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