Fe XIII coronal line emission in cool M dwarfs PDF
Preview Fe XIII coronal line emission in cool M dwarfs
Astronomy&Astrophysicsmanuscriptno.AA0464 February2,2008 (DOI:willbeinsertedbyhandlater) ⋆ Fe XIII coronal line emission in cool M dwarfs B.Fuhrmeister, J.H.M.M.SchmittandR.Wichmann HamburgerSternwarte,UniversityofHamburg,Gojenbergsweg112,D-21029Hamburg e-mail:[email protected] thedateofreceiptandacceptanceshouldbeinsertedlater 4 0 0 Abstract. We report on a search for the Fe forbidden coronal line at 33881.1 Å in a sample of 15 M-type dwarf stars 2 coveringthewholespectralclassaswellasdifferentlevelsofactivity.AcleardetectionwasachievedforLHS2076duringa majorflareandforCNLeo,wherethelinehadbeendiscoveredbefore.Forsomeotherstarsthesituationisnotquiteclear.For n a CNLeoweinvestigatedthetimingbehaviour oftheFelineandreportahighlevelofvariabilityonatimescaleofhours J whichweascribetomicroflareheating. 8 Keywords.stars:activity–stars:coronae–stars:late-type 1 v 2 1. Introduction those latter wavebands are extremely difficult since the coro- 0 nal emissions are much fainter, and especially in the optical, 1 Thesolar coronacanbe studiedatextremelyhighspatialand 1 the weak coronal emission has to be detected above the usu- spectralresolutionoveralmosttheentirerangeoftheelectro- 0 ally much brighter optical photospheric emission. The most magneticspectrum.Withtypicalcoronalplasmatemperatures 4 promising candidates to search for optical coronal emission 0 of a few million degreesthe bulk ofthe Sun’s coronalenergy are clearly late-type M dwarf stars, which can be as X-ray / lossesisemittedintheX-rayrange,theenergylossesatshorter h brightasorevenX-raybrighterthantheSun,butwhosephoto- p andlongerwavelengthsbeingconsiderablysmaller.Therefore sphericemissionisratherfaintinparticularatnearUVwave- - observationsofthesolarcoronaintheopticalarequitedifficult o lengths. The detection of coronal emission in the optical was because of the faintness of the corona compared to the pho- r recently accomplished for the active M-star CN Leo, where st tosphere in this wavelengthrange. This problem can be over- Schmitt&Wichmann(2001)wereabletodetecttheFefor- a comeatleastforobservationsabovethesolarlimbifthepho- bidden coronalline at 3388.1 Å. This successful detection of : tosphericlightis blocked,e.g.by the Moonduringan eclipse v coronal emission in one star raises the question to what ex- Xi or–artificially–byacoronograph.Opticalobservationsofthe tentsuchdetectionsarepossibleforothercoolstarsaswellor solarcoronacarriedoutinthatfashionwerethefirsttoreveal whetherCNLeoisauniqueandsingularcase. r thetruenatureofthecoronaasahotplasmawithtemperatures a Inthispaperwewilldiscusstheproblemsofdetectingthe muchhigherthantheunderlyingchromosphereorphotosphere. 3388.1Å forbiddencoronallineinasmallsampleoflate-type However,thesolarcoronacouldonlybeobservedoffthelimb starsandpresentananalysisofthetemporalvariabilityofthis intheoptical,andonlythroughsatellite-basedimagingobser- line in the ”proto-type” CN Leo. Our paper is structured as vationsofthecoronaatX-rayandEUVwavelengthsthecorona follows: In section 2 we describe the VLT data used for our atlargecouldbeobservedandstudied. analyses and the sample of investigated stars. In section 3 an TheexistenceofcoronaesimilartothatoftheSunaround overviewover the spectral range under investigationis given, essentially all late-type main sequence stars with outer con- whileinsection4wedealwiththerotationalvelocityofthean- vection zones has been established by extensive X-ray stud- alyzedstars.Insection5theresultsofoursearchfortheFe ies (e.g. Schmitt(1997)). As is the case for the Sun,the bulk line forindividualstarsarepresentedandthetimingbehavior of the energy losses for stellar coronae also occurs in the X- ofthelineforCNLeoisdiscussedinsection6.Section7deals ray range, and consequently this spectral band is the most withtheX-raytoFelineratioforCN LeoandLHS2076. natural one for the study of stellar coronae. However, coro- Insection8wedescribetheresultsofoursearchforotherfor- nal emission from stars can also be observed in the radio biddencoronallinesinCNLeo. range (e.g. Berger (2002) or Gu¨del (2002)) and in the op- tical (Schmitt&Wichmann (2001)). Coronal observations at 2. Observationsanddataanalysis Sendoffprintrequeststo:B.Fuhrmeister ⋆ Based on observations collected at the European Southern The observationsreported in this paper were obtained in vis- Observatory,Paranal,Chile,68.D-0166A. itor mode with ESO’s Kueyen telescope at Paranal equipped 2 B.Fuhrmeister,J.H.M.M.SchmittandR.Wichmann:FeXIIIincoolMstars withtheUltraviolet-VisualEchelleSpectrograph(UVES)from In order to give an overview of the nature and quality of March, 13th to 16th in 2002, with the exception of UV Ceti, our spectra, we show (in Fig. 1) the spectral range between ProxCen,LHS292andoneoftheCNLeospectra,whichwere 3370 Å- 3390 Å for most of our program stars. LHS 292 is observedinservicemodeduringthewinterseason2000/2001. not shown since its spectrum is very similar to DX Cnc, and For the March 2002 run the instrument was operated in a LHS 428 and LHS 6158 are not shown since they are both dichroicmode,yielding33echelleordersinthebluearm(spec- double stars with very complicated spectra that could not be tral coverage from 3030 to 3880 Å) and 39 orders in the red disentangled and were therefore excluded from the analysis. arm(spectralcoveragefrom4580to 6680Å). Forthe runsin The spectra shown in Fig. 1 are sorted by decreasingspectral the winter 2000/2001 a monochroic setup was used, provid- type.Onenotesa distinctsequencefrom“early”M dwarfsto ingusonlywiththebluepartofthespectrum.Inthedichroic “late”Mdwarfs.Inthe“early”MdwarfslikeGl229AandAD setuptheredpartofthespectrumisrecordedontwoseparate Leo one still recognizes a clear photospheric spectrum in the CCDs; therefore there is a spectral gap from ∼ 5640 to 5740 range 3370 Å- 3390 Å with some additional chromospheric Å, resulting from a spatial gap between the two CCDs. As a emission lines, while in the later type M dwarfs like CN Leo consequenceofthevariousinstrumentalconstraintswecannot andthe(flaring)LHS2034analmostpureemissionlinespec- observethelinesfromH uptoH oftheBalmerseries,nordo trum appears;these latter spectra are obviouslydominatedby 3 8 wecovertheCaHandKlines.Thetypicalresolutionofour thestellarchromospheresratherthantheirphotospheres. spectra is ∼ 45000, typical exposures times were 20 minutes The Fe line at 3388.1 Å detected in CN Leo by exceptfor the brighter among our sample stars. Normally for Schmitt&Wichmann(2001)is–unfortunately–blendedwith eachstarthreeexposuresweretakeninsequenceinordertofa- a Ti chromospheric line at 3387.846 Å.This particular Ti cilitatetherecognitionofcosmics,againwiththeexceptionof line and other emission lines from the same multiplet are thefourspectraof CN Leo,UV Ceti, ProxCen andLHS292 presentinallrecordedspectrabesidesthenon-flarespectraof takenduringthewinterseason2000/2001. LHS2034andLHS2076andthespectrumofDXCnc.InGL A list of the observed stars and the available spectra for 229ATiappearstobepresentinabsorption. each star is providedin Table 1, giving the observation dates The stars earlier than M4.5 have a rather high pseudo- andtotalexposuretimes.Moreoverweprovidesomebasicpa- continuumintheconsideredwavelengthrangewhichisclearly rameters of the sample stars, including the highest detected dominated by overlapping absorption lines. This continuum Balmer line in our data as an estimator of activity. Note that stronglydecreasesforthespectraltypesM3.5-M4.5starsby during the March 2002 run CN Leo was observed nightly to afactoroften,accompaniedbythevanishingofthestrongab- carry out an investigation of its chromospheric and coronal sorption features in this regime. Consequently, a detection of variability. the Fe line should be much easier for spectral types later All data were reducedusing IRAF, includingflat-fielding, than M4.5. Therefore the most promising search for coronal orderdefinitionandscatteredlightsubtraction.Thewavelength lineemissioncanbemadeforstarsinthespectralrangeM4.5 calibrationwascarriedoutusingThorium-Argonspectrawith - M6 since for later stars even the normally strong chromo- anaccuracyof∼0.03Åinthebluearmand∼0.05Åinthered sphericTiemissionlinescannotbedetectedanymore. arm.InadditiontherearephotometricdatafromtheUVESex- posuremetertakenforengineeringpurposesandthereforenot 4. Determinationofrotationalvelocities flux calibrated.Still, these data were usefulto assess whether thestarwasobservedduringquiescenceorduringamajorflare. A critical issue for the successful separation of the chromo- The spectral line fits were carried out with the CORA fit spheric Ti II emission line at 3387.846 Å and the coronal program (Ness&Wichmann 2002). This software tool was FeXIIIemissionlineat3388.1Åaretheintrinsiclinebroaden- originallydevelopedforanalyzinghighresolutionX-rayspec- ingprofilesoftheselines.Thewavelengthdifferencebetween tra, but the fit algorithms employed by CORA are also well thoselines(≈0.26Å)correspondstoavelocityof≈23km/sec, suited for the modeling of well-defined chromospheric and easily within the reach of thermal and rotational velocities. coronalemissionlines.Theprogramprovidesanaccurateerror Obviously,thethermalbroadeningprofilesdiffersubstantially analysis.All fits werecarriedoutusingGaussian line profiles because of the different origins of these lines, and rapid ro- aftershiftingthewavelengthtothestars’restframe. tation is expected at least for the more active stars. Since we clearly need accurate values for vsin(i) for disentangling the 3. DetectionoftheFe line TiIIandFe XIIIemissionlines,wedecidedtomeasureitfor some of our program stars. In order to determine vsin(i) we The prime target of our observing program was the spectral used 18 of the red arm orders that show no strong emission regionaround3388Å.Thespecifictransitionunderconsider- lineseitherfromthestarorfromterrestrialairglow.Asatem- ationis 3s23p2 3P −1D . Flower&PineaudesForets(1973) plate we decided to use CN Leo since CN Leo is known to 2 2 give a detailed discussion of the atomic physicsof Fe and rotateveryslowly(vsin(i)< 2.9kms−1(Delfosseetal.1998)). provide level population calculations for a range of tempera- ThemeasuredspectrumofCNLeowasspunupwithrotational tures and densities, considering direct excitation due to colli- velocitiesfrom3upto45kms−1.Thebestfitvaluefortherota- sions with electronsand protonsand indirectexcitation of al- tionvelocityvsin(i)ofthespunuptemplateandthestarunder lowedtransitionsviacollisionswithelectronsfollowedbyra- studywasdeterminedwithaχ2-testforeveryorderused.The diativedecayandfindratherlowdependenceontemperature. final rotationalvelocity was determinedby averagingthe val- B.Fuhrmeister,J.H.M.M.SchmittandR.Wichmann:FeXIIIincoolMstars 3 Fig.1.Theconsideredwavelengthrangeofselectedprogramstars.Somestrongabsorptionlinesareidentifiedinthetopspec- trum.TheTiemissionlinesaremarkedinthespectrumofAdLeo. 4 B.Fuhrmeister,J.H.M.M.SchmittandR.Wichmann:FeXIIIincoolMstars Fig.1.(continued)ForLHS2034andLHS2076thespectraareshownduringflareandduringquiescentstate.Notethatthese fourspectraandthespectrumofUVCetarenotaveragedspectraandthereforesomeremainingcosmicswerereplacedmanually byastraightline. B.Fuhrmeister,J.H.M.M.SchmittandR.Wichmann:FeXIIIincoolMstars 5 Table1.Basicobservationsparametersoftheobservedstars. name other spectral logL 1 observations Tilines highest X name type Balmerline LHS1827 GJ229A M1 27.13a 2002-03-154spectra1200s absorption - LHS428 M3e 28.75a 2002-03-152spectra1200s absorption H 18 LHS6158 M3.5 28.76c 2002-03-152spectra2400s weakabsorption H 18 LHS5167 ADLeo M3.5 28.92a 2002-03-133spectra1800s emission H 18 2002-03-162spectra1200s HD196982 ATMic M4.5 29.55c 2002-03-162spectra2400s emission H 20 LHS1943 YZCMi M4.5e 28.67b 2002-03-133spectra3600s emission H 24 LHS2664 FNVir M4.5 27.92b 2002-03-133spectra3600s weakemission H 18 LHS324 GLVir M5 27.65a 2002-03-133spectra3600s weakemission H 19 2002-03-162spectra2400s LHS36 CNLeo M5.5 27.78a 2002-03-136spectra7200s emission H 24 2002-03-144spectra4800s 2002-03-156spectra7200s 2002-03-166spectra7200s 2001-01-061spectrum3120s LHS2076 EICnc M5.5 27.60a 2002-03-154spectra4800s - H 18 2002-03-161spectrum1200s LHS49 ProxCen M5.5 27.26a 2001-02-021spectrum3120s weakemission H 18 LHS10 UVCet M5.5 27.31a 2000-12-171spectrum3120s emission H 18 LHS248 DXCnc M6 26.60a 2002-03-163spectra3600s - H 11 LHS2034 AZCnc M6 28.89c 2002-03-146spectra6000s emission H 17 2002-03-162spectra2400s LHS292 M6.5 2001-02-021spectrum3120s - H 15 1inerg s−1 aSchmittetal.(1995) bDelfosseetal.(1998) cHu¨nschetal.(1999) dSchmitt&Liefke(2002)X-rayluminosityisvariable ues obtained in the different orders. We mostly used only the Table2.Rotationalvelocitiesfor(someof)ourprogramstars. redpartofthespectrumsinceinthebluepartofthespectrumit LHS428isomittedbecauseitisadoublestarandcouldnotbe isratherdifficulttofindlargerregionsunaffectedbyemission separated.LHS6158evenseemstobeatriplesystem.GJ229A lines.Wetriedtoavoidthisproblemofthebluespectrabyus- issaturatedintheredarm.ProxCenwasusedastemplatefor ingonlyverynarrowspectralranges(∼10Å)ofeveryaperture, UVCet,FNVirandGLVirinthebluearm.Allmeasurements yettheissuesofwhichlinesareinemission,whichshowemis- inkms−1. sioncoresandwhicharepurelyinabsorptionareverydifferent star vsin(i) vsin(i) literature for each program star. Therefore the template has to be very redarm bluearm similar to the test star and the used wavelengthintervalshave ADLeo 7.6±1.2 6.2±0.81 tobedeterminedindividuallybuteventhenthescatterforthe ATMic 11.7±0.9 wavelength intervals is bigger than in the red arm. Therefore YZCMi 6.7±0.6 7.0±2.02,6.5±1.11 we determined the rotational velocity for only three stars in FNVir 17.4±1.4 13.1±3.7 16.8±2.11 the blue arm. For these stars we used Prox Cen as template GLVir 15.3±0.8 16.0±2.0 9.2±1.91 sinceitsspectrumisverysimilartothesestars.CNLeocould UVCetB 32.5±2.02 not be used as template because it is much more active than UVCetA - 25.9±8.6 themeasuredstars.Therotationalvelocitiesdeterminedinthis DXCnc 8.9±0.8 11.0±2.02,8.1±1.11 fashionarelistedinTable2.Ingeneral,thevaluesdetermined LHS2034 7.9±2.8 by us agree with previouslydeterminedones foundin the lit- 1Delfosseetal.(1998) eraturewiththeexceptionofGLVir,wherewefindasubstan- 2Mohanty&Basri(2003) tiallylargerrotationalvelocitybothintheredandbluearmthan Delfosseetal.(1998).Acarefulvisualinspectionandcompar- ison of the spectra of GL Vir with those obtained for FN Vir ForthedoublestarUVCetwemeasurearotationalveloc- and AD Leo shows a very high similarity to the faster rotat- ityof25.9±8.6kms−1 whichagreeswiththevaluefoundby ingspectrum.Moreoverweusedatemplatestarverysimilarto Mohanty&Basri (2003) for UV Cet B. Since UV Cet A has GL Vir in spectral type while Delfosseetal. used a synthetic asimilarrotationvelocity(G.Basri,privatecommunication)it K0III spectrum. Thereforewe do favor our (larger)rotational cannot be decided from the rotationalvelocity which compo- velocity. nentofUVCetdominatesourspectrum. 6 B.Fuhrmeister,J.H.M.M.SchmittandR.Wichmann:FeXIIIincoolMstars 5. Observationsofindividualstars series (see section 6). The central wavelength of the absolute line position is a bit on the blue side but well within the 2- Since the spectral propertiesof our sample stars are very dif- σerror,whiletheabsolutepositionsofthethreesinglefitTi ferentwe appliedspecific methodsforthe individualstars for linesseemtobeslightlyredshifted(butonlytowithin1-σerror a detection or non-detectionof the Fe line. These are dis- each).The3387.84Å Tilineunfortunatelyhasalargeerror cussedindetailbelow. initscentralwavelength.Giventhissituation,wecannotdraw any meaningful conclusions about possible wavelength shifts 5.1.CNLeo andremarkthatduringaflare lineshiftsfrequentlyoccurdue tomassmotions. WereportacleardetectionoftheFelineforCNLeo,which is also found to be variable on a timescale of hours, as will be discussed in section 6. Thus the Fe XIII line detection of 5.3.GLVir Schmitt&Wichmann(2001)isfullyconfirmed. The continuum for GL Vir is low as for CN Leo, but un- like CN Leo the Ti at 3380 Å line is barely detectable. 5.2.LHS2076 Neverthelessthereisabroademissionlineat3387.81±0.14Å with a half width of 0.15 Å and an amplitude of 141.7±28.9 AcleardetectionoftheFe3388.1Å emissionlinehasbeen counts. If this emission is attributed to the Ti line, the mea- obtained during a flare on LHS 2076. LHS 2076 is a double sured amplitudewould contradictthe atomic data, whichpre- star known for its flare activity and separated by 3′′.4 with dicts the same intensity as for the 3380 Å line (which is not both components being late-type (Pettersen 1985). Our spec- seenatall,cf,Fig.1).If,ontheotherhand,thisfeatureisiden- trumreferstoonesystemcomponentwithsomeminorcontri- tified with the Fe line, it is clearly blue shifted. Since the bution from the second star. Duringthe last of the four expo- starisinaquiescentstate(ascanbeseenfromthelightcurve, suresonMarch,15thadouble-peakedflarecanberecognized whichisnotshownhere)itisunlikelythatablueshiftiscaused inthephotometerlightcurveasshowninFig.2. bymassmotions.However,theblueshiftcouldalternativelybe OutsidetheflareintervalnosignificantTiemissionisde- causedbyabsorptionattheredwingoftheFelineduetoa tectableinthespectra.Theemissionlinepropertiesoftheflare CoIlineat3388.17Å whichisalsoseeninotherspectra(see spectrum are listed in Table 3. The emission feature at 3387 Fig.1).MoreevidenceforaninterpretationinvolvingtheFe Å is quite broad and was fitted with a double line compo- linecanbegainedfromtheamplitudeofthe3383.71Å lineof nentwith amplitude,centralwavelengthand FWHM for both 65 ± 15 counts. The 3387Å Ti II line should have a smaller componentsasfreeparameters.TheotherTilineswerefitted amplitudeaccordingtotheatomicdataandthemeasuredspec- as single lines with all three parameters (λ ,A and σ) free. cen traofCNLeo.Ontheotherhand,thehalfwidthsofthe3383.71 Inspecting the properties of these three Ti lines (cf., Table Å line(0.13±0.02Å)andofthe3387.81Å line(0.15±0.01) 3) one finds that the fitted half widths of all three lines agree are consistent with each other. The Ti II line at 3372.73Å is verywellwitheachotherasexpectedforlinesfromthesame unfortunatelytoodeformedtoallowanyconclusionsonitshalf multiplet.Thehalfwidthofthenarrowcomponentofthe3387 widthoramplitude.Thereforethe questionariseswhetherthe Å line also agrees well with the half widths of the three Ti halfwidthsofthetwolinesat3383.71Å and3387.81Å canbe lineswhilethehalfwidthofthebroadcomponentismorethan causedbyrotation.Therotationvelocityvsin(i)forGLVirwas threetimeslarger.Wethereforeconcludethatthenarrowcom- measuredby Delfosseetal.(1998) as9.2± 1.9kms−1,while ponent of the broad emission feature at 3387 Å must be the we determined15.3± 0.8kms−1 (see section4).Thus,if our fourthTilineinthemultiplet,whilethebroadcomponenthas rotationalvelocityiscorrect,thehalfwidthsσcanbeexplained to be identified with the Fe line. Further evidence for this byrotationalbroadening,however,if vsin(i) =9.2kms−1,the interpretationcanbefoundfromthelines’amplitudes.Atomic measuredhalfwidthisasomewhattoolargetobeexplainedby dataavailablethroughtheNISTatomicspectradatabase1 for therotation.SothedataofGLVirarenotunambiguous. theselinespredictequalrelativeintensitiesforthetwolinesat 3372.80 and 3383.77Å and equal relative intensities for the twolinesat3380.28and3387.84Å loweredbyafactoroffour 5.4.LHS2034 comparedtotheformertwolines.AccordingtoTable3theam- The flare star LHS 2034 was observed during a longer flare plitudesofthe3372.80and3383.77Å lineagreetowithinthe onMarch,14th,lastingoverhalfanhourascanbeseenfrom errorsandthetwootherlinesareweaker.Thereforethebroad component cannot be attributed to Ti since its amplitude is the light curve of the photometer (not shown here). While only the very strongest chromospheric emission lines can be more than twice as large as that of the 3372.80 or 3383.77 seeninquiescence,amultitudeofemissionlinesbecomesvis- Å lines. On the other hand, both amplitudes and half widths ible in the spectrumtaken duringflare maximum.Indeed,the fit perfectlyif the broad emission componentis identified the Feline;also,thefittedhalfwidthsagreeverywellwiththe flarespectrumofLHS2034isverysimilartothespectrumof half width determined for the Fe line in the CN Leo time CN Leo with a rather flat continuum without strong absorp- tion lines and dominated by strong chromospheric emission 1 availablevia lines. Surprisingly,however,we did notfind anyevidencefor http://physics.nist.gov/cgi−bin/AtData/main asd Feemissioninthisspectrum.Thesameanalysisprocedure B.Fuhrmeister,J.H.M.M.SchmittandR.Wichmann:FeXIIIincoolMstars 7 Fig.2. To the left the light curve of the flux of LHS 2076 on March, 15th, correspondingto a single spectrum. On the x-axis theuniversaltimeisgiven.Thefluxisinarbitraryunitssincethephotometerisusedforengineeringpurposes.Theupperlight curvecorrespondstotheredarmofthespectra,whilethelowerlightcurvecorrespondstothebluearmandisscaledrelativeto theredfluxforconvenience.Twoshortdurationflarescanbeseeninboththeredandthebluearmofthephotometerfollowing eachotherrapidly.TotherightthespectrumofLHS2076duringtheflarearound3388Å showinganbroademissionfeatureat 3387.92Å. Table3.PropertiesoftheTiandtheFelineduringtheflareonLHS2076.Givenaretheamplitude(inelectrons),thecentral wavelength and half width σ of each line for the best fit. For the 3380.28 Å line no error estimation for the half width was possibleduetothelowsignaltonoiseratio. Ti(3372.80Å) Ti(3380.28Å) Ti(3383.77Å) Ti(3387.84Å) Fe(3388.1Å) amplitude 134.3±19.2 67.3±17.3 115.1±17.3 11.7±16.0 305.5±35.1 centralλ 3372.85±0.05 3380.33±0.15 3383.81±0.04 3387.81±1.3 3387.93±0.1 σ 0.06±0.01 0.06 0.05±0.01 0.04±0.01 0.20±0.02 as applied for LHS 2076 was used for the flare spectrum of LHS2034.AllfourTiIIlinesarefoundtohaveaboutthesame FWHM as expected but also the same amplitude contrary to expectationfromatomicdataandwhatwasfoundforCNLeo and LHS 2076. This fact remains unexplained. In particular, we cannot attribute – as in LHS 2076 – the excess emission to Fe XIII, since this would contradictthe measured FWHM. Obviously,efficientchromosphericheatingofLHS2034does occurduringthe flare,butthe coronaiseithernothotenough ortoohotforproducingadetectableFelineflux. 5.5.DXCncandLHS292 Fig.3.SpectrumofCN Leo witha signalto noiseratioartifi- ciallyreducedtothatofDXCnc.Butdespitethelowsignalto These two stars and LHS 2034 are the latest-type stars in the noiseratioclearlytheTiandFelinescanberecognized. sampleandhaveaverylowsignaltonoiseratio(whichapplies for the quiescent spectrum for LHS 2034 as well) which can causethenondetectionofFe.Inordertoanswerthisques- 5.6.UVCet tionwetookthespectrumofCNLeoasatemplateandreduced it to the lower signal to noise ratio foundin our DX Cnc and UV Cetisa binaryflarestarwellknownfromtheradiotoX- thequiescentLHS2034spectra.Theresultsofthisexercisefor ray band (Stepanovetal. 1995) and has been resolved in the DX Cnc are shownin Fig.3. Clearly theTi andFe lines X-rayforthefirsttimeonlyrecently(Audardetal.2003).Our canberecognizedinthisexample.Carryingoutalargernum- UVES spectrum was taken with both components in the slit, ber of such simulations we found that the Fe need not al- butwewereunabletoresolvethespectraofthetwostars(due waysberecognisable,buttheTilineshouldalwaysbefound totheseeing).Thereforethespectrumshouldbedominatedby in lower SNR spectra.Thusthe levelof activityin these stars thebrighterA-component.Theemissionlinesseeninthespec- mustbelowermakingthepersistentpresenceoftheFeline trumofUV Cetseemtohavesuperimposedasecondcompo- unlikely. nentofnarrowemissionlineswhichareslightlyredshifted.We 8 B.Fuhrmeister,J.H.M.M.SchmittandR.Wichmann:FeXIIIincoolMstars Fig.4.ThespectrumofUVCet3388.1Å (grey)incomparison tothespectrumofCNLeo(black)broadenedartificiallytothe rotationalvelocityofUVCet. tentativelyinterpretthissecondsetofemissionlinesasanac- tive chromospheric region occupying only a part of the stars surface,sinceitistoonarrowtobeascribedtothesecondcom- ponentofthebinarysystem. SearchingfortheFelinewecomparedthebroadcom- ponentofthespectrumofUVCetwithanartificiallybroadened spectrumofCNLeo(seeFig.4).TheFelinedisappearsin Fig.5. Top panel: Logarithm of the line flux (in electrons) of thebroadenedspectrumsince itbecomestotallyblendedwith the 3387 Åline plotted versus the logarithm of the line flux the Ti line at 3387.84Å. Thereforea detectionof theFe of the 3380 Å line for the spectra of the CN Leo time se- lineintheUV Cetspectrumcanonlybeobtainedthroughin- ries,LHS2034,LHS2076duringtheflare,YZCMi,ADLeo, direct reasoning as follows: First, the broadened spectrum of ATMic.Thestraightlinemarkstheratioofunity.Thesquare CN Leo fits very well to the UV Cet spectrum in this wave- marks the flux of LHS 2076 where the line is blended with lengthregion,although-admittedly-inotherspectralregions Feandthefitcannotdisentanglethetwolineswell.Bottom there is less similarity. Second, we know from CN Leo and panel:Logarithmofthecombinedfluxofthe3387Å andthe atomicdatathatthefluxintheTilineat3387.84Å shouldbe Felineplottedversusthe3380Å line.Thetrianglesdenote lessthanthatcontainedinthe3372.80Å line,butforUVCet UV Cet with respect to the combined set of emission lines. wefindjusttheopposite.Inordertodeterminelinefluxes,we Clearly AD Leo, AT Mic, YZ CMi and LHS 2034 show no fitted each spectral line simultaneously with a narrow and a Fe line emission, while the UV Cet line ratios are in the broadcomponent(seeTable4).Ifthebroadcomponentcomes regimeofstarswithadditionalFeflux(note,thatthesquare fromthe whole surface of the star and the narrowcomponent denotingLHS2076hasmovedabovethelineofunity). froman active region,the combinedamplitudesmustbe con- sidered and the combinedamplitude of the 3387.84Å line is higherthantheamplitudeofthe3372.80Å linethusproviding evidenceofadditionalemissioninthelines.Thesameapplies and AD Leo: The Ti lines at 3372, 3380 and 3383 Å have foracomparisonwiththeTilineat3380.28Å forwhichone a width of about0.05 Å whereasthe line at 3387Å shows a expectsequalfluxratios.Ifoneplotsthelineratiosforthetime widthof0.1Å.ThiscanbeduetotheFelineblendedbythe series of CN Leo and other stars, one finds, that the UV Cet absorptionfeatureat3388.17Å,alternatively,leftovercontin- lines are the only one with a ratio that needs additional flux uummayalsobeanexplanationforthebroaderTilinesince in the 3387.84 Å line (see Fig. 5). Given the high degree of thespectraofthethreestarsdoshowmanyabsorptionlines.In activity on UV Cet it is suggestive to attribute this additional ordertotestthishypothesisweusedthespectrumoftheearliest emissiontotheFeline,butgiventhecomplicatedsituation MdwarfGJ229asatemplateforthephotosphericpartofthe withthenarrowandbroadsetofemissionlinesweconsiderthe spectrumapplyingthespunupandscaledspectrumasa“back- detectionofFeXIIIinUVCetastentative. ground”fortheemissionlinefitwithCORA.Forthethreestars ATMic,YZCMiandADLeothismethodyieldsabestfitwith anarrowTilineofabout0.05Å halfwidthσ,consistentwith 5.7.ADLeo,ATMic,YZCMi,andFNVir thehalfwidthfoundfortheotherTilinesinthesestars.The FortheotherstarsinoursamplewithspectraltypeM4.5-M6 fitted spectra are displayed in Fig. 6. Therefore the apparent thesituationisnotasclearduetothepresenceoftwoabsorp- widthofthe3387Å lineseemstobeduetophotosphericcon- tionlinesat3387.45(NiI)and3388.17(Co I)Å in thespec- tinuumandthereisnoneedtoinvokeFeemissioninthese trum.ThissituationappliesforthethreestarsATMic,YZCMi threestars. B.Fuhrmeister,J.H.M.M.SchmittandR.Wichmann:FeXIIIincoolMstars 9 Table4.PropertiesoftheTiofUVCetmeasuredwithCORAusingabroadandanarrowcomponentsimultaneously.Given arealwaysthevaluesforthenarrow(first)andthebroadcomponent(second). Ti(3372.80Å) Ti(3380.28Å) Ti(3383.77Å) Ti(3387.84Å) amplitude 380and2416 329and1394 662and4665 520and5393 centralλ 3372.80 3380.31 3383.82 3387.88 σ 0.04and0.23 0.05and0.30 0.07and0.53 0.05and0.46 Fig.7.LightcurveofthefluxofCNLeoonMarch,16th,dur- ing the second observationblock butotherwise like in Fig. 2. Clearly a major flare canbe seen in boththe red andthe blue armofthephotometer. 5.8.ProximaCentauri ForProximaCentauritheTilinesareclearlyseeninemission, butagainthetemplateGJ229deviatessignificantlyifoverlaid tothespectrum.Inadditiontothis,thecontinuumisilldefined andthereforeafitofthelineverydifficultalthoughespecially theTilineat3387Å isclearlyseen.Althoughthebumpon theredsideoftheTilineat3387.84Å issomewhatsugges- tiveofaFeemissionlineblendedwiththeabsorptionline of Co I, this could also be due to leftover continuum. Since we were not able to obtain meaningful fits, the situation for ProximaCentaurimustremainopenandwemustrefrainfrom drawinganyconclusions. 6. TimingbehavioroftheFelineinCNLeo Every night during the observation run in March 2002 (with the exception of March, 14) two series of spectra were taken Fig.6. The best fit of the 3387 Å line for the stars AD Leo, ofCNLeo,eachconsistingof3spectrawith20minutesexpo- ATMicandYZCMiwiththerotationalbroadenedspectraof sure.OnMarch,14thonlyonespectralseriesofonehourdu- GJ229asbackground(blackline). rationwastaken,onesinglespectrumtakenabout2hourslater wascombinedwiththeotherstoonlyonetimeseries. We av- eragedthespectraofeachseriestoimprovethesignaltonoise ratio and constructed a total of 7 spectra of CN Leo. During thesecondspectralseriesonMarch,16thCNLeoamajorflare For the case of FN Vir this method cannot be used, be- occurredascanbeseeninthephotometerlightcurveinFig.7; causethetemplateGJ229deviatessignificantlyfromthepho- onMarch,14thCNLeowasveryquiet,showingverylittleof tospheric absorptionlines seen in FN Vir. However,since the theflickeringseeninthelightcurvesontheotherdays. Tilineat3372Å isfoundtohavethesamehalfwidthσ(0.11 WeusedtheCORAlinefittingprogramtomeasuretheTi ±0.01Å)asthe3387Å line(0.12±0.01Å),thereisagainno lines and the Fe line fluxes as described in section 5.2; a needtoinvokeanyFeemissionforthisstar,either. listingofourfitresultscanbefoundinTable5.TheFeline 10 B.Fuhrmeister,J.H.M.M.SchmittandR.Wichmann:FeXIIIincoolMstars was detected in all spectra. However, for the spectrum taken trumon2002-03-15.FortheFelinefluxofGLVirwecom- on March, 16th during the flare the fit results are ambiguous. puted3.9·10−15ergcm−2s−1,andfortheflarespectrumofLHS One can obtain a good fit with two narrow lines or a narrow 2076 the measured flux is 9.8 · 10−15ergcm−2s−1. Using dis- and a broad line, where the broad line has nearly the same tancesof 2.4pc forCN Leo 6.5pc for GL Vir and 5.2pc for central wavelengthlike the narrow line (i.e. the Fe line is LHS2076(Oppenheimeretal.2001)thesefluxesresultinline blueshifted). We assume that the fit with the lineshift of the luminosities of 2.0· 1024 up to 9.6· 1024ergs−1 for CN Leo, Felineisthecorrectone,sincethereisdefinitelyasecond 1.9·1025ergs−1forGLVirand3.2·1025ergs−1forLHS2076. lineandaphysicalinterpretationwithtwonarrowlinesoneof Comparing these numbers with the non-simultaneously mea- whichisascribedtoFewithahalfwidthσofonly0.1Å is sured X-ray luminositiesone then finds a ratio of broad band notmeaningfulduetotemperaturebroadening.Thereforeafit X-raytolineluminosityintherange3012to628forCNLeo, with two narrow lines implies that there is no Fe present. 235forGLVirandof124forLHS2076. Thiscanbepossible,whentheFeisionizedtohigherion- If one takes CN Leo as a prototype showing Fe emis- isation stages during the flare. This is quite unlikely though, siontheratiosofGLVirandLHS2076aretoolow.However, since we find no Fe emission at 5303 Å in CN Leo (see forLHS2076theFewasmeasuredduringaflare,whilethe section8)evenduringtheflare.Moreoverthesecondemission X-rayluminositywasmeasuredduringquiescenceandisthere- linehastobeexplainedotherwise.We favourthereforeanin- fore presumablytoolow leadingto an incorrect(lower)ratio. terpretation with a narrow Ti line and a broad blue shifted InthecaseofGLVirthelinewasmeasuredduringquiescence Feline.Lineshiftsareknowntooccurduringflares,when and the low ratio adds thereforeto the interpretation,that the theemittingmaterialisraisedintheatmosphere.Thevelocity line is actually the Ti line broadened by the higher rotation towardsthe observerin the lineof sightis about20 kms−1 in ratewefoundforGLVir. thiscase.AblueshiftedFelineduringtheflarelinesinwith theblueshiftoftheFelineweobservedinLHS2076during 8. Searchforotherforbiddencoronallinesin theflare. CNLeo Although the photometer data for the spectra outside the major flare are rather constant, the corona must have under- After the detection of the Fe line in CN Leo the question gone some variability as can be seen from Fig. 8, where the arisesif thereisevidenceforotherforbiddencoronallines.A averagedspectraofeachtimeseriesareshown.TheFeline catalog of such lines can be found e.g. in Allen (1973). Out ismostclearlyseeninthespectrumofthefirstseriestakenon of this we investigated the following two lines more closely: March, 15th, where the continuum is relatively high and the Caat3327Å andFeat5303Å. Ti line is very intense, although in the photometer data no UsingtheCHIANTIsoftwarepackage(Youngetal.2003) notableflux variabilityis seen besidesthe usualflickering. In we computed emissivity ratios for the lines mentioned above theotherspectrathelinedoesnotshowthatclearly,infactthe to the Fe line, which are proportionalto the flux ratios of line would be undetectable if it were only a bit weaker or if the lines. All computations were done assuming solar photo- one chose a higher backgroundlevel. Thus even for CN Leo spheric abundances since we do not know anything specific theFelineneednottobeapersistentfeature,andthehigh aboutthecoronalabundancesofverylowmassstars.Theion- variabilityduringquiescenceissuggestiveofthemicroflaring isationbalancewascomputedusingtheionisationratiosfound as proposed for the heating of stellar coronae (Kashyapetal. byMazzottaetal.(1998).Thecomputedfluxratiosprovedto 2002).AlsothechromosphericTilineshowsmajorvariabil- be quite sensitive on the ionisation balance whose choice can ityevenonatimescaleofhours.Forexample,onMarch13th influencethecomputedfluxratiosbymorethanafactoroftwo the amplitude of the Ti lines increased by a factor of more whilethechosenabundanceshavelessinfluence. thantwooveratimespanofonlytwohours.Thereforealsothe chromospheremusthaveundergonerapidandlargechanges. 8.1.AnalysisoftheCaline 7. X-rayandFelinefluxes A plot of the region around the Ca line is shown in Fig. 9. Besides the two Ti lines the other faint emission features UsingthestandardstarHD49798forfluxcalibration,wemea- are unidentified, but none of them is likely to be caused by sured absolute fluxes in the Fe 3388.1 line for the stars Cabecausethelinewidthistoonarrowforalineformedat CN Leo, GL Vir and for LHS 2076 (during the flare). Since temperaturesofaboutlogT∼ 6forwhichonewouldexpecta the standard star was observed only once per night, we esti- halfwidthof0.2Å.Whilealineconsistingof50countsanda mate an error in this absolute flux calibration up to a factor halfwidthof0.2Å caneasilybehiddeninthespectrumaline oftwo.BroadbandX-rayfluxesforthesestarswereobtained with100countsshouldbepossibletodetect.Thisleadstore- frombroadband countrates and shouldhave (systematic) er- strictionsforthepossibletemperaturesanddensities.Sincewe rors of about 50 %, resulting from the adopted count rate to measured200countsintheFelineat3388Å forCN Leo energy flux conversion. Also note that for none of our stars averagingallourspectra,thehighestpossibleratiobetweenthe dowe havesimultaneousopticalandX-rayobservations.The twolinesthatisinagreementwithanon-detectionisabout0.5, measured average fluxes for the one hour duration spectra of whichexcludestemperatureshigherthanlogT=6.4andlower CN Leo range from3.2·10−15 cm−2 s−1 in the first spectrum thanlogT=6.0.Amoredetaileddiscussionisgiveninthenext takenon2002-03-13to1.4·10−14ergcm−2s−1inthefirstspec- subsectiontogetherwiththerestrictionsdrawnfromFe.
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