A CO J=1-0 Survey of common optical/uv absorption sightlines

0.7 MB·English

by  H. S. Liszt

Astronomy&Astrophysicsmanuscriptno.10525-xar c ESO2009
(cid:13)
January23,2009
⋆
A CO J=1-0 Survey of common optical/uv absorption sightlines
H.S.Liszt1
NationalRadioAstronomyObservatory,520EdgemontRoad,Charlottesville,VA,USA22903-2475
9
receivedJanuary23,2009
0
0
ABSTRACT
2

n Context.Comparisonofoptical/uvabsorptionlinedatawithhigh-resolutionprofilesofmm-waveCOemissionprovidescomplemen-
a taryinformationontheabsorbinggas,astowardζOph.OverthepastthirtyyearsawealthofobservationsofCOandothermolecules
J inoptical/uvabsorptionindiffusecloudshasaccumulatedforwhichnocomparableCOemissionlinedataexist.
8 Aims.Toacquiremm-waveJ=1-0COemissionlineprofilestowardasubstantialsampleofcommonly-studiedoptical/uvabsorption
linetargetsandtocomparewiththepropertiesoftheabsorbinggas,especiallythepredictedemissionlinestrengths.
] Methods.UsingtheARO12mtelescope,weobservedmm-wavelengthJ=1-0COemissionwithspectralresolutionR 3 106and
A spatialresolution1 towardasampleof110linesofsightpreviouslystudiedinoptical/uvabsorptionlinesofCO,H ,C≈H,e×tc.
′ 2
Results.InterstellarCOemissionwasdetectedalong65ofthe110linesofsightsurveyedandthereisageneralsuperabundanceof
G
COemissiongiventhedistributionofgalacticlatitudesinthesurveysample.Muchoftheemissionisopticallythickorveryintense
. andmustemanatefromdarkcloudsorwarmdensegasnearHIIregions.
h
p Conclusions. Judging from the statistical superabundance of CO emission, seen also in thetotal lineof sight reddening, the OB
staroptical/uvabsorptionlinetargetsmustbephysicallyassociatedwiththelargequantitiesofneutralgaswhoseCOemissionwas
-
o detected,inwhichcasetheyareprobablyinfluencingtheabsorbinggasbyheatingand/orphotoionizingit.ThisexplainswhyCO/H
2
r and12CO/13COratiosdiffersomewhatbetweenuvandmm-waveabsorptionlinestudies.Becausethelinesofsighthavebeenprese-
st lectedtohaveAV<1mag,relativelylittleoftheassociatedmaterialactuallyoccultsthetargets,makingitdifficultforCOemission
a lineobservationsto∼isolatetheforegroundgascontribution.
[
Keywords. interstellarmedium–molecules

1
v
6
1. Introduction. exhaustively studied in the diffuse interstellar medium (ISM).
1
With much scatter, N(CO) N(H )2 for 1012 cm 2 < N(CO)
.11 TclhoeudpsreasteAncVe<of1gmasa-gphwasaes csharobwonnbmyoSnmoxitihde&(CStOec)hienr d(1iff9u7s1e) 3×F1r0o1m6ctmhe−2C,O10c1o9lu<mNn(Hde2n∝)s<∼iti2es×a21n0d2r1octamti−o2n.alex−citationtem-
1 shortly after ∼the discovery of interstellar CO itself via the
peraturesobservedin absorption,it is straightforwardto calcu-
0 intense mm-wave CO J=1-0 rotational emission in the Orion
latethattheintegratedbrightnesstemperatureofCOJ=1-0rota-
9 Nebula(Wilsonetal.,1970).Althoughthefractionalabundance
iv:0 oNaf(fCeCwOO)p/Niesr(cHree2nla)tti<ovfelt1yh0e−s5mfrsaeolelgtihnaast-dpiNffha(uCsseeOc)cal<robu<odnsN,i(stCyap+ci)tcuaaalnlldyly1ait0n−mC7oOs<t, taKisoknstmarlosen−mg1ia×sssNi1o(0nC-1Oin2)/dK1iff0ku1m5secsm−g1a−s(2LsfhiosorzutN,ld2(C0v0aO7r)y).<∼aTs1hW0is1C6wOcam=s−aR2ctTaunBaddllvyW≈oCbO1-
X the presence of even this much CO has proved a consistent served when comparing CO emission and absorption at mm-
r challenge to interstellar chemistry (Glassgold&Langer, wavelengths toward distant extragalactic background sources,
a 1976; Black&Dalgarno, 1977; Bally&Langer, 1982; where separating foreground and background gas is not an is-
Glassgoldetal., 1985;VanDishoeck&Black,1988;Leeetal., sue (Liszt&Lucas, 1998). It is also correct along the archety-
1996). pal line of sight toward ζ Oph at N(CO) = 2.2 1015 cm−2.
×
ProportionalitybetweenW andN(CO)isaverygeneralcon-
It is only quite recently that substantial numbers of sight- CO
sequenceofweakexcitation,i.e.T <<T (Goldreich&Kwan,
lines have been studied with high sensitivity in uv absorption ex K
1974) but the normalization is determined by ambient physi-
with HST and FUSE (Sonnentruckeretal., 2007; Burghetal.,
2007; Shefferetal., 2007, 2008) and in mm-wave absorption calconditions.Asdiscussedbelow(seeSect5),WCO/N(CO)is
against extragalactic continuum sources (Liszt&Lucas, 1998) about50timeshigherindiffusecloudsthantowardOrionAor
at the PdBI so that the formation, excitation and fractionation typicaldarkclouds.
ofCOindiffusecloudscanbeconsideredinasystematicfash- The CO emission from absorbing foreground material in
ion(ibid andLiszt(2007));comparewiththemuchearliersur- diffuse clouds should be readily detectable if N(CO) > 3
vey of Federmanetal. (1980). Notwithstanding the long inter- 1014 cm−2 and rather strong, comparable to the emissio∼n from×
val since its discovery, carbon monoxide is now the molecule dark clouds, whenever N(CO) > 1015 cm−2. Morever, if CO
whose chemistry and abundance relative to H are the most emissionisobservable,thehigh∼spectralresolutionavailablein
2
mm-wavespectra,typicallyR 3 106,providescomplemen-
≈ ×
Sendoffprintrequeststo:H.S.Liszt taryinformationtothatavailableinlower-resolutionabsorption
⋆ BasedonobservationsobtainedwiththeAROKittPeak12mtele- spectra(Knapp&Jura,1976;Liszt,1979;Wannieretal.,1982;
scope. Langeretal.,1987;Wilsonetal.,1992;Nehme´etal.,2008).As
Correspondenceto:[email protected] well, the identificationof absorbinggasin CO emission would
2 H.S.Liszt:ACOJ=1-0Surveyofcommonoptical/uvabsorptionsightlines
offer the prospect of relatively high spatial resolution imaging Table1.SightlineslackinginterstellarCOJ=1-0emission1
oftheabsorptionlinehostbody.Thisbeingthecase,itseemed
reasonable to survey CO J=1-0 rotational emission toward a Target l b v rms N(CO) N(H )
tel 2
substantialsampleofcommonly-observedoptical/uvabsorption HD ◦ ◦ kms−1 K cm−2 cm−2
sightlines.Asmallbutnominallysimilarsurveywasperformed 13994 134.39 -3.42 -6.17 0.098
atamuchearlierepochbyKnapp&Jura(1976),withsomewhat 21856 196.46 -50.96 -29.36 0.124 20.04
22951 158.92 -16.70 -14.08 0.087 14.22 20.46
mixedresultsowingtoconfusionbetweentelluricandinterstel-
23408 166.17 -23.51 -18.11 0.098 19.75
lar emission ( Appendix A). The results are somewhat mixed
23480 166.57 -23.75 -17.98 0.169 20.12
hereaswellowingto confusionbetweenforegroundandback-
23630 166.67 -23.46 -18.32 0.088 <12.34 19.54
ground material (see Nehme´etal. (2008)), combined with the
23850 167.01 -23.23 -18.56 0.162
propensityforearly-typeabsorptionlinetargetstarstobeasso- 23840 159.46 -15.03 -33.41 0.055
ciated with but in frontof large amountsof neutralmaterialas 24912 160.37 -13.11 -12.78 0.066 13.49 20.53
discussedinSect.4and5. 30614 144.07 14.04 -14.38 0.073 14.49 20.34
Section 2 here describes the data which were taken to im- 34078 172.08 -2.26 -6.72 0.126
plementthe surveyandthemethodsofsightlineselection,data 35149 199.16 -17.86 -14.71 0.107 <13.00 18.30
reductionandpresentation.Section3describestheobservational 36486 203.86 -17.74 -14.17 0.116 <12.04 14.74
37128 205.21 -17.24 -14.13 0.085 <12.30 16.28
results.Section4reportsthesurveystatisticsanddiscussessome
42087 187.75 1.77 -30.60 0.098
evident biases. Section 5 discusses the apparent difficulties in
57060 237.82 -5.37 -5.79 0.157 <12.67 15.78
separatingforegroundandbackgroundgasinemissionandSect.
57061 238.18 -5.54 -5.89 0.158 <12.61 15.48
6 is a summary.AppendicesA-Cdiscussvariousobservational
58510 235.52 -2.47 -23.78 0.138
aspects of the implementation of the present survey, including 121968 333.97 55.84 29.95 0.145 <12.30 18.70
telluricCO emission,spectralbaselineremovalanddeconvolu- 143275 350.10 22.49 18.28 0.099 12.49 19.41
tionoffrequency-switching(respectively). 144217 353.19 23.60 35.21 0.116 13.63 19.83
144470 352.75 22.77 34.93 0.120 12.95 20.05
145502 354.61 22.70 34.88 0.120 12.76 19.89
2. Sampleselectionandoverlapwithpriorwork 164353 29.73 12.63 10.47 0.086 13.04 20.26
166937 10.00 -1.60 24.39 0.131
Thegoalofthisworkwastoobserveasmanytargetsaspossible 167971 18.25 1.68 5.93 0.129
givenpriorabsorptionlinesurveysofimportantspeciesH2,CO, 170074 75.06 23.71 14.80 0.090
OH, CH, CH+ and H +. Owing to terrestrial geography, only 177989 17.81 -11.88 -1.67 0.123 14.64 20.23
3
targetsabove-25odeclinationwereincluded;giventhatabsorp- 181615 21.84 -13.77 -2.31 0.153
tion lines of H and CO are observedfrom space and that that 184915 31.77 -13.29 -2.70 0.101 20.31
2
the center of the Galaxy is in the southern sky, this criterion 192639 74.90 1.48 2.07 0.131 14.13 20.69
193237 75.83 1.32 2.23 0.102
eliminated many sightlines. The final target list numbered 110
197592 30.70 -32.15 -13.21 0.121
andtwosightlinesnearOrionBwereaddedtotheobservational
198478 85.75 1.49 1.09 0.102
program(butexcludedfromcalculationofthestatistics)ascon-
198781 99.94 12.61 6.23 0.066 15.22 20.56
trolled examples of heated gas along sightlines to well-studied
199579 85.70 -0.30 0.23 0.112 20.36
HIIregions(Petyetal.,2007). 201345 78.44 -9.54 -4.69 0.064 <12.40 19.43
FirstprioritywasgiventosightlinesobservedinuvCOab- 203064 87.61 -3.84 -1.71 0.092 20.29
sorptionbySonnentruckeretal.(2007),Burghetal.(2007)and 206773 99.80 3.62 0.55 0.101 14.20 20.47
Shefferetal. (2007). These references also tabulate or provide 209339 104.58 5.87 1.96 0.105 13.95 20.25
values or new measurementsof N(H ). Sightlines havingmea- 209975 104.87 5.39 1.80 0.080 20.08
2
sured N(H ) not included in the CO surveys were taken from 217035 110.25 2.86 -0.26 0.090 14.57 20.95
2
217675 102.21 -16.10 -10.33 0.135 12.75 19.67
theworkofSavageetal.(1977)andRachfordetal.(2002).CH
218915 108.06 -6.89 -5.68 0.134 13.64 20.15
column densities for sources surveyed in CO absorption are
224572 115.55 -6.36 -5.99 0.113
included in the work of Sonnentruckeretal. (2007) and some
1ForexplanationoftableentriesseeSect.3
further sightlines studied in CH were taken from the work of
Allcolumndensitiesarelogarithmic.
Craneetal. (1995) 1. Finally, a few sightlines were included
which have recently between observed in CH+ by Stahletal.
(2008) (CH+ was also surveyed by Craneetal. (1995)) and in
H + absorptionbyMcCalletal.(2002).
3
beamwidth of the 12m antenna is 65 . Line brightnesses from
′′
the12mtelescopeareonaT scaleandshouldbescaledupward
∗r
3. Observationsanddatareduction bya factor0.85/0.72= 1.2toderivecorrespondingmain-beam
brightness.
Thenewdatadiscussedhere,profilesofJ=1-012CO and13CO
Inanall-skysurveyofthissortitisnotpracticabletosearch
emission,wereacquiredattheARO12mtelescopeduring2007
for nearby off-source positions which are free of emission at
Decemberandtoalesserextentin2008JanuaryandFebruary,to
low levels. Therefore, the raw data were taken in a frequency-
makeupfortimelosttoweatherandobjectstooclosetotheSun
switchingmode(Liszt, 1997) with the 6144-channelmm-wave
earlier.Theobservationsaresomewhattime-specificbecauseof
autocorrelator(MAC)at 24.4kHz resolution(0.0635kms 1 at
the presence of telluric emission in the 12CO spectra. At the −
the12COrestfrequency).Thethrowofthefrequency-switchwas
frequency of the 12CO J=1-0 line, 115.3 GHz, the half-power
1MHzformostspectraand 2MHzforafewotherswhichat
± ±
1 Several of the lines of sight in the work of Craneetal. (1995) 1MHzwerehardtoreconstructfaithfully.Typicalintegration
±
are denoted by their HR numbers, specifically HR1156 = HD23480, timeswere12-18minutesineachoftwopolarizationsresulting
HR1178=HD23850,HR2135=HD41117andHR6812=HD166937 inatypicalrmsof0.1Kafterthespectrawereco-addedinthe
H.S.Liszt:ACOJ=1-0Surveyofcommonoptical/uvabsorptionsightlines 3
twopolarizationsandhanning-smoothedtoafinalresolutionof 3.1.Datainmachine-readableform
48.8kHz(0.127kms 1for12CO).
−
Individual spectra and zip files of all spectra having detected
Thebaselinesinfrequency-switcheddataatsomemm-wave
interstellar CO emission are available in CLASS FITS format
telescopesarebadenoughtopreventreliablereconstruction(un-
on NRAO’s anonymousftp server as http://tinyurl.com/45ps73
folding)ofbroad-bandspectra,butthoseatthe12mweresums
and http://tinyurl.com/5pgju9 for 12CO and 13CO spectra, re-
oftwoorthreepuresine-waves,typicallydominatedbyperiods
spectively.
of approximately 156 and 17.4 MHz (400 and 35 kms 1, re-
−
spectively for 12CO), making baseline subtraction simple. This
isillustratedinAppendixBandFig.A.2. 3.2.Detectabilityofinterstellaremissionandpresentationof
The data were unfolded using the EKHL algorithm (Liszt, upperlimits
1997)illustratedinFig.A.3anddiscussedinAppendixC.This
technique,analogousto thedual-beamswitchingprocedurefor The spectraacquiredhere easily detecttelluric emissionwhich
spatial mapping, allows recovery of spectra with frequency- appearswithWCO 1Kkms−1/sin(el)andFWHM=0.8kms−1
≈
switch intervalssmaller than the extentofthe emission profile, (seeAppendixA);thenominalsignal/noiseofsuchadetectionis
as long as the spectral baselines are flat. In turn, because the nearly20:1forachannel-to-channelrmsof0.1K.Fora4kms−1
switchingintervalmaybemadesmaller,thebaselinesmayalso interval,typicalofwhatisneededtobracketasinglehypotheti-
improve.Thereisapenaltytobepaidintermsoftimebecause calinterstellarcomponent,the3σrmsnoiseinWCO withsingle
the rms noise may not decrease much after unfolding, but this channelrmsnoise0.1Kis0.22Kkms−1.AsdiscussedinSect.
is negligible compared to the amount of time which would be 5(seeFig.5atbottom),interstellaremissionwasdetectedinev-
spenthuntingforasuitableoff-sourcereferenceposition(which ery case, save one, where N(CO) exceeded 4 1014 cm−2 and
would be followedby observingin a position-switchingmode, emissionwasexpectedatalevelWCO>0.4Kk×ms−1.However,
withlessthanhalftheobservingtimespenton-sourceanyway). bothindividualcomponentsandlinesofsightwithdetectedin-
Thepenaltyis alsonegligiblecomparedtothe processdetailed terstellar lines at levels WCO < 1 K kms−1 are rare. Because
inFig.8ofMcCalletal.(2002)wherespectratakenwith3fre- no threshold for CO emission is expected empirically (WCO
∝
quencythrowstowardHD183143didnotyieldaspectrumshow- N(CO),seeFig.6ofLiszt(2007)),theactualaposteriorisensi-
ing all the emission present along the line of sight (see Fig. 2 tivityofthesurveymaybesomewhatpoorerthansuggestedby
here). thedetectabilityofthenarrowtelluricemissionorthe0.1Ksin-
The velocity scale of all the spectra is with respect to the glechannelrmsnoise.WhereupperlimitsappearintheFigures
LSR.Thedataoutputfromthe12mareonaT∗r scale,thatis,cor- (4and5),theyareshownsymbolicallyatWCO <0.8Kkms−1.
rectedforabeamefficiencyof0.85correspondingtotheforward Thenon-detectionsinTable1shouldbereliableatalevelWCO
responsefallingwithintheareaoftheMoon.Toputtheinterstel- <0.5-0.8Kkms−1.
lar data on a main-beamscale, the spectra should be scaled up
byafactor 1.2(0.85/0.7)correspondingtothefullmainbeam
efficiency. ≈ 4. Surveyresults
ThesurveyresultsaresummarizedinTables1-4.Table1de-
4.1.Statisticsandbiases
scribes sightlines lacking detected interstellar emission; it pro-
vides the galactic coordinates, the (date-specific) mean lsr ve- Giventhatthepurposeofthesurvey(whichwasachieved)wasto
locity at which telluric emission appeared in the spectra (la- providelineprofilestowardapre-definedbutratheradhocgroup
belled v ), the channel to channel rms of the reduced, coad- oftargetsightlines,thestatisticalpropertiesoftheCOemission
tel
ded and smoothedARO spectrumand the log of the absorbing foundinthesurveymightnotseemtobeofmuchintrinsicvalue.
CO andH columndensitiesknownfromotherwork.Tables2 However,theyservetoshowthatthetargetsareassociatedwith
2
and3summarizethelinesofsightwithdetectedinterstellarCO. thegasdetected(makingthegasatypicalofthediffuseISMasa
Foreachsourcetheyprovidegalacticcoordinates,thereddening whole)andthatCOemissionsurveyswhichseektheforeground
outtoinfinityfromtheworkofSchlegeletal.(1998),thefore- gascontributiontowardbrightstarsareheavilycontaminated(if
groundreddeningcopiedfrom earlier references,v , followed not totally corrupted) by material lying behind the absorption
tel
by the peak and integrated T , the mean velocity of the emis- targets.
∗r
sion(whichinafewcasescontainsanindistinguishabletelluric This contamination occurs for several reasons. First is the
contribution),thecorrectionV -V whichmaybesubtracted innateinabilityoftheemissionspectratodiscriminatebetween
lsr hel
fromthelsrvelocityscaletoconverttoheliocentricvelocity,fol- foregroundandbackgroundplacementof materialnearthe tar-
lowedbythelogoftheabsorptionlinecolumndensitiesofCO get in the absence of other information (for instance, but not
andH .Table4providesrelevantinformationforthe14linesof necessarily conclusively, the velocity profile of the absorbing
2
sightwhere13COwasalsoobserved.Integrationtimesforthese gas). Second is the fact that when substantial amounts of gas
linesofsightweresomewhatlonger,typically24minutes. arepresentalongsuchlinesofsight,theywillpreferentiallyoc-
Thumbnail plots of the spectra with detectable interstellar curbehindthestars,whichwereaprioriselectedtoobservethe
CO emission are shown in Figures 1-3; where 13CO was ob- diffuse cloud regime A < 1 mag. Finally, this bias is greatly
V
served,thosespectraareshownoverlaid.When v > 10 15 enhanced by the propensi∼ty for the target stars to occur in re-
tel
kms 1 and the galactic latitude is a few degrees| or|more f−rom gions of high gas density, presumably because they are gener-
−
thegalacticplane,overlapwithinterstellaremissionismuchless ally of earlyspectraltype andwere deemedto be “interesting”
likely.Conversely,thereareexampleswherethetelluriclinein- forspectroscopicstudies.
extricably appears within the velocity range of the interstellar TheassociationbetweenthetargetsandtheCOemittinggas
emission and the spectra can only be understood in complete canbedemonstratedinseveralways.Itisobviousfromthespec-
detail with reference to the tabulated telluric velocities v . In trathatseveralofthesources(HD23180,HD281159,HD37043,
tel
generalthetelluricandinterstellaremissioncanbeseparatedby HD37903, HD147889) are seen against very strongly-emitting
observingatseveral-monthintervals. molecular gas characteristic of natal clouds near giant HII re-
4 H.S.Liszt:ACOJ=1-0Surveyofcommonoptical/uvabsorptionsightlines
Table2.SightlineswithdetectedinterstellarCOJ=1-0emission1
Target l b E E v rms T W <V > V -V N(CO) N(H )
∞BV BV tel ∗r CO lsr lsr hel 2
mag mag kms 1 K K Kkms 1 kms 1 kms 1 cm 2 cm 2
◦ ◦ − − − − − −
HD2905 120.84 0.14 1.90 0.01 -2.72 0.179 4.12 8.40 -52.42 7.86 20.27
HD21483 158.87 -21.30 1.04 0.56 -16.46 0.083 7.59 22.76 6.06 -6.41
HD23180 160.36 -17.74 4.54 0.30 -14.71 0.108 11.46 19.62 8.51 -6.49 14.83 20.60
HD281159 160.49 -17.80 10.02 0.85 -14.74 0.059 27.97 41.98 8.35 -6.54
HD24398 162.29 -16.69 0.32 0.32 -14.61 0.086 0.97 2.13 8.05 -6.95 14.86 20.67
HD24534 163.08 -17.14 0.43 0.45 -14.92 0.073 2.55 5.31 7.60 -7.23 16.20 20.92
HD24760 157.35 -10.09 0.95 -10.71 0.074 4.57 22.68 -5.45 -4.75 11.93 19.52
HD27778 172.76 -17.39 0.60 0.37 -15.24 0.080 3.38 7.17 5.74 -9.98 16.08 20.79
HD29647 174.05 -13.35 2.31 1.00 -13.09 0.090 5.84 16.35 6.14 -9.98
HD283809 174.16 -13.35 2.33 -13.11 0.114 5.08 16.74 5.95 -10.01
HD283879 175.72 -12.61 1.50 -12.71 0.095 6.79 13.23 5.92 -10.36
HD36371 175.77 -0.61 1.54 0.43 -6.29 0.137 1.26 3.60 -18.90 -9.04
HD36861 195.05 -11.99 0.45 0.12 -11.62 0.119 1.22 1.13 5.12 -14.95 12.59 19.12
HD37043 209.52 -19.58 1.51 0.07 -15.17 0.106 20.15 58.57 8.01 -17.83 12.43 14.69
HD37742 206.45 -16.59 4.20 0.08 -13.88 0.096 5.42 18.58 10.06 -17.28 <12.72 15.86
HorseHead 206.96 -16.79 0.98 0.00 0.069 15.92 31.80 9.79 -17.37
NGC2024#1 206.53 -16.38 58.85 -37.16 0.064 31.46 156.18 9.95 -17.28
HD37903 206.85 -16.54 2.05 0.35 -37.35 0.069 41.20 134.38 9.46 -17.34 14.00 20.92
HD41117 189.69 -0.86 1.67 0.45 -32.27 0.050 1.75 3.54 4.51 -12.62
HD43384 187.99 3.53 0.81 0.58 -29.67 0.114 1.31 1.19 2.41 -11.63
HD46202 206.31 -2.00 0.82 0.49 -31.43 0.102 1.38 0.32 9.53 -16.03
HD46711 208.59 -2.40 2.17 1.04 -31.02 0.103 1.75 2.74 23.23 -16.43
HD47129 205.87 -0.31 0.72 0.36 -30.65 0.099 1.35 1.17 20.08 -15.75 20.54
HD47839 202.94 2.20 6.31 0.07 -3.21 0.090 12.45 47.99 6.87 -14.92 <12.81 15.54
HD48099 206.21 0.80 1.13 0.27 -30.18 0.095 1.49 1.21 15.98 -15.66 20.29
HD52382 222.17 -2.15 1.99 -28.05 0.123 2.45 4.86 39.30 -17.94
HD53367 223.71 -1.90 6.89 0.74 -27.43 0.109 13.32 47.45 17.22 -18.02
HD53974 224.71 -1.79 1.99 0.22 -27.37 0.067 5.29 12.55 15.48 -18.07
HD54662 224.17 -0.78 1.61 0.35 -27.21 0.062 4.21 16.63 14.19 -17.91 20.00
HD147165 351.31 17.00 1.11 0.38 32.57 0.113 2.16 0.46 3.12 9.55 12.48 19.79
HD147888 353.65 17.71 1.26 0.47 33.10 0.112 3.14 3.49 2.71 10.25 15.30 20.48
HD147889 352.86 17.04 15.13 1.07 32.76 0.122 16.74 30.86 3.13 9.98
HD147933 353.69 17.69 1.23 0.45 32.85 0.124 2.93 3.39 2.63 10.25 15.28 20.57
HD148184 357.93 20.68 0.49 0.55 34.41 0.105 1.80 1.10 1.14 11.58 14.58 20.63
HD148605 353.10 15.80 2.03 0.10 32.26 0.123 2.04 0.99 3.67 9.94 18.74
1ForexplanationoftableentriesseeSect.3.Allcolumndensitiesarelogarithmic.
gions, while others among the few observedin 13CO show the Table4.Sightlinesobservedin13CO1
optically thick CO emission typical of dark clouds (HD21483,
HD24760,HD29647,HD283809).HIIregionsanddarkclouds Target rms T∗r W13 <Vlsr > W12/W13
arerarelyfoundbyaccidentawayfromthegalacticequator.In HD K K Kkms−1 kms−1
anycase,thestrongCOlinesindicateveryhighcolumndensities 21483 0.038 2.23 3.47 5.93 6.56
23180 0.025 0.44 0.84 8.37 23.46
ofmaterialbehindwhichthestarswouldnotbesuitabletargets
281159 0.033 9.20 13.38 8.61 3.14
foroptical/uvabsorptionlinestudiesofdiffuseclouds.
24534 0.017 0.22 0.29 7.77 18.49
Theexistenceofanoverendowmentofgasalongthechosen 24760 0.023 1.01 3.02 -4.97 7.52
linesofsightisapparentinatleastthreeways:theamountofred- 27778 0.021 0.19 0.43 5.96 16.86
deningalongtheensembleoflinesofsightismuchhigherthan 29647 0.031 3.79 5.84 5.88 2.80
expected;COisdetectedtoofrequentlyandtheCOemissionis 283809 0.025 2.27 4.05 5.96 4.13
on average too strong. Each of these is discussed separately in 37043 0.038 4.14 13.92 7.87 4.21
37742 0.030 0.33 0.95 10.10 19.52
thefollowingsubsections.
47839 0.027 4.98 10.14 7.00 4.73
52382 0.024 0.48 0.63 39.32 7.71
53367 0.047 5.94 13.98 17.41 3.39
4.2.Reddeningandthetotalgascolumn
53974 0.022 0.29 0.91 16.15 13.82
1ForexplanationoftableentriesseeSect.3
Table5givessomestatisticsofthereddeningandintegratedCO
emission.Foraseriesofminimumseparationsfromthegalactic
equator b = 3o- 20o, the columns of Table 5 show the sub-
<
| |
sumed number of lines of sight; the number among them with
E > 1mag;themeanreddeningperkpcofequivalentpathin gratedCOemissionperunitlength(calculatedasfortheredden-
∞BV
the galactic plane < E > / < l >, where l = 0.1 kpc/sin(b) ingperunitlength);theratioofthereddeningandCOemission
∞BV | |
correspondingtoahalf-thicknessofthedustlayerof100pc;the perunitlength,convertedtoanequivalentcolumndensityofH ;
2
numberofsightlineswithdetectedCOemissionandmeaninte- andfinally,fourcolumnsgivingthemeanforegroundandlimit-
H.S.Liszt:ACOJ=1-0Surveyofcommonoptical/uvabsorptionsightlines 5
Table3.SightlineswithdetectedinterstellarCOJ=1-0emission1
Target l b E E v rms T W <V > V -V N(CO) N(H )
∞BV BV tel ∗r CO lsr lsr hel 2
mag magkms 1 K K Kkms 1 kms 1 kms 1 cm 2 cm 2
◦ ◦ − − − − − −
HD149757 6.28 23.59 0.56 0.32 35.83 0.102 1.60 1.66 -0.07 13.72 15.40 20.64
HD166734 18.92 3.63 1.52 1.39 7.74 0.094 2.43 3.90 6.15 14.92
HD168076 16.94 0.84 25.94 0.78 6.29 0.094 5.99 38.82 0.88 14.19
HD168607 14.97 -0.94 8.07 1.61 5.16 0.165 4.77 74.31 38.38 13.55
HD169454 17.54 -0.67 13.94 1.12 4.35 0.222 7.72 12.12 5.55 14.12
HD172028 31.05 2.83 2.97 0.78 6.45 0.118 7.65 23.03 7.58 16.83
HD179406 28.23 -8.31 0.49 0.33 -1.34 0.083 1.26 3.09 2.46 14.82 15.64 20.73
HD183143 53.24 0.63 3.75 1.27 2.80 0.129 3.03 15.90 -22.61 18.28
HD192035 83.33 7.76 0.46 0.28 4.86 0.071 1.16 2.13 5.58 17.10 15.14 20.68
HD195592 82.36 2.96 2.75 1.18 2.34 0.062 1.14 3.85 2.28 16.74
CygOB2#8 79.89 0.96 5.83 1.11 0.057 4.95 50.36 -66.24 16.82
CygOB2#5 80.12 0.91 3.46 1.99 1.33 0.072 2.01 23.26 -3.90 16.79
CygOB2#12 80.10 0.83 4.18 3.31 0.97 0.063 4.13 27.44 1.93 16.78
VICYG#8A 80.22 0.79 6.46 1.04 0.062 3.63 27.49 -10.74 16.76
HD200120 88.03 0.97 2.67 0.18 0.62 0.061 1.76 9.69 -38.87 15.63 12.18 19.30
HD203374 100.51 8.62 1.09 0.60 3.89 0.076 1.18 3.19 2.20 14.04 15.41 20.70
HD203938 90.56 -2.23 1.55 0.74 -1.36 0.073 2.90 4.97 5.15 14.76
HD204827 99.17 5.55 1.05 1.11 1.39 0.088 2.81 11.21 -66.86 14.02
HD206165 102.27 7.25 0.70 0.47 2.96 0.090 1.27 1.86 -0.36 13.51 15.18 20.78
HD206267 99.29 3.74 0.98 0.52 1.19 0.084 1.54 2.91 -2.24 13.79 16.04 20.86
HD207198 103.14 6.99 0.66 0.62 2.67 0.153 1.61 2.63 -26.88 13.29 15.50 20.83
HD207308 103.11 6.82 0.59 0.50 2.75 0.123 1.88 4.31 -2.18 13.28 15.93 20.86
HD207538 101.60 4.67 1.19 0.64 1.27 0.082 1.33 2.19 -1.89 13.39 15.40 20.91
HD208440 104.03 6.44 0.66 0.34 2.28 0.133 1.22 1.10 -1.06 13.02 14.20 20.34
HD210121 56.88 -44.46 0.22 0.40 -21.62 0.140 2.00 2.91 -6.32 7.76 15.83 20.75
HD210839 103.83 2.61 1.64 0.56 -0.12 0.107 1.14 4.16 -77.96 12.64 15.44 20.84
HD216532 109.65 2.68 3.73 0.86 -0.11 0.085 2.35 9.00 -98.72 11.22 15.15 21.10
HD216898 109.93 2.39 2.86 0.85 -0.66 0.084 0.97 4.59 -9.89 11.11 15.04 21.05
HD217312 110.56 2.95 1.04 0.66 0.07 0.154 1.60 3.09 -78.91 11.02 14.38 20.80
HD218376 109.95 -0.78 1.51 0.25 -2.14 0.138 1.47 7.58 -45.05 10.70 14.80 20.15
BD+631964 112.89 3.10 1.70 1.00 -0.05 0.115 1.38 2.73 -10.86 10.43
HD229059 75.70 0.40 13.52 1.71 1.05 0.055 10.00 74.64 -52.72 17.24
1ForexplanationoftableentriesseeSect.3.Allcolumndensitiesarelogarithmic
ingreddening,tabulatedseparatelyforsightlineswithandwith- at high latitude. The mean integrated brightnesses per kpc of
outdetectableCOemission.Toeliminatetheinfluenceofafew pathinthissurveyat b > 3o areA =14-39Kkms 1 kpc 1,
CO − −
| |
outliers(seeTables2-3),themeanreddeningperunitlengthwas generallyincreasingwith thelowerboundofthe latituderange
calculatedusingsightlineswith E < 5magandthe meanCO considered. This may be compared with a value A 5 K
emissionperunitlengthwascalcu∞BlVatedforWCO<50Kkms−1. kms−1 kpc−1attheSolarCircleinsurveysofthegalaCcOtic≈plane
The table entries for either quantity would have been 50-75% (Burton&Gordon,1978).
higheriftheselimitswerenotobserved.
For a local mean reddening at z = 0 of 0.61 mag kpc 1
− 4.4.Foregroundvs.backgroundreddening
(Spitzer,1978),linesofsightwithE >1magshouldbecon-
∞BV
finedwithinabout4o ofthegalacticplane,whichismanifestly The final four columns of Table 5 give the mean foreground
not the case here. Moreover, the mean limiting reddening per andlimitingreddeningforsightlineswithandwithoutdetected
unitpath in Table 5 rangesfrom2 to 5 times the localaverage CO emission. The sample mean foreground reddening for the
depending on the lower latitude limit. This can be contrasted 71 lines of sightis 0.35mag correspondingto A 3.1E =
V BV
with the opposingtendencyforabsorptionline surveysto have 1.1mag,observingthediffusecloudregimeA <1≈mag.Those
V
smaller than average reddening per unit length when only the sightlinesshowingCOemissionhaveasomewh∼atlargeroverall
foregroundmaterialiscounted(Savageetal.,1977). meanforegroundreddening,0.48vs.0.21magandamuchlarger
mean limiting reddening, 1.87 vs. 0.48 mag. This is a quanti-
tative demonstration of the bias noted earlier whereby associ-
4.3.FrequencyofoccurenceandstrengthofCOemission
atedmaterialpreferentiallyoccursbehindearly-typetargetstars
Inthepresentsurvey,31ofthe55linesofsightat3o <b< 20o whose foreground extinction has been a priori selected to fall
inthediffusecloudregime.Moreover,COemissionispreferen-
had detectable emission (56%) compared to 10 of 48 lines of
tiallydetectedinthemoreextremecases.
sight (22%) in the same latitude range in a truly blind sur-
vey for galactic CO emission toward extragalactic mm-wave
radiocontinuum sources (Liszt&Wilson, 1993; Liszt, 1994).
4.5.RelationshipsbetweenW andE
Furthermore,theemissionlinesfoundin theblindsurveywere CO ∞BV
much weaker, illustrating that the presentwork also includesa The endowments of limiting reddening and CO emission are
superabundanceofintegratedCOemission(notjustdetections) generallyproportionalandtheirratio,convertedtoanequivalent
6 H.S.Liszt:ACOJ=1-0Surveyofcommonoptical/uvabsorptionsightlines
HD2905 HD21483 HD23180 30BD31 643 1HD24398
4
10
3 20
5
2
5
10
1
0
0 0 0 0
-60 -40 -20 0 0 10 0 10 0 5 10 -40 -20 0 20
HD24534 5HD24760 HD27778 HD29647 HD283809
6 5
4 3 5
2 4
3 4
2 3
3
1 2 2
2
1
1 1 1
0 0 0 0 0
-1 -1
0 10 -10 0 0 10 0 5 10 0 5 10
)
N
VI 7HD283879 HD36371 HD36861 20HD37043 6HD37742
L 6 5
E 1 1
K 5 4
* ( 4 10 3
r 3
T 2
2
1 1
0
0 0 0 0
-1 -1
0 5 10 -20 0 -20 0 0 10 0 10 20
2
HorseHead NGC2024#1 HD37903 HD41117 HD43384
30 40
30
10 20
1
20
10
10
0
0 0 0 0
0 10 -10 0 10 0 10 0 10 -10 0 10
0.3HD46202 2HD46711 1HD47129 HD47839 HD48099
0.2 1
10
0.1
1
0 5
-0.1
0
0 0 0
-0.2
0 10 20 20 30 15 20 25 0 10 0 10 20
-1
V (KM S )
LSR
Fig.1.ObservedCO J=1-0emission profiles.Velocityresolutionis generally0.13kms 1 buta few moreextensiveprofileshave
−
beensmoothedto0.25kms 1
−
columndensityofH inTable5followingSavageetal.(1977), is no plane-parallel stratification of the CO-emitting medium.
2
is relativelyconstant. It is also comparableto typical valuesof Unlike in the blind survey, the reason now is not the galactic
theCO-H conversionfactorusedtoconvertCOintensitytoH structure of the local bubble but rather the association of CO-
2 2
column density, 2 1020 H (K kms 1) 1, suggesting that the emittinggaswiththetargetstars.
2 − −
×
materialrepresentedbyE islargelyinmolecularformforthe
∞BV
largervaluesofE whichdominatetheensemblemean.
∞BV
AtbottominFig.4,W isplottedagainstthelimitingline
CO
Line of sight effects are expressed differently in Fig. 4. At ofsightreddeningouttoinfinityE fromSchlegeletal.(1998).
∞BV
top, Fig. 4 plots the distribution of W with csc(b); as ob- AssuggestedbythenearconstancyofW /E inTable5there
CO | | CO ∞BV
servedintheblindsurveysforCO emissiontowardextragalac- isasubstantialsubsetoflinesofsightwhichshowaproportion-
ticcontiuumsources(Liszt&Wilson,1993;Liszt,1994),there alitybetweenW andE .
CO ∞BV
H.S.Liszt:ACOJ=1-0Surveyofcommonoptical/uvabsorptionsightlines 7
15 6
HD52382 HD53367 HD53974 HD54662 0.4HD147165
5 4
2
10 4 3
3
2
1 5 2 0
1
1
0 0 0 0
-1
30 40 50 10 20 10 20 0 10 20 0 10
HD147888 HD147889 3HD147933 0.5HD148184 0.5HD148605
3
2 10 2
1 1 0
0
0 0 0
0 10 -5 0 5 10 -5 0 5 10 -10 0 10 0 10
)
N
VI HD149757 HD166734 6HD168076 5HD168607 HD169454
L
E 1 2 5 4
K
( 4 3 5
* 3
Tr 1 2 2
1 1
0 0 0 0 0
-1
0 10 0 10 0 20 0 20 40 60 0 50 100
HD172028 HD179406 HD183143 HD192035 HD195592
3
1 1
1
5 2
1
0 0 0 0 0
0 10 -10 0 10 -20 0 20 40 0 20 -20 0
CygOB2#8A CygOB2#5 CygOB2#12 4VICYG#8A 2HD200120
5 2 4
4 3
3
3 2 1
1 2
2
1 1 1
0 0 0 0 0
-100 -50 0 -40 -20 0 20 -40 -20 0 20 -40 -20 0 20 -60 -40 -20 0
-1
V (KM S )
LSR
Fig.2.ObservedCOJ=1-0emissionprofiles,asinFig.1
At higher latitudes where the foregroundreddening toward 5. Tracingabsorbinggasinemission?
the target stars seldom exceeds 0.35 mag correspondingto the
diffusecloudregime,thepresentsurveyseldomfindsCOemis-
sionexceptalongsightlinesatE >0.5mag.Thisissimilarto Althoughitwasanoriginalgoalofthisworktotestwhetherthe
what occurs along the line of si∞BgVht toward ζ Oph (HD149757) behaviour predicted for absorbing gas seen in emission could
where W = 1.7 K kms 1, E = 0.32 mag and E = 0.55 actuallybeobserved,theprofoundcontaminationofthesurvey
mag, butCtOhe inherent likl−ihoodBVof finding lines of s∞BiVght with by background emission from associated material complicates
such E at high latitude is much higher in the survey sample ormootsthisissue.Giventhebuilt-inbiasforthetargetstarsto
heretha∞BnVforrandomlychosenlinesofsight. be situated in front of the associated material, the question be-
comesoneofascertainingwhethertheemissionfromforeground
absorbing CO may even be reliably identified in arcminute-
diametermm-waveemissionfanbeams.
8 H.S.Liszt:ACOJ=1-0Surveyofcommonoptical/uvabsorptionsightlines
3HD203938 3HD204827 HD203374 HD206165 HD206267
1
1
2 2 1
1 1
0 0 0 0 0
-20 0 -20 0 -10 0 10 -10 0 10 -20 0
1.5
HD207198 2HD207308 HD207538 HD208440 HD210121
2
1
1
1
1 1
0
0 0 0 0
-10 0 10 -10 0 10 -10 0 10 -10 0 10 -20 0
)
N
VI .5HD210839 HD210839 HD216532 1HD216532 1HD216898
EL 1 2
K
(
*
r 1
T
0
0 0 0 0
-80 -70 -20 0 -105-100 -95 -20 0 -20 0
HD217312 HD218376 HD229059 BD63 1964
1 10
1
1
5
0
0 0 0
-80 -70 -60 -40 -20 0 -20 0
-1
V (KM S )
LSR
Fig.3.ObservedCOJ=1-0emissionprofiles,asinFig.1
Thebehaviorpredictedforemissionfromtheforegroundab- Theexpectationforthediffuseforegroundgasisthatthedat-
sorbinggastakestwoforms,asshowninFig.6ofLiszt(2007). apointsshouldlie onorslightlybelowthe modelresultswhich
Most importantly, the integrated intensity should vary as W areshown(seeFig.6ofLiszt(2007))andthereisacomponent
CO
=R TBdv 1Kkms−1 N(CO)/1015 cm−2 withnothresholdin of the data, about one-third of the detections, for which this is
N(CO) (wh≈ich varies as N(H )2) and little dependence on the thecase.Itistheselinesofsightforwhichthecontributionfrom
2
theforegroundgashasmostlikely,notdefinitely,beenisolated.
ambientnumberdensityinthehostgas.Theproportionalitybe-
However,manymorelinesofsightwithdetectedCOarehighly
tweenW andN(CO)isaconsequenceofweakrotationalex-
CO
overluminousin the ratio W /N(CO). Given that this ratio is
citationasoriginallyshownbyGoldreich&Kwan(1974).This CO
actuallymaximizedindiffuse gas–comparewith W /N(CO)
behaviorwasactuallyseeninCOemissiontowardthesampleof CO
=500Kkms 1/3 1019cm 2 15Kkms 1/1018cm 2 0.02
extragalacticmm-wavebackgroundsourceswhereallthegasis − − − −
Kkms 1/1015 cm×2 towardOri≈onAoratypicaldarkclo≈ud,re-
intheforeground(Liszt&Lucas,1998). − −
spectively–contaminationbybackgroundgasistheonlypossi-
Figure 5 at bottom shows the comparison between emitted bleexplanationforsuchbehaviour.
W and absorbing N(CO) together with some model results Fortheabsorbingforegroundgas,theCO-H conversionfac-
CO 2
which serve to sketch the expected emission locii for the ab- torW /N(H )shouldbesmallforlowerN(H ),onlyapproach-
CO 2 2
sorbinggas.COemissionwasdetectedalongeverylineofsight, ingvaluesW /N(H ) = 1K kms 1/ 2 1020 cm 2 forN(H )
CO 2 − − 2
saveone,forwhichN(CO)> 4 1014 cm 2,correspondingap- > 5 1020 cm 2/(K kms 1). Fig. 5 at to×p shows the observed
− − −
proximatelytoanexpected∼emis×sioncontributionW > 0.4K i∼nteg×rated intensity plotted against the absorbing column den-
CO
kms 1.InSect.3.2weestimatedaroughapriori3σsen∼sitivity sity of H . The contribution from any underluminous diffuse
− 2
limitof0.22K,makingitappearthatCOemissionwasactually foregroundgasispresumablyrepresentedbythenon-detections
detectedtotheextentexpected,buttheemissioniscontaminated at N(H ) < 5 1020 cm 2 but there is not much of a margin
2 −
byanuncertaincontributionfrombackgroundgas. of detecta∼bility×. What is true is that essentially every line of
H.S.Liszt:ACOJ=1-0Surveyofcommonoptical/uvabsorptionsightlines 9
b=12o
100 100
1] 1]
-s -s
m m
k k
K 10 K 10
[O [O
C C
W W
1 1
19 20 21
1 10 100 10 10 10
|1/sin(b)| N(H ) (absorption) [cm-2]
2
100 100
1] 1]
-s -s
m m
k k
K 10 K 10
[O [O
C C
W W
b<3o
1 b>3o 1
0.1 1 10 12 13 14 15 16
∞ 10 10 10 10 10
E [mag] -2
BV N(CO) (absorption) [cm ]
Fig.4. Variation of the integrated intensity of the CO emission Fig.5. Variation of the integrated intensity of the CO emission
W .Top:vs.thecosecantofthegalacticlatitude.Bottom:vs. W .Top:vs.thecolumndensityofH seeninabsorption.The
CO CO 2
the reddening out to infinity from Schlegeletal. (1998) where straightgrayed(blue)linerepresentsaCO-H conversionfactor
2
linesofsightatlowerlatitudesareshownasredopendiamonds. W /N(H )= 2 1020 cm 2/(Kkms 1).Bottom:W plotted
CO 2 − − CO
×
Linesofsightlackingdetectableemissionareindicatedsymbol- againstthecolumndensityofCOseeninabsorption.Thedotted
ically. (red)lociiatbottomcorrespondtothemodelresultsforn(H)=
128and256 cm 3 showninFig.6ofLiszt(2007)andindicate
−
theexpectedbehaviourforforegroundgasobservedinemission.
sightwithdetectedCOemissionhasW /N(H )> 1Kkms 1 Linesofsightlackingdetectableemissionareindicatedsymbol-
CO 2 −
/1 2 1020 cm 2 . Perversely,mostofthe very∼strongestCO ically.
−
− ×
emissionisfoundalonglinesofsightwithverysmallabsorbing
N(H )andN(CO),asuresignofcontaminationbybackground
2
gas. The sole line of sight with appreciable N(H2) and no CO fourteen profiles of 13CO. Thumbnail sketches of the spectra
emission,HD217035,haslogN(H )=20.95andlogN(CO)=
2 are shown in Figs. 1-3 and the observationsare summarizedin
14.57,a verysmallfractionalCO abundance,andaninferreda
Tables1-4.
foregroundemissioncontributionW <0.4Kkms 1.
CO − The CO emission results were comparedwith other line of
sightparameterssuchasthetotalreddeningouttoinfinity,E ,
∞BV
and the absorbing column densities of H and CO, see Sect. 4
6. Summary 2
and 5. In doingso we showedthat the surveysightlinesgener-
To test the predictions made by absorption line observations, allyarebiasedtowardveryhighmeandensityofmaterial(total
andtoprovidecomplementaryinformationsuchasveryhighly reddeningperunitlengthtypically2-5timesthelocalgalactic
velocity-resolved line profiles, we conducted a survey of the average,seeTable5andSect.4)andcomparablylargeamounts
mm-wave J=1-0 12CO emission toward 110 stars which have ofCOemission.
inthepastservedasprimetargetsforoptical/uvabsorption-line This can be understoodif the targets are physically associ-
studiesoftheinterstellarmoleculesH ,CO,CH,CH+ andH +. atedwiththeneutralmaterialcausingtheover-density,presum-
2 3
Thesampleofincludedsightlineswasconstructedonthebases ably because they are of early spectral type and were further
ofinclusivenessandpracticality,i.e.δ > 25o.Asdiscussedin selected because they returned interesting spectra. In this case
−
Sect. 3.1, all of the profiles with detected interstellar emission the targetstars mustin generalalso be interactingwith and in-
are online in machine readable form along with an additional fluencing the associated material, which to this extent can not
10 H.S.Liszt:ACOJ=1-0Surveyofcommonoptical/uvabsorptionsightlines
Table5.StatisticsofE ,E andW 1
BV ∞BV CO
COundetected COdetected
b N E >1mag <E >/<l> CO? <W >/<l> aE /2W 2 <E > <E > <E > <E >
| |< # ∞BV # m∞BaVgkpc 1 # KkmCOs 1kpc 1 H cm 2∞BV(KkmCOs 1) 1 mBaVg m∞BaVg mBaVg m∞BaVg
− − − − − −
3o 71 21 1.4 34 14.1 2.8 1020 0.21 0.48 0.48 1.87
5o 63 16 1.66 29 21.3 2.3×1020 0.19 0.41 0.45 1.98
10o0 49 14 2.4 21 35.7 1.9×1020 0.16 0.38 0.41 2.46
15o 36 11 2.8 16 38.6 2.1×1020 0.13 0.42 0.39 2.76
20o 16 1 1.5 4 30.0 1.5×1020 0.12 0.28 0.46 0.58
×
1ForexplanationoftableentriesseeSect.4.2and4.3
2a=5.8 1021Hcm 2mag 1
− −
×
be entirely typical of the interstellar gas as a whole. The enor- 197592
mous scatter in N(CO) at fixed N(H ) which is typical of uv
2 2.5
absorption line surveys may be exaggerated owing to the pho-
toionizing radiation of the target stars. This could also explain 181615 144217
why selective photodissociation and fractionation of 13CO are 1] 143275
- 144470
observed in somewhat different degrees in absorption line sur- S
2.0
veys toward stars and toward extragalactic continuum sources M
(althoughsmall-numberstatisticsmayalsoplayarole). K

Becausethetargetstarshavealsobeenselectedtoobeythe K
[
diffuse cloud limit AV < 1 mag, any overendowment of asso- O) 1.5 198781
ciated neutral material ∼must occur preferentially behind them. C
Thisgeneralscenarioismanifestedinthedetectionofsomevery (

l
strong(20-40K)COlinestypicalofnatalgiantmolecularclouds e
t
near H II regions, and some heavily saturated lines typical of W 1.0
darkclouds,behindanyofwhichthetargetstarswouldnothave
detected
been suitable candidates for optical/uv absorption line studies limits
in the first place. Lines of sight with detectable CO emission
are somewhat more highly reddened in the foreground of the
1.0 1.2 1.4 1.6 1.8 2.0 2.2
targetstar (0.25mag) but much more heavily reddenedbehind csc(el)
(1.5mag).Theeffectofassociationandpreferentialforeground
placementcreatescomplicationsforobservationsofCOinemis- Fig.A.1.IntegratedintensityofthetelluricCOemissionvs.the
sion which lack an innate ability to discriminate between fore- cosecant of the elevation. Lines of sight with detected and un-
groundandbackgroundmaterialinthevicinityofthestar. detectedinterstellaremissionareshownasopenrectanglesand
One of the original goals of this survey was to compare (red)diamonds,respectively.A few linesof sightfromTable 1
direct observations with the prediction, based on the observ- withoutobviousinterstellar emission aredistinguishedbyrela-
able properties of absorption line gas, that WCO 1 K kms tivelystrongtelluricemissionandarelabelled(inblue).
N(CO)/1015 cm 2, with little dependence on the ≈number den-
−
sityofthehostgas.Thephysicaloriginofsuchaproportionality
resides in very general properties of the weak-excitation limit
perhapsuniquely,theopportunitytoimagethehostbodyofthe
but the normalization depends on the particular properties of
absorbinggascolumnwithhighspatialandspectralresolution.
thediffusegas.Forinstance,W /N(CO)issome50-100times
CO Thesearelaudablegoals,but,aswehaveshownhere,greatcare
higher in diffuse gas than when the CO rotation ladder is ther-
mustbeexercizedindecidingwhethertheyareactuallyachiev-
malized,whetherindarkclouds(W =15Kkms 1,N(CO)=
CO − ableinanyindividualcase.Wearecurrentlytryingtofindsight-
1018 cm 2)ortowardOrionA(W =500Kkms 1,N(CO)=
− CO − linestowardbrightstarswhicharegoodcandidatesformapping
3 1019cm 2).
− theforegroundgas.Inthemeantime,amorefruitfulapproachto
×
HoweveritseemsmorelikelythattheratioofW observed
CO mappingdiffusecloudsinCOmightbetoconcentrateonthose
inemissiontoN(CO)seeninabsorptionisbestusedasagauge
which occult extragalacticbackgroundsources, for which con-
of whether the detected emission can reasonably be associated
fusion between foreground and background material is not an
with the foreground absorbing material. Along many lines of
issue.
sightsurveyedhere,whichhaveW /N(CO)muchhigherthan
CO
even the already very large value which applies to diffuse gas
(seeFig.5)thisismanifestlynotso,owingtobackgroundcon-
AppendixA: Telluricemission
tamination. In these cases, the observed CO emission contains
information on the environent of the star, but not necessarily Anarrow(FWHMof300kHzor0.8kms 1)telluricCOemis-
−
on the foreground gas column. Along other lines of sight, the sion line appears with W = 1-2 K in all frequency-switched
CO
observedemissionmaycontaina substantialthoughnotneces- spectra.Insomespectra,especiallyatlowgalacticlatitude(for
sarilyexclusiveorevendominantcontributionfromforeground instance toward HD169454), the telluric emission is insepara-
material. ble fromthat originatingin the Galaxy. In principle,this could
Whentheforegroundgascanbeisolated,COemissionoffers conceal an interstellar line if observationsare not made at two
the prospectofveryhighlyvelocity-resolvedline profiles, and, epochs widely spaced during the year. In practice the telluric