197Lapj...168L..53P the Astrophysical Journal, 168:L53-L58, 1971

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197lApJ...168L..53P 1213 1332 1332 1 _1 1 14-2 contract withtheNationalScience Foundation. relative abundanceofthe sulfurisotope.TheterrestrialC/ratiois 89andthe in ^C^Swhichoughttobemoreintensethanfor CSowingtothegreaterexpected ratios. Unfortunately,anequipmentfailureprevented observationofthesametransition sources, OrionAandW51,inordertoobtainopacity informationfromthelineintensity corresponding transitioninCSat138,738MHz weremadeinthetwobrightest and DR21.TheresultsaresummarizedinTable 1.Inaddition,measurementsofthe them. Thegalacticcenterwas,unfortunately,not observableduringthetimeofthese observations. Thesourcesinwhichwehavefound CSareOrionA,W51,IRC+10216, / =3states49,147,and294GHz,respectively,abovethegroundstate.Thedecayrate has beendetectedininterstellarspace.Theobservationofthisline,originatingfroma made withthe36-footantennaofNationalRadioAstronomyObservatoryatKitt the interstellarmedium.Themoleculeisasimplerotorwith/=1,2,and level ofhighexcitation,makesCSausefulvehicleforthestudyphysicalconditionsin by 1Mz(2.04kms).Carbonmonosulfideisthefirstmoleculecontainingsulfurthat of theobservedtransitionis Peak. Weusedalinereceiveroffortychannels,each2MHz(4.08kms“)wideseparated The AstrophysicalJournal,168:L53-L58,1971September1 observed carbonmonoxideemissionatcorrespondingvelocities.Observationswere in carbonmonosulfide,CS,thedirectionsoffoursourcesfromwhichwepreviously the observedlineintensitiesareusedtoderivedensitiesforcentralregionsofsources. sources. Typicalcolumndensitiesarenear10moleculescm.Theexcitationratesrequiredtoproduce © 1971.TheUniversityofChicago.Allrightsreserved.PrintedinU.S.A. 1 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem TheNationalRadioAstronomy ObservatoryisoperatedbyAssociatedUniversities, Inc.,under We investigatedfoursourcesknowntocontain molecules andfoundCSinallof We haveobservedlineemissionfromthe146969.16-MHz/=3to2transition We haveobservedlineemissionfromthe146969.16-MHz/=3to2transitioninCSfour INTERSTELLAR CARBONMONOSULFIDE Bell TelephoneLaboratories,Inc.,MurrayHill,NewJersey Bell TelephoneLaboratories,Inc.,Holmdel,NewJersey Bell TelephoneLaboratories,Inc.,Holmdel,NewJersey An 2 Ihâ , 647tVm Received 1971May25 II. OBSERVATIONS Columbia University I. INTRODUCTION P. M.Solomon K. B.Jefeerts R. W.Wilson A. Penzias ABSTRACT 6-1 L53 = 6.5X10"s. AND 197lApJ...168L..53P hms /// L54 interstellar ratioisthoughttobeclosethisvalue(Penzias,Jefferts,andWilson1971) Low (1967)whichislocatedsome17secondsof arctothesouth.Noemissionpeak latter ratioshouldbeanimportantby-productoffutureCSstudies. while theterrestrial^S/^Sratiois25.Adeterminationofinterstellarvaluesforthis was foundfromthedirectionofinfrarednebula associatedwiththeTrapezium with thehalf-intensitycontoursome3'(decl.)X 9'(R.A.).Themostintensefeature Eliasson 1967)coincidentwiththeinfraredpoint sourceofBecklinandNeugebauer. position ofoursecondary maximum. It isdifficult,however,toexcludeunambiguously the infrarednebulaofKleinmannand approximately centeredonthisposition.Thisisthe OHmaserposition(Raimondand occurs inthescantakenatR.A.53347,decl.—5°24 16 (Fig.\a).Fromtheintensi- stars (NeyandAllen1969). However,NeyandAllenreportedasouthward extensionof ties obtainedinadjacentdirections,wefindthe sourceofmaximumintensitytobe CO emission(Wilson,Jefferts,andPenzias1970), theCSemissionissharplypeaked the KleinmannandLow objectsomewhattotheeastofTrapezium, and nearthe © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The OrionNebulawasstudiedmostextensively (Table 1).Incontrastto115-GHz Orion DR 21(OH). DR 21 W51 IRC+10216. Source hr 20 37 20 37 19 21 532 9 45 R.A. A. PENZIASETAL. l 55* 51 47 47 51 51 47 47 47 47 51 47 47 47 47 43 43 43 47 31 35 39 43 31 23 27 27 27 13. 14.8 13. CS Sources a) OrionA TABLE 1 -5° 24'16" 42 1159.9 42 0859.9 14 2430 13 3040 Decl. 24 26 23 25 27 23 21 20 24 23 29 28 26 25 24 22 25 24 24 26 24 24 19 23 24 25 (° K) 0.4 0.9 0.5 0.4 0.4 3.0 0.8 0.9 2.2 2.1 2.6 2.4 1.5 0.8 2.1 0.7 2.8 0.9 1.7 2.3 1.6 1.8 1.4 1.8 0.6 1.0 1.2 1.8 1.1 1.4 1.1 Ta -A_1 (km s) +12 + 10 + 10 + 10 + 9 +10 + 10 +10 + 10 + 10 + 9 + 9 +10 + 9 + 8 + 9 +10 + 9 + 9 + 9 +10 + 9 + 8 +60 +57 +57 +56 +57 -22 - 4 - 2 v Av 24 12 5 5 5 4 4 5 6 5 5 5 5 5 5 5 5 5 5 5 6 6 7 9 5 5 6 8 6 6 7 Vol. 168 CU 00LO co - No. 2, 1971 INTERSTELLAR CARBON MONOSULFIDE L55

\—II b) W51 ^ W51 (Fig. 1¿>) was measured at five locations spaced 1 arc min apart in the form of a cross. The central spectrum was peaked at 58 km s_]—in good agreement with our previous CO studies of the source. The weaker 70 km s-1 CO feature (Penzias et al. 1971, Table la) is completely absent in CS. However, the asymmetry of the CS spectrum suggests a component corresponding to a lower-velocity (~53 km s-1) feature present in the CO data, which has also been detected in 6-cm H2CO absorption (Scoville and Solomon 1971). The angular extent of the CS emission also corresponds closely to 2-mm H2CO measurements for the source. c) IRC+10216 This radio source has a CS line with the same velocity, width, and shape as the CO emission (Solomon et al. 1971) with about one-third the intensity. We have detected emission in this object from four molecular lines (12CO, 13CO, CN, CS), and a discussion of the results including the CS observations will be presented in a later paper.

d) DR 21 In DR 21 we note that the emission is stronger at the position of the OH emission than at the continuum peak. This association with OH sources was illustrated by failure to obtain data from W3. We had planned to investigate the OH source in W3 for CS emission, but we inadvertently pointed the antenna to the W3 continuum position, which is separated from the OH position by some 20 arc min, and found no emission. Measurements of 13C32S were made at 138,738 MHz for Orion and W51. In both cases, residuals between 0.1° and 0.2° of antenna temperatute were obtained. We expect to pursue this investigation in the near future and for the present regard these values as merely representing upper limits.

m. ABUNDANCES Unless the emitting gas is known to be optically thin, one cannot obtain unambiguous column densities from emission spectra without additional opacity information. We may, however, deduce useful limits to the density from the data at hand.

Fig. la.—Orion peak. R.A. 5h33m478, decl. — 5"24'16" (1950). The feature is broadened by the 2 MHz (4 km s-1) width of the individual channels. Fig. 1&.—W51. R.A. 19h21m278, decl. 14°24'30" (1950).

© American Astronomical Society • Provided by the NASA Astrophysics Data System 197lApJ...168L..53P 1213 1332 216 13 be populatedaccordingtotheirstatisticalweights,the7=3stateat the minimumexcitationtemperatureconsistentwithobservedantennatemperature, lower limittothetotalcolumndensityofCSbyassuming/=0,1,and2states C/ =89.Underthisassumption,r^C^S)<6forOrion,andlessthan10W51. using therelationbetweencolumndensityandantennatemperaturevalidfor These valueswouldcorrespondtoaCSantennatemperatureof0.2°K.Inthiscase, emission wemayobtainupperlimitstotheopacitybyassumingarealisotoperatio and allstateswith7>3tobeunpopulated.ThisgivesN2N.Combiningthis optically thincasewhentherotationaltemperatureTz>hv/K.Wethenhave L56 A.PENZIASETAL.Vol.168 Av isthelinewidth.Wecanmaximizefractionofmoleculesin/=2andobtaina where ristheopacityandTWexcitationtemperatureoftransition.Inthis where Tbgisthetemperatureofmicrowavebackground.Inourcomputationwe take Tbtobe2.8°K,althoughneglectingitentirelywouldincreaseourfinalvaluesby result withtheabove,wehave case ofhighopacityisaminimumandcomputedfromtherelation where rjisthebeamefficiencyofantenna(0.6),Ttemperature,and 0-1 and1-2transitions.Bothofthesehavetheeffectoverestimatingpopulation with increasing7.Therefore,ifweassignTztoalllevels,areassuredofoverestimat- and forcollisionalexcitation,theexcitationtemperatureeachpairoflevelsdecreases only some10percent.Foranobservedantennatemperatureof3°K,andr>1,this ing theexcitationoflevelsabove7=3aswellunderestimating equation givesT=8.3°K. of theotherlevelsrelativeto7=2level: cm” whereasforW51wehave2.1X10>A" 4.3 X10forallthesourcesobserved. than CNandlessabundant thanHCN.Carbonmonoxide,ontheotherhand, isatleast 2 2 four ordersofmagnitude moreabundantthanCS. Inserting theexpressionforN,wehave g Carbon monosulfideisthus apparently,likeformaldehyde,somewhatmore abundant A 2 2Z 2 N =2X-iV2.3Xmolecules 2 j g2y*=o _\hv) T A ^I¿[exp(hu/kT)-1](hi>/kTl]f’ nbg 2 SwgvtAv N =2 2 c*gAv[1-exp(-W¿r)]’ 323 N> 3.IX10«AvT. a 7 STgivkTAàv N <3.5X10tAv. N > 21 “hcgzAz2V 197lApJ...168L..53P 19 -2 6 24 -2 43 162 -5 13 -14 o l4 upper limitof10ontheopacityisjustinrange wherethiseffectbeginstobeimpor- optically thick,photontrappingwillsomewhatreduce therequireddensity.Theobserved about 1arcminuteareregionsofveryhighdensity. Wenote,however,thatiftheCSis be removedonlywhenaccuratecross-sectionsarecalculated. be comparedwith3X10carbonmonoxidemolecules cm'reportedbyPenziasetal. kpc andZ>(Orion)=0.5kpc,angularextents of~0Í7and^1Í5,givesilf(W51)= we obtainwh=3.5X10forW51.Thusthecoreofmolecularcloudssubtending the excitationcross-section.Weadoptatypicalkineticcross-sectionvaluefor= where T*isthekinetictemperature,wh,densityofmolecularhydrogen,and(W51)=7 very lowionizationheatingrate,andthereforeelectrondensity.Apreliminary molecular, sincetheyhaveneverbeendetectedat21cm,withessentiallyallthehydro- 2 X10“cm.Thisintroducesanuncertaintyofapproximatelyafactor3whichcan with theobservedbrightnesstemperaturecanthenbedeterminedfrom gen intheformofH.Theelectrondensitymustbeextremelylowviewfast paper.) TheCOemissionintensityisobservedtodecreasegraduallyoveraverywide analysis ofthisionizationequilibriumgivesn/n^<10;therefore,theexcitationwill cloud. ThelowkinetictemperatureobtainedfromtheCOobservationsalsoindicatesa collision ratescaleswiththepermanentdipolemomentandwouldgiveapproximately taking placebycollisionwithneutralatomicormolecularhydrogensincetheelectron- which istoosmalltothermalizethelevels.Thisstronglyimpliesthatexcitation peak brightnesstemperatureneartheCSpositionof40°,andisclearlyheavily tant. be dominatedbyneutralcollisions. recombination rateofmolecules,intheabsenceanyionizingradiationmolecular equal excitationtoHCN,CS,andCO. thick sinceitsCisotopeisobserved,showsamuchlowerexcitationtemperaturethan Buhl 1971),whichalsohasashortlifetime(^l=4X10s)butisapparentlyoptically exceeds theCStemperaturebyatleastafactorof8.EmissionfromHCN(Snyderand area withanangularextentofaboutI.InallpositionstheCObrightnesstemperature saturated. (AcompletediscussionoftheOrionregionwillbepresentedinaforthcoming s, raisesinterestingquestionsastotheexcitationmechanismforthisprocess.The the COline.Thisindicatesthatlowerobservedtemperatureofshort-lifetime (1971) forthesamesource. Ifweassumethatallthecarboninsource is inCOand 2.6-mm /=1to0COlinewhichoriginatesfromthesamelocationinOrionhasa (high dipolemoment)transitions(CS,CN,HCO,HCN)isduetoanexcitationrate some 10wavenumbersabovethegroundstateandwithalifetimeA~ofonly1.6X No. 2,1971INTERSTELLARCARBONMONOSULFIDELS7 2 0 2 et 2 0-32 2 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The cloudscontainingtheobservedmoleculesareundoubtedlyalmostcompletely Substituting inTk=40°K,wethenhavewh4.3X10cmOrion.Similarly, The densityofmolecularhydrogensufficienttogivetheminimumn^/nzconsistent Asimilarresultfortheformaldehyde caseisgivenbyKutnerelal.(1971). The massesofthecentralregionsW51andOrion cannowbeestimatedfromthe The observationofCSemissionwithastatetemperatureT^z>8.3°K,fromlevel 2 nz gzVrimwexp(—hv/kTK)i’ ih _&l~Az2n^xirvI IV. EXCITATION L58 A. A. PENZIAS ET AL. that the ratio of hydrogen to carbon is 3.3 X 103, we can use the GO data to predict a column density of 5 X 1022 hydrogen molecules cm-2, or about a factor of 30 below the estimate obtained from CS excitation.

v. CONCLUSION Carbon monosulfide is a relatively abundant and widely distributed molecule, roughly comparable to formaldehyde in these respects. The observation of the / = 3 to J = 2 transition implies a high excitation rate which, we have argued, in turn implies a high density (>105 hydrogen molecules cm-2) for the central regions of the sources.

REFERENCES Kleinmann D. E., and Low, F. J. 1967, Ap. J. {Letters), 149, LI. Kutner, M., Thaddeus, P., Jefferts, K. B., Penzias, A. A., and Wilson, R. W. 1971, Ap. J. {Letters), 164, L49. Ney, E. P., and Allen, D. A. 1969, Ap. J. {Letters), 155, L193. Penzias, A. A., Jefferts, K. B., and Wilson, R. W. 1971, Ap. J., 165, 229. Raimond, E., and Eliasson, B. 1967, Ap. J. {Letters), 150, LI71. Scoville, N., and Solomon, P. M. 1971, in preparation. Scoville, N., Solortion, P. M. and Thaddeus, P. 1971 (submitted to Ap. J.). Solomon, P. M., Jefferts, K. B., Penzias, A. A., and Wilson, R. W. 1971, Ap. J. {Letters), 163, L53. Wilson, R. W., Jefferts, K. B., and Penzias, A. A. 1970, Ap. J. {Letters), 161, L43.