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