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