areremovedbythecharge transferreaction and israpidlyremovedbyphotodissociation temperature T=800K(seeFigs.7and9).TheOHradicalis endothermicity ofreaction(R3).TheOHmoleculesproduced a reactionwhichpossesseslargeactivationbarrier,andby efficient productionofmanymoleculesandmolecularionsin a crucialintermediaryinthechemistryofhotgas,andits visual extinctionA=0.6whereh(H)/az0.2andthegas OH abundancereachesamaximumvalueof6X10~at sociation which isthemajorsourceof protonsinthehotgas.Thepro- very largeabundanceintheH/Htransitionlayerleadsto high andtheOHdensitydecreasessharply.Inourmodel molecules whosevibrationalenergiesareusedtoovercomethe and isdestroyedbyphotodissociation 2 2 v2 2 0 isefficientlyproducedbythefastneutralreaction A crucialreactionintheHizoneandH/Htransition H0 isproducedinthehotgasby 2 2 2 + OH +HfH0H,(R8) 0H +H-^H0H,(R7) 2 OH +C-►COH (R13) 2 H0 +H-^0H(R10) 2 + 0H +H-^0H.(R6) H0 +vOH(R9) 2 2 0H +0-*0H(Rll) O +H-*0 (R14) 2 OH +j/^OH(R5) 0 +v—►O(R12) 2 569 1995ApJS ... 99. .565S + and isremovedbydissociative recombination and byphotoionization layer byreactions(R23),R24andR25). a largeHdensitymaybemaintainedintheH/Htransition and dissociative recombinations tion layers.AlthoughtheHionsarerapidlyremovedby may bemajorsourcesofHionsintheIandH/Htransi- followed by processes inthehotgas.Forexample, 32 and recombinations tion reactions(R15)-(R17)isinterruptedbythedissociative of H0inthehotgas.InIzonewheredensityis The dissociativerecombinationofH0isanegligiblesource small andtheelectrondensityislargesequenceofabstrac- 3 570 which initiatesthesequence 32 2 3 + 0 isproducedintheHIzoneby Photodissociation ofH0andmaybeimportant 2 23 © American Astronomical Society • Provided by theNASA Astrophysics Data System + + + + H0 +^,(R17) 23 + + + + OH +H0,(R16) H0 +HJOH,(R24) H0 -hi/-*-+O,(R23) 2 + Hi +c^HHH,(R27) 3 H +£-►(R26) 2 + + H0 +e->OH.(R19) H0 +e-+,(R18) 32 H0 +£-►O,(R21) H0 -he->OH+H,(R20) 32 32 2 2 0H +0—OiH(R28) OH +c—OH.(R22) 0 +HOH,(R15) H +(R25) 2 23 0ï +e->00 (R30) 0 +v-+0}e (R29) 2 STERNBERG &DALGARNO + + -8 + + + OH, isalsoanimportantintermediary inthehotgaschemistry, to thelocationofOHdensity peak(seeFig.12).CH,like CH abundancereachesamaximum valueof6X10close the hotpartofH/Htransition layer,andinourmodelthe in theHizonewhereatomichydrogendensityislarge. the reverseofreaction(R33) ( R34)areremovedbyreactions(R35andR41),also by in theHizone.TheCHionsformedbyreactions(R33) and and tions This sequenceisinterruptedbythedissociativerecombina- initiates thesequence 2 and by The rapidformationofCHbyreactions(R33)and(R34) chemistry isinitiatedbytheendothermicabstractionreaction the HizoneandH/Htransitionlayer,carbon maintain mostofthecarboninsinglyionizedformthroughout molecules andthephotoionizationofatomiccarbon sition layer(seeFigs.9and10). H0, 0,andneartheOHdensitypeakinH/Htran- and (R28)maintainlargedensitiesofOH,H0 and photodissociation 2 2 23 + A largeCHdensitypeakisproduced byreaction(R33)in Rapid molecularphotodestructionofthecarbonbearing The sequence(R14)-(R17)andreactions(R7),(Rll), + + CH +HCHJH,(R36) CH +H,(R35) + + + 2 2 + C +H*CHH.(R34) C +H-*CH(R33) CH +CH(R41) CHÍ +e^CH,(R40) CHÎ +e—CHH,(R39) CH +H-^CH(R42) 2 CHj +^-^CHH.(R38) CH +eH,(R37) 2 + 2 2 32 3.1.2. CarbonFamily Oj +ï,—00.(R31) + C +^Cé?(R32) Vol. 99 1995ApJS ... 99. .565S + No. 2,1995CHEMISTRYINDENSEPHOTON-DOMINATEDREGIONS571 by photoionizationandphotodissociation tion andphotodissociation todissociation ofCHTv—►+H,(R59) of CHinthehotgas.The moleculesarerapidlyremoved The dissociativerecombination ofCH5isanegligiblesource is amajorsourceofprotons. eral molecularspeciesintheH/Htransitionlayer. and itslargeabundanceleadstotheefficientproductionofsev-CH+^£,(R57) photodissociation in theHIzone. and by and bytheendothermicabstractionreaction (R37), (R38),and(R39),areremovedbyphotoioniza- (R41 )isthedominantsourceofatomiccarbon,andpho- and bythereverseofreaction(R49) followed by in theHizone. 42 3 4 2 4 + The moleculesCHandareproducedbyreactions Near theCHdensitypeakdissociativerecombination+y-►H,(R58) The CHmoleculesareremovedbyphotoionizationand CH isproducedbyradiativerecombination CH4 isproducedbyradiativeassociation 2 43 3 3 © American Astronomical Society • Provided by theNASA Astrophysics Data System CH5 +CHH,(R55) CH +H|-^H(R50) CH?+ í?-*.CH4+H. (R56) CH +H-►(R53) 32 23 + CHÎ +H->-CH?i'(R54) + CH +H(R49) 32 CH +i/->H.(R47) CH +e,(R46) CH +—H CH +ï>-^CHJ£,(R51) 2 23 2 2 32 3 CHl +e^Cnv(R48) CH +y-►e,(R44) CH +ï/^CH,(R45) CH +v-+CH(R43)í/-^CHH.(R60) 3 42 (R52) + + + + + the photoionizationofatomicsulfur and CHnearthedensitypeakinhotpartofH/ CH, CH,andsmallerdensitiesofCH5, (R50), (R54),and(R56)maintainlargedensitiesof the HIzoneandH/Htransitionlayer,sulfur chemistry isinitiatedbytheendothermicreaction maintains mostofthesulfurinsinglyionizedformthroughout action The formationofSHisfollowedbyanotherendothermicre- and by tions (R62)and(R64)isinefficientaugmentedbythe and by and bythereversesofreactions(R62)(R64) The SHandHSionsareremovedby radiative association The productionofHSbythesequenceendothermicreac- H transitionlayer(seeFigs.12and13). in theHizone.Thedissociative recombinationofSHisthe 4 23 2 2 2 2 The sequence(R35)-(R38)andreactions(R48),(R49), Rapid photodestructionofthesulfurbearingmoleculesand + + + + SH +H*HS. + HS +^SH, (R70) 2 sh +h—hs + 2 + + + 2 SH +H-►S (R71) S+ +H*-*SHH.(R63) 2 S +H-►v.(R66) HS +^^S,(R68) HS +e-»-SHH,(R67) S +H—SH(R62) 2 2 2 2 Sir +e—SH,(R69) 3.1.3. SulfurFamily + S +pe(R61) (R65) (R64) 1995ApJS ... 99. .565S try isinitiatedbytheendothermic abstractionreaction H IzoneandH/Htransition layer,andthenitrogenchemis- maintains mostofthenitrogen inatomicformthroughoutthe (see Fig.15). small densitiesofSHandHSaremaintainedinthehot gas reactions (R74),R75andR76)isinefficient,only very of SHbyreactions(R67),(R72),and(R73),HS by of atomicsulfurisverysmallintheHIzone,formation in theHizone. and bythereversesofreactions(R72)(R75 and and photodissociation The SHandHSmoleculesareremovedbyphotoionization and and bytheendothermicreaction 2 dominant sourceofatomicsulfurinthewarmpartH/ HS isproducedbyradiativerecombination 2 2 and reaction 2 572 H transitionzone. 2 2 Rapid photodestructionofthenitrogenbearingmolecules Because reaction(R62)isendothermicandtheabundance SH isformedbyreaction(R67)andalsotheendothermic © American Astronomical Society • Provided by theNASA Astrophysics Data System + SH +H + 3.1.4. NitrogenFamily 2 SH +H—S + HS +SH N +H-^NH (R83) HS +SH HS Tv—►+^ 2 2 2 2 2 HS +e-►v 2 SH +HS, S +H?-*•SHH. SH +ï/S-hH, SH +ye, 2 S +H—SH 2 HS +H. 2 STERNBERG &DALGARNO (R82) (R81) (R80) (R79) (R78) (R77) (R76) (R75) (R74) (R73) (R72) the endothermicreaction and by and alsobythephotodissociations recombinations In theH1zonethissequenceisinterruptedbydissociative The photodissociationofNHisthedominantsourceatomic nitrogen inthehotgas. and bythereverseofreaction(R83) removed byphotodissociationandphotoionization The NHmoleculesformedbyreactions(R83)and(R84)are and by NH isproducedbyreactions(R92)and(R93)also 2 The photoionizationofNHinitiatesthesequence + + + + NH, +HNHJH,(R90) NH£ +H->►NHJ,(R89) NH +H-*NHJH,(R88) 2 2 2 NH +H—NH, (R103) NH +o-»■NH.(R101) NH +!'-*■+H,(R100) NHJ +r-*•H,(R99) NHJ +yNH3H,(R98) 2 NH +é?—NH,(R97) NH¿ +é>-*NH,(R96) 2 NHJ +e-»-NHH,(R95) NHJ+ í?^NH+H,(R94) NHJ +c->NHH,(R93) NH +H-*NH (R102) 2 2 2 2 NHJ +e-+NHH.(R92) NH4 +e^*-NHH,(R91) + 2 3 N +H*-»-NHH.(R84) NH +H->-NH.(R87) NH +p-►e(R86) NH +v—NH,(R85) 2 Vol. 99 1995ApJS ... 99. .565S + + + the photoionizationofatomic silicon very smallinthehotgas(see Figs. 19,20,and21). NH, NHJ,andinthehotpartofH/Htran- sition layer.However,thedensitiesofNandHremain (R88)-(R92), maintainlargedensitiesofNH,NH, , zation and isremovedbydissociativerecombinationphotoioni- NH isproducedby and isremovedbyphotodissociation and thereverseofreaction(R107) and isremovedbyphotodissociation and by reaction No. 2,1995 in theHIzone. and isremovedbyphotodissociationphotoionization and bythereverseofreaction(R102) 32 2 2 2 Rapid photodestructionofthe siliconbearingspeciesand Reactions (R83),(R102),and(R107),thesequence N isproducedbythereactions NH isproducedbyreaction(R91)andtheendothermic 2 3 © American Astronomical Society • Provided by theNASA Astrophysics Data System + + + NH +NH NH +H-*H. 2 32 NH +e^NH, NH +^N. NH +H^H 2 2 23 NH +H^ NH +H^ 23 2 NH +vH NH +e NH +vH NH +NH, 3.1.5. SiliconFamily 32 NO +N^O 2 2 2 CN +NC, 2 2 N +*/—. 2 CHEMISTRY INDENSEPHOTON-DOMINATEDREGIONS (R117) (R116) (R115) (R114) (R113) (R112) (Rill) (R109) (R108) (R110) (R107) (R105) (R104) (R106) + and isremovedbyphotodissociation HCO isproducedby tion layer(seeFig.18). followed by density ismaintainedintheHizoneandH/Htransi- tion theproductionofSiHbyreaction(R122)isinefficient, source ofatomicsiliconinthehotgas.OnlyaverysmallSiH rather thanthedissociativerecombinationofSiHismajor and radiativerecombination Because thesiliconchemistryisinitiatedbyradiativeassocia- sociation and and by SiH isformedbyreaction(R122)andremovedphotodis- lowed by because reaction(R120)isextremelyendothermicandre- the HIzoneandH/Htransitionlayer.Thesiliconchem- mains ineffectiveeveninthehotgas.Reaction(R119)isfol- istry isinitiatedbyradiativeassociation rather thanbythehydrogenabstraction maintain mostofthesiliconinsinglyionizedformthroughout 2 2 2 CO isefficientlyproducedinthehotgasbyreaction + 3.1.6. OxygenIntermediates + + CO +H^ 2 HCO +yCOH + + + 2 CO +H-*►. + OH +CCOH OH +C-►COH SiH +£SiH Si+ HSiH 2 2 Si +H^SiH 2 SiH +y-►SiH. + + Si +ey Si +y—►e (R128) (R127) (R126) (R124) (R123) (R122) (R125) (R121 ) (R119) (R120) (R118) (13) 573 1995ApJS ... 99. .565S + + + and photoionization the HIzoneandinH/Htransitionlayeronlyverysmall (see Fig.14). densities ofSOandS0aremaintainedinthesecloudlayers However, becausethedensityofatomicsulfurisverysmall in nant sourceofatomicsulfurinthehotgas(seeFig.14). layer, andthedissociativerecombinationofSOisdomi- A largedensityofSOismaintainedintheH/Htransition and isremovedbyphotodissociation and areremovedbyphotodissociationphotoionization and photodissociation and and isremovedbydissociativerecombination peak intheH/Htransitionregion(seeFig.11). 2 large densitiesofCO,CO,andHCOneartheOHdensity 2 Reaction (R13)andthesequence(R125)-(R129)maintain which isanadditionalsourceofCOinthehotgas.The molecules areremovedbyphotodissociation and dissociativerecombination 2 2 574 STERNBERG + NO isefficientlyproducedby SO andS0areproducedby SO isefficientlyproducedbythereaction 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System + + + + S0 +0H-*S0H(R135) OH +SSOH(R131) HCO +e->COH(R129) + 2 S0 +^S00.(R138) OH +N-►NOH NO +^e SO +ySO.(R133) 2 SO +^SOe,(R137) SO +í^SO,(R136) OH +S^SOH(R134) CO +i/^CO.(R130) NO +y-►NO S0+ +e-+S0(R132) (R141 ) (R140) (R139) & DALGARNOVol.99 + + + peak intheH/Htransitionlayer.However,density of A moderateSiOdensityismaintainedneartheOH Si0 remainsnegligibleinthispartofthecloud(seeFig.17). and isremovedbyphotodissociation and Si0isproducedby todissociation andphotoionization in thehotgas. in theHIzone.Thisreactionisamajorsourceofsiliconatoms This sequenceisinterruptedbythedissociativerecombination 2 which initiatesthesequence 2 transition region(seeFigs.19and21). ties ofNOandneartheOHdensitypeakinH/H 2 Reactions (R139),R141andR142)maintainlargedensi- The NOionsareremovedby which isthemajorsourceofNOinhotgas.Anadditional which isfollowedby source is 2 + + SiO isproducedbyreaction(R147)andremovedpho- SiO isproducedrapidlybythereaction CS israpidlyproducedbythereaction + + + SiOH +e->SiOH.(R147) SiOH +eSiOH,(R146) SiO +HSiOHH,(R145) 3.1.7. CarbonIntermediates + 2 + OH +N->NOH.(R142) + SiO +OH^Si0H(R151) + OH +Si—SiOH(R144) 2 + Si0 +*/-*Si00.(R152) SiO +p-►e,(R150) SiO +*—SiO(R149) + 2 CH +S—CSH NO +é>—NO.(R143) SiO +£-►SiO(R148) (R154) (R153) 1995ApJS ... 99. .565S + in theHizone. and which isinterruptedbythedissociativerecombinations and israpidlydestroyedbyphotodissociation layer (seeFig.16). sity peaksneartheCHdensitypeakinH/Htransition No. 2.1995CHEMISTRYINDENSEPHOTON-DOMINATEDREGIONS575 These reactionsinitiatethesequence Reactions (R153)-(R155)produceCS,CS,andHCSden- dissociation andphotoionization CS isproducedbyreaction(R155)andremovedphoto- and and photodissociation This sequenceisinterruptedbydissociativerecombination and and onlyasmalldensityofHCSismaintainedinthehotgas. 2 + HCS isinefficientlyproducedby CN isformedby © American Astronomical Society • Provided by theNASA Astrophysics Data System + + + HCN +H^CN,(R165) 2 HCN +HCN,(R166) + HCN +^(R167) 2 2 + + + CN +HHCN,(R164) 2 + + CH +N^CNH.(R163) NH +CCNH(R162) HCS +v-+CSH.(R157) HCN +e-+CNH(R168) HCS +e->CSH.(R155) + CH +S->HCSH(R160) HCS +p-►CSH,(R161) + 2 CS +v-*CSe.(R159) CS +CS(R158) + CN +CN (R169) CS +CS(R156) + A= 1.7(seeFigs.8and11). C iizoneispresentatcloud depthsbetweenA=0.7and which thehydrogenisfully molecular. Inourmodelsucha zone existsinwhichmostofthe carbonissinglyionizedandin deeper intothecloudthandoesHizonebecauseof the which mostofthecarbonissinglyionized.Thislayerextends by theincidentFXJVfieldmaintainsanextendedcloudlayer in H/H transitionlayer(seeFig.22). CN, HCNCN,andneartheCHdensitypeakin effective self-shieldingofthehydrogenmolecules.Thus,a Cn in theHizonewhereatomichydrogendensityislarge. throughout theHizoneandH/Htransitionlayer.The (R174) v and HCN moleculesarealsoremovedbythereverseofreaction tion and by The CNandHCNmoleculesareremovedbyphotodissocia- by theendothermicreaction v and 2 HCN isproducedbyreaction(R166),thereactions and 2 by The photoionizationofatomiccarbon Reactions (R162)-(R175)maintainlaigedensitiesof CN isproducedbyreactions(R167)and(R168)aswell + HCN +H^,(R173) + CN +H*—HCNH.(R175) CN +H^.(R171) CN +H-►HCN(R174) 3.2. CnandSnZones HCN +H—CN(R178) 2 CH +N-►HCNH(R172) 2 2 HCN +j/^CNH(R177) CH +N-^CNH(R170) + CN +yCN(R176) C +e(32) 1995ApJS ... 99. .565S = + + + 576 than carbon.ThusaSIIzoneexistsinwhichthesulfurissingly by photoionizationandphotodissociation,isinitiated does thelayerofionizedcarbonbecausesulfurislessabundant between^4 T7andA=3.7(seeFigs.8,11,and14). lecular. InourmodelsuchaSIIzoneispresentatclouddepths ily inCOmolecules),andwhichthehydrogenisfullymo- ionized butinwhichthecarbonisneutral(andpresentprimar- is singlyionized.Thislayerextendstolargerclouddepthsthan maintains anextendedcloudlayerinwhichmostofthesulfur as wellthechargetransferreaction tions (R32)and(R61),dissociativerecombinationre- gas iscoldwithtemperaturesT^100K(seeFig.7),andpro- production inthehotHIzoneandH/Htransitionlayerbe- drogen moleculesandtheabundantatomsionsO,N,C, radiative associationandabstractionreactionsbetweenthehy- these cloudlayers.Electronsareproducedbythephotoioniza- ceed onlyinreactionswithFUVpumpedH*molecules. come muchlesseffectiveintheCIIandSnzoneswhere (R3), (R33),(R62),and(R83)whichcontrolthemolecular Si, andS.Theendothermichydrogenabstractionreactions and theformationofOHisdominatedby cited hydrogenmoleculesthroughouttheCnandSzones, mains rapidintheCIIandSnzones. molecular photodissociationandphotoionizationthroughout and Siizones,butthechemistryisstillmoderatedbyrapid and In theSiizoneadditionalsourcesofOHare vibrationally excitedhydrogenmoleculesbecomesverysmall. The OHmoleculesareremovedbyphotodissociation even attheinneredgeofSHzonewheredensity also effectivelyremovedby but wheretheCdensityis still large,theOHmoleculesare throughout theCnandSzones.Nearinneredgeof the C iizonewheretheFUVradiationfieldispartiallyattenuated, Kv 2 The molecularchemistryintheCnandSzonesisdriven FUV-pumping maintainsapopulationofvibrationallyex- The FUVradiationfieldispartiallyattenuatedintheCn Similarly, thephotoionizationofatomicsulfur © American Astronomical Society • Provided by theNASA Astrophysics Data System + + HS +O^OHSH.(R181) 2 OH +CCOH (125) NH +O^OHN(R180) 3.2.1. OxygenFamily + 0 +H2^0HH(4) C +S-►(R179) + OH-h^-^OFH (5) S +y—>S£(61) STERNBERG &DALGARNO tons areproducedbycosmic-raydissociativeionization tons intheCiizone. H ionsareproducedbycosmic-rayimpactionization which intheCnzonecontinuestoinitiatesequence and by ated bytheprotontransferreaction and thehydrogenabstractionsequence(R16)-(R19)isiniti- dothermic. TheprotondensitybecomessmallintheSiizone, are removedbyradiativerecombination Reaction (R13)continuestodominatetheproductionofpro- rather thanbythechargetransferreaction(R14)whichisen- rather thanbyreaction(R13).InthecoldSnzoneprotons throughout theCiiandSnzones.Nearinneredgeof the C iizonetheH0moleculesarealsoremovedby followed by and isremovedbyphotodissociation (rather thanbyreaction[R14]followed[R15])wherethe 3 2 + + In theSiizone,Cdensitybecomessmallandpro- The protonsareremovedby H0 isformedbythedissociativerecombinationof 0 isformedby 23 2 + + + H0 +,(17) + H0 -fC^HCO+. (R186) 23 + 2 + + OH +H^0,(16) 2 H +c.r.->£(R182) + OH +C-►COH.(13) 2 + H0 +e->OH.(19) H0 +e->,(18) 32 32 0 +H?^0HH(R184) + 2 0 +H->OHH,(15) H +-►.(25) 2 23 H0 +z/-*0H(9) H +c.r.e(R185) 2 O +H-►0(14) 2 + H +ev(R183) Vol. 99 1995ApJS ... 99. .565S No. 2,1995CHEMISTRYINDENSEPHOTON-DOMINATEDREGIONS throughout theCnandSzones. and isremovedbyphotodissociation throughout theCnandSzones. bined actionof zone andH/Htransitionlayer(seeFigs.910). less efficientinthecoolCnandSIIzonesthanhotHI and theradiativeassociationreaction reaction (R187). is verysmallthecarbonchemistrycontrolledprimarilyby and In theSiizonewheredensityofFUV-pumpedmolecules which leadstotheformationofCH,CH,and . ionization These moleculesareremoved byphotodissociationandphoto- 2 23 4 The formationofmolecularspeciestheoxygenfamilyis The carbonchemistryintheCnzoneisinitiatedbycom- 0 isformedanddestroyedby Reactions (R34)andR187arefollowedbythesequence 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System + + CHj +H-^CHi+H,(36) CH +H->CHJH,(35) CH +H-*►CH5p,(54) + + 2 2 + 32 CH +pe, (44) OH +O0H,(28) C +HCH(34) C +H-►CHp.(R187) CHJ+ —CH+H,(37) CH +£-*►p,(48) CHj +£-^CHH,(38) 2 CH5 +eCHH,(55) 2 CHÍ +^^CHH,(56) 2 2 3 2 32 4 3.2.2. CarbonFamily OH +O0H(11) 2 Oj +e^OO(30) 0 +y-►O(12) 2 0 +^►^,(29) 2 + + + tiating reaction(R187)proceeds moreslowly(seeFigs.12 through theS11zoneasC densitydecreasesandtheini- tion field.IntheS11zonewhereCdensityissmall the throughout theC11andSzones.Nearinneredgeof and 13). through theCiizoneasdestructiveFUVradiationfield is ties ofCH,CH,andincreasewithincreasingdepth dominant sourceofcarbonis bearing moleculesarestillbeingdestroyedbytheFUVradia- part ofthecloud. to theefficientproductionofcomplexhydrocarbonsinthis CH isformedintheC11andS11zonesbothbyreactions(R48) C iizonetheCHmoleculesarealsoremovedby attenuated. However,thedensities ofthesemoleculesdecrease FUV-pumped HmoleculesandCionsdecrease.Thedensi- CH5 decreasethroughtheCIIandS11zonesasdensities of inner edgeoftheC11zonebyreaction(R189)whererate zone. Alargedensityofatomiccarbonismaintainednearthe reactions oftheform and (R55),aswellbythephotodissociationofCH. rather thanreaction(R189). of carbonphotoionizationisreducedbutwherethe is thedominantsourceofneutralatomiccarboninC11 near theinneredgeofC11zone.Thesereactionswilllead 234 3 2 2 4 + The densitiesofthemolecularionsCH,and The carbonhydridesarealsoremovedbyinsertion Radiative recombination 23 + + CH„ +CC.HJ-!H CH +*/—CHH,(47) CH +*/-*CHÍ+e,(51) CH +ï/-►e,(46) OU +p-^CHte,(57) CH +^CHH,(52) CH +í'-^CHH,(59) CH +ï/—CHH,(58) CH +^CHH(60) 2 3 2 32 42 43 42 CH +C^.(R188) CH +v-+CH,(45) 2 + CH +i/^CH(45) C +e^>Cp(R189) 577 1995ApJS ... 99. .565S + + the efficientproductionofcomplex hydrosulfidesinthispart of thecloud. reactions oftheform The SHmoleculesarealsoremovedby and near theinneredgeofS11 zone. Thesereactionswillleadto near theinneredgeofCnzone. the combinedactionof and byphotoionization quence areremovedbyphotoionizationandphotodissociation and theSHHSmoleculesthatareformedbythisse- is small. reaction (R61)dominatesintheSnzonewhereCdensity duction ofSintheCnzonewheredensityislarge,and in theCnandSzones.Reaction(R179)dominatespro- sity ofFUV-pumpedhydrogenmoleculesisverysmall. Reaction (R63)islesseffectiveintheSnzonewhereden- and radiativeassociation 578 2 The hydrogensulfidesarealsoremovedbysulfurinsertion Sulfur ionsareproducedbythechargetransferreaction Reactions (R63)andR66arefollowedbythesequence The sulfurchemistryintheCIIandSnzonesisinitiatedby © American Astronomical Society • Provided by theNASA Astrophysics Data System + SH* +SHJH + + + + 2+1 + + + + + HS +e-+SH,(67) HS +e^Hv,(74) HS +£-►,(68) S +Hv.(66) HS +^SHH.(80) HS +i^Hé>,(79) SH +CCSH(R190) S +H*SHH(63) 2 2 2 2 2 2 SH +£-►SH,(69) SH +y—SH,(78) SH +ve,(77) + 3.2.3. SulfurFamily C +S-►(179) + S +y£(61) STERNBERG &DALGARNO + + quence areremovedbyphotodissociationandphotoionization The NHandmoleculesthatareproducedbythisse- tains mostofthenitrogeninatomicformthroughoutCn with increasingclouddepth(seeFig.15). these cloudlayersisthereforeradiativerecombination than intheHIzone.Thedominantsourceofatomicsulfur throughout theC11andSzones.Nearinneredgeof the small theproductionofNHisaugmentedby In theSIIzonewheredensityofFUV-pumpedHbecomes followed by and Siizones.Thenitrogenchemistryismediatedby (R69)-(R74) proceedsmoreslowlyintheCnandSIIzones removed bythefastneutralreactions field isattenuated,theNH,NH , andNHmoleculesarealso and neartheinneredgeof S11zone,wheretheradiation rather thanthedissociativerecombinationofSH. C iizonetheNHmoleculesarealsoremovedby S IIzonesasthedestructiveFUVradiationfieldisattenuated 23 2 23 2 + Photodestruction ofthenitrogenbearingmoleculesmain- The densitiesofSHandHSincreasethroughtheCII The productionofSHandHSthereactionsequence Reactions (R86)andR192initiatethesequence 2 2 + + NH +C^HCNH, (R193) 2 NHÎ +H^NH?H,(89) NH +H,(88) NHJ +H-►,(90) + 2 2 2 + NH +vNHJH,(105) NH +*/-^NHH,(104) N +HNH.(R192) 2 2 NHJ +£-►NHH.(93) NHJ +e-+NHH,(92) NHÍ+ e—NH3+H,(91) 2 3.2.4. NitrogenFamily 2 2 NH +v^e.(86) N +H*-►NHH(84) H +^NH(109) 32 + S +e^>Sv(R191) Vol. 99 1995ApJS ... 99. .565S + + + than inthehotHIzoneandH/Htransitionlayer(seeFigs. throughout theCnandSzones.Additionalsourcesof Si ions arethechargetransferreactions of thenitrogenfamilyislessefficientinCnandSIIzones near theinneredgeofCn zone,and throughout theCIIandSnzones.NHionscontinuetobe throughout theCIIandSnzones. produced by The Nmoleculesareremovedbyphotodissociation and intheSnzonebyreaction reaction and bythechargetransferreaction The NHionsareremovedbydissociativerecombination and in theCnzone.InSzonetheseionsareproducedby No. 2.1995CHEMISTRYINDENSEPHOTON-DOMINATEDREGIONS579 19, 20,and21). 2 2 2 2 With theexceptionofNformationmolecularspecies Silicon ionsareproducedbyphotoionization In theCiizoneproductionofNisdominatedby 2 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System + + + N +H^H.(R197) 2 NH +N-►H(115) NH +SNH3.(R196) 2 3 NH +O^OH(R195) + 2 + NH +O-^NOH(R194) NH +O—OHN(180) NH-bé>^N +(116) + 3.2.5. SiliconFamily N0 +N->N0.(113) CN +N-►C,(112) 2 2 2 C +Si (R198) + S +Si (R199) N +p>(114) Si +v-+e(118) 2 + + + + becomes thedominantsourceofHCO.Theions are and reaction(R127)dominates theremovalofCOions removed bydissociativerecombination Near theinneredgeofSnzonereaction followed by the CiizoneHCOisalsoproducedby and thereactions are majorsourcesofHCOinboththeCnandSiizones.In rather thanbyreactions(R13)or(R123).Chemi-ionization and COisproducedbythereactions and decreases (seeFig.18). through theCiiandSnzonesasphotodestructionrate and theSiHdensityincreaseswithincreasingclouddepth The SiHmoleculesareremovedbyphotodissociation followed by form throughouttheCnandSregions. in theSnzone.Mostofsiliconremainssinglyionized Atomic oxygenremainsabundantintheCnandSzones SiH isproducedbytheradiativeassociationreaction + + + + CO +HHCO.(127) 3.2.6. OxygenIntermediates 2 + CRt +OHCOH(R204) CH +OHCOH,(R203) CO +HHCO(R205) 2 2 + 32 + HCO +eCOH, (129) + OH +CCOH(125) CH +OHCOe(R202) SiH +£-►H.(122) SiH +eSiH,(121) 2 2 CH +O-►COH(R201) Si +HSiHv(119) 2 2 CH +O-^COH(R200) SiH +i/^SiH,(123) 1995ApJS ... 99. .565S + and by destruction ratesdiminish(seeFig.14). molecules increasewithincreasingclouddepthasthephoto- throughout theCnandSzones,densitiesofthese throughout theCnandSzones.TheCOmoleculesarere- carbon isdrivenintotheCOmolecules(seeFig.11). and isremovedby large neartheinneredgeofCnzoneasallgas-phase come smallintheCnandSzones.TheCOdensitybecomes in theCnandSzones.TheCOHCOdensitiesbe- moved byphotodissociation and SOS0areproducedby are removedbyphotodissociationandphotoionization by reaction(R206)intheSnzone.TheSOandS0molecules density becomessmall.TheproductionofS0isdominated association An additionalsourceofS0intheCnzoneisradiative and rather thanbyreactions(R134)and(R131)becausetheOH and 580 2 2 2 2 + + NO isproducedintheCIIandSiizonesby In theSIIzoneSOandareproducedby In theCIIzoneSOisproducedby © American Astronomical Society • Provided by theNASA Astrophysics Data System + S0 +0H-*S0H.(135) 2 OH +SSOH(131) + + NH +O^NOH S0-b y^SO+O(138) OH +N^NOH(139) 2 SO +O->S0v.(R206) + SH +0-*S0H(R207) OH +S-^SOH(134) 2 SO +p-+SOe,(137) SO +j/^S+O,(136) SO +e->S0,(132) SO +v^SOe(137) CO +i^CO(130) STERNBERG &DALGARNO (194) neartheinneredge oftheCnzone. + and throughout theCnandSzonesalsoby toionization The SiOmoleculesareremovedbyphotodissociationandpho- and isremovedbyphotodissociationphotoionization followed byreaction(R147)and In theSIIzoneproductionofSiOisaugmentedby and areremovedby are formedby zones (seeFig.19). and remainsapproximatelyconstantthroughtheCnS The NOdensityislimitedbyreactions(R208)and(R113) nant sourcesofNOintheCIIzone.InSnzoneions in theSnzone.Reactions(R141)and(R209)aredomi- near theinneredgeofCnzone,andby The NOmoleculesarealsoremovedby SiO isproducedintheCnzonebysequence + + + SiOH +e-►SiOH.(147) + SiO +HSiOH,(145) 2 OH +SiSiOH,(144) SiH +O^SiOHH(R210) + + + 2 + + SiO +C— (R213) SiO +CSiCO(R212) NO +C(R208) + + NO +S-►(R209) SiH +O-^SiOH.(R211) NO +ve.(141) NO +y-►NO,(140) NO +£-►NO.(143) SiO +z/-►e(150) SiO +z/^SiO,(149) NO +NO(113) 2 Vol. 99 1995ApJS ... 99. .565S + C iiandSIIzonesisinitiatedby followed bythesequence moved by and theOCSmoleculesformedbyreaction(R214)are re- the C/Otransitionregionaselectrondensityand In theSiizoneHCSmoleculesarealsoremovedby and destroyedbyphotodissociation rate ofreaction(R155)decreaseseeFig.16). as thephotodestructionratesdiminish,butdecreasesthrough tion The CSdensityincreasestowardtheinneredgeofCnzone (R155) areremovedbyphotodissociationandphotoioniza- in theCnandSzones.TheCSmoleculesformedbyreaction produced bythesequence mains negligibleintheCnandSIIzones(seeFig.17). (R150), (R212),and(R213)decrease.TheSi0densityre- rates ofphoto-andchemicaldestructionbyreactions(R149), and removedbyphotodissociation No. 2,1995 2 + The formationofthecarbon-nitrogenintermediatesin HCS isformedby The carbon-sulfurintermediatesCS,HCSandare The SiOdensityincreaseswithincreasingclouddepthasthe Si0 isinefficientlyproducedby 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System + + 3.2.7. CarbonIntermediates + HCS +£-^CSH(155) + CS +H^HCS,(154) 2 HCS +O^OCSH,(R214) CH +S-^CSH,(153) NH +CCNH (162) SiO +OHSi0H(151) 2 + CH +S-h*HCSH(160) Si0 +^SiO0.(152) HCS +î/^CSH.(161) OCS +^COS.(R215) 2 2 CS +y-►£..(159) cs +^cs,(158) CHEMISTRY INDENSEPHOTON-DOMINATEDREGIONS + between3.7 and6(seeFigs. 8,11,14,and17). but inwhichthecarbonandsulfurarepresentneutral layers. Thus,aSinzoneexistsinwhichthesiliconisionized, tends tolargerclouddepthsthantheionizedcarbonorsulfur ( atomicormolecular)form,and inwhichthehydrogenisfully ionized. Becauseofthesmallsiliconabundancethislayer ex- molecular. InourmodelaSi ii zoneispresentatclouddepths maintain acloudlayerinwhichmostofthesiliconissingly and thechargetransferreaction ratio islargethroughouttheCiiandSzones(seeFig.22). where thereactionsequence(R162)-(R167)remains S iizoneastheCdensitydecreases.TheCN/HCN (R176) isdiminished.TheCNdensitydecreasesthroughthe effective, butwheretherateofphotodestructionbyreaction near theinneredgeofSiizone. and by by photodissociation are themajorsourcesofCN.TheCNmoleculesremoved which togetherwithreaction(R177)and The HCNmoleculesareremovedbyphotodissociation by thereaction The formationofHCNintheCIIandSiizonesisaugmented At intermediateclouddepths aSIzoneexistsinwhichmost The photodissociationofatomicsilicon The CNdensitybecomeslargeneartheedgeofCiizone + + + HCN +HCN,(165) 2 + HCN +e.(167) HCN +^HCN,(166) 2 2 CN +HHCN(164) 2? CH +N->HCNH.(172) 2 HCN +v->CNH,(177) 3.3. SiiiandSIZones CN +N-*NC(112) CN +O-^CON,(R216) + 2 CH +N-^CNH(170) + S +Si(199) CN +pCN,(176) Si +ve(118) 581 1995ApJS ... 99. .565S which isanadditionalsource ofOH.TheOHmoleculesare removed by and byphotodissociation H0 moleculesareremovedby The dissociativerecombinations(R18)and(R19)arethe do- minant sourcesofH0andOHintheSinSIzones. The between themolecularionsandoxygenatomsbeginto proton transferreactions.Theelectrondensitybecomessmall tion. Thechemistryisinitiatedbyradiativeassociationsand followed by cosmic-ray ionization ation andphotoionizationwhichproceedsmoreslowlyinthe zone. Themolecularchemistryismoderatedbyphotodissoci- while theatomicoxygendensityremainslarge,andreactions photoionization andphotodissociationcosmic-rayioniza- and SIzones,thechemistryisdrivenbyacombinationof and thechargetransferreaction(R199).Inourmodelabroad recombination cules, andrapidneutralizationofthesulfurionsbyradiative maintained byphotodestructionofthesulfur-bearingmole- of thesulfurispresentinneutralatomicform.TheSIzone and thesequence compete withdissociativerecombinationsintheSinandSI and theSiIISIzonesoverlap(seeFigs.1417). S izoneispresentatclouddepthsbetweenA=3.7and7.6, Si IIandSIzonesthaninthecloudlayersclosertosurface. 582 2 2 v The FUVradiationfieldisseverelyattenuatedintheSiII In theSinandSIzonesoxygenchemistryisdrivenby © American Astronomical Society • Provided by theNASA Astrophysics Data System + + + H0 +^H,(17) 23 H0 +SiSiOH(R217) + + 2 OH +H^0H,(16) 2 H0 +e->OH-f.(19) H0 +e->H,(18) 32 32 + 3.3.1. OxygenFamily H0 Ty-►OH+(9) Hi +H—HJ(25) 2 0 +HÍ->0HH,(184) 0H +0->0H (11) 2 H +c.r.e(185) 2 2 2 + S +e^Sv(191) STERNBERG &DALGARNO + + depths (seeFig.9). tion (Rll)andapproachesaconstantvalueatlargecloud (R12) decrease.However,theOHdensityislimitedbyreac- zones astheratesofphotodestructionbyreactions(R9)and which isthedominantsource ofatomiccarbonintheSIzone. in theSIzone. where theHeionsareproducedbycosmic-rayionization in theSiIIzone,andbychargetransferreaction and areremovedbyradiativerecombination The Cionsareremovedbythechargetransferreaction and isremovedby and by and as wellby sociation and the0formedbyreaction(Rl1)isremovedphotodis- 2 + The 0andHdensitiesincreasethroughtheSinSI The dominantsourceofCintheSIzoneisreaction Protons areproducedbycosmicrays 0 isproducedby 2 2 + He +COCO(R222) + H +c.r.e(182) + 2 + + He +c.r.e.(R223) OH +O0H(28) 3.3.2. CarbonFamily + 2 0 +H-*OÍ(R220) + 2 0 +S-^S00.(R219) + 0 +C-►COO(R218) H +S-►(R221) 2 Oi +£->00.(30) 2 C +S (179) H +eHv(183) 0 +*/-*0(12) 2 Vol. 99 1995ApJS ... 99. .565S 3 + tion andphotoionization formation anddestructionofHCO inourmodel. explicitly includeditinourcalculations sincewehavenotconsideredthe in theSi11andS1zones darkcore(seebelow)wehavenot The CH4moleculescontinuetoberemovedbyphotodissocia- also by CH4 isformedbythedissociativerecombination(R56) and and by and theCHradicalsareremovedby the neutralreactions The CH,CHradicalsformedbythissequenceareremoved reactions arefollowedbythesequence in theSIzonewhereCdensitybecomessmall.These and alsobytheprotontransferreaction tion No. 2,1995 2 3 2 3 Althoughreaction(R225)dominates theremovalofCHradicals The carbonchemistryisinitiatedbytheradiativeassocia- 3 © American Astronomical Society • Provided by theNASA Astrophysics Data System + CHJ +CO-*CHHCO. 4 + CH +1/H, CH +1/—CHJe, CHJ +H^v, CH +H—CHJ, CH +H—CHJ, 43 4 CH +NHCNH. 2 2 2 32 + CH +OHCO + 32 CH +O—COH, 2 CHJ +c-^CHH CHJ +£-»-CHj+H, CHÎ +e^-CHjr, CHJ +e-*-CHH, CHj +i>->-CHH, C +HJ-*CHH(R224) 4+ CH +OCOH, 2 2 2 2 C+ HHJy(187) 2 CHEMISTRY INDENSEPHOTON-DOMINATEDREGIONS (R227) (R226) (R225) (201) (200) (54) (57) (56) (48) (38) (36) (35) (58) (55) (37) + + transfer reaction near theinneredgeofS1zone. SHisformedbytheproton and and alsobyreactionswithatomicoxygen and areremovedbydissociativerecombination becomes small. near theinneredgeofS1zonewhereelectrondensity and alsobythechargetransferreaction ative recombination in theSi11andS1zones.Thesulfurionsareremovedbyradi- and chargetransfer and CHbecomeverysmallintheSi11S1zones(seeFigs. ever, thedensitiesofCH,CHJ,CH,CH photodestruction byreactions(R57)-(R60)diminish.How- attenuated. throughout theSi11andS1zones,by near theinneredgeofS1zonewhereradiationfieldis 12 and13). 3 2 + + HS ionsareproducedby S isproducedbyphotoionization The CHdensityincreasesthroughthecloudasratesof 2 4 + CH, +SHCSH(R228) 2 + + HS+0^-0H SH(181) CH +i>-*-H(60) CH +P->►H,(59) + 2 + 42 42 HS +0->S0(R229) + 2 + HS +c—S,(68) HS +e-►SH,(67) 2 2 S +HJSHH (R230) S +H-*H)/(66) 2 + 2 + 3.3.3. SulfurFamily S +Si—S(199) C +S-^CS(179) + + S +e-*Sv(191) S Ft'-»-+c(61) 583 1995ApJS ... 99. .565S ani -9 584 SH isformedbyreaction(R67)andremoved*chargetransfer and removedbyalso removed byphotodissociationandphotoionization drops sharplybeyondtheedgeofSIzoneasdensity destruction byreaction(R80)isreduced.TheHSdensity reaction (R231)remainsefficient,butwheretherateofphoto- tained neartheinneredgeofSIzonewhereformation throughout theSiIIandSIzones.AlargeHSdensityismain- The NHandmoleculesproducedbythissequence are which initiatesthesequence and 15). atomic sulfurdecreasesandtheisdrivenintoSO and isremovedbyphotodissociation driven bythereaction value of6X10atavisualextinctionA=1.6(seeFigs.14 S0. InourmodeltheHSabundancereachesamaximum 2 2 23 v 2 Nitrogen isfullymolecular,andthenitrogenchemistry HS isefficientlyproducedbythechargetransferreaction 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System + + He +N-►(R232) 2 NH3 +HNHJ,(90) NH +H^NH3,(89) NH +H—NHJ+H,(88) 2 2 2 + + NH +y-^NHH, (109) NH +*/-^NHÎé>, (105) NH +z/-*NHH,(104) HS +(R231) NHJ +c^NHH.(93) NH4 +e-+NHH,(92) NH4 +e-+NHH,(91) 32 2 2 2 N +HNH,(R233) 3.3.4. NitrogenFamily 2 2 3 + 2 SH +O-^SOH.(207) HS +vSH(80) SH +SH.(69)NH0-*0H(195) 2 2 STERNBERG &DALGARNO + and alsoby tion followed by in theSi11andS1zones.SiHisproducedbyradiativeassocia- and bychargetransfer The NHmoleculesareremovedprimarilyby The SiHmoleculesareremovedbyphotodissociation Fig. 19). creases withclouddepthastheelectrondensitydecreases(see constant values(seeFig.21).ThedensityofNHionsin- ( R195),R180andR194theirabundancesapproach as therateofphotodestructionbyreaction(R109)decreases. and areremovedbydissociativerecombination However, thedensitiesofNHandarelimitedbyreactions in theSi11andS1zones. Reaction (R195)isthemajorsourceofNHinSIzone. in theSIzone. 2 2 + + NH ionsareproducedbythereaction Si isproducedbyphotoionization The NHdensityincreasesthroughtheSi11andS1zones 2 3 + + NH +S^NH?S.(196) N +H3H(197) + 3 2 + SiH -he->+H.(122) SiH +£SiH,(121) 2 2 Si+ HSiHj*>(119) NH +0-*NOH(194) NH +O-^OHN,(180) 2 NH +é^N(116) + 3.3.5. SiliconFamily 2 + S +Si(199) SiH +iz-^SiH (123) Si +ye(118) Vol. 99 1995ApJS ... 99. .565S + + + + Near theinnerpartofSIzone thereactions and isremovedby SO andS0areproducedbythereactions and anegligibleCOdensityismaintainedintheS1zone. and israpidlyremovedby duced by density decreases(seeFig.11).COisveryinefficientlypro- zone astheSidensitydecreases(seeFig.18). and The HCOdensityincreaseswithclouddepthastheelectron and isremovedby uated. TheSiHdensitydecreasesbeyondtheedgeofSin near theinneredgeofSizonewhereUVfieldisatten- No. 2,1995 and areremovedbyphotodissociationphotoionization 2 + + HCO isproducedintheSIzonebysequence SO isproducedby © American Astronomical Society • Provided by theNASA Astrophysics Data System + + 3.3.6. OxygenIntermediates CO +H3HCOH,(205) 2 + CO +H^HCO(127) HJ +H^JH,(25) + H +c.r.-►HJe,(185) 2 2 + 2 HCO +e-+COH.(129) CO +H-►(R234) + 2 SH +O^SOH(R235) + SiH +O-^SiOH(211) SH +O-*►SOH,(207) S0 +*/-+S00.(138) SO +O^S0v(206) SO +^SOe,(137) SO H-pS+O,(136) 2 SO +C^COO (R236) 2 SO +£-►SO.(132) CHEMISTRY INDENSEPHOTON-DOMINATEDREGIONS + and isremovedbyphotodissociation which isthedominantsourceofNinSi11andS1zones. Si0 isproducedby and NO isinefficientlyproducedby and isremovedby (R136), (JU37),and(R138)decrease(seeFig.14). densities andtheS0/S0densityratioincreasethrough Si iiandS1zoneastheratesofphotodestructionbyreactions dominate theremovalofSOmolecules.TheandS0 dissociation andphotoionization and OH intheSIzone.TheSiOmoleculesareremovedbyphoto- The dissociativerecombination(R146)isamajorsource of and followed by and by Fig. 19). 2 2 approximately constantthroughtheSi11andSIzones(see 2 The NOdensityislimitedbyreaction(R113)andremains 2 and isremovedby NO isproducedby SiO isproducedbythereactions + + + H0 +Si^SiOH(217) 2 + SiH +OSiOHH(210) + 2 SiOH +e-+SiOH.(147) + SiO +OH—Si0-fH (151) 2 SiOH +eSiOH(146) NO +S—NO+(209) + SiO +v^e.(150) OH +N-►NOH(139) N0 +é>^N0.(143) N0 +N-^N0(113) SO +C-^CSO(R237) 2 SiO +ySiO(149) 585 1995ApJS ... 99. .565S -6 and and removedbyphotodissociation OCS isproducedby tion lowed by dominates theremovalofprotons. a majorsourceofatomiccarbon,andthechargetransferreac- of 8.5X10atavisualextinctiony4=8(seeFig.16). (R206). InourmodeltheCSabundancereachesapeakvalue comes aninefficientsourceofatomiccarbon,andwherethe yond theinneredgeofSIzonewherereaction(R179)be- and (R159)arediminished.TheCSdensitydropssharplybe- SO moleculesarepreferentiallydriventoS0byreaction and wheretheratesofphotodestructionbyreactions(R158) charge transferreaction zone whereatomiccarbonisstillefficientlyproducedbythe and isremovedbyphotodissociationphotoionization The CSdensitybecomeslargeneartheinneredgeofSi ( R152)diminishseeFig.17). rates ofphotodestructionbyreactions(R149),(R150),and The SiOandSi0densitiesincreasewiththeclouddepthas 586 K 2 2 + HCN isproducedbythereactions CS isformedprimarilybyreaction(R159)whichfol- Near theCSdensitypeakphotodissociationofisitself CS isformedby © American Astronomical Society • Provided by theNASA Astrophysics Data System + + 3.3.7. CarbonIntermediates HCS +eCSH.(155) CS +HHCS,(154) 2 CH +N-►HCNH (226) + 32 CH +N^HCNH (172) 2 + CS +H(R238) Si0+ ^8100.(152) OCS +*/—COS.(216) 2 + CO +S^OCSi/(R239) CS +ye.(159) CS +ï^CS,(158) SO +C-^CSO(237) C +S-►(179) STERNBERG &DALGARNO Gredeletal. 1989). Dalgamo, &Lepp1987;Gredel, Lepp,&Dalgamo1987; and photoionization(Prasad &Tarafdar1983;Sternberg, ress 1979)andalsobycosmic-ray-induced photodissociation are removedbyfastion-molecule reactions(Prasad&Hunt- abundant molecularions.Thefractionalionizationismuch of saturatedmolecularionsbecomelarge.Neutralmolecules smaller inthedarkcorethanPDR,andabundances small, andmostofthepositiveelectricchargeiscarriedby the drogen andhelium. where thechemistryisdrivenbycosmic-rayionizationof hy- depth Aæ10.darkcloudcoreexistsatgreaterdepths dominate orinfluencethecloudchemistry.Inourmodel the inner edgeofthephoton-dominatedregionoccursatacloud completely attenuated,andtheexternalphotonsnolonger which initiatesthesequence and The CNmoleculesareremovedby CN isformedbyreaction(R177)andalso and isremovedbyphotodissociation and decreases (seeFig.22). with increasingclouddepthastheHCNphotodestructionrate sources ofCNandHCNintheSiIISIzones. However, reactions(R166)and(R167)areonlyminor v + In ourmodelthedensityofnonreactivemetalions is The formationofCNisdominatedby At sufficientlylargeclouddepthstheincidentFUVfield is The HCNdensityincreasesandtheCNdecreases + + + HCN +H^CN,(165) 2 + HCN +e-►.(167) HCN +e^HCN,(166) 2 2 CN +HHCN,(164) + 2 CH +N-►CNH(163) HCN +*/^CNH.(177) CN +N—NC.(112) CN +O-^CON(216) NO +C-^CNO(R240) 2 C +N-►CNv.(R241) 3.4. DarkCore Vol. 99 1995ApJS ... 99. .565S and areremovedby and by OH isformedbyreaction(R19)andremoved No. 2.1995CHEMISTRYINDENSEPHOTON-DOMINATEDREGIONS587 in thesequence and bycosmic-ray-inducedphotodissociation followed by SI zones)drivenbycosmic-rayionizationandprotontransfer 0 ionsareproducedbycosmic-ray-inducedphotoionization and by moved bythereactions Reaction (R18)isthedominantsourceofH0whichre- The 0formedbyreaction(R11)isremoved 2 2 2 The oxygenchemistryinthedarkcoreis(asSinand © American Astronomical Society • Provided by theNASA Astrophysics Data System + HCO +H0—CO,(R242) 23 + + H0 +-^Hl-H,(17) + 23 + + 0H +H->-H0H,(16) + 2 HJ +H0—(R243) H0 +e—OH.(19) H0 +H,(18) + 23 Hí +H^JH,(25) H0 +v-+OH.(R244) 32 32 H +c.r.-*•Hje,(185) 0 +C^-C0(R247) + 2 2a 2 2 0 +Hj-*0HH(184) 3.4.1. OxygenFamily 0H +0—H.(11) 2 0 +Si-*Si00,(R246) 0 +H-»• (220) 2 2 2 0 +S—SOO,(219) 2 0 +v-*-02+e(R248) 2a 0 +i'O(R245) Oî +e—00 (30) 2cr The CH,CH,andmolecules areremovedby and core. Thecarbonchemistryisdrivenbythereactions and areremovedbyreaction(R220) CH, andbyreactions(R37),(R55),(R56)isaug- atomic oxygeninthedarkcore. and thesequence which arefollowedbyradiativeassociation and isremovedbyreactions(R19)(R184).Mostofthe mented bythereactions oxygen remainsatomicinthedarkcore(seeFig.9). in thedarkcore. and The initiatingreaction(R222)isanadditionalsource of 23 34 The electrondensityissmallandtheproductionofCH , Atomic oxygenisproducedbyreactions(R245)andR246 Most ofthecarbonispresentinCOmoleculesdark Protons areproducedbycosmicrays 2 + + CH4 +HeCHH He .(R252) 3 + CH5 +CO—CHHCO,(227) + 4 + He +CO-►CO(222) + CH5 +O—CHH0,(R251) + + 23 CH3 +H-►CH5v,(54) CH +H-►CH3,(36) H0 +->-H(R250) 2 + 2 He +c.r.-►e,(223) H +c.r.—H^CHH,(55) CH? +é>—CH4H.(56) CH3 +£CHH,(38) CHÎ +é?-^CHH,(37) 2 32 2 2 C +H-^CHJ+*(187) 3.4.2. CarbonFamily 2 1995ApJS ... 99. .565S + tion The sulfuratomsaresupplied bythedissociativerecombina- and rapidlyremovedby and arerapidlyremovedbythepairofreactions cules, andHSionsareinefficientlyproducedbythereactions ties oftheotherspeciescarbonfamilyremainsmall(see Figs. 11,12,and13). The carbonatomsareremovedby cosmic-ray-induced photodissociationofcarbonmonoxide which isanadditionalsourceofCH,aswellby todissociation 2 The CHmoleculesareremovedbycosmic-ray-inducedpho- and by 2 588 STERNBERG 4 + SH ionsareinefficientlyproducedby In thedarkcoremostofsulfurisdrivenintoS0mole- The CHdensitybecomeslargeinthedarkcore.densi- The dominantsourceofatomiccarboninthedarkregionis 2 4 © American Astronomical Society • Provided by theNASA Astrophysics Data System + He+ S0—SSO,(R258) 2 + + + + + + HS +0->-S0.(229) HS +0—OHSH,(181) + CH +H^.CHÎH.(R256) CH +H—CHÎ,(R255) 2 2 CH +IU—CHJH,(R254) HCO +S-►SHCO(R259) 4 42 42 S +H-»•v(66) CH +N—HCNH.(226) CH +O^HCOH,(225) CH +0-^C0H,(201) 2 32 SH +OSOH. (235) 32 2 CH +vH(R253) CH +O—COH,(200) 4cr2 0 +C->C00.(218) 3.4.3. SulfurFamily CO +íVr-^CO.(R257) 2 & DALGARNOVol.99 The NHmoleculesproduced inthissequenceareremovedby which drivesthesequence 2 nitrogen chemistryisinitiatedbythereaction small inthedarkcore(seeFig.15). and isremovedby photoionization SH isformedby and isremovedbycosmic-ray-inducedphotodissociation and areremovedby + Most ofthenitrogenismolecularindarkcore,and The densitiesofSH,HSSH,andbecomevery HS isinefficientlyformedby 2 2 + + HS +NH(R261) + + 23 He +N(232) 2 SH +CHHS,(R260) NHJ +H—NHJH,(90) NHJ +H—NHÎH,(89) NH +H—NHi+H,(88) 423 2 2 2 + + + NH +0-*OH (195) 2 NHJ +c^NHH. (93) NHÎ +c^NHH,(92) HS +^c.(R263) NHÎ +c->NHH,(91) HS +r—SH,(R262) 2 2 N +HNHH,(233) 2 3 2cr HS +S^H,(231) 3.4.4. NitrogenFamily 2 2 SH +O-►SOH.(207) + OH +S^SHO(R264) 0 +S—SOO.(219) 2 SO +c-*-SO(132) 1995ApJS ... 99. .565S + and isremovedby and thedensityofSiHbecomesverysmall(seeFig.18). becomes inefficient.TheSiHmoleculesarerapidlyremoved by No. 2.1995CHEMISTRYINDENSEPHOTON-DOMINATEDREGIONS589 in thedarkcore,andformationofSiHbyreaction(R122) The NHdensitybecomeslargeinthedarkcore(seeFig.21). NH isproducedbyreaction(R91)andremoved which isthedominantsourceofNH.TheNHmoleculesare The SiHionsarepreferentiallyremovedby and removed by which arefollowedbythesequence core. Siionsareproducedbythereactions the reaction and, becausetheelectrondensityislow,theyareremovedby rather thanbydissociativerecombinationinthedarkcore. 3 3 2 + + HCO isformedby NH ionsareproducedby Most ofthesiliconispresentinSiOmoleculesdark 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System + + + H0 +NHNHJ.(R266) 32 + He +SiOSiO,(R268) HCO +NH^NHJCO(R265) + 3 + NH +CO-►HCO(R267) + 3.4.6. OxygenIntermediates 2 + SiO +C^SiCO(213) CO +HHCO (205) SiH +OSiOHH(210) 32 N +HJ^H,(197) + 2 2 SiH +e-+H.(122) SiH +eSiH,(121) Si -bH-*►SiHj+v,(119) 2 2 SiH +O^SiOH,(211) NH +O-^OHN.(180) 2 3.4.5. SiliconFamily + + + + + CO ionsareveryinefficientlyproducedby and by duced by which isthemainsourceofNindarkcore.NOpro- and isremovedby and arerapidlyremovedby becomes smallinthedarkcore(seeFig.14). The S0densitybecomeslarge,andtheS0/S0ratio and isremovedby and isremovedby which isanimportantsourceofatomicoxygen,andby duced bycosmic-ray-inducedphotodissociation and theSOdensitybecomeslargeindarkcore.ispro- and by and isremovedby CO densityremainsnegligible(seeFig.11). The HCOdensitybecomeslargeinthedarkcore,while 2 2 + NO isproducedbyreaction(R232)followed SO ionsareproducedby + + HCO +H0^CO.(242) 23 He +S0SOO(R271) + 2 CO +H^HCO.(127) 2 + + + S0 +CSOCO(R270) + 2 + HCO +e—COH(129) 0 +C-►COO(R269) 0 +NNOO (249) 2 + 2 OH +NNOH(139) NO -bN+O. (143) SO +OS0v.(206) S0 +*/SO(R272) 2 NO +N-►O(113) 2cr SO +SO,(132) 2 0 +S-*S0(219) 2 1995ApJS ... 99. .565S + 3 + and produced byreactions{Kill)and(R278)areremoved than attheedgeofSIzone(seeFig.16).TheHCS ions The CSdensityisabout10~timessmallerinthedark core and tion and isremovedby A largerSi0densityismaintainedinthedarkcorethan which isthedominantsourceofSiOindarkcore.Si0 outer cloudlayers(seeFig.17). and isremovedslowlybycosmic-ray-inducedphotodissocia- formed by The atomicsiliconisremovedby which isthemainsourceofatomicsiliconindarkcore. and and bycosmic-ray-inducedphotodissociation Reactions (R273)and(R274)arefollowedby in thedarkcore(seeFigs.19and21). The NOdensityandthe/NOratiobecomelarge 2 2 590 CS continuestobeformedby The SiOmoleculesareremovedby © American Astronomical Society • Provided by theNASA Astrophysics Data System + + + HCO +SiO^SiOHCO,(R273) + H0 +CSHCS.(R278) 32 H0 +SiOSiOH(R274) + 32 HCO +CS-h*HCSCO(R277) 3.4.7. CarbonIntermediates HCS +O—HCOS. (R279) + + + SiO +OH-►Si0H(151) Si0 +ï/ctSiOO.(R276) SiOH +eSiOH(146) SiOH +SiOH(147) 2 2 HCS +£->CSH (155) SiO +i/cr-^SiO.(R275) 0 +Si-*Si0(246) 2 SO +C^CSO(237) STERNBERG &DALGARNO + + + species atdifferentdepthsthrough thecloud.Ourresultsare processes thatleadtotheformation ofatomicandmolecular used ourmodelinordertoillustrate thegas-phasechemical molecular exposedtoanintense FUVradiationfield.Wehave structure ofaphoton-dominated regionproducedinadense density ratioremainslargeinthedarkcore(seeFig.22). The CN/CNdensityratiobecomeslarge,andtheHCN/CN and isremovedby CN isproducedby moved by CN isproducedbyreactions(R167)and(R282)re- and isremovedby and removedby Reaction (R283)isfollowedbythesequence and by OCS isformedby and arerapidlyremovedby CS ionsareveryinefficientlyproducedby We havepresentedadetailedcomputationofthechemical HCN isproducedby + He +HCNCNH(R284) + + + + HCN +H^CNH,(165) + 2 HCN +e^>.(167) HCN +H.(R283) HCN +e-+HCN,(166) CN +H^HCN.(164) 2 + 2 2 CS +H^HCS.(154) 2 CH +N-►HCNH(226) 32 + HCN +vCNH(R282) OCS +^^COS.(R281) cr CN +O-^CON.(216) SO +CCSO(R280) CO +SOCSv(239) 4. SUMMARY Vol. 99 1995ApJS ... 99. .565S + + + + + + + + + + + + + + 6-35 -17-1 in theC11zoneleadsto efficientformationofC,CH, tion islessrapidthaninthehot H1zone.ThelargeCdensity HCN. Beyondtheedgeof C11 zonetheCdensitybecomes CH, CH3,CH,CS HCS,CNHCNCN,and zone. Thegasiscoldinthesecloud layersandmoleculeforma- lecular. ThecarbonissinglyionizedintheC11zone,but is large CHdensityleadstotheefficientproductionof , neutral (andpresentprimarilyinCOmolecules)the S 11 oxygen andnitrogenareatomic,thehydrogenisfully mo- HCN, CN,andHCN. NH densitiestogetherleadtotheefficientproductionofCN , CHJ, C,CH,CH,CSandHCSthelarge and tion ofH,OH0HJ,0,OJ,CO , densities becomeverylargeinthewarmpartofH/Htran- the oxygenandnitrogenareatomic.ThemoleculesOH,CH, is atomic,thecarbon,sulfur,andsiliconaresinglyionized, HCO, CO,NO,NOSiOandSiOHinthehotgas. The sition layer.ThelargeOHdensityleadstotheefficientproduc- NH, andSHarerapidlyproducedinthehotgas,their zones, andadarkcore.Thelocationsofthesezonesinour hot HIandH/Htransitionlayer,coldC11,S11,SiS1 cloud. Weidentifythesezones(inorderofclouddepth)asa entially producedindifferentchemicalzonesthroughthe model cloudaregiveninTable2. summarized inTable2whichliststhespeciesthatareprefer- No. 2,1995 with atotalhydrogenparticledensityn=10cm,whichisexposedtoFUVradiationfieldx2Xandcosmic-rayionization rateof$*=5X Carbon intermediates. Oxygen intermediates. 2 2 Nitrogen. Sulfur Carbon . Hydrogen . 2 23 2 Silicon Oxygen 2 10 s.ThetotalelementalabundancesarelistedinTable1. T a In theCiiandS11zonessulfursiliconaresinglyionized, In thehotH1zoneandH/Htransitionlayerhydrogen Thistableliststheatomicandmolecularspeciesthatarepreferentiallyproducedindifferentchemicalzonespresentaphoton-dominated region 2 Family © American Astronomical Society • Provided by theNASA Astrophysics Data System + + + + + + + + + + + + + + + CN HCNH CN HCN CS HCS SiO SiOH No NO CO HCO SO NH NH N CHÍ CHt CH CHJ CH cc o Si S SH OH H0 0 OHH H HfHJ 2 23 2 3 23 2 23 2 CHEMISTRY INDENSEPHOTON-DOMINATEDREGIONS A= 0^0.7 Hot HiH/H v 2 a Atomic andMolecularSpeciesinDensePDRs + + + + + + + CN HCN CN HCN CS HCS CO HCO CH CHJ CC Si N S CHJ CH5 CH CH OOH h 2 23 2 0.7 -*►1.7 Cu TABLE 2 + + + + + the chemistryisdominatedby cosmic-rayionizationandthe mation anddestruction.The gas-phasecarbon,sulfur,nitro- incident FUVphotonsnolonger influencethemolecularfor- et al.(1991)andJansen (1994). dances inFUV-illuminatedcloudshasbeenpresentedbyZhou clouds inadditiontobeingatracerofdensemolecular gas. depths. TheCSmoleculemaybeatracerofFUV-illuminated Evidence forphotochemicallyenhancedCSandHSabun- larger neartheinneredgeofS1zonethanatdeepercloud duced intheS1zone.ThedensitiesofHandCSaremuch ions HS,S,SO,CS,HCS,andHCSareefficientlypro- where theSidensityislarge.Themoleculesandmolecular by Puenteetal.(1993).ThesulfurhydridesSHandHSare tion ofHCN)alsobecomesmall.TheCN/HCNdensityratio hydrides SiHandareefficientlyproducedintheSi11zone ized intheSi11zoneandisatomicS1zone.Thesilicon sity ratiosinFUV-illuminatedcloudshasbeenpresentedby Evidence ofenhancedCOdensitiesandlargeCN/HCNden- is largeintheC11zoneanddecreasesatlargerclouddepths. are atomic,andthehydrogenismolecular.Thesilicon ion- is lockedinCOmolecules,andthesulfur,oxygen,nitrogen efficiently producedintheS11zonewheredensityre- Latter etal.(1993),Störzer,Stutzki,&Sternberg(1995),and small, andthedensitiesofallthesespecies(withexcep- mains large. 2 2 2 2 2 At sufficientlylargeclouddepths adarkcoreexistsinwhich + + In theSi11andS1zones(whichpartiallyoverlap)carbon CO N S SHH Si o H 2 2 1.7 —3.7 SII + CO SiH N s Si o h 2 2 3.7 —6 Si ii + + + + OCS HCN CS HCS so CO N Si NN S Hhs O H 2 2 2 3.7 —7.6 Si + + + + + + + + + OCS HCN HCS SiOH NO CO HCO SiO Si0 so so nh nh NH4 NN cm ch o ho oho H 2 2 23 2 4 2 3 2 23 Dark Core A> 10 v 591 1995ApJS ... 99. .565S 9 4 + + + + + + the cloudsurfaceto3X10indarkcore(seeFig.8). dark core.Thefractionalionizationrangesfrom3X10near and bycosmic-rayionizationofhydrogenheliuminthe toionization ofcarbon,sulfur,andsiliconthroughthePDR, become verysmallatlargeclouddepths. remain largethroughtheSiizone.Allofthesedensityratios through theCnzone,and0H/H0,CN/HCNratios zones. VerylargeSO/SOandSiOiOdensityratiosare 0H/H0, COandNH/NHratiosremainlarge produced inthehotHIzoneandH/Htransitionlayer.The CN/HCN densityratiosarelargestintheouterchemical which maybeusedasdiagnosticsofthedifferentchemical zones inthecloud.Thistableshowsthat0H/H0,OH/ large inthedarkcore. ions H0,0,O?,HCONHJ,NHNSO gen, andsiliconarepresentinCO,S0,NSi0mole- S0, NONO,SiOHSiO,HCSHCN,andOCSbecome core. Thedensitiesofthesaturatedmoleculesandmolecular rapidly andthefractionalionizationbecomessmallindark positive chargeiscarriedbymolecularionswhichrecombine remains primarilyinatomicform.Inourmodelmostofthe cules respectively.TheexcessoxygennotdrivenintoCO 592 H0, CO/SO+/SO,SiOSiO,NH/NHand 2 3 2 2 32 2 2 3 The fractionalionizationiscontrolledbythesequentialpho- In Table3welistasetofspecificmoleculardensityratios + b 6-35 + + 17 CN/HCN 4.71.1(1)8.43.3(-l)1.3(-3)1.8(-4) NH/NH 7.8(4)2.2(5)9.2(3)9.3(-1)1.7(-2)4.9(-4) CO/HCO 5.1(-2)3.51.5(-4)1.7(-6)2.9(-7)2.0 0H/H0 4.5(l)2.3(1)9.39.2(-2)4.4(-3)3.4(-4) hydrogen particledensityn=10cm,whichisexposedtoaFUVradiationfieldwithx2Xandcosmic-rayionizationrateof^ S07S0 1.72.3(—1)2.8(—3)6.4(-4)1.2(-4)5.7(-4) 0H7H0 1.81.37.5(—2)9.6(-3)1.3(-3)1.4(-5) SiO/SiO 1.6(-1)5.7(—3)3.0(—5)1.2(-7)1.8(-9)1.7(-10) 5 X10“s".ThetotalelementalabundancesarelistedinTable1. 3 2 T 3 b1 8 = Numbersinparenthesesareexponents.Forexample,4.5(1)=4X10. Selectedmoleculardensityratiosatspecificlocationsinthedifferentchemicalzonespresentaphoton-dominatedregionwithtotal Density Ratioy4^=0.6A=1.535l> v © American Astronomical Society • Provided by theNASA Astrophysics Data System Hot HiH/HCiiSSinDarkCore 2 8 Selected MolecularDiagnostics STERNBERG &DALGARNO TABLE 3 tion throughgrantAST-93-01009. ical ObservatoryandpartlybytheNationalScienceFounda- work hasbeenpartlysupportedbytheSmithsonianAstrophys- Jaffe, H.Störzer,andJ.Stutzkiforusefuldiscussions.This We thankE.F.vanDishoeck,R.Genzel,A.I.Harris,D.T. transfer program,andD.Jonesforhisassistanceinusingit. elsewhere. cosmic-ray ionizationrate.Wewillpresentaparameterstudy locations andsizesofthevariouschemicalzones, driven chemistry.However,ourmodelisillustrative.The scattering properties,thetotalelementalabundances,and eral, sensitivetothetotalhydrogengasdensity,intensity density ratiosexistwhichmaybeusedasdiagnosticsofphoto- inated regionsofmolecularclouds,andthatspecific in Table3)thedifferentchemicalzones. cloud collapsetimes(Shu,Adams,&Lizano1987;McKee of theincidentFUVradiationfield,grainabsorptionand abundances oftheatomicandmolecularspeciesare,ingen- ular speciesmaybesignificantlyenhancedinthephoton-dom- molecular abundancesanddensityratios(suchasthoselisted The ambipolardiffusionrates,andthemagneticallyregulated 1989), arethereforesensitivetotheclouddepthand We thankW.G.Robergeforprovidinguswithhisradiative We havedemonstratedthattheabundancesofmanymolec- Vol. 99 1995ApJS ... 99. .565S -2+ Integrated columndensitiesNi(cm)ofH,H,H*,HJ,ande. 2 + Fig. 8.—{a)Abundancesx,=ni/n(wherenisthedensityofhydrogennucleiandriiaredensitiesspecies/)ofH,H,H*,Hj,e.(b) 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System + + + Fig .10.—(a)AbundancesofO, OH,H0andO.(6)Columndensitiesof , H0andO. 23 32 Fig. 9.—{a)AbundancesofO,OH,H0,and0.{b)Columndensities 2 Fig. lOa Fig. 106 Fig. 8û8/? Fig. 9a9b 1995ApJS ... 99. .565S + Fig. 12.—(û)AbundancesofCH,CHJ,andCHJ.{b)ColumndensitiesofCHCH¿,CHt © American Astronomical Society • Provided by theNASA Astrophysics Data System + Fig .11.—(a)AbundancesofC,C,CO,COandHCO{b)Columndensities Fig .13.—(a)AbundancesofCH, CH,and.(b)ColumndensitiesofCH,CH4. 234 Fig. \\aMb Fig. \2a\2b 594 C/} LO

+ + + + Fig. 14.—(a) Abundances of S , S, SO, and S02, and SO . (/>) Column densities of S , S, SO, S02, and SO .

Fig. 15a Fig. \5b + + + + Fig. 15.—(a) Abundances of SH, H2S, SH , and H2S . (b) Column densities of SH, H2S, SH , andH2S .

Fig. 16a Fig. 166 Fig. 16.—(a) Abundances of CS, HCS, OCS, CS+, and HCS+. (6) Column densities ofCS, HCS, OCS, CS+, and HCS+.

595

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1995ApJS ... 99. .565S © American Astronomical Society • Provided by theNASA Astrophysics Data System + Fig. 18.—(a)AbundancesofSiH,SiH,SiOandSiOH.(6)Columndensities 2 + Fig. 19.—(a)AbundancesofN,N , NO,andNF1.(6)ColumndensitiesofN,H + 2 Fig .17.—(a)AbundancesofSi,Si,SiO,andSi0{b)Columndensities 2 Fig. \laMb Fig. 18a186 Fig. 19a Fig. 196 596 1995ApJS ... 99. .565S + + Fig. 22.—(a)AbundancesofCN, HCN,CN,HCNandH.(b)ColumndensitiesofCN, CN,HCNandH 2 2 + Fig. 20.—(a)AbundancesofN,NHNHJ,andNHJ.(b)Columndensities 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System + Fig. 21.—(a)AbundancesofNH,NH,andNO.(b)Columndensities 23 Fig. 20a20b Fig. 22a Fig. 22b 597 1995ApJS ... 99. .565S x+ x -2 -22-2 x 5 these moleculesdissociatebecomeopticallythick(Federman,Glassgold,&Kwan1979;Sternberg1988;vanDishoeck &Black by Robergeetal.butforwhichFUVabsorptioncrosssectionsareavailable.InTable4welisttheratesofatomicphotoionization independent crosssections.InTable5welistthevaluesof(ft,,andftthatusedforeachphotoreactions includedin and molecularphotodissociationforthesespeciesinanunattenuatedDrainefield.ForwithunknownFUVabsorption cross in ordertocomputethecoefficients(ft,aandft(forAy=100)severalspeciesC,Si,S,H,CO,NOH) not considered tions weadoptedattenuationparametersappropriateforopticallythickcloudswithAv=100.Weassumeagas-to-dust massratio by Draine&Lee(1984)fora“Mathis-Rumpl-Nordsieck”mixtureofgraphiteandsilicategrains(Mathis,Rumpl,Nordsieck & Dalgamo1989),andthatthedepth-dependentHphotodissociationrateisgivenby our calculations. sections weseta=2.55andß—1.65X10whichcomputedusingtheRobergeetal.programassuming wavelength- suchthatT= 4.75X10whereV"isthetotalcolumndensity(cm)ofhydrogennucleifromcloudsurface. atomic ormolecularFUVphoto-absorptioncrosssections,thegrainproperties,andtotalcloudthicknessAv. In ourcalcula- presented byRobergeetal.(1991)fortheresultingdepth-dependentphotoionizationandphotodissociationrates.In theseexpres- photoionization andphotodissociationdiminish.Inourmodelweadoptedthegrainscatteringabsorptionpropertiescalculated Draine’s (1978)fittothespecificphotonintensityofaverageinterstellarFUVfield sions isthevisualextinctionmeasuredfromcloudsurface,andC*,a:/ftareattenuationparametersthat dependonthe multiplied byanintensityscalingfactorx=2X10. 1988; Abgralletal.1992).InourcalculationsweassumedthattheFUVlineandcontinuumattenuationareseparable (Sternberg t2 1977), andweusedtheanalyticbi-exponentialexpressions 2 F 598 We usedthe“sphericalharmonics”radiativetransferprogramandfittingproceduresdevelopeddescribedbyRoberge etal. The photodissociationratesofHandCOarealsodiminishedby“self-shielding”astheFUVabsorptionlinesthrough which As theFUVradiationpenetratesclouditisscatteredandabsorbedbydustgrainsratesofatomicmolecular In ourmodelweassumethatasemi-infiniteplane-parallelcloudisexposedtoanisotropicfar-ultravioletradiationfieldgivenby In thisappendixwedescribesomeofthebasicinputdataandapproximationsthatemployedincarryingoutourcalculations. 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System 4 1/1.068X10~ r/ x — Lho X A1.1. PhotoionizationandPhotodissociationRates j— + + + + tion crosssectiondataareavailable. SeeTable todissociation andatomicphotoionization in erge etal.1991forwhichFUVphotoabsorp- cies notincludedinthecompilation ofRob- an unattenuatedDrainefieldforseveral spe- OH +i/HO CO +v^CO n +^n C +i/->Cí? 5 forreferences. Si +^e S +y—►c H +v—► -1 2 2 ire me a r, =xCiexp(—oiiAvßiA\)s(A2) 2 Thistableliststheratesofmolecularpho- STERNBERG &DALGARNO 1.719 X10“6.853X10“‘\ 2 r Photoreaction 2 h(v)fM(Nn) 2 3 2 +3 Photoreaction Rates X j Al. CHEMISTRY APPENDIX TABLE 4 2 , aAß„1 uvHV 4.3 (-10) 4.9 (-11) 2.0 (-10) 2.1 (-10) 2.4 (-10) 3.1 (-9) 1-2 (—11) 21 photons cmsHzsr (Al) (A3) 1995ApJS ... 99. .565S + + + + + + + + + + + + + + + + + + + + + + + + + + + CHJ +z/-^CHJH CHJ +z>H CHJ +z;—H CHJ +z;H CH +z;^H CH -h+H CH +v-+CHJH CH +z/-^HC 0 +y-►Oo H0 +z;OH... H0 +z/^OH.... H0 +v... H0 +z;... H0 +z/^OH.... H0 +Z/ H0 +V-+H2+o OH +^HO H +z/-► H+ z;HOH.... H +z/ H +z;-► OCS +z/^COS HCN -hv^CN-hH CN +z/^CN HCS +z/^CSH NO +v^£ NO +z/—NO CS +z;—►e CS +z/—CS CO +z/—CO SO +v—*■-he NH +z;^H N +z/^N NH +v-+e NH +z/e NH +z/^H NH +z/^NH S0 +z^—►SOO SO +z>-►os Si0 +v—►SiOO SiO +V-+--he SiO +z;—SiO SiH +z/—SiH Si +v^Sie NH +z;^-he HS +z/-►SH CH +Z/^CHJ-he CH +z/^CHH CH +z/e HS +v^-he SH +ve SH +z/—SH CH +z/—CHH CH +z;^H CH +H CH +z/^CHH CH +z/^CHH CH +z>-►CH S +v-+S-he CH +v-+-he CH +v-+e C +v-+Ce 0 +z'^ 0 +v—►O H0 +z/^e H0 +i/-►OH H0 +v—►O OH +i/—OH 2 2 32 2 3 2 32 32 32 32 2 2 2 2 2 32 32 32 2 3 2 2 2 2 2 4 42 3 2 43 42 32 32 2 2 2 2 2 2 2 + H Photoreaction CaßReference © American Astronomical Society • Provided by theNASA Astrophysics Data System Photoreaction AttenuationParameters 2.50 (-10)2.55 2.50 (-10)2.55 2.50 (-10)2.55 2.50 (-10)2.55 2.50 (-10)2.55 2.50 (-10)2.55 2.50 (-10)2.55 2.50 (-11)2.55 7.50 (-12)2.55 2.50 (-11)2.55 2.50 (-11)2.55 2.50 (-13)2.55 8.12 (-11)3.71 2.04 (-12)3.35 2.50 (-13)2.55 2.25 (-10)2.55 5.00 (-11)2.55 5.00 (-11)2.55 2.00 (-10)2.55 6.50 (-11)2.55 5.00 (-11)2.55 5.00 (-11)2.55 3.44 (-10)2.69 3.55 (-10)3.71 5.00 (-10)2.55 2.00 (-9)2.55 2.00 (-10)2.55 6.76 (-11)3.86 5.17 (-10)2.30 3.00 (-10)2.55 3.53 (-11)3.47 5.00 (-11)2.55 9.65 (-10)2.37 5.00 (-11)2.55 7.50 (-11)2.55 5.27 (-11)3.35 3.05 (-10)2.16 7.32 (-10)2.38 5.00 (-12)2.55 2.94 (-10)2.61 2.50 (-11)2.55 2.50 (-11)2.55 2.50 (-10)2.55 2.50 (-11)2.55 2.43 (-10)1.74 1.50 (-10)2.55 3.00 (-10)2.55 5.00 (-10)2.55 7.45 (-11)3.51 2.32 (-10)2.23 9.35 (—11)2.13 1.19 (-9)2.05 1.31 (-10)2.07 1.00 (-10)2.55 5.00 (-12)2.55 1.50 (-9)2.55 1.46 (-11)1.90 1.68 (-10)2.37 1.85 (-10)3.30 1.22 (-10)3.13 1.50 (-10)2.55 1.00 (-10)2.55 1.50 (-10)2.55 1.50 (-10)2.55 1.00 (-10)2.55 1.82 (—11)3.83 1.62 (-10)2.10 1.00 (-11)2.55 1.33 (—11)3.78 TABLE 5 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -2.05 (-2)2 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -8.16 (-2)10 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -3.73 (-2)2 -1.28 (-2)2 -8.14 (-3)2 -1.65 (-2)1 -1.65 (-2)1 -1.40 (-2)2 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -2.30 (-2)2 -1.65 (-2)1 -1.31 (-2)9 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.62 (-2)6 -1.47 (-2)8 -7.71 (-3)7 -1.58 (-2)2 -1.71 (-2)2 -6.20 (-3)2 -1.65 (-2)1 -1.22 (-2)2 -2.61 (-2)2 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.61 (-2)5 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -2.03 (-2)2 -1.65 (-2)1 -1.65 (-2)1 -1.65 (-2)1 -2.21 (-2)2 -1.58 (-2)4 -9.95 (-3)2 -1.54 (-2)2 -1.65 (-2)1 -1.65 (-2)1 -1.63 (-2)2 -1.66 (-2)2 -1.04 (-2)3 1995ApJS ... 99. .565S -10_I =2 _1 -2 1 1 photodissociation rateequalto 2.0X10sinaunitDrainefield(vanDishoeck&Black 1988). computed (usingtheRoberge etal.program)assumingopticallythinCOdissociatingabsorption lines.Equation(A5)yieldsaCO by vanDishoeck&Black(1988) asfunctionsoftheHandCOcolumndensities7Vh andVcoThebi-exponentialfactorin equation (5)accountsforgrain attenuationoftheCOphotodissociationrate.Weseta o 3.47andßu=—1.31X10“whichwe predissociating fractions(Letzelteretal.1987;vanDishoeck &Black1988).COlineshieldingoccursduetooverlapwithbroadH where faretheFUVabsorptionlineoscillatorstrengthsto predissociatinglevelsinexcitedelectronicstatesofCO,andt]arethe absorption linesandopacityin theCOlinesthemselves.Inourmodelweusedshielding function,0(Vco>Nh)>computed unit Drainefield. clouds) assumingopticallythinHabsorptionlines.Equation (A3)yieldsaHphotodissociationrateequalto4.9X10sin attenuation. WeadoptedtheHattenuationparametersa =3.78andß—1.04X10whichwecomputed(forAV”100 The bi-exponentialterminequation(A3)accountsforthe decreaseintheHphotodissociationrateduetoFUVcontinuumgrain used theanalyticformulaeofFedermanetal.(1979)tocomputeself-shieldingfactorsasfunctionsHcolumn density. where JVaretheequivalentwidthsofabsorptionlines,andA'h,isHcolumndensitymeasuredfromcloud surface.We radiative dissociations(Stephens&Dalgamo1972),andxL(v)aretheincidentFUVphotonfluxesatabsorption-line transition the excitedB-„andf1electronicstates,r/,.arefractionsofFUVabsorptionsintolevelsrthatleadto spontaneous 600 frequencies. Thelineself-shieldingfunctionsaregivenby and WernerbandFUVtransitionsbetweenthegroundvibrationalofX2electronicstate levels,v,of In thisexpression,/,aretheHabsorptionlineoscillatorstrengths(Allison&Dalgamo1970)ofenergeticallyaccessible Lyman 2 2 C 2 2 v v 2 2 2 2 Hl 2 2 v2 g 2 Similarly, weusedadepth-dependentCOphotodissociation rategivenby © American Astronomical Society • Provided by theNASA Astrophysics Data System + + + + _1 + + + + + + + + + + + + + CN +v-»•CN5.00(-11)2.55-1.65(-2)1 CS+Tr —S+CI.OO(-IO)2.55-1.65(-2)1 CO +i’^C01.50(-11)2.55-1.65(-2)1 NHj +1/-*NH+H2.50(—11)2.55-1.65(-2)1 Henry 1972;(9)Letzelteretal.1987,andvanDishoeck&Black1988;(10)SaxonLiu1986. Chapman &Henry197LandDilletal.1975;(6)KirbyGoldfield1991;(7)Starketal.1992;(8)Chapman* 6' =0(seeRobergeetal.1991). G =xCexp(—aA—ßAl)s.ForA>15theparametersaandßshouldbereplacedby«'+7.5X HCN +j/-^HCN5.00(-11)2.55-1.65(-2)1 HCN+1/-► CNH5.00(—11)2.55-1.65(-2) HCS +V-*CSH2.00(-10)2.55-1.65(-2)1 SiOH +!'-*■SiOH1.50(-11)2.55-1.65(-2)1 SiO+ ?—SiO1.50(-11)2.55-1.65(-2)1 SO+Tr —S+05.00(-11)2.55-1.65(-2)1 HCO+ v—COH1.50(-10)2.55-1.65(-2)1 NH+ r—5.00(-11)2.55-1.65(-2)1 NHJ +i/—NHÎH2.50(-10)2.55-1.65(-2)1 NHÎ +k^-NHÎH2.50(—10)2.55-1.65(-2)1 NHJ +1/-»■+H1.75(—10)2.55-1.65(-2)1 NHJ +i/—NHH2.50(—11)2.55-1.65(-2)1 NHJ +><-»■NHHJ2.50(—11)2.55-1.65(-2)1 NHJ +v-*-NHH2.50(—10)2.55-1.65(-2)1 NH +r—NH5.00(—11)2.55-1.65(-2)I HS +!'-*•SH2.00(—10)2.55-1.65(-2)1 SH +1>-»■S+H5.00(—13)2.55-1.65(-2)1 2 v 2 2 2 2 2 2 a Thedepth-dependentatomicandmolecularphotoionizationphotodissociationratesaregivenby References.—( 1)Estimate;(2)Robergeetal.1991;(3)Allison&Dalgamo1970;(4)Cantu1981;(5) Photoreaction CaßReference Tco -X we me STERNBERG &DALGARNO 2 I(v)fvVv v gv(N) H2 TABLE 5—Continued rco,l 0(AH,Acok““^'‘V (A5) 21 2 TTg rdWy me 7dN Hl Vol. 99 (A4) 1995ApJS ... 99. .565S _7-0 5 3-1 -1 pilation ofMillaretal.(1991)forwhichthechemicalreactantsandproductsareatomicmolecularspeciesincludedinour tons maybewrittenas(Gredeletal.1989) of HandothermolecularspeciesinthewarmpartH/Htransitionlayer.Inourmodelweneglectedgas-phase Hformation (R26) andHj+^-^HH(R27)forwhichweadoptedrapidratecoefficientsequalto1X10(T/300) cm swhere adopted thechemicalratecoefficientslistedbyMillaretal.(1991),exceptfordissociativerecombinationsH3+e->H+H we haveincludedinourcalculations.Inmodeladoptedagrainalbedoco=0.5. model. Only210ofthesereactionssignificantlyinfluencethecloudchemistryasdiscussedin§3andillustratedFigures1-6.We molecular FUVphotoabsorptioncrosssections.InTable6welistthevaluesofpforcosmic-ray-inducedphotoreactionsthat where fisthecosmic-rayionizationrate(s)ofhydrogen,cograinalbedo,and/?afactorthatdependsonatomicor photodissociations. Therates,Æ,ofatomicandmolecularphotoionizationorphotodissociationbythecosmic-ray-inducedpho- No. 2,1995CHEMISTRYINDENSEPHOTON-DOMINATEDREGIONS (Prasad &Tarafdar1983).Thesephotonsareabsorbedbythedustgrainsandatomsmoleculesinphotoionizations molecules producedbyHcollisionswiththeenergeticsecondaryelectronsgeneratedfollowingcosmic-rayionizationof Tis thegastemperature(K)(Amano1990;Dalgamo1993). 2 2 2 cr 2 The HformationrateisacrucialparameterwhichinfluencesthesizeandgastemperatureofhotHIzone, densities In ourcalculationsweincludedallofthe772ion-moleculeandneutral-neutralchemicalreactionslistedin(UMIST)com- A fluxofFUVphotonsismaintainedinthe“dark”coresmolecularcloudsbydecayelectronicallyexcitedhydrogen 2 American Astronomical Society •Provided bythe NASA Astrophysics Data System + + + + + ■ C+I'd--►e (3) Langer&Glassgold1990;(4)Estimate. in unitsofthecosmic-rayionization rateofhydrogen(seetext). Ctr +^cr—CH NH +Vçr-*-H.. NH +vt-*■NHJe... ch +h.. OCS +I'd-~cos HCN +l'a-—CNH... NO +^cr—e NO +Vqt—>•NO CO +i>CO MLj +i/c^NHÎ+e... SÍO2 +^cr^SÍOO.... SÍO +Ver—-SÍO 502 +^—800 SO +—>■SO NH +I'd--►H. NH +^crH... HS +e.... HsS +^SHH CH +I/—CH 0 +^crO H0 +Vcr-+-OH.... SÍ Ver—*■Si+e S +Vqj;—>e 0 +^crC>2£ OH +^^OH 32 2C 42 cr 32 2 2 CT 2 2 2 a Thecosmic-ray-inducedphotodestruction ratesaregiven References.—(1) Gredeletal.1989; (2)Gredeletal.1987; A1.2. Cosmic-Ray-inducedPhotodestruction Cosmic-Ray-induced PhotodestructionRates Photoreaction A1.4. HFormationandExcitation 2 A1.3. ChemicalRateCoefficients *cr = TABLE 6 1 —CO 2986 5371 2272 5072 1500 1500 1995 1500 1325 1555 1000 430 427 610 979 500 541 756 730 F 543 510 522 183 81 88 10 Reference (A6) 601 1995ApJS ... 99. .565S 4-3 -13/2 -3 -1 -17832 molecules (Sternberg&Dalgamo 1989).Weconsideredgascoolingbyatomicandionic fine-structurelineemission,molecular ionization of“largemolecules” (Lepp&Dalgamo1988;Verstraeteetal.1990),andcollisional de-excitationbyFUV-pumpedH gas temperatureequalto20K. field dominatesthegasheating. Atlargerclouddepths,wherethegasheatingmechanisms arelesscertain,weadoptedauniform deactivation ofHmoleculesinthev=1levelbecomeequal atacriticalhydrogenparticledensityequalto5X10cmK. density inthev=1levelislargeduetorapidcollisional excitationthatoccursinthehotpartofH/Htransitionlayer(see We neglectedcollisionswhichinducetransitionsfor Av>Fortheseratecoefficientstheratesofradiativeandcollisional Fig. 8).Theexcitationofthehigherlyingvibrationallevels isdominatedbyFUVpumping(Sternberg&Dalgamo1989). transitions andcollisionswithH.WeadoptedapproximateH-H-Hcollisionalde-excitationrate coefficients 2 of detailedbalance (Sternberg &Dalgamo1989;MandyMartin1993;Sun1994),andupwardratecoefficientsgivenby thecondition Qv^v-\ =5.4X10ri;(cms)whereTisthegastemperatureandvvibrationalquantumnumberof upperlevel collisions withthermalhydrogenatomsandmolecules,vibrationaldeexcitationsby(opticallythin)quadrupole radiative level distributionswithineachofthe15vibrationalmanifolds.WeconsideredexcitationsbyFUV-pumping andby ( depth-dependent)equationsofstatisticalequilibriumfortheHvibrationallevelpopulations,jc(d),butweneglected therotational reaction endothermicitydefinedsuchthatforgroundstateHmoleculestheratecoefficientsareequaltokQXQ(-E/kT). Sternberg 1988;&Dalgamo1989;Burtonetal.1990;Goldshmidt1995).Inourcalculations wesolvedthe where x(v)andEarethepopulationfractionsenergiesofHmoleculesinvibrationallevelsv.Inthisexpression Eisthe even atlowgastemperatures(Jonesetal.1986).Forthesereactionsweadoptedratecoefficients (suprathermally )excitedmoleculesmaybeusedtodriveendothermichydrogenabstractionreactionsoftheform respectively. given byHollenbach&McKee(1979).ThefactorsSandfdependonboththegastemperaturegrainT.For and thefraction temperatures (Buch1989;&Zhang1991),andinourmodelweadoptedthestickingcoefficientgivenbyBurkeHollenbach 2 (1983) do notevaporatefromthegrainsurfacesbeforemoleculeformation.TheHformationefficiencyisuncertainatlargegasand 2 In thisexpressionTisthegastemperature(K),5gas-grainstickingcoefficient,and^fractionofhydrogenatomsthat hydrogen particledensity,n(cm)istheatomicandRH-grainformationratecoefficientgivenby processes andassumedthatthehydrogenmoleculesformongrainsurfacesatarateperunitvolumeRnrin(s)wherenistotal 2 602 T* =30KtheHformationratecoefficientequals1.7X10and7.7cmsatagastemperatureofK, 2 20 v2 ag 2 u2 2 The dominantgasheatingmechanisms thatweincludedaregrain-photoelectricemission (deJong1977;Draine1978),photo- In ourmodelweexplicitlycomputedtheequilibriumgastemperatures oftheoutercloudlayerswhereincidentFUVradiation In Table7welistthetotalcolumndensitiesofvibrationally excitedHmoleculesproducedinourmodel.Thetotalcolumn Detailed studiesofHexcitationinPDRshavebeenpresentedelsewhere(Black&Dalgamo1976;BlackvanDishoeck 1987; The densitiesofvibrationallyexcitedhydrogenmoleculesareimportantcloudquantitiessincetheinternal energies of 2 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System 1/2-36 S=[l +0.04(T+7^)+2X10r8T](A8) {E)/kT3l k =ko'Zx(v)e~~vcms~(All) -18/23 STERNBERG &DALGARNO R =3X10/STcms.(A7) 4v a ^ 1+10exp(-600/7^)' A2. THERMALBALANCE Qv-+v-1 X+H-^XH +H(A10) v=0 2 ^{E-E^/kT = Vol. 99 (A12) 1995ApJS ... 99. .565S bination ontothegrainsurfaces andphotoelectricemissionfromthegrainsvanishes.In ourcomputationsweassumedthatB= photoelectric yield,Eistheenergy oftheejectedelectrons,andf(E,e)\senergydistribution functionfortheemittedelectrons. where Bisthephotoelectric threshold, ¿Ke)isthephotonradiationintensityforphotons withenergye(seeeq.[Al]),Y(€)isthe The grainpotentialUisdetermined bytherequirementthatinequilibriumtotalgrain currentduetoelectronandionrecom- 8 eV. parameters tocomputethephotoelectricheatingratewhich isgivenby No. 2,1995 0 0 lenbach &McKee1989).Wedescribetheseprocessesbelow. rotational andvibrationallineemission,gas-graincollisions(Tielens&Hollenbach1985;SternbergDalgamo 1989;Hol- energy transfersis Williams 1986).However,theseprocessesdonotcontribute significantlytothegasheating(Sternberg&Dalgamo1989). in collisionaldeexcitationsisgreaterthantheenergylostexcitations. where w(H)istheHdensity,x(v)arefractionsofmoleculesineachvibrationallevelsv,Eenergy oflevelv,and vibrational excitationfollowedbyradiativequadrupoledecaycoolsthegas.Thenetheating(orcooling)ratedueto thecollisional q{v ^v')arethecollisionalratecoefficientsfortransitionsfromvtov'.Gispositivewhentotalenergytransferred tothegas 1978; Tielens&Hollenbach1985;SternbergDalgamo1989). InourcalculationsweusedDraine’s(1978)formalismandgrain 2v H2 Photoelectric emissionofelectronsfromgrainsurfacesis a majorheatsource(Watson1972;Jura1976;deJong1977;Draine FUV pumpingoftheHfollowedbycollisionalde-excitationvibrationalmolecularlevelsheatsgas,and collisional We havealsoincludedgasheatingbyHphotodissociationandmolecularformation(Stephens&Dalgamo1973; Duley& 2 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System H 31 CHEMISTRY INDENSEPHOTON-DOMINATEDREGIONS Gpe =47tf^{E-eU)dEY(e)f(E,e)g(e)deergscm' s",(A14) =3 ^h «t«(H)2x(v)q(v->v’)(E-E>ergscms*,(A13) 2v JEq JeQ 1 =5 6-3 Xelectronic stateofHthatare the 15vibrationallevelsofground 4.. . 0... produced inacloudwithtotalgas 6.. . 2... FUV fieldwithx2X10. density n=10cmexposedtoa 9... 8... 7 ... 5 ... 3.. . A2.1. HHeatingandCooling 14. 13. 12. 11 . 10. 1 ... 2 T 2 A2.2. PhotoelectricEmission a V v' Totalcolumndensitiesineachof Vibrational HColumn 2 51 Densities TABLE 7 2 At, (cm) 4.5(14) 6.8(14) 4.6(15) 4.3(13) 2.8(13) 6.2(13) 2.2(14) 8.6(13) 3.1(14) 1.6(14) 1.2(15) 1.6(17) 1.1 (22) 1.7(13) 1.2(14) 603 1995ApJS ... 99. .565S -1 4-82 -10/2 parameters recommendedby Hollenbach&McKee(1979).DetailedtreatmentsofCO lineemissionfromPDRshavebeenpre- the criticaldensityforlevel whoseenergyequalskT,andcisafineopticaldepth factor.WeusedtheCOandOHcooling sented byBurtonetal.(1990), Kösteretal.(1994),andWolfire1993). where that areproducedinourmodelclouduptoadepthAy=10. the atomicdataweusedtocomputefine-structurecoolingrates.Mostofthesewereobtainedfrom compilation of the transitionfrequency.Weusedan“escapeprobability”methodtocomputecoolinginopticallythicktransitions, andineq. n(X) isthemoleculardensity, AandEaretheradiativetransitionprobabilityenergy ofthelowestrotationaltransition,nis function significant indeeperandcoolercloudlayers. rotational linecoolingisparticularlyimportantinthehotpartofH/Htransitionregion,andH0CO become Hollenbach &McKee(1989).InTable8wealsolistthetotalopticaldepthsandlineintensitiesofvariousfine-structure lines center opticaldepth.WecomputedthelinedepthsassumingDopplerparametersb=2kms.InPDRs atomicand where isthevolumedensityofupperfine-structurelevel/,^4Einsteincoefficientforspontaneoustransitions, andvis ionic fine-structurelevelsareexcitedprimarilybyneutralimpactcollisionswithhydrogenatomsormolecules.In Table 8welist (A16) ß(r)isthephotonescapeprobabilityforstaticplaneparallelclouds(deJong,Chu,&Dalgamo1975)where ristheline- ion Xis etal. 1990). total PAHabundancen=10~V•ForthischoicetheheatingratesGandarecomparable(Lepp&Dalgamo1988; Verstraete & McCray1972;TielensHollenbach1985;SternbergDalgamo1989).Thecoolingrateduetoatransitioni-►j ofanatomor (1990) weset(is)=2.2eV,2X10/r,andH2.7exp(—2.55A+1.65^4i-).Inourmodel weadopteda PAHs, n(e)istheelectrondensity,and(ß^eU/kTwhereUelectricpotentialofionizedPAHs.FollowingVerstraeteetal. is theneutralPAHfraction,wherek=1.3X10r(1+0)ratecoefficientforrecombinationofelectronswithionized the photoelectrons,n\stotalvolumedensityofPAHs,and 604 T as afunctionofclouddepth.InthisexpressionIVisthephotoionizationrateneutralPAHs,(£)averagekineticenergy data providedbyVerstraeteetal.(1990)forlargeplanarorganicmoleculestocomputethePAHphotoionizationrate polycyclic aromatichydrocarbons(PAHs).Suchmoleculesbeenproposedasthecarriersofseveralbroadnear-infraredemission regions (Lepp&Dalgamo1988;Wolfireetal.1995).Inourmodelweassumethatlargemoleculesarepresentintheformof 0 cr 2 features innebulae(Leger&Puget1984;d’Hendecourtetal.1982)althoughtheidentificationsremainuncertain.Weused PHPE PAv P P In ordertocomputetheOHandCOcoolingratesweused Hollenbach &McKee’s(1979)analytic“universal”molecularcooling Rotational lineemissionbythetracemolecularconstituentsOH,H0,andCOcontributessignificantlytogas cooling. OH Fine structurelineemissionfromtraceatomicandionicspeciesisamajorgascoolantinphoton-dominatedregions (Dalgamo Discrete photoionizationoflargemoleculesorverysmallgrainsmaybeamajorheatingmechanisminphoton-dominated 2 © American Astronomical Society • Provided by theNASA Astrophysics Data System Lrot 2 2(kT)An(X) 0 3 Eo G =Tpnpf^E')—ergscms(A15) P 31 Lfj =niAijhvijßiTij)ergscms,(A17) STERNBERG &DALGARNO A2.4. Fine-StructureEmission ym =ln A2.3. PAHPhotoionization A2.5. MolecularCooling 1 +c(n/n)l.5(n raCT /= 1 + 2 +y0.6y^ Tp +kn(e) m P 1 +Winer/n)’ kn(e) P 1/2 31 ergs cms Vol. 99 (A19) (A18) (A16) 1995ApJS ... 99. .565S + + column densitiesNi(z).WeusedtheNewton-Raphsonmethodtosolvesetofnonlinear(algebraic)equations (1),(2),and molecular columndensities. (3), andweemployedtheBulirsch-Stoeradaptivestep-size extrapolationalgorithm(Pressetal.1986)tocomputetheatomicand produced inourmodelcloud. No. 1,1995 and b=2kms"istheemissionlineDopplerparameter.InTable9welisttotalOH,H0,COrotational intensities for thetotalwaterUneemissivityinstaticclouds.Inthisexpressionn(H0)andAoarevolumecolumn densities, between levelsiandj. the collisionalratecoefficientsaregivenbelow. Fe s.. o. C . Si 1992; (6)Aannestad1973. 2 2H2 e= d-1 -2-1 cß3-1 b 3 Species Equations (1)-(4)(in§§2and3)areasetofnonlinearcoupledordinarydifferentialequationsfortheatomic molecular For thepolyatomicmoleculeH0weusedexpressionprovidedbyNeufeld&Melnick(1987) Tisthefine-structureline-centeropticaldepth(atAy10). Thetotalfine-structurelineintensitiesproduced(withinaclouddepthÁy=10)inourmodelcalculationaregivenas(cgs)unitsofergss cmsr. Collisionalde-excitationratecoefficientsforcollisionswithatomicormolecularhydrogenareqH=aTcms.^istheradiativetransition probability Upperlevelenergiesaboveground. Mostofthefine-structuredatalistedinthistableweretakenfromcompilationpresentedbyHollenbach&McKee1989.Individual referencesfor References.—(1) Launay&Roueff1977a;(2)Flower1977;(3)HollenbachMcKee1989;(4)1977b;(5) Jaquetetal. 2 3 6 © American Astronomical Society • Provided by theNASA Astrophysics Data System 6 6 3 Pi d 9/2 r 3 D/2 d Pi ^2 9 v2 2 ■*1/2 n 1/2 6 6 6 3 3 3 3 3 3 3 3 r A/2 d D-j/2 Po Pl Fo Po Pl P Pl 3^3/2 5/2 2 P2 3/2 h CHEMISTRY INDENSEPHOTON-DOMINATEDREGIONS 407 414 961 255 228 826 554 571 326 (K) E 98 63 24 92 39 ■i'H-.O 229.9 609.2 369.0 145.6 1.7(-5) 157.7 2.4(-6) (iim) 44.2 26.0 25.2 63.2 35.4 56.6 34.8 15.0 17.4 X 29 1 +V«hVo/(1.4XlO'^T-) H2 -29l95 6 -3 5 -1 -1 3.67 X107’'«rc(H0) H2 2.5 (-3) 2.7 (-7) 7.1 (-8) 9.0 (-5) 2.0 (-14) 2.1 (-4) 7.9 (-8) 3.0 (-4) 1.5 (-9) 1-6 (-3) 1.4 (-3) 1.0 (-10) exposed toaFUVradiationfieldwith produced inacloudwithn=10cm X =210. sion lineintensities(ergsscmsr ) CO 6.7(-3) H0 1.4(-4) OH 5.9(-3) T 3 2 3 A 3 Fine-Structure CoolingData (s-) Molecular CoolingEmission Totalmolecularrotationalemis- A3. 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