Future MmSubmm Instrumentation and Science Opportunities Example of Deuterated Molecules

During the next decade a tremendous advance will take place in instrumentation for spectroscopy of the interstellar medium. Major new facilities (ALMA, SOFIA, APEX, LMT, Herschel and others) will be constructed and commissioned, so that the science opportu

1

FUTUREMM/SUBMMINSTRUMENTATIONANDSCIENCEOPPORTUNITIES:EXAMPLE

OFDEUTERATEDMOLECULES

ThomasG.Phillips1andCharlotteVastel1

CaliforniaInstituteofTechnology,320-47,Pasadena,CA91125,U.S.A.

a

r

X

i

v:astro-ph/0211610v1 27 Nov 2002

Abstract

Duringthenextdecadeatremendousadvancewilltakeplaceininstrumentationforspectroscopyofthein-terstellarmedium.Majornewfacilities(ALMA,SOFIA,APEX,LMT,Herschelandothers)willbeconstructedandcommissioned,sothatthescienceopportunities,inthe eldofastrochemistry,willincreasebyahugefactor.Thiswillbeenhancedbythenewreceiverswithgreaterbandwidthandsensitivity.Thenewopportunitieswillbeintheareaofastrochemistryofdistantobjects,throughgreatersensitivity,ornewspectralrangesduetotheplat-formsabovetheEarth’satmosphere.

Variousaspectsofnewspectralrangesarediscussed,withemphasisonH2Olines,featurespreviouslyhiddenun-derH2OorO2lines,lighthydridesandparticularlyondeuteriuminmolecules.Recently,multiplydeuteratedspecieshavebeendetected,e.g.ND3,incolddensere-gionsoftheinterstellarmedium.Itisarguedherethatitispossiblethatsomuchdeuteriumcouldbetrapped,bythefractionationprocess,intoheavymoleculessuchasND3,etc...,andspeciessuchasH2D+andpossiblyD2H+,thatDandHDmightbedepleted.Thiswouldbethemechanismforthelargedispersionof[D]/[H]valuesfoundintheinterstellarmedium.Lightmolecules(hydridesanddeuterides)generallyhavelargefundamentalrotationfre-quencies,oftenlyingintheHIFIbands.Thedeuteridesareaspeciallysuitablecase,becausethespeciesexistmainlyincolddenseregions,wherethemoleculesareinthegroundstatesandTHzobservationswillbestbecar-riedoutbyabsorptionspectroscopyagainstbackgrounddustcontinuumsourcessuchasSgrB2andW49N.Keywords:ISM:molecules–Astrochemistry–Deuterium–Submillimeter

Proceedingsoftheconference“ChemistryasaDiagnosticofStarFormation,”UniversityofWaterloo,Canada,21-23August2002(C.L.Curry&M.Ficheds.).

During the next decade a tremendous advance will take place in instrumentation for spectroscopy of the interstellar medium. Major new facilities (ALMA, SOFIA, APEX, LMT, Herschel and others) will be constructed and commissioned, so that the science opportu

2Phillips,Vastel:Futuremmandsubmm

instrumentation

1

ATMOSPHERIC TRANSMISSION

01

10002000FREQUENCY (GHz)

3000

Figure1.Illustrationoftheprincipalspectrallinesex-pectedatsubmillimeterwavelengths,shownsuperposedonthedustcontinuumemissionofa30Kinterstellarcloud.

parisonoftheatmospherictransmissionsbetweentheskyvisibleatCSO(withastandardprecip-itablewatercolumnof1mm)andtheskythatwillbevisi-bleforSOFIA(withastandardprecipitablewatercolumnof10µm),bothatanelevationof45o.

Athird,andpossiblydominantreasonfortheunder-developednatureofthe eldisthelackofmajorinstru-ments.Becausethemillimeter/submillimeterobserversaredevelopingtheir eldlater,intime,comparedtotheoptical,radioandX-raycommunities,wehavehadtowaitourturnformajorfunding.Thewaitisalmostoverandthefullpotentialofthisexciting eldisabouttoberealizedinthecomingdecade!

tothegasphase.Here,wediscussthisonlyfromthepointofviewofgasspectroscopy.

Table1listsmanyofthemajornewtelescopesandinter-ferometerswhichwillbeavailablewithinthenextdecade(hopefully).

Comparedtowhatwehavetoday,thiswillbeastag-geringnewcapabilityandthe eldwillleadastronomyinthatitwilldiscoverandde nethenewobjectsandnewconceptstobeinvestigated.Onemightworrythatthere2.NewInstruments

willnotbeenoughtrainedpeopleinthe eldtofully

Weareconcernedwiththenewopportunitiesforinter-utilizesuchacornucopiaoffacilities,souniversity-basedstellarchemistry,whichincludeinvestigationofmoleculesfacilitiesshouldexpandtheire ortsinthis eldnow!andatomsinthegasphase,dustgrainsandsurfacechem-ALMArepresentsthebiggeststepforward,providinganistryandtheinteractionsbetweenthegasanddustphases,unprecedentedspectroscopiccapability,throughouttheincludingaccretionofmoleculesontograins,andtheop-atmosphericwindows,fornearbyanddistantobjects.Thispositeprocess,wheretheaccretionmaterialis

returnedhugesensitivitywillprobablymanifestitselfmostdra-

During the next decade a tremendous advance will take place in instrumentation for spectroscopy of the interstellar medium. Major new facilities (ALMA, SOFIA, APEX, LMT, Herschel and others) will be constructed and commissioned, so that the science opportu

Phillips,Vastel:Futuremmandsubmminstrumentation3

Table1.Newfacilitiesforinterstellarchemistry.

SOFIAHerschelALMACARMASMAe-SMALMTAPEXSouthPoleGBT2.5m3.5m64×12m6×10m8×6m

(8×6m)+15m+10m

50m12m8m100mAirborne(747)Space(L2)Atacama(Chile)

WhiteMts(California,USA)MaunaKea(Hawaii,USA)MaunaKea(Hawaii,USA)SierraNegra(Mexico)Atacama(Chile)SouthPole

WestVirginia(USA)2005200720102005200320052004200520062002

During the next decade a tremendous advance will take place in instrumentation for spectroscopy of the interstellar medium. Major new facilities (ALMA, SOFIA, APEX, LMT, Herschel and others) will be constructed and commissioned, so that the science opportu

4Phillips,Vastel:Futuremmandsubmminstrumentation

3.NewSpectralOpportunities

WhileALMAprovidessensitivityandangularresolutionbreakthroughs,SOFIAandHerschel(HIFI)generatetheopportunityforobservinginnewspectralranges,duetotheavoidanceofsomeoralloftheatmosphericH2O.HIFIwillprovideanovelviewofmuchofthesubmillimeterspectrum,totallyunperturbedbyatmosphericH2O.ItwasintendedthatHIFIwouldhavetotalfrequencycov-eragefromabout480GHzto2.7THz,butthetechnicaldi cultyofproductionofthelocaloscillators,combinedwiththelimitedbudgetand xedlaunchschedule,hasforcedtheabandonmentofthehighestfrequencychannel,sothehighestfrequencynowis1.9THz(thefrequencyoftheC+line).

3.1.Line-Surveys

MuchoftheHIFIdatawillbetakenintheformofline-surveys.Thesehavebeenspectacularlysuccessfulonground-basedtelescopes(seeFigure3),andthe

method-ologyhasbeenre nedbymeansofsimulations(Comito&Schilke2001),tomakeitsuitableforrapidfrequencyscanning,suchthatafullspectralsurveyofTHzband-widthcanbecompleted,byHIFI,forstrongsourcesinadayorless.Thiscompareswiththemanyweekstakenonground-basedtelescopestoobtainonly100GHzofdata.Thefactor～100ofimprovementintimeismostlyduetotheautomationnecessaryforspaceandthevastlyimproveddesignsofmixersandlocaloscillators.

3.2.H2O

SomeH2OlineshavebeenobservedfromthegroundandfromtheKAO,butthebulkofobservationsisofthe557GHzlinefromSWASandalsoODIN.Figures4and5showsomeamazingSWASH2Ospectra,indicatingacomplexstructuretothelineshape,oftenincludingselfabsorption.Thethreesourcespresentedinthese guresareSgrB2(Neufeldetal.2000),W51(Neufeldetal.2002)andW49N(Plumeetal.inpreparation).ThesearedistantcompactHIIregions,whereabsorptionlinesobservationscantracethewatervaporcontentincloudsalongtheirlineofsight.

HIFIwillhaveavailablemanytransitionsofH2OandH218O,mostofwhichhavenotbeenobservedpreviously,includingthoseintheimportantexcitationpathtotheground-state(Figure6).

ThesemanyavailabletransitionswillprobethegascoolingpropertiesofH2Oinobjectswithawiderangeofdensities,suchascollapsingprotostellarenvelopes(Cec-Figure3.Line-surveysofOrionwiththeCSOtelescope.carellietal.1996),whereH2Obecomestheprimarycoolantasthedensityincreasesintheinnerregions.

3.3.HiddenSpecies

BesidestheH2Olinesthemselves,thereexistsabodyofspectralfeaturesneverobservedbecauseoftheirproxim-ityinfrequencytotheH2OandO2lines.SomeexamplesarelinesofH2D+(e.g.21,1→21,2at1.112THz),CH+(e.g.1→0at836GHz),NH(e.g.N=1→0,J=0→1at946GHz)andLiH(e.g.1→0at444GHz).MolecularlinessuchasHCl(1→0)at626GHzandSiHat627GHzarehardtoobservefromtheground,beinginthewingsofH2Olines.Manyofthe“hardtodetectfromthe

During the next decade a tremendous advance will take place in instrumentation for spectroscopy of the interstellar medium. Major new facilities (ALMA, SOFIA, APEX, LMT, Herschel and others) will be constructed and commissioned, so that the science opportu

Phillips,Vastel:Futuremmandsubmminstrumentation

5

Figure6.WatertransitionsavailabletoHIFI.Bands1-5coversthefrequencyrange472-1280Gz;Band6covers1400-1904GHz;Gap5-6isthefrequencyrangebetweenBand5andBand6,whichmaynotbecovered.ground”speciesarelight,fastrotators,typi edbythedi-atomichydrides.Someofthese,e.g.NaH,CaH,MgH,...havebeensearchedforinJ=0→1transitionsusingatmosphericwindowsforground-basedtelescopes,withnegativeresults,butcanbesearchedfornow,within-creasedsensitivity,intwoways.The rstmethod(canbecarriedoutfromthegroundinfavorablecases)makesuseofcoldcloudsonthelineofsighttopowerfulcontinuumsources,suchasSgrB2.There,inthecolddarkclouds,themoleculeswillbeinthegroundstateandwouldbede-tectedinabsorption.ThismethodavoidstheconfusioninthespectraofGMCs,sincethelineforestislargelymadeupofhighlyexcitedmolecularlines,which,ofcourse,isnotthecaseforthehydridesincoldclouds.Agoodex-ampleisseenintheSWASdetectionoftheground-stateH2OlineinSgrB2(Figure4).Thesecondmethoddoessu erfromGMClineconfusion,butmakesuseofthenewhighfrequencycapabilitiesforHIFIandSOFIA,wherethehighJlinesofthevarioushydrideswillhavemuchhigherlinestrengthsthan1→0;hopefullythiswillover-cometheline-foreste ectinthehightemperatureand

highdensitysources.

Generally,above1THzthealmostzeroatmospherictrans-missionhaspreventedstudiessofar,butseveralwindowsoflimitedsizeandstrengthhavebeenrecentlydetectedonhighsites.SpaceplatformswillallowstudiesofHD(1→0)at2.67THz,HeH+(1→0)at2.01THz,N+at1.46and2.46THz,HFat1.23THz,FeHat1.41THz,OHat1.83THz,OH+at2.46THz,etc...SomespecieshavebeendetectedbyISO,e.g.CH+inthehighJtran-sitions(Cernicharoetal.1997),HF(Neufeldetal.1997)andHD(Wrightetal.1999,Cauxetal.2002).HIFIwillbeagreatimprovementcomparedtowhatusedtobeavailableonISO.

4.Deuterium

Oneofthemostimportantaspectsofmolecularspec-troscopyoftheinterstellarmediumisthestudyoftheabundanceofdeuterium.Thenewfacilitieswillprovideamuchwiderrangeoffrequenciesandsensitivities,sothatthedeuteriumstudiescanbeextendedtonewspeciesandnewobjects.Thefundamentalaspectofdeuterium

During the next decade a tremendous advance will take place in instrumentation for spectroscopy of the interstellar medium. Major new facilities (ALMA, SOFIA, APEX, LMT, Herschel and others) will be constructed and commissioned, so that the science opportu

6Phillips,Vastel:Futuremmandsubmm

instrumentation

Figure4.H162OandH18

2OlinedetectionsbytheSWASsatellitetowardSagittariusB2.

chemistryisthatthechemicalfractionationprocess(see

below)forcesdeuteriumatomsintoheavymolecules,attheexpenseofhydrogen,wheneverthemediumiscold,withamazingresults.

Deuterium-bearingmoleculeshavebecomethetargetofmanyobservationsin

recentyearsandseveralmodelshavebeendevelopedtoaccountforthem(Tielens1983,Roberts&Millar2000a,Roberts&Millar2000b).Morethantwentysuchmoleculeshavebeendetectedtodateininterstellarcloudswithabundances,relativetothenon-deuteratedcounterpart,rangingfrom10 1to10 4.ItisacceptedthatdeuteriumisproducedduringtheBigBanganditisgenerallybelievedthatsincetheBigBang,deuteriumhasbeendestroyedbutnotcreatedinnuclearreactionsoccurringinsidestars.

ThemostreliabledeterminationsoftheD/HratioarebasedonspectroscopicmeasurementsofLymanseriesultravioletabsorptionlinesfromforegroundinterstellargas.InourGalaxy,thishasbeenobtained(viaDIand

)

K( *ATVLSR (km/s)

Figure5.TwospectacularexampleswithW49N,adis-tant(～11.4kpc)compactHIIregionwhoselineofsightcrossestwicetheSagittariusspiralarm,andW51locatedat～7kpc.

HIobservations)withsatellitessuchastheInternationalUltravioletExplorer(IUE),Copernicus,theExtremeUl-traVioletExplorer(EUVE),theHubbleSpaceTelescope(HST)andrecentlytheFarUltravioletSpectroscopicEx-plorer(FUSE).MeasurementsoftheD/Hratiotowardhigh-redshiftsystemslikequasars(e.g.Tytleretal.1999)seemedtoshowmoredispersionthanexpectedandanin-versecorrelationofthisabundancewithHIcolumnden-sity.Ifthisisreal,itwouldsuggestthatinthesehighHIcolumndensitysystems,someprocessingofD/Hmusthaveoccurred(Vidal-Madjar2000,Fieldsetal.2001).Itisinterestingtonotethatabundancesofdeuteriummea-suredintheinterstellarmediumalsoappeartoshowcon-siderabledispersion.Frompublishedvalues,D/Hrangesfrom～5×10 6to～4×10 5.FUSEobservations

During the next decade a tremendous advance will take place in instrumentation for spectroscopy of the interstellar medium. Major new facilities (ALMA, SOFIA, APEX, LMT, Herschel and others) will be constructed and commissioned, so that the science opportu

Phillips,Vastel:Futuremmandsubmminstrumentation7

ofsevenwhitedwarfsandsub-dwarfsleadtoaD/Hra-tioof(1.52±0.08)×10 5(Moosetal.2002),tobecomparedwiththevalueof(1.5±0.1)×10 5(Lin-sky1998)determinedfromHSTobservationsoflate-typestars.Bothmeasurementsrefertowarminterstellargas,locatedwithin100pcoftheSun.However,di erencesbyafactoroftwohavebeenderivedfromCopernicusandHSTdatatowardstarslocatedbetween100and500pcfromtheSun.Theratioseemsconstantwithin100pc,butseemstovaryatconsiderablygreaterdistances.Thereisnoidenti edprocesswhichcanexplainsuchlargevariabilityandwithoutanunderstandingitisnotjusti- edtouseanaverageD/Hratiotorepresenttheprimor-dialdeuteriumabundance.Wewillargue,below,thatthechemistryoftheinterstellarmediumcouldberesponsi-ble,inthatitcanextractlargeamountsofdeuteriumwhichbecomestrappedinmoleculesandongrains.

4.1.BasicChemistry

Deuteriumbearingspeciesaregoodprobesofthecoldphasesofmolecularcloudspriortostarformationandmanyrecentobservationspointtothefactthattheirabundancerelativetotheirhydrogenatedanaloguesarelarger,byafactorupto10000,thanthesolarneighbor-hoodvalueof～1.5×10 5(seereferencesabove).

Thereforetherelativeabundanceofisotopomersdoesnotmeasuretherelativeabundancesoftheisotopesthem-selves.Thechemicalfractionationprocessarisesfromdif-ferencesinthemolecularbindingenergiescausedbythedi erentzero-pointvibrationenergy.Almostincredibly,thiscanleadtoadetectablequantityofthetriplydeuter-atedammonia(seesection4.2).

Inmolecularclouds,hydrogenanddeuteriumarepre-dominantlyintheformofH2andHDrespectively.SotheHD/H2ratioshouldcloselyequaltheD/Hratio.Sincethezero-pointenergiesofHDandH2di ergreatly(seeFigure7),thechemicalfractionationwillfavorthepro-ductionofHDcomparedtoH2.

DeuteriumisinitiallyremovedfromtheatomicphasethroughchargeexchangewithH+,followedbyreactionwiththeabundantH2.HDcouldfurtherinteractwithD+againtogiveD2:H++D←→H+D+(1)D++H2←→HD+H++ E1(2)D++HD←→D2+H++ E2(3)

Thereactions2and3areexothermicassubstitutinganHatomversusanDatominapolyatomicmolecule

generallyleadstoagaininenergy.Theseenergiesmaybecomputed(at0K)bythedi erencesbetweenthezero-pointenergiesoftheproductsandthereactants.Theen-ergies E1and E2arequotedin gure7.

separation

H

HD Ε ～ 410 Κ1D

Ε ～ 500 Κ2

Figure7.H2,HDandD2potentialenergydiagram. Ei

isthedi erencebetweenthezeropointenergiesrelativetotheminimumofthemolecularpotentialcurve.Inthedense,coldregionsoftheinterstellarmedium,Dwillbeinitiallynearlyallabsorbed+intoHD.Theabun-dantionavailableforinteractionisH3,whichgivesH2D+

:

H+3+HD←→H2D++H2+ E3

(4)

where E3/k～230Kforatypicaltemperatureofadarkcloudofabout10K(e.g.Millaretal.1989).Thereversereactiondoesnotoccure cientlyinthecolddensecloudswhereobviouslythetemperatureismuchlowerthan E3.Therefore,thedegreeoffractionationofH2D+becomesnon-negligible.

Thisprimaryfractionationcanthengiverisetoasecondfractionation:

H2D++CO←→DCO++H2

(5)←→HCO++HD

(6)IndarkcloudsH+3givesrisetoHCO+

viathereaction:

H++3+CO←→HCO+H2

(7)

4.2.MultiplyDeuteratedMoleculesThestudyofdoublydeuteratedinterstellarmoleculeshasbeenboominginthelastfewyearssincethesurprisingdiscoveryofalargeamount(～5%)ofdoublydeuteratedformaldehydeinthelowmassprotostarIRAS16293-2422(Ceccarellietal.1998,Loinardetal.2000).ThisismorethanoneorderofmagnitudehigherthaninOrionKLwhereD2COwas rstdetectedbyTurner(1990).This

Energy

During the next decade a tremendous advance will take place in instrumentation for spectroscopy of the interstellar medium. Major new facilities (ALMA, SOFIA, APEX, LMT, Herschel and others) will be constructed and commissioned, so that the science opportu

8Phillips,Vastel:Futuremmandsubmminstrumentation

rstdiscoverywasfollowedbymanyotherstudieswhichcon rmedthepresenceofverylargeamountofdoublydeuterated

formaldehyde(D2CO)aswellasammonia(ND2H)(e.g.Roue etal.2000,Loinardetal.2001).Gasphasechemicalmodelsaccountrelativelywellfortheobservationsoftheabundancesofsinglydeuteratedmoleculesbutmaynotbeabletocompletelyreproducethelargedeuterationsobservedformultiplydeuteratedmolecules.Thelargedeuterationscouldalsobeaprod-uctofactivechemistryonthegrainsurfacesaspredictedbyTielens1983withtwoprocesses:

1.Deuterationduringthemantleformationphase;

2.Evaporationofthemantlesicesresultingfromtheheatingofthenewlyformedstar,withinjectionintothegasphaseofthedeuteratedspecies.Theyear2002appearstobethecornerstoneinthestudyofdeuterationprocesseswiththe rstdetectionofatriplydeuteratedmolecule,ND3.Untilnow,thepos-sibilityfordetectingtriplydeuteratedmoleculeswassoremotethattheirlineswereomittedinthespectroscopiccatalogsforastrophysics.Theground-staterotationaltran-sitionat309.91GHzofND3hasbeendetectedwiththeCSOtowardstheBarnard1cloud(Lisetal.2002,seeFigure8)andtheNGC1333IRAS4Aregion(VanderTaketal.2002),withabundanceratios[ND3]/[NH3]～10 3and[ND3/H2]～10 11.Theyconcludethatreac-tionsinthegas-phasearemorelikelytoproducethesehighdegreesofdeuteration,ratherthangrainsurfacechemistry.Howevertheycannottotallyruleoutthepos-sibilitythatsurfaceprocessesalsocontributetothefor-mationofND3(cfRodgers&Charnley2001).Moreover,therecentdiscoveryofdoubly-deuteratedmethanolto-wardsIRAS16293-2422(CHD2OH/CH3OH～0.2,Pariseetal.2002)cannotbeaccountedforgas-phasemodels.Themostpromisingrouteforsuchmethanoldeuterationseemstobebysurfacechemistry.RecentobservationsoftheextendedD2COemissiontowardsL1689N(Cecca-rellietal.2002)pointedoutthedi cultyinexplainingtheD2COabundance,eitherwithgasphasemodelsorwithgrainsurfacechemistry.Themanyroutestodeuter-ationarenotfullyunderstoodandmoreobservationsarenecessarytoelucidatethesesprocesses.Veryrecently,doublydeuteratedhydrogensul dehasbeendetectedbyVasteletal.(inpreparation)withtheCSOtowardstheBarnard1cloudandtheDCO+(3→2)emissionpeakofNGC1333IRAS4A.Figure9showsasanexamplethe rstdetectionofD2S(11,1→00,0)togetherwithHDS(10,1→00,0).

Figure8.Spectrumofthe309.91GHzND3linetowards

Barnard1observedattheCSO.Theredlinesshowthe tstothepositionandstrengthofthehyper necompo-nents.

Figure9.HDS(10,1→00,0)andD2S(11,1→00,0)detec-tionswiththeCSOtowardstheDCO+(3→2)emissionpeakofNGC1333IRAS4A(Vasteletal.,inprepara-tion).

4.3.DepletionofCO

Gasphasespeciesareexpectedtobedepletedatthecen-tersofcold,darkclouds,sincetheytendtosticktothedustgrains.Aseriesofrecentobservationshasshownthat,insomecases,theabundanceofmoleculeslikeCOdecreasestowardthecorecenterofcold,denseclouds(L1498:Willacyetal.1998;IC5146:Krameretal.1999,

During the next decade a tremendous advance will take place in instrumentation for spectroscopy of the interstellar medium. Major new facilities (ALMA, SOFIA, APEX, LMT, Herschel and others) will be constructed and commissioned, so that the science opportu

Phillips,Vastel:Futuremmandsubmminstrumentation9

Berginetal.2001;L977:Alvesetal.1999;L1544:CaselliallowedrotationalstateoftheH+3moleculepermittedbyetal.1999;L1689B:Jessop&ward-Thompson2001;Bac-thePauliexclusionprinciple(～92K),thesevaluesare:mannetal.2002).Thesedecreasesinabundancehave Ea=～230K, Eb=～180Kand Ec=～230K.beeninterpretedasresultingfromthedepletionofmoleculesontodustgrains(see,e.g.,Bergin&Langer1997;Charn-ley1997).Itisnowclearthatthesedropsinabundance

separationaretypicalofalldensecores.Theremovalofthesere-activespeciesa ectsthegas-phasechemistryandpar-ticularlythedeuteriumfractionationwithinthecloud.+

H gy

Indeed,theremovalofspecieswhichwouldnormallyde-stroyH+3(e.g.CO;Roberts&Millar2000a)meansthattheH+3ismorelikelytoreactwithHDandproduceH2D+

(reaction4).

H2D+

hasbeendetectedtowardthelowluminosityproto-starsNGC1333IRAS4A(Starketal.1999)andIRAS16293-2422(Starketal.inpreparation)withaH2D+/H2ratioof3×10 12intheformer.Recently,Casellietal.(inpreparation)detectedtheH2D+

372GHz

linetowardstwopre-stellarcores,L1544andL1521F,bothprobablyonthevergeofcollapse.

4.4.ModificationoftheDeuteriumChemistry

Thelargedi erencesfoundinthevaluesofthedeuteriumabundanceintheinterstellarmediumposethequestionastothemechanismsresponsibleforthesevariations.Apos-sibleansweristhechemicalfractionationprocess,which,

incoldregionsoftheinterstellarmedium,steadilyforces

thedeuteriumintotheheavymolecules.Thetrend,isto

minimizethefreeenergy,whichimpliesformingtheheav-iestspecies,withtheleastuncertaintyenergy(seeFigure10).Asshownintheprevioussection,intheverycold(～

10K)regionswhere,e.g.ND3isdetected,COisinfactheavilydepletedbyaccretionontograins.Forexample,[CO/H2]～5×10 6(Bacmannetal.2002),leavingHDat[HD/H2]～5×10 5

asthemostabundantmolecule

availableforreactionwithH+3andH2D+

.Anotherexam-pleisthecaseofHD112µmabsorptiontowardsacoldmolecularcloudinthelineofsightofW49whereCauxetal.(2002)foundthatHDis12timesmoreabundantthanCO.Theresultmaybetheproductionofmorehighdeuteriumcontentmolecules:

H++HD←→H2D+

3+H2+ Ea(8)H2D++HD←→D2H++H2+ Eb(9)D2H++HD←→D+

3+H2+ Ec(10)

where Ea, Eband ingthezero-pointenergiescomputedbyCarney(1980),andtheenergyofthe rstH D+~ 550 KD H+~ 600 KD +~ 640 KFigure10.H+3,H2D+,D2H+

andD+3potentialenergydi-agram.

ThespeciesD2H+mayindeedbedetectableinsuf- cientlycold,COdepletedregions.Becauseofthelowtemperatureofthehighlydeuteratedregions(～10K),

thelowestlyingtransitionofH2D+andD2H+willbemostappropriateandcanbesearchedforinabsorptionagainstSagittariusB2,possibly:–H2D+101+ 000at1370.15GHz

–H2D211 212at1111.74GHz

–D2H+111 000at1476.60GHz

–D2H+220 211at1370.05GHz

Itappearstobepurelychancethatthesefrequenciesaresoclose.4.5.D,HDDepletedbyFractionation?

Itwouldbeadi culttasktomakeaninventoryofallDsubstitutedmolecularspeciesthroughouttheGalaxy,toseeifthemeasuredvaluesofthedeuteriumabundancecouldbea ected.Thefractionationmightworkinfavorofmetalspecies(e.g.ND3)inhighmetallicitygalaxies,

orinfavorofpurelyhydrogenicspecies(e.g.H2D+andD2H+)inlowmetallicityobjects.Atanyrate,wecanaskifitisphysicallypossiblefortheformercasetooc-cur.IfwetakethelocalvalueofC,NandOabundances

andcountthetotalnumberofbondsavailableforsub-stitutionofHbyD,weget3×10 3[H].But[D]/[H]is

only～10 5,sotheredoesexistthepossibilityofsuchane ect.Infact,iftheproductofthefractionofavail-ablebondsactuallyoccupiedbyD(fD)timesthefrac-

Ener

During the next decade a tremendous advance will take place in instrumentation for spectroscopy of the interstellar medium. Major new facilities (ALMA, SOFIA, APEX, LMT, Herschel and others) will be constructed and commissioned, so that the science opportu

10Phillips,Vastel:Futuremmandsubmminstrumentation

tionoftheinterstellarmediumwhichiscoldenoughforstrongfractionationandCOdepletion(fC)approaches10 3,thentherewillbeasigni cante ect:fD×fC≥10 3

(11)

WeknowthatfD～10 1,ormoreinsomecases,soiffC≥10 2,measuredvaluesofthedeuteriumabundancemaybesigni cantlya ected.

Acknowledgements

WeacknowledgesupportfromtheNSFundercontractAST-9980846.WethankEricHerbst,JohnPearson,TomMillarandDieterGerlichforhelpfulconversations.Wearealsograte-fultoPaulGoldsmithforprovidingusthe guresofSWASobservationstowardsSgrB2,W49NandW51.

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