1982ApJ. . .255. .564D The AstrophysicalJournal,255:564-576,1982April15 © 1982.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. in adarklanecrossingthenebula.Herbig(1956,1971) in Perseusmeasuringabout10'acrossontheredprint We willusethisestimateincalculations inthispaper. Pease (1917)whodrewattentiontoafaintcentralstar of thePalomarSkySurvey.Itwasfirstphotographedby region (S222),andassuch it wasincludedinseveral underestimate ifthesestarslieabovethemainsequence. stars, Herbig(1971)estimatedthedistancetoNGC searches forradioemission.Weak emissionat1.4GHz in redlight(Herbig1971)whichshowsthestarclearly. The reflectedemissionshowsonlytheHandKUnesof rich inemissionUneswhichmatchedthoseofthenebula. (A~\l mag),possessedaspectrumbrightinHaand unidentified spectralfeatures.Figure1isaphotograph found thatthisstar,LkHalOl,thoughheavilyobscured Ca iiinabsorption.Inthenear-infraredHerbig(1971) 1579 tobe800pcbutnotedthatthismayan found manynarrowemissionUnesandanumberof v NGC 1579isanirregularlyshapedreflectionnebula From UBVmagnitudesoftwonearbynebulousB Sharpless (1959)cataloged the nebulaasanHii LkHa 101isspectroscopicallyaveryunusualstar. © American Astronomical Society • Provided by theNASA Astrophysics Data System -1 _I 20- velocities —4to6kms(LSR)withanabsorptioncomponentonmostprofilesnear1.The 2'X3'5 by0.67kmsinradialvelocity)showacontinuumsourceof176mJycenteredonthestar greatest extensionoflow-brightnessemissiontothenorthwest.Thegasisdistributedoverradial obscured emission-linestarLkHa101.Newaperturesynthesisobservationsat21cm(resolution dark cloud. and ~85Mofatomichydrogeninasurroundingcloud~3.5pcacrossattheassumeddistance Subject headings:nebulae:individual—reflectionradiosources:21cmradiation other evidencethatthestaranditsHniregionsarenewlyformed. associated withmorediffuseHnregions,noexpansionoftheatomicgasisdetected.Thisreinforces from thecentralstarindicatethatdensityinmediumismuchhigheronnearsideof absorption, ofopticaldepth~3,maybeduetoacoolHishellonthenearsideassociated 800 pc.ThehighestcolumndensitiesofHi(13X10cm)arenearthestellarpositionwith star thanontheother,inagreementwithobservedextendeddistributionofCO.UnlikeHizones the samedirections,whichindicatesthatthereisnodetailedmixingofCOandatomicgas. 0 _1 The reflectionnebulaNGC1579issituatedinadarkcloudandilluminatedbythehighly The widthsoftheHiemissionprofiles(~7kms)aremorethantwicethoseCOin Models oftheHiemissioncloudbasedontwo-stepdissociationbyultravioletradiation 2 I. INTRODUCTION THE HiCLOUDSURROUNDINGEMISSION-LINESTAR Dominion RadioAstrophysicalObservatory,HerzbergInstituteofAstrophysics stars: early-type—emission-line LkHa 101INTHEREGIONOFNGC1579 Received 1981July15;acceptedOctober5 P. E.DewdneyandR.S.Roger ABSTRACT 564 (1972), butnofluxdensityorsizewasgiven.Spencer by CohenandDewhirst(1970). Itsinfraredcolorindex very compactradiosource(
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35° 05
4h 27m 4h26m 4h27m 4h26m Right Ascension Right Ascension
Using this equation and keeping the observable quan- tities constant, one can generate a family of profiles, each member of which has a different spin temperature. With sufficiently accurate measurements it would be possible to determine Tsf and oT by fitting the profiles to the observations. In the present case the profiles are very noisy, and it is possible only to derive rough limits. The profiles do not seem flattened enough to allow Ts{ to be much higher than 11 K. This corresponds to a peak optical depth of 3 and a velocity half-width of 3 km s-1 -1 (aT = 1.29 km s ). These parameters lead to an estimate of mean column density for the absorbing gas, 1.9 X 10 20 cm-2. We suggest that the absorbing gas is actually associ- ated with the region containing NGC 1579 for the following reasons: 1. The absorption feature is seen only in the emission spectrum of the cloud associated with LkHa 101 and not against the extended emission component (see § TV a). This implies that it is beyond the bulk of local H Fig. 4.-32' X32' maps similar to the front face of Fig. 3 but i in this velocity range. integrated over the three velocity ranges AKa, AK¿,, AFC (cf. Fig. 3). 2. There is widespread optical obscuration in the AKa and AKC cover the bulk of the emission on either side of the “velocity gap” which itself is the range “AK¿.” The contour region of NGC 1579, an outline of which, derived from intervals are 0.375XIO20 cm-2 equivalent column density or 50 star counts on a photograph (Herbig 1956), is shown in Kkrns“1. Figure 6. The dark cloud is elongated N-S, but the H i emission appears associated with an E-W bulge in the obscuration. That some of this concentration of dust where and gas is on the near side of the nebula is suggested by 2 2 t( t>) = wexp( - ü /2ctt ), fine scale obscuring features crossing the nebula itself, gas may be seen in Figures 1 and 7. Tbo(v) is observed brightness temperature profile, Tbb 3. The absorbing gas and LkHa 101 itself (Herbig -1 the observed background brightness temperature, 7¡f is 1971) share the same radial velocity (+ 1 km s ), about the foreground spin temperature, r(v) is the optical 6.6 km s-1 more positive than that predicted by the depth at velocity v, and © American Astronomical Society • Provided by the NASA Astrophysics Data System 1982ApJ. . .255. .564D -1 20- 570 bly duetoanatomiccomponentofforegroundgas and theabsorptionfeature,indicativeofcolddensegas, also hasthisvelocity. absorption (see§IVe)inthedirectionofLkHa101; associated withthedustresponsibleforgeneral obscuration intheregionofNGC1579. individual spectra,andspecificcorrectionsforabsorp- kms. we havesufficientsignal-to-noiseratiotoallowfits density ofgascorrectedforabsorptionoverthewhole et al.1976)isshownforcomparisonwiththeHispectrum.Inparticular,absorptioncomponentsofbothCOandareatabout +1 spectra bandcare3'west6'west,respectively.TheindividualfittedcomponentsinemissionabsorptionshownlightUnes, with asinglemeanscalingfactorof1.75whichmini- regions werecorrectedbyscalingtheobservedvalues emission cloud.Inthecentralpartofcloud lustrates therelativescaleof thereflectionnebulaand regions. Figure7showsthecorrectedmapsuperposed mized thediscontinuitybetweencentralandperipheral and thefittedresultisshowninheavyUnessuperimposedonmeasuredspectrum.Theresidualbelow.COspectrum(Knapp on Herbig’s(1956)photographofthenebulaandil- tion weremade.Thecolumndensitiesintheperipheral equivalent columndensityof 13X10atomscmand the Hicloud.Thecorrected distributionhasapeak a totalmassofHi85M. Bothestimatesaresubject 0 We conclude,then,thattheHiabsorptionisproba- It isusefultohaveanestimateofthetotalcolumn Fig. 5.—SomerepresentativeHispectraillustratingthetwocomponentspectralfits.SpectrumaisatpositionofLkHa101,and © American Astronomical Society • Provided by theNASA Astrophysics Data System d) TheEmissionCloud DEWDNEY ANDROGER 2 pc, thistotalmassmustbescaledasD. cally thin.Ifthedistanceisotherthanassumed800 to theassumptionthatemissionclouditselfisopti- process inwhichthemoleculeabsorbsaphoton capable ofdissociatingHmoleculesbyatwo-step Wilhams 1967). Lyman-Werner bands.Subsequentradiativedecayusu- ally leadstoaboundvibrationalstate,butabout11%of violet radiationfieldrather thanthatofasingle grains. Thebasicdynamicequilibriumtheoryofdissoci- the decaysleadtodissociatedstate(Stecherand context ofadensecloudembeddedindiffuseultra- ation andreformationhasbeendiscussedbymany from Hiatomsoccursefficientlyonthesurfacesofdust dependent casesofdissociation aroundanevolvingHn Hollenbach (1978),andLondon (1978)discusstime- authors (Hollenbach,Werner,andSalpeter1971;Jura star. Hollenbach,Chu,and McCray (1976),Hilland 1974, 1915a,b).Theseauthorsderivethetheoryin 2 The ultravioletemissionintherange912-1120Ais In thesameregionsformationofHmolecules 2 e) ADissociationModelfortheEmissionCloud around LkHa101 Vol. 255 Q KO LO No. 2, 1982 H i CLOUD SURROUNDING LKHa 101 571 found by setting dn2/dt=0 in equation (Al) of Hill and Hollenbach (1978): Rn nH — ^ (2) 2 2Rnt + I0(r/r0) ßN^y cxp ( — KNt) and n 2n nHi = ,- n2, where R is the H2 formation coefficient, nn nHi, and -3 nH ! are particle densities (cm ), Nt, AH2, and NH , the equivalent radial column densities (cm-2), r is the radius (cm) from the star to any volume element, and r0 is the inner boundary of the dissociation cloud. 70 is the -1 photoabsorption rate at r0 (s ); ß, the Lyman self-shielding parameter (cm-1); and AT, the dust extinc- tion parameter (cm2). The numerator and the first term in the denominator are formation terms for H2. (Note Fig. 6.—A comparison of relative extinction in the region that the formation rate for H2 is j for a constant around NGC 1579 with the emission cloud of H i. Star counts gas to dust ratio [Hollenbach, Werner, and Salpeter were made using a 4'64 reseau on Herbig’s (1956) photograph (cf. 1971].) The second term in the denominator is the Fig. 7) excluding the 24 brightest stars. The region of least extinc- dissociation term. It contains a factor ßN^y2 ap- tion {shaded) has an average count of 18 stars per element. The region of most extinction {unshaded) has an average count of 4 propriate to the square root portion of the curve of stars per element and is outlined at 8 stars per element. The growth of the Lyman absorption band (Jura 1974). contour map of H i emission, corrected for absorption, is the same Under equilibrium conditions in a constant density as in Fig. 7. The position of LkHa 101 is marked by a star. environment, the hydrogen tends to be completely disso- Coordinates are for epoch 1950. ciated out to a particular radius, and then to revert in a short distance to the molecular form. When integrated region. Hollenbach, Chu, and McCray (1976) treat stel- as column density along the Une of sight, projection lar-wind-driven shells of H2 around O and B stars. Hill effects soften this change to some extent, but still keep and Hollenbach (1978) discuss the evolution of a dis- the outer radius fairly constant. sociation shell from the time the H n region first begins In general, this relation must be integrated along all its expansion to an age about 105 years later. They trace radii from the ionization boundary outward to obtain the expansion of a rapidly advancing dissociation wave the volume distribution of H i. To provide comparison followed by a shock which eventually overtakes the with observations this distribution must then be in- dissociation wave. Dissociated H i is predicted to be tegrated along lines of sight from the observer, a process observable only in absorption against the continuum which is equivalent to solving an integral equation for radiation from the H n region. London (1978) examines one or more of the dependent variables in the equation. heating resulting from dissociation and predicts H i in Even with some simplifying assumptions an exact solu- emission but only as a very thin shell around the H n tion would be difficult to obtain. Instead we fit a simple region. model to the observed parameters using no more de- Roger and Pedlar (1981) and Roger and Irwin (1981) grees of freedom than necessary. have applied the simpler equilibrium model to explain The corrected map of the emission cloud (Fig. 7) can the broad density distribution of H i in the regions be characterized as roughly oval shaped with the peak around NGC 281 and IC 5146, cases in which there is a offset to one side along the major axis. A method single exciting star. Since the H I zone of LkHa 101 is illustrated in Figure 8 was used to reduce the observa- also broad and since there is no evidence for shocked tions to cuts along the major and minor axes. The gas, it is appropriate to apply the equilibrium model in observed column density profiles (see Fig. 9) were de- the present situation. rived from the corrected map of H i emission by in- The process assumes dynamic equilibrium between tegrating over 90° sectors. The unshaded sectors shown the dissociation rate and the reformation rate for H2 in in Figure 8 apply to the AA' plane; the shaded ones, to each element of volume. The amount of dissociation the BB' plane. This technique improves the signal-to- depends upon the spectral type of the star, the absorp- noise ratio of the column density curves but may add tion due to dust, and the density of hydrogen. The low-level wings to them. The position angle of BB' is following equation (Roger and Pedlar 1981) can be about 32°. © American Astronomical Society • Provided by the NASA Astrophysics Data System 1982ApJ. . .255. .564D planes. model wascalculated.ThetwoplanesintersectatthepositionofLkHa 101.Alsoshownaresimplifiedcolumndensityprofilesinthetwo reflection dustnearthecenterofNGC1579.Adarkobscuringbandinfront1579appearstocausemostextinctionLkHa Herbig (1956)intherange3300-5000À.LkHa101isinsidecentralcontourdisplacedslightlysoutheastfromtriangularpatch of 101 (~11mag). 20- Fig. 8.—AsimplifieddiagramoftheHIemissioncloudshowing major(AA')andminor(BB')axisplanesinwhichthedissociation Fig. 7.—MapoftheHiemissioncloudcorrectedforabsorption.Thecontourintervalis2.25XIOcm.photographfrom © American Astronomical Society •Provided bythe NASAAstrophysics Data System 1982ApJ. . .255. .564D 20- 6 1 21 17 31 2 AA' axisisthemodelcurve{a)shiftedby0.5pctobestfitobserved(c)asexplainedin§We. column densityare10cm.InboththeAA'andBB'planesmodelcurveshaveaboutsamehalf-widthextentasobserved curves. Thewingsontheobservedcurves(particularlyBB')resultfromsectoraveragingillustratedinFig8.dashedcurve{b) the AA' andBB'(Fig.8)whichwouldreproducetheob- distribution oftotalparticledensity{n)intheplanes but asasingleindependentvariablewithK/Rconstant. H icloud,r,wassetat0.1pc.Atthisdistancefrom to beofstellartypeB0.5ZAMS.Theinnerradiusthe served Hicolumndensitycurves.LkHa101isassumed such astar,thephotoabsorptionrate7is2.64X10“ K andthereformationcoefficientRareallowedtovary, Roger andPedlar(1981),thedustabsorptioncoefficient s“ (Hollenbach,Chu,andMcCray1976).Following York etal(1973)weestimateKtobeabout3X10“ estimates RtobebetweenIX10“and5X10“ Using CopernicussatellitemeasurementsofHUnesin cm s“.FrommeasurementsbyBohlin(1975)and ratio intheregion. cm. BothKandRaredependentonthedust-to-gas the ultravioletfromnearbystars,Jura(1974,1975a) major axis.Thehalf-widthinthesoutheasterlydirection distribution oftotaldensitywasinsimulatingtheob- density isroughlysymmetricalwithahalf-widthof served asymmetryincolumndensityofHIalongthe t to thenorthwest.Alongminoraxiscolumn along thismajoraxisis~0.75pcwhereasit~1.7 distribution ofHIisoneinwhich thereisvariationonly about 1.0pc.Thesimplestcompatible three-dimensional about thisaxis. along Unesparalleltoanaxis andcircularsymmetry 0 0 2 Fig. 9.—Acomparisonofthemodelcolumndensitycurves{thinlines)andobserved{thicklines).Theunits The goalofthemodelfittingwastofindsimplest The maindifficultyencounteredindetermininga © American Astronomical Society • Provided by theNASA Astrophysics Data System H iCLOUDSURROUNDINGLKHa101 3 3 2173 1 3 provide initialestimatesofKandR. tion alongthelineofsightyieldsaninitialestimatefor together withanestimateoftheextentdistribu- minor axiswasmatchedinaconstantdensitymodelto preferred alignment,thisaxishasbeenassumedtobeat the totaldensitynThewidthofprofilealong 45° totheUneofsight.(Thecolumndensitycurveisnot reproduce thecolumndensitycurveAA'.Toavoid AA' hadbeenachieved,thismodelwasusedtoreesti- along theaxisofsymmetryweretriedinorderto The bestfitsobtainedwerewiththeprofileofdensity was repeateduntilnofurtherimprovementpossible. mate thefitinsectionBB'.Thismodelingsequence respectively. Thedensitytothenorthwestofstar a strongfunctionofthisangleexceptnear0°and90°.) dechnes monotonicallyfrom~100atomscm“nearthe shape ofthecolumndensity curveforAA',butadis- rises sharplywithin0.5pcto~200cm“.Foradis- these fitsare8X10“cm“andcms“, shown inFigure10.Thepredictedcolumndensitiesare placement ofthecurvetoward thehighdensityside tance tothecloudotherthanassumed800pcthese shown inFigure9.TheparametersKandRusedfor H idensitiesshouldbescaled asD“. star to~20cm“at4pc.Tothesoutheastdensity r 20 The peakcolumndensity,13X10atomscm“, A numberofdifferentcurvesnversusdistance After areasonablefittothecolumndensitycurve t Under theaboverestrictions it waspossibletofitthe 573 1982ApJ. . .255. .564D 1316 16-2 13 2 13 -2 -1 574 profile isacutintheplaneAA'whichLkHa101at is novariationinthedirectionperpendiculartoplaneAA'.The origin. Theangleofthecutisabout45°tolinesight.There corresponding tothemodelcolumndensitycurvesinFig.9.This steep gradienttotheSEofLkHa101maybeassociatedwith the planeAA'. the obscuringdustbandrunningapproximatelyperpendicularto (/= 10)spectrawithinanarea5'by8'nearLKHa of emission.TheprofilessuchastheoneinFigure5 spectrum atthestellarpositionexhibitsanenhancement ^ =9.1magfromacomparison ofmeasurednarrow- From thispairofspectra,onecanroughlyestimatethe corresponding spectrumforC0wasalsomeasured. absorption. InthedirectionofLKHa101itself show acomponentinemissionandnarrowerone justified. measured byDickman(1978)andBohlin,Savage, Milman etal(1975),as2.8X10cm.Withratios column densityofCO,usingthemethoddescribedby bly undercorrectedforopticaldepth,thisabsorption Drake (1978),weinferacolumndensityof2.1X10 have estimatedA=14.2magfromtherelativeextinc- may beunderestimated.Mostoftheextinctioninthis possible ifthestarislocallyembeddedingasofhigh remains (seeFig.9).Aresolutionofthisanomalyis band scannercolorswith appropriatetoaB0.5 front ofLkHa101forwhichThompsonetal(1977) direction probablyoccursinthedarkobscuringband tion (^4^)of11mag.BecausetheCOprofileisproba- atoms cmforHandacorrespondingvisualestinc- 101 witharesolutionof65"by0.26kms.The density andaconicalopeningexiststowardthelow tion ofinfraredemissionlines andCohen(1980)derives escape andproducethelongtailtonorthwest.The density sideallowingsufficientdissociatingphotonsto star. density sideoftheobservedcolumncurveindi- fact thatthestarisslightlydisplacedtowardhigh cates thatsomesortofthree-dimensional“wrapping complicated model,however,wouldprobablynotbe around” ofthedistributionmayexist.Fittingamore v 2 -3 1216 © American Astronomical Society • Provided by theNASA Astrophysics Data System Fig. 10.—Themodeltotalparticledensity(ncm)profile t Knapp etal(1976)havemeasuredeighteenC0 f) AComparisonofCOandHiEmission DEWDNEY ANDROGER -1 12 1 -1 21- -3 5- 3 integrated emissionisstrongestonthesouthand positions wherethevisualextinctionis>1mag.The position anglesfromthestar)whereHIemissionis on theHiemissioncloudwithresolution2'by0.6km declining mostrapidly(cf.Figs.6,7)andweakesttothe s. GenerallyspeakingtheCOemissionisdetectedat sured 89COprofileswithina12'byareacentered dense andrarefiedgas. expect fromtheequilibriummodeldescribedin§IVc greatest extension.Thisispreciselywhatonewould northwest wherethethreeinnermostcontoursshow considering thepenetrationofdissociatingphotonsin southeast ofLkHa101,justinthosedirections(i.e., near 0kms~inradialvelocity,theHiprofileshavean macroscopic scale. nent residingindense,coldclumpsorconcentrations closely associatedwiththemolecularhydrogencompo- of theCO.Wesuggest,then,thatCOisprobablymost average widthof7.4kms,approximatelytwicethat and coexistingwithanydissociatedHionlyona ponent ispossiblyanexposedouterlayerofthemainly hydrogen columndensity.Thisdistributedatomiccom- hydrogen columndensityof~4X10atomscm. (private communication)hasfoundthatthemeanvisual largely molecular(H)withconcentrationsofvarious molecular regionwhichiskeptdissociatedbydiffuse nent ofcoldHiwhosecolumndensityfromabsorption molecular gasisoforder200atomscm.Likeother we estimatethatthemeandensityofmuchdiffuse Drake (1978),onewouldinferfromthisameantotal obscuration overmuchoftheareaaroundNGC1579is outlined inFigure6.Fromrecentstarcounts,Christie region. An estimateofitswidth,togetherwiththecolumn ultraviolet radiationfromexternal0andBstars(e.g., measurements ismuchless(afactorof20)thanthetotal dark clouds(e.g.,Knapp1974),theregionhasacompo- Using thewidthofregionasameasureitsdepth, and dustapparentasanelongatedareaofobscuration temperatures ofvariousobservedcomponentsthe gests densitiesof~10cm inthisband.LkHa101 density derivedfromthe~11 magofobscuration,sug- dark obscuringbandseenextendingacrossthenebula. Hollenbach, Werner,andSalpeter1976). centration, andmeasurements ofthecorecomponent appears tohaveformedon the faredgeofthiscon- sizes anddensities.Oneverydenseconcentrationisthe ~2 mag.WiththeratioN/AofBohlin,Savage,and 2 tv Christie (privatecommunication)hasrecentlymea- Although theCOandHiemissionarebothcentered Table 3summarizesthesizes,densities,massesand On thelargestscaleisaregionofmedium-densitygas Inside thisextendedregionthegasprobablyremains V. ASUMMARYOFTHESTRUCTUREREGION AROUND LkHa101 Vol. 255 1982ApJ. . .255. .564D 3- 7-3 -3 No. 2,1982 the Hiisurroundingstar(Brown,Broderick,and pc) oftheionizedgasaresomewhatlessthan10cm. radius of20AU.Bycomparison,ouranalysisshows Knapp 1976)yieldsdensitiesof10cmwithina region detectedbyHarvey,Thronson,andGatley(1979). Possibly relatedtothisgasisthefar-infrared(100/tm) that densitiesinthemainextendedcomponent(r<0.1 They postulateanorigininthethermalemissionfrom source ofabouttwicethediameterextendedHn circumstellar dustleftoverfromthestar’sformation. of order100-200atomscm,similartothemeantotal density inthediffusedarkcloud.Theasymmetryof H iemissionisinthesamesenseasasymmetryof to distancesof1-3pc.OnthisscalethedensityHiis dark obscuringband.Ahigherdensityofmoleculargas the reflectionnebula,extendingtonorthwestof density tothenorthwest,asindicatedbyCOobserva- eling indicatesthatasteepgradientofdensityexistsin to thesouthandsoutheastofLkHa101alower its brightnesstemperatureandlessthan1200K,in- tions, wouldexplaintheasymmetry.Ourdetailedmod- (CO) componentsaresimilar.Thespintemperatureof the immediateneighborhoodofstar. 101 hasdissociated—85Mofmolecular(H)gasout and themeanvelocitiesofatomicmolecular relation betweensizeandapparentmagnitude(Herbig nebula ismuchlargerthanexpectedfromHubble’s ferred fromthewidthofHiprofiles. the Hiinemissionismorethan70Kasconstrainedby to —11mag,accountsforthefactthatreflection for thereflectionnebula,consistentwithobserved the HubblerelationforblueplatesofPalomar ing LkHa101andfindthatthe massandextentofeach size. Sky Survey(Martin1978),weestimatearadiusof-3' 1971). UsinganMof—2.8andtheabsoluteversion 02 v Beyond thesecircumstellarsources,wefindthatLkHa No systematicmotionoftheatomicgasisapparent, The heavyobscurationofLkHa101itself,amounting We havemeasuredtheHnand Hiregionssurround- © American Astronomical Society • Provided by theNASA Astrophysics Data System 5 b 3674 Obscuring band...>327.6X100.2X>0.6 HI absorption...602.11 H iiextended0.088303.40.168000 HII core~2.5X10~~10~3410~ Darkcloud ...-200-40.6X>15 HI emission85100-20013.2X370