1968ApJS...15..131G -8-7 medium. OftentherelationofOHtootherconstituents oftheinterstellarmediumcan portance oftheisotropicbackgroundandgalacticisstressed. frequency is6kHzlessthanRadford’svalueandessentiallyremovesthediscrepancyin plet havebeenobserved.Mostofthesourcesshownormalintensityratiosinmainlines.Exceptfor be obtainedregardingmolecularprocessesand excitationconditionsintheinterstellar vestigation istoextendsuchstudiesbyexaminationofthespectranorthernhemisphere useful toolforstudyingconditionsintheinterstellarmedium.Thepurposeofthisin- sources inthattheemissionisathigh-velocitysideofoneabsorptionlines. W42, W43,W44,W47,W66,W67,W69,W72,W73,W80(NGC7000),andW81(CasA).Fourofthese be ascertained.Finally,insomecasestheOHlines canserveasatoolforthestudyof radio sourcesforOH18-cmabsorptionlinesarising fromtheground-stateAdoublet. W12 andW22allthesourcesshowemissionin1612MHzand/or1720satellitelines.Evenfor sources containOHemissioninthemainlines(1667MHzand1665MHz)additiontoabsorption: W22 (NGC6357),W28,W29(M8),W30,W31,W33,W356604),W37(M16),W38(M17),W41, These observationsprovidethefollowinginformation: (1)thegalacticdistributionof field (assumedtobe3°K).Thecalculationspredictanexcitationtemperatureofabout20°K,while OH hasadistributionsimilartothatofthéHi. the Hiabsorptionlinesfoundbyotherobservers.Thegeneralagreementinvelocitysuggeststhat tion lines.The158aHelinefromthestrongeroftwoHnregionshasbeendetected. sum rule. W12 andW22theintensityratiosareincompatiblewithauniqueexcitationtemperatureforAdoublet. Orion Aat1667and1665MHz,W33W42W43 requirements forthePh.D.degree. galactic structure. the widthsofindividualDopplerfeatures. Fromsuchinformationknowledgecan OH; (2)theprojecteddensitiesofOH(divided by theexcitationtemperature);and(3) are intherange10-10. sible causesforthisdiscrepancyarediscussed. tion iscontrolledby(1)collisionswithpositiveionsandelectrons,(2)the18-cmisotropicradiation the observationalevidenceindicatesthatexcitationtemperatureislessthanabout10°K.Twopos- 1665 MHz.TheW42,W33,andOrion(1667MHz)sourcesdifferfromthepreviouslydiscoveredemission (W7 andCygA):W1(NGC7822),W9(TauA),W10(OrionW122024),W14(IC443), 100-channel receiverwithfrequencyresolutionsof10kHz(1.8km/s)and2(0.36km/s). the 85-footHatCreektelescopeofUniversityCalifornia.Theobservationsweremadewith ture is3°K(thelowerlimit).BasedonthisassumptiontheratiosofOHtoHiprojecteddensities The W12resultssuggestthattherestfrequencyof1720MHzshouldbe1720.527±0.003MHz.This It haslongbeenrecognizedthatneutralhydrogenabsorptiontechniquesprovidea The solutionoftheequationtransferappropriatetoOHabsorptioncaseispresented.im- In manycases,itispossibletocomparetheOHabsorptionlinesobservedinthisinvestigationwith A surveyofnorthernhemisphereradiosourcesfor18-cmOHabsorptionhasbeencompletedusing The majorprocessesestablishingtheexcitationofOHinHiregionsareconsidered.levelexcita- Two HnregionswhichlieinthedirectionofW43havebeenidentifiedfromtheir158arecombina- For thesourcesW10,W12,W22,W28,W41,W43,W44,W51,andW81allfourlinesofOHmulti- For thepurposeofestimationOHprojecteddensities,itisassumedthatexcitationtempera- Galactic OHabsorptionlineshavebeenfoundin26galacticsourcesandtwoextragalactic * BasedonathesissubmittedtotheUniversityofCalifornia, Berkeley,inpartialfulfilmentofthe f NowatC.S.I.R.O.Radiophysics, Sydney,Australia. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Radio AstronomyLaboratory,UniversityofCalifornia,Berkeley OH ABSORPTIONINTHEGALAXY* Received April26,1967 W. MillerGossf I. INTRODUCTION ABSTRACT 131 1968ApJS...15..131G o 2 2 private communication).Inaddition,OHemissionlineswerefoundinthedirectionof W3 andNGC6334arediscussedbyWeaver,Dieter,Williams(1968).Fiveofthe These sourcesareGasA(Weinreb,Barrett,Meeks,andHenry1963),W43W51 and twoextragalacticsources(CygAW7).Theabsorptionlinesinthedirectionof twenty-eight sourceshavebeenreportedelsewhereandareherediscussedinmoredetail. this survey.SectionHIisasummaryoftheobservations.IVdiscussion 132 W.MILLERGOSS ten sources. fines inCasA—themost unfavorablecasebecauseofitslargeantenna temperature. lowed bya100-channelreceiver.Bandwidthsof10kHz(correspondingto1.8km/s)and versity ofCalifornia.Alow-noiseparametricamplifierwasusedasapreamplifierfol- faulty componentinthepreamplifier. using theexpressiongivenbyMezgerandHenderson (1967).ThevaluesofAat1612 switching wasemployed;askyhornutilizedasreferenceloadforthe18-cmcon- the equationoftransferappropriatetoOH.In§Vwediscussproblemexcita- tion temperatureoftheOHAdoublet.Theconclusionsfollowin§VI. (Williams 1965),W75(Weaver,Williams,Dieter,andLum1965)CygA(Menon, in thisdiscussion. Deviations fromlinearity werefoundtobelessthan10percentand will beneglected less than1'atthesedeclinations. ing theMoonatlowdeclinations (^—28°)haveindicatedthatthepointing errorsare and Lohman(1966).FurthertestsbyWelch(private communication)madebyobserv- tion ofthesystemduringlatterpart secondobservingperiodwasduetoa half-power beamwidths,0i/,werederivedfrom scansofthebrightOHemissionfinesin munication) in1965atafrequencyof1515 MHz withthesamefeedsystem.The non-thermal sourcesCasA,CygTauandVirgoAusingthefluxesmeasuredby tions. tinuum observations.ThepositionangleoftheE-vectorwas90°duringtheseobserva- 2 kHz(correspondingto0.36km/s)wereused.FortheOHobservationsfrequency Baars, Mezger,andWendker(1965).Theeffectivearea,A,apertureefficiency,tja, discharge tubewasusedasasecondarystandard;itcalibratedagainstthe temperature wasintherangeof130-200°Kduring theseobservations.Thedeteriora- and 1720MHzwerefoundtobe320±30 330±30m,respectively.Thesystem W3 andW49.Fromthevaluesof0i/2tja beamefficiency,tjb,wascalculatedby agrees quitecloselywiththevalueof304mobtainedbyW.J.Welch(privatecom- of theantennaweredeterminedbycalibratingnoisetubewithahot-coldload. e 2 e Section IIisadescriptionoftheinstrumentationandnumericalparametersusedin Galactic OHabsorptionlineshavebeenfoundinthespectraoftwenty-sixgalactic The linearityofthesquare-law detectorswascheckedonthestrongOH absorption The pointingofthe85-foottelescopehasbeendiscussed indetailbyWelch,Thornton, The observationsweremadewiththe85-footHatCreekradiotelescopeofUni- During eachoftheobservingruns(April-MayandOctober-November,1966)agas- The instrumentalparametersaresummarizedinTable1.derivedvalueofA e © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem II. INSTRUMENTALANDNUMERICALPARAMETERS Instrumental Parametersat1666MHz 2 dii2 3l!4±COinEplane dij2 35Í6±COinEplane A 300±10m 7]b 0.96¿0.07 e 0.57+0.02 TABLE 1 1968ApJS...15..131G provided bytheothersourcesinvestigated;useofderivedrestfrequencyimproves four lines,i.e.,thesumof1667and1665MHzfrequenciesis1kHzlargerthan cates, thisrevisionessentiallyremovesthediscrepancyinfrequencysumrulefor sum ofthe1612and1720MHzfrequencies.Furtherevidencefavoringthisrevisionis W12 (§III)thereisastrongindicationthatRadford’s(1964)valuefortheF=2to son andMcGee(1967).Fromtheresultsobtainedinthisinvestigationforsource This revisedfrequencyhasbeenadoptedinthepresentinvestigation.AsTable2indi- F =1line(1720MHz)shouldbedecreasedbyabout6kHzto1720.527±0.003MHz. the velocityagreementinmanycases.TheCasAresults(§III)forlinesat1667and quencies usedwere1651.5416and1652.214MHz,respectively. the Orionarmlinesagreementinvelocitiesisbetterthan0.1km/s,i.e.,0.5kHz investigation forW43.Column(12)indicates thetypeofsource;“E.N.,”“Gal.,” while column(11)showsthe158aH-linevelocity takenfromDieter(1967)andthis sorption andemission,respectively.Thenumbers indicatetheupperlimitsinantenna follows in§§Illa-IIIaa. observations. Thetablesarefirstdescribed;thediscussionofindividualsources adopted distanceofthe galactic sources;inmostcasesthekinematical distances derived source, supernovaremnant, andinfraredstar,respectively.Column (13)givesthe optical depthisdiscussedin§Illaa.Column (10)givestherangeofOHvelocities, temperature, while“ul”indicatesthattheupper limitofantennatemperatureand/or “N.T.S.,” “S.N.R.,”and“I.R.STAR”indicate emissionnebula,galaxy,non-thermal searched withafrequencyresolutionof10kHz. Insomecases(W12,W22,W43,and are thoseofWesterhout’s(1958)surveyat22cm. Column(5)showsthevelocityinterval the DownesandRinehart(1966)5000MHzsurveyisutilized.Incolumns(3)(4) detected inthe158aH-line (§IHr).Columns(14)and(15)indicatethe totalantenna solved, bothdistancesare given.ThetwodistancesforW43refertothe twoHnregions by Dieter(1967)arelisted. Forsourcesforwhichthedistanceambiguity cannotbere- we listthegalacticlongitudeandlatitudeinsystem IIcoordinates.Thepositionsused Cyg A),andoneinfraredstar(CIT11)wereobservedforOHabsorptionemission. 1665 MHzindicatethatRadford’sfrequenciesforthemainlinesarequiteconsistent;in Cas A),selected36km/sintervalswereinvestigated withthe2kHzfilters.Columns temperature (Ftot)and thebackgroundtemperature(Fag)observedat theWesterhout (6)-(9) indicatewhichofthefourOHlineswas investigated;“A”and“E”indicateab- (1) andotherdesignations(2).FortheCygnusXsources(W66-W75)notationof The problemoftherestfrequenciesforfourOHlineshasbeendiscussedbyRobin- Thirty-seven galacticsources,fourextragalacticsources(W7,W21,VirgoA,and The sourcecharacteristicsaresummarizedinTable3.Tables4-8summarizetheOH In Table3,columns(1)and(2),weidentifythesourcesusingWesterhout’snotation For theobservationsof158alineshydrogenandheliuminW43restfre- © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem E=2—>1 F=2—>2 F= 1—>1 F= 1—>2 2 Rest FrequenciesofOHn/2aDoubletLines 3 Transition OH ABSORPTION III. OBSERVATIONS Radford (1964) TABLE 2 1612:231 ±2 1720:533 +2 1667:358 +2 1665:401 ±2 (kHz) 1720:527+3 This Work (kHz) 133 1968ApJS...15..131G 2 maximum antennatemperatures,areas,andfullwidthsathalf-maximumintensitywere positions. Finally,column(16)indicatestheremarkswhicharegivenatendof Table 3.Thereferencesareindicatedbythenumbersinparenthesesandpresented determined byfittingGaussianfunctionstotheprofiles.Theprogram each ofthelinesobservedmeanvelocitieswithrespecttolocalstandardrest, at theconclusionoftable. for theabsorptionlinesifmeasuredfullwidthsathalfmaximum,exceeded developed byMrs.DieterandMr.AlanEbertwasused.Ashortdescriptionofthispro- in column(9).Whereanambiguityexists,the leastlikelydistanceisinclosedinpa- jected densitydividedbytheexcitationtemperature. Theexponentofthepower10 rentheses. Whennochoice ispossible,bothdistancesareinparentheses. Column(10) with respecttothelocalstandardofrest, opticaldepths,andthecorrectedhalf- parentheses. Incolumn(3)theidentificationnumbers ofthefeatureswhichappearon rection isunwarrantedifAvl<2Avr.)Thecorrectedhalf-width,Av,wasobtained cedure isgivenbyKaper,Smits,Schwarz,Takakubo,andvanWoerden(1966).Owing 134 W.MILLERGOSS of thetableissimilarto thatofTable4.Columns(5)and(6)givethe peakantenna of theprojecteddensityisgiveninparentheses. Thekinematicaldistancesareindicated a lowerlimit.Incolumn(6)theobservedhalf-widths, Apr,areinclosedinbracketsfor widths, Apd.Incolumn(4)thevalueforoptical depthisinclosedinbracketsifitonly some oftheprofilesinFigure1aregiven.Columns (4),(5),and(6)givethevelocities distance ofthebackgroundsourceisknown;inafewcasesmethodproposedby uncertain, theamountofcorrectionis<12percentforAvl^2Avr. errors foreachofthecomponents.Acorrectioninstrumentalbroadeningwasmade to thesmallnessofsignals,only“obvious”numberGaussiansconsistentwith temperatures andthemeasured linewidths,respectively.Table6gives thecomparison identifies theremarksto Table4. Weaver wasused.(SeeRemarkIIIAtoTable3.) is limitedbythesetwolattereffects.ForthesourceswithR<10kpc,anambiguityin motion orfrompeculiarmotions.Inallcases,theaccuracyofkinematicaldistances ured velocities;theydonotincludethecontributionsfromdeviationscircular for <10kpc.Inapplyingthelattercurvedistancescalewasexpandedto= for R>10kpcandtheKwee,Muller,Westerhout(1954)galacticrotationcurve is unjustified,alowerlimittothemeasuredopticaldepthwasobtained. half-width oftheequivalentGaussianwhichdescribesmultichannelfilter.Thecor- from thehalf-widthoffilterpassband,i.e.,Av=^/(Av—),whereis twice thefilterbandwidth(deJagerandNeven1966).(Theseauthorsshowthatacor- the signal-to-noiseratioofeachprofileswasused.Thefittingprogramgivesrms those caseswhereacorrectionforinstrumental broadeningisnotmade.Incolumn(7) AT(v) andTtotgiveninTable3(§IV,eq.[8]).Forthosecasesforwhichacorrection the areaofabsorptionfeaturein°K*kHzis given,whilecolumn(8)showsthepro- the distanceestimatesexists.Inmanycasesambiguitycanberesolvedsince The errorsinDwhichareshownTable4thosedueonlytothemeas- arm canleadtooverestimatesofup0.8kpcinthederivedkinematicaldistances.) the measuredvelocitiesusingAustralianConferenceModel(quotedbyBok1965) rection tothepeakabsorption(madeiîAvl>2Avr)wasfoundbychord-construc- tion methodderivedbyBracewell(1955).Althoughthesecorrectionsmaybesomewhat (2) indicatestheOHlinewhichwasobserved.The filterbandwidthinkHzisgiven 10 kpc.(AsMiller[1968]pointsout,deviationsfromcircularvelocityinthePerseus D DLrR a The absorptionandemissionobservationsaresummarizedinTables45.For Table 5summarizesthe emissionlinesfoundinthisinvestigation.The organization Table 4summarizestheabsorptionlines.Column(1)givesWnumber;column The kinematicaldistances,19,totheOHabsorptionfeaturesweredeterminedfrom For theabsorptionlines,valuesof
TABLE 3—Continued 3 © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem - * In the text f is denoted by ( t ). TABLE 4—Continued © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem TABLE 4—Continued © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 00 Ï—I LO \—i o TABLE 4—Continued h- © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem ä° rH 00 m oo o o o O OO^-1 o vO NO vO CM o vo -H 6.4(13) ± 20% 4.07 ± .03 (14.12 CTi ^£> 00 00 ï—i LO \—i o TABLE 4—Continued Q © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 142 > oo \—I LO \—I o TABLE 4—Continued © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 143 1667 (10) 1 5.8 ± 0.3 [-079 ± .016] [12 ±2] 33 ± 2 A.0(13) ± 14% .43 ± .02 oo \—I LO \—i o TABLE 4—Continued © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 144 TABLE 4—Continued © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 145 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 146 1720 6 -37.1 ± 0.30 .0027 ± .0004 OH ABSORPTION 147 REMARKS TO TABLE 4 I. This is an impossible circular velocity for this longitude. II. This distance is greater than the kinematical distance of the source. III. No optical depth or area is given for the satellites due to the low signal-to-noise ratio. IV. Blending problems exist in features (5), (6), and (7). V. The 1665 MHz line in features (7) seems to have a smaller AvD than 1667 MHz. VI. The 1720 MHz data are taken from the 2 kHz profile. VII. Low signal-to-noise ratio. VIII. There may be a line at +45.5 km/s. This is confused by the strong emission line at +58.3 km/s. IX. The line at +67 km/s is confused by the strong emission line at +58.3 km/s. X. 64.2 km/s is a forbidden velocity at this l since 61.5 km/s is the velocity of the tangential point. Thus the distance of the tangential point (6.5 kpc) is accepted. XI. There is some hint of absorption in the 1612 MHz line at 3-4 km/s. XII. This could be a blend of two lines. There is not enough signal to noise to separate. XIII. Feature (3) is blended with feature (4). XIV. This may be a blended line since Avl ^ 10 kHz. XV. The kinematical distance is greater than the adopted distance to Cas A (3.4 kpc). involved in the H i-H n interaction process. The photometric distance given by Oster- brock (1957) (0.9 kpc) agrees closely with the kinematical distance given by Dieter (0.8 kpc). b) W7 (3C 123) Figure 1.2 shows the 1667 MHz profile. The line is quite narrow; the velocity agrees closely with the H i absorption line found by Clark (1965). c) W9 (Tau A) In Figure 1.3 the four profiles obtained in the direction of Tau A are presented. Only at 1667 MHz can we clearly see a line. The appearance of the H i absorption profile in Tau A is very similar to the OH profile, as reference to Figure 3 of Clark (1965) will indicate. Clark (1965) has suggested that the core of the +13 km/s line has a dispersion equivalent to a kinetic temperature of ^40° K. The OH line should be investigated with higher resolution than used in this survey. The optical depths in the 1667 MHz lines are among the smallest found in this survey. d) WIG {Orion A) The 1665 MHz emission in Orion was announced by Weaver et al. (1965). The 1665 and 1612 MHz emission lines have been discussed in detail by Palmer and Zuckerman (1967) and Weaver et al. (1968). Figure 1.4 shows the four profiles in Orion A. Narrow absorption lines are evident at 1667 and 1720 MHz, and emission at 1667 MHz appears at the high-velocity side of the absorption. The 1667 MHz emission is probably composed of many blended features. In Table 5 only one component is listed. The kinematical distance of the +5.5 km/s absorption feature of 0.55 kpc agrees quite closely with the accepted photometric distance of the Orion Nebula; however, the velocity of the 158a H-line is an impossible circular velocity for this longitude. Table 6 shows the comparison with the H i absorption results of Clark (1965). e) W12 {NGC 2024, Orion B) The continuum center of W12 (see Remark I of Table 3) is 5' west and 2' south of NGC 2024, which is bordered on the south by the Horsehead Nebula (IC 434). The dis- tance estimates for NGC 2024 vary somewhat; Mezger and Höglund (1967) adopt 0.4 kpc, which agrees with the Johnson and Mendoza V. (1964) photometric distance. Dieter (1967) (see Table 3) has a derived a kinematical distance of 0.7 kpc. The exciting star of NGC 2024 (an OB star) has been shown by Johnson and Mendoza V. (1964) to have 9.4 mag of extinction in V; additionally they show that the interstellar extinc- © American Astronomical Society • Provided by the NASA Astrophysics Data System 148 W. MILLER GOSS tion curve is quite similar to that found in the Trapezium. Hall and Iriarte (1964) find about 0.2 mag of polarization in the visual region of the spectrum in this star; they sug- gest that the extinction and polarization occur in regions of space close to the nebulosity. The OH lines in W12 have the largest measured optical depth, TABLE 5 OH Emission Identifica- V AT w Line tion a Avjj Remarks (MHz) Number (km/s) (° K) (kHz) 10. 1667 (10) 29.0+0.3 0.15+0.02 64 ±2 28. 1720 (10) 10.7+0.1 9.3 ±1.0 20 ±2 (1667 (10) 39.1+0.3 2.0 ±0.40 15 ±2 11665 38.8+0.2 4.4 ±0.70 15 ±2 33. 11667 62.6+0.3 0.66+0.09 16 ±2 (1665 62.7+0.2 0.88 + 0.13 22 ±2 1612 (10) 20.8+0.4 0.61+0.06 26 ±2 1612 50.0+0.4 0.49 + 0.03 25 ±2 41 1720 2.1 + 1.2 0.17 + 0.09 17 ±8 1720 94.4+0.6 0.12+0.04 27 ±4 4667 (10) 64.9+0.3 0.75+0.07 22 ±1 1667 73.8+0.3 0.42+0.04 23 ±1 42, 1667 81.0+0.3 0.23+0.02 34 ±1 1612 22.0+0.7 0.41+0.12 22 ±4 1612 64.7+0.7 0.23+0.03 75 ±4 4667 (2) 37.0+0.2 1.0 ±0.20 7.0 + 1.0 1665 36.9+0.2 1.3 ±0.20 7.0 + 0.9 1612 40.3+0.1 7.4 ±0.40 7.0 + 0.3 43, 1665 27.1+0.7 .43+0.17 15 ±5 1612 27.4+0.2 1.9 ±0.30 8.6 + 1 1720 89.8+0.3 0.99+0.09 26 ±2 1720 7.2 + 1.2 .28+0.10 40 ±8 II 4612 (10) 13.5+0.8 .41+0.23 12 ±5 44. 1720 44.1+0.2 2.4 ±0.10 25 ±1 4665 (10) 58.6+0.4 1.7 ±0.20 19 ±2 1612 59.6+0.6 .39+0.15 7 ±2 III 51, 1720 58.3+0.3 1.17+0.10 30 ±1 1720 5.5+0.4 .70+0.13 20 ±3' 75 1665 (10) 0.3+0.4 1.03+0.15 11 ±1 81 1720 (2) -0.4+0.2 .72+0.10 15 ±1 REMARKS TO TABLE 5 I. Low signal-to-noise ratio. II. 10 kHz data. HI. Low signal-to-noise ratio. Weaver et al. (1968) show this line on their 2 kHz profile. © American Astronomical Society • Provided by the NASA Astrophysics Data System 1968ApJS...15..131G 38. . 37. . 43 29. . 22.. 0.14), —11.5(r«0.7),and—32.70.25)km/s.Any OH linescorrespondingtotheseHiwould points out,reliableexpectedprofilesinHiaredifficultto obtainforlarge-diametersourcessuchasW22, and +4.5km/s. have {r)’s«0.005andwouldprobablynotbedetected nearthetwostronglines. km/s iscomparedwiththesumoftwoOHfeatures between+50and+60km/s.AsClark(1965) corresponds totheOHlineat+68.7km/s. 57 W43, andW51. 81 51 the Hilineat+61km/s(fromprofileobservedby MenonandWilliams,privatecommunication) The OHlinesat+7.3and+12.3km/sarecomparedwiththeHiline+12.7km/s;inaddition OH linesat+81.6and+89.1km/sarecomparedwiththeHiline+83.6km/s.Itmaywellbethat 10.. § III.ForthecomparisoninTable6,sumoftwo features foundbyCRWbetween+50and+60 9.. 7.. I. ThecomparisonwiththeHiresultsofClark(1965)isachievedbyaddingfeaturesat+2.5 V. ThisisbasedontheassumptionthatOHhas sameAvlastheHi. VI. ThecomparisondoesnotincludetheweakH-lines at7.9(r«0.1),3.4—4.8~ IV. TheproblemofthecomparisonOHand H iprofileobservedbyCRWisdiscussedin III. TheproblemofthecomparisonOHandHiprofilefoundbyClarkisdiscussedin§HI. H. TheproblemofthecomparisonHiandOHisdiscussedin§III. * Single-dishresults. w © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 1+13.2 /+ 2.3 1+52, +64 /+ 5.8 1+ 5.7 f- 0.2 f- 3.1 +20.3 +94.2 +89.1/ +81.61 +12.3/ + 7.31 +19.6 +10.6 + 5.5 + 5.0 + 3.5 - 1.6 -37.0 -40.9 -46.3 -48.2 (km/s) Loh 1-85.1/ /+ 3.61 1+ 4.5 /+ 2.5 + 4.9 + 3.9 +47.5, +54.9 +83.6 +12.7 +20.7 +18.0 + 6.3 +10.9 + 5.1 +94.0 + 7.2 - 1.3 -38.01 -45.0 -48.0 - 1.5 - 0.4 -37.4/ -41.9 Comparison ofOHandHiAbsorption (km/s) E h REMARKS TOTABLE6 —0.7 —1.5 >1.3 >1 >1.2 >1.4 >1.5 >1.5 TABLE 6 T H 0.65 0.86 0.82 0.35 0.73 0.56 0.7 0.4 0.17 0.27 2.5 6.7 1.61 1.8 1.41 149 Avd (Hi) —20 <12 '20 (kHz) 43 25 30 51 25 27 20 17 15 13 17 14 19 15 17 14 14 19 8.7 7.3 <1.6(—7) <2.7(—8) <1.8(—7) <4.3(—8) <1.5(—7) <1.7(—7) <7(—8) 0.54(—8) 0.95(—8) 4.3(—7) 4.7(—9) 4.4(—8) 9.0(—8) 8.2(—8) 2.8 (-8) 2.4(—7) 7.2(—8) 1.4(-7) 1.5(-7) 1.0(—8) 1.7(-8) l.K-7) Noh/Nh Clark (1965) Clark (1965) Clark (1965) Clark (1965) Clark (1965) CRW* CRW* Clark (1965) Clark (1965) Clark (1965) Clark (1965) Clark (1965) Clark (1965) Clark (1965) Clark (1965) Clark (1965) Clark (1965) Clark (1665) Clark (1965) CRW* Clark (1965) CRW* Reference for Hi marks Re- II I VI III II II IV 1968ApJS...15..131G 44. 38. 28. 22. 12. * Thisratioisobtainedfromthe10kHzprofiles.TheotherratiosforW12are2results. emission nebulosityIC405. ably themosthighlyreddenedstarknown.Thevisualextinctionis « 13.5mag. Becklin (1966).Wisniewski,Wing,Spinrad,andJohnson(1967)report thatthestar(F0la)isprob- 18 16 W © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem § NGC1999inOrionisnearseveralHerbig-Haroobjectsreportedby Herbig(1951). * CIT11isaninfraredstarreportedbyUlrich,Neugebauer,McCammon. Leighton,Hughes,and Î Münch(1964a)findsprominentCHlinesinthespectrumofAEAurigae whichisinthereflection- f Münch(1964J)findsstrongCNlinesinthestarHD206267IC 1396. w (km/s) 42.4 19.8 12.5 19.8 V 9.3 Virgo A NGC 1999§ NGC 2244 IC 405J IC 1396t CIT 11* Source 11667:1612 11667:1720 J1667:1665 /1667:1665 -5667:1612 /1667:1665 11667:1612 fl667:1665 [1667:1720 1667:1665 1667:1720 1667:1665 1667:1612 1667:1665 1667:1612 (MHz) Lines Negative Results Line Ratios \1665 /1667 JT667 \1665 TABLE 7 Line TABLE 8 1667 1667 1667 1667 150 9.0 +1.6 3.12+0.20 3.6 +1.0 3.0 +0.30 3.67+0.32 2.40±0.14 2.70+0.20 3.9 +0.30 2.22+0.21 1.07+0.06* 1.4 +0.30 1.31+0.11 1.3 +0.10 1.66+0.12 1.5 +0.10 Integration Equivalent Ratio of Widths 55^44 16.42 Time 0.88 0.88 0.88 0.88 6.16 2.64 (Peak to Peak) 0.5 0.04 (° K) >20 AT .5 .5 .5 .15 .08 .2 4.3 (+0.5,-0.4) 0.5 (+0.4,-0.4) 3.5 (+0.4,-0.3) 0 0 (+0.5) 4.5 (+1.0,-0.8) 4.1 (+3.4,-1.6) 2.3 (+co,-2.0) 3.5 (+2.5,-1.5) 2.8 (+0.4,-0.4) 5.4 (+0.9,-0.5) 7.3 (+2.0,-1.0) 1.5 (+0.8,-0.8) 10.5 (+2.6,-1.8) t(1667) Upper Limit 0.008 0.015 to + 10 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 153 +8 km/s +8 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem o 154 + 10 km/s +10 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 155 CM O VO d 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 156 0 km/s 0 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 157 km /s 0 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 158 + 50 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 159 + 30 km/s 6 CTi 00 00 ï—i 0 W35 (NGC 6604) 10 kHz © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 1968ApJS...15..131G © American Astronomical Society Provided bytheNASA Astrophysics DataSystem 161 +40 km/s +40 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 162 + 30 km/s +30 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 163 + 50 km/s +50 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem o 164 OJ o i ^r CO uo — +70 km/s +70 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 165 + 73 km/s +73 km/s 1968ApJS...15..131G en o C\J © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 166 + 37 km/s +91 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 167 +73 km/s +73 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 168 + 50 km/s +5° km/s 1968ApJS...15..131G © American Astronomical Society 169 Provided bytheNASA Astrophysics DataSystem 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem < 170 J I1L — ööööooö ro —CT)lOrO—;fO 10 — 6 to o i—r i + 50 km/s +50 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 171 + 50 km/s +50 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 172 - 40 km/s -40 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 173 + 10 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 10 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 175 10 km/s +1° km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 176 0 km/s 0 km/s oo \—I LO \—I O Cas A Relative Intensities © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 177 -20 km/s -20 km/s 1968ApJS...15..131G © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 178 0 km/s 0 km/s 1968ApJS...15..131G (/) OJ Ö CO < < o © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 179 -44 km/s -44 km/s 180 W. MILLER GOSS and 1720 MHz lines would have a peak absorption of ^ Io K. (See § IV for a discussion of the difference between (r) and r.) Table 7 summarizes the line ratios in W12. These results indicate that the assumption of a unique Ts for the four hyperfine levels is incorrect. The value of 3lm 30 29 28 27 26 25 24 23 22 21 20 19 18 17 Fig. 2.—The 18-cm continuum distribution near W22 as measured with the 85-foot Hat Creek tele- scope. The positions of the three sources observed by Mezger and Henderson (1967) are indicated. f) W14 (IC 443) Figure 1.6 shows the 1667 MHz profile in the direction of W14, the supernova remnant IC 443. The velocities are both negative; hence kinematical distances of the clouds cannot be obtained since the source is at ¿ = 189°. The relation between the OH and H i in the direction of IC 443 is ambiguous. Radhakrishnan (1960) finds two velocity features near IC 443 with pronounced self-absorption. On the source itself he finds no absorption due to the continuum source. Locke, Galt, and Costain (1964) and later Akabane (1966) found negative velocity “clouds” in emission at ^ —2 km/s. The OH at —3.5 km/s may refer to this feature. In addition, Akabane has suggested that there is am H i “cloud” at —7 km/s. g) W22 (NGC 6357) Figure 2 shows the 18-cm continuum distribution near W22. The location of the three thermal sources found by Mezger and Henderson (1967) with improved angular resolu- tion are shown. Figure 1.7 shows the four OH profiles. Feature (2) seems to be absent at 1720 MHz. Although the measured © American Astronomical Society • Provided by the NASA Astrophysics Data System OH ABSORPTION 181 agree quite closely; in addition feature (2) has an impossible circular velocity for this longitude. These facts suggest that feature (1) is quite close to the source while feature (2) is close to the Sun. The comparison with the Hi results of Clark, Radhakrishnan, and Wilson (1962; hereinafter referred to as “CRW”) is somewhat difficult. The profile they observed con- tains at least four features; the two major ones agree approximately with the OH lines. There are no OH lines corresponding to the —12.7 and +11.4 km/s H i lines. Fig. 3.—The 18-cm continuum distribution near W43. The plus signs mark the positions where OH profiles were obtained. In addition to the profiles on the source center a 5 X 5 grid (spaced 15' between points) was obtained at the frequency 1667 MHz. Within the accuracy that the signal- to-noise ratio permits (which is not very great) the ratios of the two lines and the measured optical depths are the same over the grid. As Figure 3 indicates, practically all of the grid points fall on a portion of W22. h) W28 Rieu’s (1963) investigation of W28 with a 20' beam has shown that the source con- sists of three parts: +, ^42, Az (M20). These contribute 76, 16, and 8 per cent of the flux, respectively, yli is a non-thermal source with a spectral index of —0.37 ± 0.10. Courtès, Véron, and Viton (1964) have discussed optical evidence which suggests that is a supernova remnant. M20 is 0?4 northeast of the W28 position. Figure 1.8 shows the four profiles. The 1720 MHz emission velocity is +3.4 km/s greater than the 1667 MHz absorption velocity. The width of the 1667 MHz line at + 132 kHz is the greatest observed in this survey; with improved signal to noise this line may be found to consist of several blended features. CRW (1962) mention a weak H i © American Astronomical Society • Provided by the NASA Astrophysics Data System 182 W. MILLER GOSS absorption line at ~ + 6 km/s. Menon and Williams (private communication) find a sharp (—25 kHz wide) H i absorption line at +7.5 km/s. In determining the distance to the OH we have assumed that the distance of W28 is similar to that of M20. The line ratios are summarized in Table 7. i) W29 (M8) Figure 1.9 shows the W29 1667 MHz profile. The optical depth of the OH line is quite small compared with other sources near the plane. In addition to a line at +6 km/s, Clark (1965) finds a line at 0 velocity. Because of the low signal-to-noise ratio of the OH profile, we find it impossible to say whether an OH line exists at this velocity. j) W30 Figure 1.10 shows the W30 1667 MHz profile. The resolution of the distance am- biguity for the OH is impossible (see Note HI A of Table 3). The 158a H velocity is ^2 km/s larger than the OH feature at +33.5 km/s. k) W31 Figure 1.11 shows the W31 1667 and 1665 MHz profiles. The over-all appearance is similar to the W30 profile. Here the 158a H-line velocity is 7 km/s smaller than the +27.6 km/s OH feature. As stated in Note HI B of Table 3 this situation can be interpreted in two ways: (1) the H n region W31 is at the far distance or (2) the OH and/or H n region have a net peculiar motion of — 7 km/s. T) W33 Figure 1.12 shows the four W33 profiles. The velocity of the OH emission at +38.8 km/s agrees quite closely with the 158a velocity (+38.6 km/s). The question marks on the 1720 MHz profile imply doubt as to the identification with features (1) and (2) at 1667 MHz and 1665 MHz. As pointed out in Remark HI C of Table 3 the distance ambiguity cannot be resolved. m) W35 {NGC 6604) Figure 1.13 shows the W35 1667 MHz profile. The OH velocity is 2 km/s less than the 158a H velocity. n) W37 (Ml 6) Figure 1.14 shows the four W37 profiles. In Table 6 the Hi absorption found by Clark (1965) is compared with that found for the OH. In addition to the ^ + 20 km/s feature, Clark finds lines at +2.1 and +35.9 km/s. As in W30, W33, and W35 the 158a H velocity is 2 to 3 km/s larger than the highest-velocity OH feature. ó) W38 (M17) Figure 1.15 shows the W38 profiles. The line ratios seem to be incompatible as Table 7 indicates. There is no 1612 MHz line of comparable magnitude to the 1720 MHz line. At ^ + 20 km/s in the line at 1612 MHz there is some hint of both absorption and emis- sion lines. The OH absorption velocity is ^ 1 km/s greater than the 158a H velocity. The agreement with the H i profile determined by Clark (1965) is only partial. The H i profile has two strong lines—one at +6.9 and the other at +20.7 km/s. The +6.9 km/s line has no obvious counterpart in OH. p) W41 Figure 1.16 shows the OH profiles for W41. The features with question marks have low signal-to-noise ratios. There is a suggestion of absorption in 1612 MHz at +52 and +95 km/s which is not associated with absorption in the main lines. W41 is the only © American Astronomical Society • Provided by the NASA Astrophysics Data System 1968ApJS...15..131G hms 5 / hms points. Neartheextremities ofthesourceratiolow-velocity tohigh-velocity lines increaseswhilethe measuredopticaldepth,(r),inthelow-velocity lineremains found inthecontinuumsurveytobeata(1950) =184458,5(1950)—2°02'.The essentially constantasseen inFigure1.20.(Thedesignationssignifypositions displaced of theabsorptionlines at ^+10,and+90km/sremainconstantover theinnergrid measure is^10.Mezger’s(privatecommunication) investigationsat2cmwitha2' measured velocity.)Ifweassumetheelectron temperature andsizeoftheHnregion circled. Theemissionat+37km/sisslightlydisplaced fromthecontinuummaximum are indicatedbycrossesinFigure3.Thecenter ofthegrid(theWesterhoutposition)is under discussionarethesameasfor+97km/s Hnregion,themaximumemission 5(1950) =—2°10±10'. Withinthelimitationsimposedbynoise, opticaldepths emission seemstobeconcentratedthesouth ata(1950)=184500±40and region is14.0±0.3kpc.(Theerrorconsidered tobecausedonlybytheerrorin km/s forthefarpartofSagittariusarm. Thus, thedistancechosenforthisHn H-line is0.5km/slessthanthatfoundforfeature(3). in thedirectionofW43showsemissionat+40km/s;thisisconfinedlatitude results ofKerrmentionedbyMezgerandHöglund(1967).findsthattheHiprofile H iiregionanda158aH-linefromannat+44km/s.Nohydrogenlinesofthe velocity (MezgerandHöglund1967;Kerr,privatecommunication).Figure3shows suggests thatthesetwolinesareatthenearpointofl=39°arm;hencecloser sion linesareband-width-limitedwiththe10kHzfilters.Thepositionof+45and noise ratio;theemissionistreatedastwolinesat+22and+65km/s.The1667km/s has beendiscussedinNoteIIIDofTable3.Theradialvelocityderivedfromthe158a beam haveindicatedthatW43consistsofseveral components. can probablybeassociatedwiththeHnregionat+44km/s. line at+97km/s,twoadditionallinesarepresent:the158aHe-lineofkm/s MHz emissioninW43. MHz profilenear+90km/sareshown.Theemissionlineat1720 four lines.Inadditiontothefeaturesindicatedbynumbers,thereareabsorption without mainlineabsorptionatthesamevelocity.Thedistanceproblemforthissource case foundduringthissurveyinwhichthereisanypossibilityofsatellitelineabsorption the methodsuggestedbyWeaver(RemarkHIAofTable3)indicatesavelocity+44 and henceisbelievedtobelocatedinthefarpartofSagittariusarm.Inaddition, has a=26kHz;thisemissionlineisquitebroadcomparedwiththe1665and1612 detail inFigure1.19.The1667,1665,and1612MHzlinesnear+40km/sthe1720 distances areused. at ^+68km/sinthe1667and1665MHzlines.Theemissionlinesareshownmore tion marksindicatedoubtabouttheassociationofdifferentfeaturesforeach +52 km/sabsorptionlinesinthev,ldiagramderivedbyWeaverforneutralhydrogen emission istreatedasthreelinesat+65,+75,and+81km/s.Undoubtedlytheseemis- the 18-cmcontinuum. tance ambiguityisresolvedsinceHiabsorptiondoesnotextenduptothetangential e(A7z =5)seriesorstrongerappearinthisfrequencyrange.Thusfeatures(3)and(4) The 158aH-lineinthedirectionofW43isshownFigure4.Inadditiontostrong As inW22,a25-pointgridat1667MHzwasobtained; thepositionsofgridpoints Information aboutthedistanceof+44km/sHnregionisprovidedbyi Figure 1.17showsthefourW42profiles.The1612MHzprofilehasalowsignal-to- W43 isalargeHnregioninthel=32°spiralarmatdistanceof7.5kpc.Thedis- In Figure1.18the10kHzprofilesindirectionofW43areshown.Hereques- © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem OH ABSORPTION183 q) W42 r) W43 184 W. MILLER GOSS from grid center, e.g., E1/S2 means 15' east and 30' south. The antenna temperature in the continuum, 7c, is given at each point. This is Ttot in the notation of Table 3.) This behavior can be caused by either or both of two effects: (1) The high-velocity (and hence more distant) OH may have a smaller angular size than the low-velocity OH. (2) Near the extremities the main contributor to the continuum is the background ; hence the percentage of the background absorbed, K(D) (§ IV), for the nearby OH is larger than K(D) for the distant OH. Thus the local OH absorption is favored. This point is discussed further in § III?. Clark (1965) has obtained an H I absorption profile for W43. The agreement in shape between this H i absorption and the OH absorption found in this study is only partial. The profile observed by Menon and Williams (private communication) shows much better agreement in velocity and in shape; e.g., they find H i lines at +12.9, +61, +81, and +91 km/s. s) W44 The properties of the continuum source W44 have been summarized by Hollinger and Hobbs (1966). They have shown that W44 consists of a non-thermal source and an H n Fig. 4.—The 158a H- and He-lines in W43. The dots are the observed points and the heavy line is a Gaussian fit to the helium and hydrogen lines. © American Astronomical Society • Provided by the NASA Astrophysics Data System OH ABSORPTION 185 region. The former shows shell structure and is presumably a supernova remnant. At 18 cm the non-thermal source contributes the major portion of the flux. Figure 5 shows the 18-cm continuum map. Figure 1.21 shows the four OH profiles in the direction of W44. As in W28 the main lines appear normal while the satellite lines appear in emission; the +14 km/s line at 1612 MHz and the +44 line at 1720 MHz appear in emission with the latter the stronger. The line ratios are summarized in Table 7. The resolution of the distance ambiguity for the OH lines follows from a consideration of the properties of W44. The diameter to half-power of the continuum source is ~ 30' from the map prepared by Hollinger and Hobbs (1966). We assume that W44 has the same intrinsic properties as two other old supernova remnants, IC 443 and the Cygnus Loop. The resultant distance for W44 is then: 3 kpc (IC 443) and 4.6 kpc (Cygnus Loop). If the OH line at +42 km/s were at the far point (+13.5 kpc), W44 would be beyond this distance and therefore would have an intiinsic power at 400 MHz greater than 100 times IC 443 and 400 times the Cygnus Loop. Thus the choice of the near point for the OH is made (e.g., 3.0 kpc for the +42.4 km/s feature). Since the l = 39° arm at « +70 km/s and at a distance of «5 kpc does not appear in absorption, W44 is probably in the Sagittarius arm at a distance of « 3 kpc from the Sun. This example shows that in some cases it is possible to use the OH absorption lines to determine the distances to continuum sources. For large sources, this method may be more reliable than the use of H I absorption lines for which there is a difficulty in obtain- ing the expected emission profile. Figure 1.22 shows the 1667 MHz profile obtained at Z = 36°, 6 = 0° which is ^ 1?5 northeast of W44. The antenna temperature of 5° ± 3° K was obtained from an extrap- olation of the W44 continuum map. This value agrees with Rieu’s (1963) continuum survey of this region at 1430 MHz. Furthermore, Rieu shows that there are no discrete sources within 30' of this position. Thus the continuum is due primarily to the galactic background. The velocities of features (1) and (2) (+13.1 and +40.0 km/s) agree approximately with the velocities in W44 (+12.5 and +42.4 km/s). The optical depth in (1) is ap- proximately the same as in W44, while the optical depth in (2) is very uncertain (^7 times smaller). The fact that this effect (the background effect) is also seen in W43 (§ IHr) and W51 (§ IILf) in places where the continuum is predominantly due to the galactic background, leads to the choice of option (2) mentioned in § Illr. Thus for this source the percentage of the background absorbed for feature (1) is ^ 1 while this per- © American Astronomical Society • Provided by the NASA Astrophysics Data System 1968ApJS...15..131G file forW44.Althoughthereissomeproblemwiththeexpectedprofile,threelinesare matical distances—(1)1kpcand(2)3kpc. seen at+12,+30,and+42km/s.ThisisinapproximateagreementwiththeOHprofile. tangential pointofthel=39°arm,andhenceitisimpossibletoresolvedistance centage forfeature(2)isThesevaluesareroughlyconsistentwiththeirkine- four thermalcomponentsandonepossiblenon-thermalsource(seeRemarkIXtoTable3). ambiguity forthisfeature. 186 W.MILLERGOSS Mezger andHenderson(1967)areindicated. Figure 6showsthe18-cmcontinuummap;twomajorthermalcomponentsfoundby and Henderson(1967)areindicated. behavior asinW28andW44,i.e.,absorptionat oneofthesatellitesandemissionat cussed indetailbyWeaveretal.(1968),whopresent 2kHzprofilesoftheemissionnear + 60km/s.Theemissionlinesnear+60km/s seem toariseoutoftheabsorptionlines cates. Theapparentexceptionisthe0/N2profile (andtheneighboringpoints).As shape. Theyfindthreelines at~+6,~+48,and«61km/s;inaddition theover-all other. form isquitesimilarto the OHcase. H iprofilefoundbyCRW (1962)isnotverygood.TheHiprofileobserved byMenon and Williams(private communication) showsimprovedagreement in velocityand Figure 6indicates,amajorcontributortotheflux atthesepositionsisthethermalsource (2) and(3).Atfrequenciesof16121720MHz, feature(1)showsthesametypeof G49 —0.4whichisat6.5kpcaccordingtoMezger andHöglund(1967). §§ Illrand111$,whichemphasizesthenearbyOH, isquiteobviousasFigure1.25indi- Menon andWilliams(privatecommunication)haveobtainedanHiabsorptionpro- Figure 1.23showstheW471667MHzprofile.TheOHlineat+83km/sisnear The continuumsourcehasbeenshownbyMezgerandHöglund(1967)toconsistof Fig. 6—The18-cmcontinuumnearW51.ThepositionsoftwothethermalsourcesfoundbyMezger A twenty-fivepointgridwasobtainednearW51. Thebackgroundeffectdiscussedin Figure 1.24showstheOHprofilesindirectionofW51.Thissourcehasbeendis- As inthecaseofW43, agreementinshapeandvelocityoftheOHprofile andthe © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem u) W51 t) W47 OH ABSORPTION 187 v) W57 (Cyg A) Figure 1.26 shows the four Cyg A profiles. The small optical depths in OH are a reflec- tion of the small optical depths in the H i absorption in Cyg A found by CRW. The —84 km/s feature (velocity marked by circled point on 1667 MHz profile) seen in H i does not have a counterpart in OH whose strength is the same order of magnitude as the +3.5 km/s line. Allowing for the band width-limited nature of the 10 kHz profiles, r ( —84) < 0.003. Table 6 indicates that there is a variation in Nob./Nb. or in T# for OH in this direction. w) W66, W67, W72, W73 (Cygnus X) Figures 1.27,1.28, and 1.29 show the Cygnus X sources exclusive of W75. The optical depths are not accurate because of the problem of correcting for the background in Cygnus X. Reference to Table 3 indicates no correlation between the 158a H velocity and the OH velocity. x) W75 The region of W75 has been studied at 5000 MHz by Downes and Rinehart (1966). Figure 7 shows the 18-cm continuum with the DR 21 and DR 23 positions marked. Fig. 7.—The 18-cm continuum near W75. The positions of the nearest Downes and Rinehart (1966) sources are shown. Figure 1.30 shows the four OH profiles. The emission line in 1665 MHz has been discussed in detail by Palmer and Zuckerman (1967) and Weaver et al. (1968). As in W51 the emission line rises out of two adjacent absorption lines. Here the velocities of the 158a H-lines from DR 21 and DR 23 (Dieter 1967) agree with the velocity of the positive- velocity OH feature. Comparison with the H i absorption found by CRW (1962) is un- certain owing to the problems with the expected profile of the H i in the direction of this source. y) W80 (NGC 7000) Figure 1.31 shows the OH profiles in W80. The 158a H-line velocity is 0.8 km/s greater than that derived from the OH line. z) W81 (Cos A) Figure 1.32 shows the Cas A profiles taken with a frequency resolution of 10 kHz. The expected ratios of 1665 MHz (5), 1612 MHz (1), and 1720 MHz (1) relative to 1667 MHz (9) for the optically thin and thermal equilibrium case are shown by the dotted © American Astronomical Society • Provided by the NASA Astrophysics Data System 188 W. MILLER GOSS lines. (The frequency dependence of the optical depth and antenna temperature are neg- lected.) The main lines seem to be in the correct ratio while the satellite lines are not; the line at 1612 MHz is too strong while the 1720 MHz line is too weak. There is some hint of emission at the high-velocity side of the Orion arm feature in the 1720 MHz line. Figure 1.33 shows the profiles taken at 2 kHz resolution which show absorption at veloci- ties characteristic of the Orion arm. The line at 1612 MHz is apparently single although there is some hint of a second line. The emission at 1720 MHz is a confirmation of Rogers and Barrett’s (1967) result. The question marks indicate some doubt in the matching up of the velocity features of the four lines. Figure 1.34 shows the lines arising from the Perseus arm. Matching up the 1720 MHz lines with the other frequencies is difficult owing to the low signal-to-noise ratio of the 1720 MHz profile. The 1612 MHz profile appears very similar to the main lines. The comparison of OH and H i is given in Table 6. In the Perseus arm the apparently large range of Noh/N-r (~ 100) may be caused partial- ly by difficulty in separating the strongly blended H i lines. aa) Negative Results Six sources which have no detectable OH are interesting from several points of view. These results are summarized in Table 8. The upper limits in optical depth were derived by applying a band-width correction factor of 2. (This empirical factor was derived from the sources for which 10 and 2 kHz observations are available.) These limits are given only for the two sources for which the antenna temperatures of the continuum were measured. The limit in optical depth derived for W16 seems to be significantly smaller than in the other sources near the plane. The Virgo A result is not surprising owing to the high latitude of this source. The remaining sources are discussed in the notes to Table 8. IV. EQUATION OF TRANSFER FOR OH The solution to the equation of transfer appropriate to the OH case differs somewhat from that derived by Hagen, Lilley, and McClain (1955) for neutral hydrogen. Modifica- tions of their solution are made necessary because of the importance of the isotropic background radiation (§ V) in establishing the excitation temperature of the OH A doublet. Following Hagen et al., we consider a simplified case. The quantities have the follow- ing definitions : Tal = antenna temperature in the line Taol = antenna temperature outside the line ATA = measured antenna temperature for frequency-switched receiver {Tal — Taol) Ts = excitation temperature of OH Tbg = brightness temperature of the galactic background Tbc = brightness temperature of the discrete source Tbb = brightness temperature of isotropic background radiation K(D) = fraction of T## absorbed (i.e., this fraction is beyond the OH at distance D) G(di) = antenna gain function in direction of cloud i, normalized to unity at 0 = 0 & mb = main beam solid angle £2oh y = solid angle of the 7th OH cloud which lies in front of the discrete source (j = 1, 2, M) ^OHi = solid angle of ith OH cloud which does not overlie the discrete source (i = 1, 2,...,N) Plc = solid angle of the source n = optical depth of ith cloud Tj = optical depth of 7th cloud © American Astronomical Society • Provided by the NASA Astrophysics Data System 1968ApJS...15..131G partially coversthesource,portiononsourceistakentobeoneofcloudsj, TVlM- ={r[1-e-^M]+[TK(D)T}e-uM} discrete sourceisopticallythininthecontinuumrange1612MHzto1720MHz. Hence, while theportionoffsourceistakentobeoneofcloudsi.(4)Thequantitiesfíon are made:(1)AllthecloudshavesameTg.(2)Noshadowingexists.(3)Ifacloud The antennaispointedtowardthediscretesource(0=0).followingassumptions where rjisthebeamefficiency.When~&mb, fifth termrepresentstheemissionfromthatportion ofthegalacticplanewhichliesin angle muchgreaterthanSImb-(6)K(D)isthesameforNandMclouds.(7)The and Qcare<&.£Imb»(5)Thegalacticbackgrounddistributionisuniformoverasolid emission fromtheMcentralclouds.Thethirdand fourthtermsrepresentthecontinuum which donotcoverthediscretesource.The second termrepresentsabsorptionand front oftheOH. emission fromtheportionsofcontinumnsources notcoveredbyOHclouds.The where (R isthefractionofsourcecoveredbyOHclouds.) sbbbg B The firstterminequation(1)representsemission andabsorptionfromtheJVclouds For anobject,X,muchsmallerthanQ,mb,itisassumedthat Since Vb ^mb © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem + T(1~R) + X + [1-K(D)]T, bc BG 3 =1 m AT a{v) T AOL Ooh= and Vb í2oh,¿£( Oj) Vb &MB ¡-^-[Ts-Tsc-TtB-KiDlTBom- e-'^l, i=l N Umb Qmb Umb Oqh &MB Q lTs[l-e-rj^] +[TTK(D)T}e~^) [T-TK(D) ][1-e~r^] T ~\~T-\-T BCBBBG sbbg bgbbb lT-K(D)T]i^~-) BBBG OH ABSORPTION189 Tax_&xTbx 1 Vb &mb Tax VB = T BX • \ ¿¿MBMMB/ — ûc M (1) (2) 1968ApJS...15..131G v v where and 190 R. Generally,theassumptionismadethatRunity.Inthiscasewemeasureamean If nodiscretesourceexists,thesecondterminequation(3)iszerosinceOc=0this where Tacistheantennatemperatureofsource.Aprioriwehavenoknowledge case. optical depth(r)whichisaveragedoverSic,he., tures. Forthiscaseanabsorptionlineisobserved: Thus (t)(v)=Rt(v)iftheopticaldepthsare<3Cl. ing opticaldepthsproposedbyWeaver(1967),comparedtotheusualmethodinvolving the antennatemperatureofsource.IfATaX»)isline1 differs atthetwofrequenciesandifTo^0.)Since if Tacisthesameatviandvi.(SeeWeaver[1967] foradiscussionofthismethodifTac defines auniquefunctionofti(v).Amoreaccurate methodofdeterminingn(0)isto assumed) andifTisthesameforeachline(i.e., thermalequilibrium),ATaX)/aX) (say, 1667MHz)andATaX)thatofline21665: use theratioofequivalentwidths(Weaver1967): (¿ =l2),where(RI)kistherelativeintensity oflinek(aDoppler-broadenedis s 5 Since T=Tbb-^t(§V),equation(2)thenbecomes In manycircumstancesweexpectTtodominatetheotherbrightnesstempera- Equation (4)indicatesthegreatadvantageofusinglineratiomethoddetermin- s BG © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem hT a{v)Ooh Vb RSI Q St MB &T(v) =-TcR[S-e-^], &MB aA j —oo J —nn We assume statistical equilibrium and solve the resulting eleven population equations for the twelve levels for a variety of hydrogen densities and kinetic temperatures. The excitation temperature, Ts, is calculated by * k \tí(N2/N1) ’ where v = 1666 MHz and N2 and N\ are the number densities of the upper and lower A doublets, respectively. Figure 9 shows as a function of riB. for four kinetic tempera- tures. Fig. 9.—The excitation temperature as a function of the hydrogen density for kinetic temperatures of 50°, 100°, 500°, and 1000° K. The processes included in this calculation are radiative and collisional processes between the two A doublets and radiative and collisional processes among the rotation levels of the 2n state. The effect of including the rotational radiative and collisional transitions is negligible compared to processes (1) and (2). For example, for a kinetic temperature of 100° K and a density of 10, the Ts including the rotational transitions is only »2 X 10-4 ° K less than if these transitions are neglected. The effect of the rotational transitions is negligible for three reasons: (1) the collisional excitation rate between the two A dou- blets is typically in excess of an order of magnitude larger than the rotational collisional excitation rates; (2) the radiative absorption rate, JB, is largest for the A-doublet 2 transition; and (3) three-step processes from the / = f and J = f( ni/2) rotational levels are much less probable than two-step processes involving the J = ■|(2Il3/2) and J = J(2Hi/2) rotational levels. Although the radiative absorption rate due to the ultraviolet is three orders of magni- tude less than the far-infrared, the ultraviolet transitions are much more efficient in the transfer from one of the A doublets to the other. If there were no collisions and no 18-cm radiation the far-infrared radiation field would lead to a Ts~ 1.0 X 10~2 °K while the ultraviolet would lead to a Ts ~ 3 X 10~2 ° K. The greater efficiency of the ultra- violet follows from the large number of possibilities for the cascade down from the 22+ state. For example, the three transitions from the lower A doublet can leave the 22+ © American Astronomical Society • Provided by the NASA Astrophysics Data System 1968ApJS...15..131G 2+ 2 5 2 -4 2 3 2+ -19 region whichincludestheA2-XIItransitions,excitesanHnregion.Theradiusof state byelevenpossibletransitionstovariousexcitedrotationallevelsintheIIi/2and H nregionweassumetobe5X10.Thesevaluesaretypicalofthestrongregions lifetime againstspontaneousemission(«10sec)ismuchlessthantheofan population equationsforthetwentyenergylevelsshownbyLitvaketat.(1966)are contribution oftheHnregiontoisotropicbackground.Ifweassumeaspherical manner discussedbyLitvaketal.(1966)isneglected. (4)Weassumeforthepurposesof neglect ultravioletabsorptionfromtheexcitedrotationallevelsinIstatesince is thatat18cmwithT^y)givenbyequation(9).(2)ForZ)>5X10pc,wecan brecht (1965)isattained.WeadoptasomewhatartificialmodelinwhichJgivenby from themaximumvalueofemissionmeasure,EJf,as represents theradiusofHnregion,andTmaximumbrightness investigated byMezgerandHenderson(1967). the staris18Ro;radius,L,ofHnregion5pc.Theemissionmeasure tion thatwouldresultfromtheultravioletand18-cmradiationheldalone.Todiscuss peter andCarroll1966).Hence,theionization anddissociationratesarequitelarge solved underthefollowingassumptions:(1)Theonlymicrowaveradiationconsidered where Disinparsecs.When34pc,theinterstellarradiationfieldfoundbyLam- nebula withauniformwecanwrite this caseweconsiderthefollowingmodel.An05starwhichhasacolortemperatureof Il3/2 states(seetheenergy-leveldiagramofLitvaketal.1966). with X<2000Â.Inaddition,theOHhasan ionizationthresholdof«13.2eV(Sal- if r=5X10°Kandthenebulaisopticallythinincontinuum. temperature oftheHnregion.FollowingMezgerandHenderson(1967)isfound where y=D/L.HereDrepresentsthedistancefromHnregioncenter,whileL 70000° Kintheultravioletandwhichhas,weassume,aflatspectrumoverspectral 196 ultraviolet. ForlargedistancesfromtheHn region,theexcitationtemperatureis as P.Solomon(privatecommunication)pointsout, theOHcanbedissociatedbyphotons absorption tothe2state.(3)Selective amongtheOHtransitionsin equation (10)forD<34pcand/=5X10>pc.Thesteady-state region thecollisionswith theprotonswouldtendtooverridethisanti-inverting tend- acting alone,Tswiththeultravioletand microwave andTs[T(y)+3]withno a functionofDisshowninFigure10.Threecurves areexhibited:Tswiththeultraviolet within theHnregion. this calculationthattheOHisnotphotodissociated withintheHnregion.However, the stronganti-inverting tendenciesoftheultravioletradiationfound byLitvaketal. are negligible.Fordistances lessthan30pc,T>3°K.Fordistances lessthan1pc B (1966) areevident.(However, eveniftheOHhadanappreciablelifetime intheHn e «3° Ksincetheeffects of theultravioletand18-cmcontinuumfrom theHnregion R R To illustratetheeffectofultravioletradiationheld,wenextconsiderexcita- The 18-cmradiationfieldcontributiontoisgivenhyTrTb,where\sthe The ultravioletradiationfromthestarcanberepresentedby Under theseassumptions,wesolvethenineteen-equation system.Theresultingas BR © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem 3 T= 1.28X10~EM, b J2 W. MILLERGOSS D 6 5.7X10- (10) 1968ApJS...15..131G -1 2+ 2 2 9 fixed. Forexample,forL—0.5,1,and10pctheheightsofmaximumin(which pendent ofdistance.Obviously,theexactshapeultraviolet+microwavecurve is sensitivetoachoiceoftheradiusHnregionifotherparametersremain doublet excitationforregionsfardistantfromHnregions.Onlyclosetothe occurs at~L)are1.5°,5°,and160°K,respectively. the Adoubletaremuchslowerthanultraviolettransfer,andTsisessentiallyinde- ency oftheultraviolet.)For The OH also agrees with the spiral arms as delineated by H n regions. In many cases for which an OH absorption line is found in the direction of a galactic H n region the velocity of one of the OH absorption lines and the velocity of the nebula agree quite closely. On a smaller scale, the structure of the OH seems to be continuous. In many cases two sources several degrees apart have OH absorption at about the same velocities. For example, NGC 6334 (Weaver et at. 1968) has lines at —4.0 and +6.1 km/s, while W22, which is 2° away, has lines at —3.1 and +5.7 km/s. (The separation of 2° corresponds to a distance of 30 pc for the negative velocity feature.) The velocities agree and feature (1) (Fig. 1.7) has the same appearance in both sources. (Gardner, McGee, and Robinson 210° 180° 150° 120° 105° 90° 75° 60° Fig. 11.—Distribution of OH projected on the galactic plane as determined from the absorption lines. The points represent the projected density divided by the excitation temperature (cm-2 0K_1). Cases for which the distance ambiguity cannot be resolved were not used. [1967] have also pointed out this agreement.) However, feature (2) is much sharper in source W22. Similar velocity agreement over a l?5-2° interval in longitude is evident in W30-W31, W37-W38, and W44, l = 36°, b = The distribution on a scale of the order of a beam width (30') is also continuous. For those sources for which profiles were obtained in a Io X Io grid around the continuum center, the OH optical depth is roughly constant over this area. This behavior contrasts sharply with the small scale of the emission (see Davies, Rowson, Booth, Cooper, Gent, Adgie, and Crowther 1967). On a scale of the order of minutes of arc the OH may well be characterized by dumpi- ness. Although the application of the equivalent-width ratio method is limited by the lack of a unique excitation temperature, Table 7 indicates that in many cases the inferred optical depth is two orders of magnitude larger than the measured optical depth. Concentration of the OH into clumps is consistent with this result. This type of small- scale structure has also been proposed for the galactic center OH by Robinson, Gardner, van Damme, and Bolton (1964). © American Astronomical Society • Provided by the NASA Astrophysics Data System 200 W. MILLER GOSS Weaver et aL (1965) have pointed out that in many cases (W3, NGC 6334, W51, and W75), an OH absorption line appears with a velocity of ^10 km/s greater than the emission line. That this is not a general pattern can be seen from the Orion A (1667 MHz), W42 (1667 MHz), and W33 (1667 and 1665 MHz) profiles where the emission is at the high-velocity side of the absorption. The existence of both possible cases strongly suggests that the relation of the OH absorption and emission velocities is kinematical and not physical, i.e., the pattern is caused by the relative locations of the emission (at the H n region) and the absorption (closer than the H n region). If we assume that the emission is associated with the H n region, the expected patterns of emission and absorp- tion agree with the observations in all cases except W51. Here, it may well be that the souce is just beyond the tangential point and that the absorption is closer to the tangen- tial point than the emission. Thus, in this case the absorption would have the higher velocity. All four lines of the multiplet have been detected in W10, W12, W22, W28, W41, W43, W44, W51, and W81. Excluding the sources with 1665 MHz emission (W10, W51, and parts of W43), these sources show normal intensity ratios in the main lines 1667 and 1665 MHz. Except for W12 and W22 all of the sources show emission in one and/or both of the satellites lines 1612 and 1720 MHz at velocities at which absorption components appear in the main lines. The 1720 MHz emission seems more prevalent and the lines appear to be significantly sharper than the absorption lines. In most cases the velocities of the satellite emission lines are quite different from the velocities of the H n regions which are background sources; hence this emission does not appear to be associated physically with the background source. This is the first evidence for anomalous OH emission which is not physically associated with an H n region. For example, the Cas A 1720 MHz emission appears to come from the Orion arm at a velocity corresponding to a distance less than 200 pc, while Cas A itself is 3.4 kpc distant. These results suggest that 1720 MHz emission may arise from regions where no discrete sources exist in the line of sight. Weaver (private communication) has investigated several regions which have no discrete sources near the Sagittarius arm tangential point at / ^ 50°; in two cases he finds 1720 MHz emission. It is interesting in this respect that in the ultraviolet pumping theory of Litvak et al. (1967) the 1720 MHz line is the first line to go into emission as the optical depth in the ultraviolet increases. The absorption is also anomalous; no cases have been found which have intensity ratios which are compatible with a single excitation temperature for the A doublet. It is interesting in this respect to see if Weaver’s (1967) suggestion of a method of enhance- ment of the 1612 MHz line with respect to the 1720 MHz line under conditions of thermal equilibrium can explain any of the observations ; the conditions for the applica- tion of the method are that To ^ 0 and that Tac be <10 TV However, two possible cases of 1612 MHz enhancement (W22 and Cas A) cannot be explained by this scheme. In several cases the 1720 MHz line is stronger than the 1612 MHz line: W12, W33, and W38. Although it is to be expected that the excitation temperature is strongly influenced by the 18-cm radiation field, we conclude that only order-of-magnitude estimates for the number densities of OH are possible owing to the uncertainty in the excitation tempera- ture of the A doublet. The investigation of this problem was first suggested by H. F. Weaver. It is a great pleasure to acknowledge the helpful criticisms of H. F. Weaver, N. H. Dieter, G. B. Field, and C. E. Heiles. Also, I would like to thank D. R. W. Williams, W. T. Lum, W. J. Welch, W. D. Gwinn, H. Spinrad, R. Minkowski, D. D. Cudaback, B. E. Turner, A. F. Setteducati, A. Ebert, M. Issel, and W. Taylor for their assistance. Support of this work by the U.S. Office of Naval Research under contract Nonr 222(66) is gratefully acknowledged. © American Astronomical Society • Provided by the NASA Astrophysics Data System OH ABSORPTION 201 Note added in proof.—Mr. P. Palmer (private communication) has pointed out that the earlier search for the 2IIi/2 (/ = |) Unes of OH (§V) was made at frequencies which are in disagreement with recently measured laboratory values. REFERENCES Akabane, K. 1966, Pub. Astr. Soc. Japan, 18, 96. Allen, C. W. 1963, Astro physical Quantities (London: Athlone Press). Arpigny, C. 1964, Ann. Pap., 27, 393. Baars, J. W. M., Mezger, P. G., and Wendker, H. 1965, Ap. J., 142, 122. Barrett, A. H., and Lilley, A. E. 1957, A.J., 62, 4. Becker, W., and Fenkart, R. 1963, Zs. f. 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Meeting, Washington, D.C. Wisniewski, W. Z., Wing, R. F., Spinrad, H., and Johnson, H. L. 1967, Ap. J. (Letters), 148, L29. Zuckerman, B., Palmer, P., and Penfield, H. 1967, Nature, 213, 1217. © American Astronomical Society • Provided by the NASA Astrophysics Data System