Future MmSubmm Instrumentation and Science Opportunities Exa(4)

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