Adv. Space Res. VoL 11.No. 11,pp. (11)37—(11)45.1991 0273—1177~1$0.00 +50Printed inGreatBritain. 1991 COSPAR

EUV AND FUV EMISSIONFROM COOL STARSAND STELLAR SYSTEMS

A. K. Dupree

Harvard-SmithsonianCenterfor Astrophysics,60GardenStreet,Cambridge,MA 02138,U.S.A.

ABSTRACT

Coolstarsrepresentarich sourceof extremeultraviolet radiationpromisinguniqueinformationonphysicalprocessesrelevantto stellaratmospheres,magneticactivity, masstransfer,andmassloss. Hot plasmamaybeassociatedwith coolstarsin stellarcbromospheresandcoronae,in disk structuressurroundingcoolstars,in magneticallyconfinedmaterial,andasaresultof pulsationandshockphenomenain stellaratmospheres.We considerfour typesof cool starsthatareof particularinterestsincerecentobservationalandtheoreticaladvancessuggestthatFUV/EUV observationswill be rewarding.Thesestarsare: cool starswith accretiondisks;starsthat flare;giant andsupergiantstars;binarysystemscontainingcool starsandextendedcoronas.

INTRODUCTION

The FUV/EUV spectralregion containsa rich assortmentof emission lines arising from collisionally-dominatedplasmaswith temperatureshigherthaniO~K. Thesespectraldiagnosticsareparticularlyvaluableto study theouter atmospheresof cool stars. As Figure 1 demonstrates,thehigh temperatureplasma(T> iO~K) showsthegreatestvariety of behavioramongdifferent solar features.ThusETJV/FUV diagnos-tics,whichariseuniquely in theseplasmas,becomeparticularlyvaluableto probethefundamentalquestionsof structureandenergy balancein theouter atmospheresof cool stars. A good exampleof thepotentialcapabilitiesof EUV/FTJV spectroscopyoccurs,in observationsof thebright binarysystemCapella.

TheTransmissionGrating Spectrometer•on EXOSATobtainedspectraof Capella, and analysisof thesespectraby Lemen et at. /1/ disclosedthat severaldifferent distributionsof emissionmeasurecould existin the outer atmospherewithin theuncertaintiesof the observations(seeFigure 2). Now, FTJV/EUVspectroscopyenablesus to determinethestructureof theouter atmospheresof starsby measuringsufficientnumbersof spectrallines to map thepresenceof plasmaat varioustemperatures.

However, spectroscopywith modest spectralresolution, namely A/L~\~ 500, will allow discriminationamongtheseveralemissionmeasuredistributions. DupreeandKenyon/2/ haveevaluatedthespectrumtobeexpectedfrom CapellausingtheDoschek/4/ codemodifiedfollowing thesuggestionsof Meweet al. /5/.Thesectionof thespectrumfrom ~110to .X140 shownin Figure 3 wascalculatedwith a resolutionof 0.5A

~ 250). The clusterof lines from )d14 to A119 is formedprincipally in the temperaturerangelogT (K)= 5.5 to 5.7 andtheir strengthreflectsthe largeemissionmeasuredefinedby the P modelin Figure2. Particularlyimportant arethehigh temperaturelines whichcanhelp to discriminateamongthevariousmodels. Lines from five sequentialstagesof iron, Fe XIX to XXIII occurin thewavelengthregion110 to140A, giving continuoustemperaturecoverage,andhencegood definition of theemission measureof theatmosphere.

Thus, theEUV/FUV spectralregionrepresentsuniquely suiteddiagnosticsto theouter atmospheresofcool stars. Spectralresolutionof ~ 500 to 1000 will benecessaryto disentangleblendsandinfer emissionmeasuredistributionswith confidence.

COOL STARS WITH ACCRETION DISKS

Circumstellardisksarebelievedto occurin manystarsandstellarsystemsof cool stars.Manyyoungstarsaresurroundedby disks of dustymaterial. Interactingbinariessuchassymbiotic starsandcataclysmicvariablestarscan also possessdisk structures. EUV emission in all of thesesystemswill arisefrom theatmospheresof thestars themselves,if theycontainplasmathat is sufficiently hot. Young stars,suchas TTauri objectsor FU Orionis starsmay haveactiveirregularmagneticactivity in their chromospheres,andwe might expectemissionsimilar to older giantsor active binaries,but on an enhancedlevel. Symbiotic

(11)37

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Fig. 1. Emissionmeasuredistributionsfor four different regionsof thesolaratmosphere.Notethesimilarityof thecurvesastheyreachlocalminimanear105K, andthediversityof theirbehaviorat highertemperatures.TheFUV/EUV regionofthespectrumcontainsstrongtransitionsof ionsformedat temperaturesaboveiO~K wheretheemissionmeasuresdisplay thegreatestdissimilarity.

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Fig. 2. Emissionmeasuredistribution for Capellabasedon Lemenet al. /1/ analysisof TGSspectrafrom EXOSATandextendedby DupreeandKenyon /2/ to fit thevalue of theemissionmeasurecomputedby DoschekandCowan/3/ from IUE observationsof N V.

CoolStarsandStellarSystems (11)39

starsconsist of a low massred giant starand a compactcompanion,the lattereither a solar-typemainsequencestar or a white dwarf companionstar. The highesttemperaturesattainedin thesesystemswillmostlikely occurneartheboundarylayerwheredisk materialfails onto thecentralstar. Themostoptimumsourcesfor EUV/FUVemissionamongsymbioticstarsappearto be asmallgroupof symbioticnovae,whicharewhite dwarfsystemsthat arelikely in themidst of thermonuclearrunawayssimilar to thoseof classicalnovae.Anotherclassof binariesthat will be rich sourcesof EUV emissionarerepresentedby cataclysmicvariables- very closebinarysystemsin whichmaterial from a cool dwarfstarflows ontoa hot white-dwarfprimary. In fact, abouthalf the luminosity of a, cataclysmicvariable systemis expectedto be emitted inthe EUV. Sincecataclysmicvariable starsarecloseby, 100 pc distant, they should be easilyobservablein theEUV. Spectrafrom all of thesestellar systemstypically ‘have threecomponentswhich areshownschematicallyin Figure4. The dominantemissionin thevisible andultraviolet spectralregionsarisesfromthecompactcompanion,herea white dwarf takenas thecentralobject in an accretiondisk, andemissionfrom thedisk itself. However, in thefar ultraviolet andEUV spectralregion, theboundarylayer of the diskdominatestheemission.It is in theboundarylayerthat theaccretingmaterial losesits angularmomentumandcanfall upon thecentralstar. Temperaturesin suchboundarylayerscan reachio~K or higher, andhenceradiateprofuselyatfar ultraviolet wavelengths.

It is expectedthat a wind may arisefrom thecataclysmicvariablewhichis illuminated by theaccretiondisk. Signaturesof sucha wind havebeenfound from IUE spectra/7,8,9/. The kinetic energyof thewindsappearsto beasignificantfractionoftheaccretionluminosity, andthemasslossrateof thewind alsorepresentsamajorcomponentof theaccretionmasstransferrate/10/. A particularlypowerfuldiagnosticoftheionization stateof thewind andtheaccretiondisk boundarylayermay bepossibleif sufficient spectralresolutionis available. Various orientationsof the accretiondisk to theobserverwill producedistinctlydifferent line profiles in theextremeultraviolet as the calculationsof Raymond andMauche /10/ haveshown.

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Fig. 3. The predictedspectrumof Capella as observedat the Earth (with no interstellarabsorptionassumed)for threedifferent distributionsof emission measureas shownin Figure 2.Vertical scalesarethe samefor all threespectra;individual zero points areindicatedfor eachspectrum.Strongemissionat )1133 is dueto Fe XXIII occurringin theTwoTempmodel.Its peakvalueof 5 x i0~ photonscm2 s~4A1 hasbeentruncatedin this Figure.

(11)40 A.K.Dupree

FLARE STARS

Flaringphenomenaoccuron starswith energeticsthat arefar in excessof thoseobservedon the Sun. Theseoccasionsof rapid releaseof radiationhavebeencaughtfor a few starswith satellitessensitiveto X-raysandultraviolet emissions,andthis allowsus to estimatewhat might be expectedin the ETJV.

Proxima Centauri, a (very) nearbydMe star fortuitously flared during a coordinatedobservationwithHEA0-2andIUE, andtheemissionmeasuresderivedby Haischandcolleagues/11/ allowedus to constructaquiet andflare model from which to evaluatetheEUV spectrum(seeFigure 6) using the line list fromDoschekandCowan /3/. Becauseof thesubstantialamountof hot materialin Prozirna Cen’s atmosphere,relativeto theamountof low temperatureplasma,the ETJV spectrumis particularly rich in high stagesofionization:Fe X, XI, XII, etc.

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Fig. 4. Schematicillustration of the emission to be expectedin four energyregions froman interactingbinarysystemconsistingof ared dwarfstar, anda white dwarfsurroundedby anaccretiondisk. Figure from Bath andPringle/6/.

In addition to the flare starsthat arecommonlyrecognized,thereis evidencefor highly energeticphenom-enaon “normal” giant stars. Recentobservationsin the optical spectralregionindicate that the flaringphenomenonmaynot berestrictedonly to flare starsor to activebinary systemssuchasthe RS CVn stars.Baliunas andcollaborators/12/ observedthe chromosphericCa II lines in giant stars,suchasa Tauri(KS III). Cool giant starsarenot believedto havehot outer atmospheres,andhenceextremeultravioletemissions.Rapidfluctuationsin the luminosityof theCaII transitionswerediscoveredthat containa totalenergyof iO~°to 1032 erg. This energyis a factor of 102 to i0~greaterthan theenergyof Ca II flaresobservedin the Sun. Such observationssuggestthat the flaring phenomenonmay bemuchmoreprevalentin normal starsthanonemight think. The EUV radiationfrom thesestarsmay prove to be greater,andtheatmospheresmore violent than anticipatedfrom studyof their ultraviolet spectra.

LUMiNOUS GIANT AND SUPERCIANTSTARS

Recentstudiesof theHe I, )~10830line mayhaveimplicationsfor EUV emissionfrom giant andsupergiantstars.The A10830 line hasbeenobservedin manytypesof stars:giants(a Boo) andsupergiants(a Aqr)/13/ andCepheids/14/, in addition to the Sun. The line is consideredto be a chromospherictransition,but studyof the helium atom andobservationsof the He II A1640 transition show that thepopulationof the lower level of the .A10530 transition, 2s

3S, is strongly influencedby the presenceof X-rays in astellaratmosphere.And, whereX-rays exist, it is highly likely that EUV emissionalso is present. Theenhancedpopulationof thelower 2s 3Slevel happensthroughrecombinationwhichfollows EUV andX-rayphotoionizationof neutralhelium.

The substantialeffect of illumination of theatmosphereby EUV andX-ray radiationcan be seenin thecalculatedprofiles of the A10830 line in the giant star a Boo, Ki III, (seeFigure 7). Constructingasemi-empiricalmodel of theatmospherebasedon a low chromosphericmodel /15/ combinedwith ahighchromosphericmodel consistentwith IUE fluxes, Dupree,Whitney, andAvrett /16/ evaluatedthe He I,A10830profile. The bestagreementwith theobservedcentraldepthis achievedwhen X-ray illuminationis included. Although X-rays havenot beendetectedfrom AlphaBoo to date,theassumedX-ray flux is

Cool StarsandStellarSystems (11)41

consistentwith theupperlimit inferred/17/ from theIPCdataobtainedwith Einstein(HEAO-2). A similarresultis foundfor the hybridsupergjant,aAquarii, althoughin this star, C IV andN V emissionarepresentin theultravioletspectrum/18/. Moreover,onenearbymemberof thehybrid class,a TrA, showsX-rayemission/19,20/.* And recently Baliunasand Robinson/22/ haveidentified tracersof surfacemagneticactivity in a and fi Aquarii through long term monitoring of thestrengthof CaII H andK emissions.Such observationscomplementthecalculationsin suggestingthat EUV radiationsmaybe presentin theatmospheresof giant andsupergiantstars.

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arisingfrom anaccretiondisk. Thetop (emission)lineprofile occurswith anedge-onpresentationof thedisk; thelowest(absorption)profile representsaface-ondisk presentation.Otherlineprofilesrepresentintermediateorientations.Thesolid curvesrepresentamassloss rateof b_bo M® yr~,andthebrokenlines indicateanorderof magnitudeless. From RaymondandMauche/10/.

An alternateinterpretationof the .X10830 profilesin thegiant star,a Boo hasbeenproposedby Curitz andLuttermoser/23/. Theysuggestedthat thestrengthof theline canbe explainedsimply by thepresence

of shockwavesin theatmosphere.Thereis substantialevidencefor aperiodicmotions in theatmosphereof this star/24,25,26/.Shocksmaycontributeto thestrengthof theline profile, but acarefulquantitativeassessmentneedsto be madeof theconsequencesof shocksupon thestrengthsand line profiles of otherchromosphericfeaturesin this star. And adeepsearchneedsto bemadefor EUV andX-ray emissionsfrombright targetssuchas these.

* Ayres /21/ suggestedthat thehybrid giant a TrA might havea companionbasedon a possibleul-

traviolet continuumexcess,but subsequentcarefulanalysis(Ayres andBurton, in preparation,quoted by/20/) revealedthat the spectrumof a TrA is typical of other hybrid stars.Thus, it is now believedthat acompanionstardoesnot exist.

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(11)42 A.K. Dupree

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Fig. 7. Profilesof the He I, A10830 transition in a Boo, with and without illumination byX-rays from 106 K plasma. Illumination by the (assumed)X-rays increasesthe centraldepthoftheline to bettermatchtheobservedprofile. Calculationfrom Dupree,Whitney,andAvrett /16/.

CoolStarsandStellarSystems (11)43

BINARY SYSTEMSOF COOL STARS

Binary systemscontainingcool starsappearto differ significantly in ‘their atmosphericstructurefrom theatmospheresof singlestars/2/. Manysystemshaveshorterperiodsthanasinglestarof similar temperatureandluminosity, andtheincreasedrotationalvelocityof thecomponentsis ‘believedto enhancethemagneticdynamoandleadto increasedemissionfrom theatmosphere.Surveysof RS CVn andW tJMa systemsdocumentthis for ultraviolet andX-Ray emissions/27,28,29/.

Much effort hasbeendirectedtowards the solar-stellarconnection in the study of binary systems,bysearchingfor theequivalentsof solarphenomenaandfeaturesin’ theemissionfrom their atmospheres.Thepresenceof regionsof chromosphericactivity correlatedwith theappearanceof starspotshasprovedfruitfulsincetheoriginal identificationof this behaviorin A And /30/. However,we believethat thesolar analogydoesnot fit well to thecoronalbehaviorof cool,starsin binarysystems.Thereis accumulatingevidenceforextended,or perhapscommonenvelopecoronaein thesesystems,andDupreeandKenyon/2/ suggestthatstellarwinds andtheir interactionbetweencomponentsof abinary systemoffer an attractivemechanismto producehigh temperatureplasmaon ascalelargecomparedto thebinarysystem.Theremaywell be a“solar” componenttoo, which by analogywith theSun is characterizedby magneticallyconfinedmaterialwith scalessmall ascomparedto thestellar radius.

In theSun, thevariability of thespatially unresolvedEUV andX-Rayflux dependsuponthe temperatureof formation of theemitting ionic species.Line emissionformedat highesttemperaturesshowsa greatervariationwith appearanceof active regionson thesolar disk than doesemissionfrom lower temperatures(seeFigure 1). Whensolarimagesin theEUV /31/ andX-rays /32/, wereobtained,theydemonstratedtheimportanceof magneticfields in definingstructuresin thesolaratmosphere.The scaleof thesestructuresrangefrom 108 to io’°cm /33/, generallyless thanthe solar radius. If structuressimilar to theseoccurin stellar atmospheres,we would expect to detecta modulation of emissionfluxes like that observedontheSun. Early observationswith’ IUE , in fact, first clearly revealedenhancementof chroniosphericMgII, andground-basedmeasuresshowedsimilar enhancementof Ca II in the RS CVn binary A And whenphotometryindicatedthat starspotswerepresent’on theactivestar/30/. However, thereareseveralsetsofmeasurementsin theultravioletandtheX-rayregionssuggestingthatemissionfromthehighesttemperatureplasmadoes not arisefrom well-definedmagneticloop structuressimilar to thoseon the Sun. In theWUrsaeMajoris eclipsingbinary system,VW Cephei,astrongsourceof X-ray emission /34,29/,a 14-hourobservationwith theEinsteinsatellite(covering2 orbital periods)revealedthat X-rays arealwayspresent,andthat they arenot modulatedin phasewith theoptical brightness/35,36/. By contrast,chromosphericlines in other contactbinary systemsshowthemodulationexpectedfrom theoptical light curve. This lackof correspondencewith theopticalandultravioletmeasuressuggeststhat thecoronaextendsto a distancebeyondastellarradius,andcanpossibly surroundthewholebinary system.V471 Tauri is anothereclipsingbinary systemin whichthe K-dwarf componentappearsto havehot structuresthat extendto a heightcomparableto a stellarradius/37/, providing direct evidencefor a new kind of extendedstructurein theatmospheresof dwarf stars.Analysisof theX-rays from AR Lac,an RS CVn-type system,showsthat oneX-ray componentoriginatesin an extendedregionthat lies outsidetheRochesurfaceof thesystem/38,39/.Theprototypeof theAlgol variables,/3 Persei,also showsevidencefor largescalestructures/40/ asinferredfrom EXOSATobservations.

Theseexamplessuggestthathot plasmacanexist in binarysystemswith asizecomparableto or largerthanthescaleof thesystem.All oftheseobservationshavebeentraditionally interpretedasarisingfrommagnetic“loops” in thestellaratmospheres.While theremaywell be a typical “solar-like” component,DupreeandKenyon /2/ proposedthat anotherphenomenonalso exists in thesesystems: namely that colliding orinteractingstellarwinds canproducehot regionson a scalecomparableto thesizeof the system. Simpleenergeticargumentsdemonstratethat for typical wind velocities andnominal massloss rates,post-shocktemperaturescanrangefrom iO~to io~K, andproduceacomponentof, if not thetotal, X-ray emissionin

suchsystems.

The configuration of sucha shockedregion can be calculatedfor variousstellar systems,and then theultraviolet, EUV, andX-ray emissionpredictedasa function of phase. The configurationof interactionmight beexpectedto be asymmetricinmost cases,unlessthestarsandtheir massloss ratesandvelocitiesweresimilar. Thus we might expect‘the shockto bendarOundthestarhavingthe weakerwind, forminganextendedinteractioncone. Our preliminarycalculations(seeFigure 8) basedon the formalismof GirardandWilison (/41/, with errorscorrected).demonstratethisbehaviorclearly. Line widths will alsobebroad,andcanbe usedto infer shockvelocities:similar to radiativebow shocks/42/, [There arehints that thehigh temperature0 VI line (A 1032)in Capella an RSCVn-type system - is surprisinglywide - morethantwice thesoundspeedat the temperatureof formation /43/.]

(11)44 ‘ A.K. Dupree

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Fig. 8. Approximatecalculationsof theshapeof thestagnationline where thedynamicalpressuresareequalfor abinary systemwhosecomponentshavewinds of varyingrelativestrengths.Thestarsarelocatedat 0 and1 onthe x-axis. thevariouscurvesare denotedby theratio of ls’Iv~for star1 to that for star2. Note thatwith equal winds (curve 1), the stagnationline is verticalandsituatedmidwaybetweenthestars,but as thewind from star1 becomesstrongerrelative tostar2, theshockedregionincreasinglybendsbackaroundstar2.

Observationsof thesesystemsat variousorbital phasescould revealthedistributionof materialwith tem-perature.Onemight expectlow temperaturespeciesto arisenearthestars,andhigh temperaturespeciestohaveboth asolar “coronal” component- arising from magneticallyconfinedmaterialassociatedwith activeregions- andanextendedenvelopearisingfrom materialmarking the interactionof thestellarwinds or acommonenvelopecorona. Line profiles would beespeciallyinteresting. In the future, ROSATandEIJVEcould provide the important observationsof high temperatureplasmaexpectedto define the interactionregionsfor sufficientlyhigh velocities andmasslossrates.

CoolStarsandStellarSystems (11)45

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