Mv. Space Ree.Vol. 13, No. 12, pp. (12)311—(12)314,1993 0273—1177193$6.00+ 0.00Printedin GreatBritain.All rights reserved. Copyright~ 1993COSPA1~

ROSATOBSERVATIONSOFTHECHAMAELEON STARFORMINGCLOUD

E. D. Feigelson,*S. Casanova,**T. Montmerle** andJ. Guibert’~’4’

* DepartmentofAstronomyandAstrophysics,Penn.sylvaniaStateUniversity,UniversityPar/c PA 16801,USA -

** Serviced’AstrophysiqueCentred’EtudesdeSaclay,91191 Gil sur-YvetteCede~France“i” ObservatoiredeParisandCentred’AnalysedesImages,61 Avenuedel’Observatoire,F-75014Paris, France

ABSTRACT

Two soft X-ray imagesof the ChamanleonI star forming cloud obtainedwith the ROSATPositionSensitiveProportionalCounterarepresented.Seventyreliable, andperhaps19 additional,X-raysourcesarefound. Up to Ninety percentof thesesourcesarecertainlyor probablyidentified withT Tauri starsformed in the cloud. Twenty to 35 are probably previously unrecognized‘weak’T Tauri (WTT) stars. T Tauri X-ray luminositiesrange from log L~<28.8 to 31.1 erg/s, or102 — iO~times solar levels,with mean < log L~>= 1029.2 erg/sin the 0.2-2.5 keV band. TheX-ray luminositiesof well-studiedChamasleoncloud membersare correlatedwith a complex offour stellarproperties:effectivetemperature,mass,radiusandbolometric luminosity. The spatialdistribution,H-R diagramlocationsof the starsindicateWTT andCTT are coeval.The total pre-main sequencepopulationof the cloud is likely to be >100 stars,with WTT starsoutnumbering‘classical’ T Tauri (CTT) starsby —‘2:1.

INTRODUCTION

Soft X-ray imagesof nearbystarforming regionsobtainedby the Einstein(HEAO-2) Observatoryrevealeddozensof low masspre-mainsequence(T Tauri) starsemitting X-rays at levels 102 — i0~times that seenin typical main sequencestars. Many of thesesourceswere found to be ‘weak-lined’ T Tauri (WTT) stars,without the strongemissionlines, UV and JR excessesof ‘classical’T Tauri (CTT) stars. From their rapid X-ray variability, hard spectraand other properties,itwasinferred that T Tauri X-ray emissionis due to solar-like magneticactivity near the stellarsurface.Thisinterpretationis supportedby photometricdetectionof hugecool starspots,enhancedchromosphericlines, and powerful gyrosynchrotronradio continuumemission. Theseresultsarereviewedin /1,2/.

The ChamaeleonI star forming cloud is an excellentsite for studyingT Tauri X-ray emission.Ata distanceof 140 pc /3/, it containsabout 60 optically identified T Tauri stars. Einstein IPCimagesshowed21 X-ray stellar sources,including severalnew WTT stars/4/. We report hereresults from two overlappingimagesfrom the ROSATObservatory.Theseimageshave15 timeshighersensitivity (2 x10~4erg/s/cm2)andup to 15 times betterspatial resolution (±4” nearthe PSPCcenter)than the earlierEinsteindata. The findings give insight into awide rangeofastrophysicalissuesconcerningpre-mainsequencestars,of which onlyafew canbementionedhere.A morecompletediscussion,with full listing of sourcesandoptical identifications,appearsin /5/.

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Figure 1: Spatialdistribution of X-ray emittingstarsaroundthe ChamaeleonI darkcloud. • =

previouslyknown CTT stars;0 = previouslyknownWTT stars; x = proposedNewT Tauristars;• = unknownor unrelatedto the cloud.

X-RAY OBSERVATIONSAND STELLARIDENTIFICATIONS

We analyzehere two ROSATPSPCimagespointed at the northernand southerncoresof theChamaeleonI cloud, obtainedin February-March1991. Eachhasexposure 6,000sec. TheROSATStandardAnalysis SoftwareSystem(SASS,version5.2) located90 sources.We modifiedthis sourcelist by: mergingisolated sourcesappearingin both fields, addingfive sourcesmissedby SASSdueto proximity to strongsource,and addingtwo weaksources.The resultinglist of 89distinct X-ray sourcesis divided into 70 confidentsourceswith S/N �3.5,and 19 fainter possiblesources.Sourcepositions wereadjustedby 4” and 10” in the two fields to correct for boresighterrors. ApproximateX-ray luminositiesL~areobtainedfrom a conversionof 1 ct/ks = 3 x 10~erg/sin the 0.4-2.5keV band,derivedfrom spectralfits of thestrongestsources.Individual sourcesshow afactor of two scatteraboutthis line duemainly to different foregroundabsorptions.

Stellar counterpartsof the ROSATsourceswerelocatedin publishedcompilationsof cloud mem-bers (e.g. /6/), the HST Guide Star Catalog (GSC),and inspectionof digitized Schmidt plates

ChamaeleonStarForming Cloud (12)313

from the southernESO/SERCSky Survey J-bandplates. Theseplates were digitized with theMAMA automaticmicrodensitometerthe Observatoirede Paris.Platesolutionscalibratedto thePPM astrometriccataloggive positional accuracyaround±0.5”.Magnitudecalibrationto pub-lishedphotometricsequencesgives R bandmagnitudeswith accuracy±0.3(90% confidencelevel).The criterion for optical identification is that theproposedstar falls insidetheboresight-correctedROSATerror circle. X-ray/optical offsets for many stars is �4”. The four known Herbig-Haroobjectsare not detected,but oneX-ray sourceis coincidentwith oneof the 20 proposedembeddedinfraredsources/3/ in the cloud.

RESULTS

The 70 X- ray sourcesdetectedwith confidencehave2 to nearly500 PSPCcts/ks,or 0.6 to 140x 1029 erg/s. The integratedX-ray emissionof the cloud sourcesis 5 x 1031 erg/s.35 of 60 (58%)of previously known cloud T Tauri stars in the fields are detected. Basedon the usual opticalspectroscopicclassificationof WTT and CTT (equivalentwidth of the H~emissionline is belowand above10 Arespectively),the detectedfractionof WTT and CTT starsareindistinguishable.18 previously unidentified,which we proposeare mostly‘New’ WTT stars,aredetectedwith highreliability, andup to 19 moremaybe present.Thelargepopulationof probableNewcloud members,and the inferreddominanceof weak T Tauri starsoverclassicalT Tauri stars,are two principalinferencesfrom this study.

The spatial distribution of X-ray emitting stars is shown in Figure 1. The X-ray selectedandoptically selected(not shown) samplesare very similar in both total population andspatial dis-tributions. Thereis no hint that selectionin the X-ray bandemphasizesanolder, moredispersedpopulationthanoptical selection.Similarly, the spatialdistributionsof CTT, WTT andproposed‘New’ starsdo not clearly differ in any respect.Thereis no indication that CTT starslie closerto the densecoresthanWTT or ‘New’ stars,as might be expectedif they formed morerecently.The distribution of X-ray luminositiesof the CTT and WTT subsamplesare quite narrow,andhavesimilar meanlog luminosities<log L~>c~29.5 erg/s (X-ray detectedstars)and ~29.2 erg/s(optically selectedstars,maximum likelihood meanincluding nondetections).There is evidencethat WTT starsareon averageslightly moreX-ray luminousthan CTT stars.

A clear,approximatelylinearcorrelation betweenX-ray andoptical luminosity (derivedfrom ourR band magnitudes)is seenovertwo ordersof magnitudesfor all identified sources.For the 35-40 Chamaeleonstarsthat havebeenwell-characterizedin the optical andinfrared bands(/6/),we find X-ray luminosity is correlatedwith a complex of four interrelatedvariables: bolometricluminosity, effectivetemperature,radius and mass. It is not clearwhich of thesecorrelationsisfundamental.Thereis someevidencethat extremeCTTs with EW(Ha � 80 A, tend to havelowert,~values,expectedfrom ‘smothering’or absorptionby the CTT wind. This mayberesponsibleforthe smalldifferencein CTT and WTT luminosity functions.We alsofind that the X-ray emissionappearscorrelatedwith the infrared luminosity of the circumstellardisk (/6/). Takentogether,the statisticallinks betweenX-ray emissionand stellarpropertiesalong in the pre-mainsequencephaseare interrelatedin a complexmanner,andtheestablishingphysically meaningfulconclusionsfrom them is difficult without additional data.

SOME BROADERIMPLICATIONS

Thereis considerableevidencefrom Einstein-erastudiesof PMS stars,combined with opticalandradioproperties,that the X-ray emissionobservedhere is dueto solar-typemagneticactivityenhancedby severalordersof magnitudes(/1,2/). It is also well-establishedthat the strengthofmagneticactivity indicatorsin late-typestarsincreasewith stellar rotation andso decreasewithstellar age (/10/). Too few rotational datawas availablefor the Chameleonstarsto test thishypothesisof a dynamoorigin of the X-ray emission. But we do find an anticorrelationof theX-ray luminositywith stellar age, inferredfrom isochroneson the Hertzsprung-Russelldiagram.

(12)314 E. D.Feigelsonetal.

On longertime scales,the shapeof the L~— t decaylaw hasbeendebated:is X-rayemissionmainlyconstantfor the first --‘ 108 years(/7/), or doesit decaycontinuouslyasapowerlaw (/5/)? Thisquestioncannot be reliably estimateduntil ROSATluminosity functionsof the Pleiadesandotheropenclustersarecomparedto the Chamaeleonresultshere.

Severalrecentinvestigationsof the pre-mainsequencepopulationof the ChamaeleonI cloudstellar(e.g. /3,6,8/)are basedon a censusof 60-80 stars found from optical and infraredstudies. Ourstudysuggeststhat the cloudpopulationis considerablylarger. ThepresentROSATimagesreveal25-38 ‘New’ identifications,mostof whichare probablenew WTT stars.More yet-unknownWTTstarsare likely to be presentbelow our ROSATsensitivity limit, on the far side of the cloud,oraccompanyingtheinfrared-excessstarsembeddeddeepin the cloud cores. Someof theseobscuredWTT starsshouldbe detectedin hard X-ray imagesusing the Astro — D, XMM and AXAFtelescopes.Altogether,we suggestthat thetotalcloud populationof M > 0.5M® is >100, possiblyeven200, cloud members. The cloud star formation efficiency (stellar mass/gas+ star mass)probablylies between10% and 30%.

We can integratethe Chamaeleonstellar census,spatial and agedistributions into a suggestedstar formation scenario. The densecloud cores (or similar coresin the past) produceCTT andWTT starsat a constantrateoveran extendedperiod of time, at least 3 x iO~’years.Initially,the ratio of WTT to CTT stars is perhaps1:1 at the upperend of the presentH-It diagram.As the CTT starsage, their inner disks becomedepletedandceaseto touch the stellarsurface,convertingthem from CTT to WTT stars.All starsdrift outwardat speedsof --‘ 0.1 — 0.5 km/s,giving asymmetricaldistribution of X-ray emitting PMS starsextendingseveralparsecsaroundthe cloud coreswith the WTT:CTT ratio increasingaway from the cores. Their X-ray emission,which averagesaroundseveralx 1029erg/s during thefirst 1 x iO~years,generallyfall below ourROSATsenstivity limit of severalx 1028 erg/sat later times. This scenariopredictsthat dozensmorenew X-ray emitting stars,bothembeddednear the cloud coresandin the peripheryaroundthe cloud,shouldbediscoverablewith in sensitiveX-ray surveys.

Acknowledgments:The work waspartially supportedby NASA grantsNAG5-1678 andNAGW-2120.

REFERENCES

1. E.D. Feigelson,M.S. Giampapaand F.J.Vrba, MagneticActivity in Pre-MainSequenceStars,in TheSunin Time,eds. C.P.Sonett,M.S. GiampapaandM.S. Matthews,Univ. Arizona,Tucson1991,p. 658.2. T. Montmerle,E.D. Feigelson,J. Bouvier andP. André,MagneticFields,Activity andCircum-stellarMaterial aroundYoung Stellar Objects,in Protostarsand Protoplanets III, eds. E.H. LevyandJ.I. Lunine,Univ. Arizona,Tucson1992, in press.3. R.D. Schwartz,The ChamaeleonDark CloudsandT Associations,in Low MassStar Formationin SouthernMolecular Clouds,ed. B. Reipurth,Euro. SouthernObs,Garching1992, p. 93.4. E.D. Feigelsonand G.A. Kriss, Soft X-ray Observationsof Pre-Main SequenceStars in theChamaeleonDark Cloud,Astrophys.J. 338, 262, 1989.5. E.D. Feigelson, S. Casanova,T. Montmerle and J. Guibert, ROSAT X-ray Study of theChamaeleonDark Cloud. I. The Stellar Population,Astrophys.J. submitted,1992.6. L.S.GauvinandK.M. Strom,A Studyof theStellarPopulationin theChamaeleonDark Clouds,Astrophys.J. 385, 217, 19927. F.M. Walter andD.C. Barry, Pre- andMain-SequenceEvolution of Solar Activity, in TheSunin Time,eds. C.P.Sonett,M.S. GiampapaandM.S. Matthews,Univ. Arizona,Tucson1991,p.633.8. T. Prusti, D.C. Whittet and P.R. Wesselius,A Study of the ChamaeleonI Dark Cloud andT-Association.V. Luminosity Functionfor Members,Mon. Not. Roy. Astr. Soc.254,361, 1992.