Fe XIII coronal line emission in cool M dwarfs

0.53 MB·

by  B. Fuhrmeister

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 oftheFelineandreportahighlevelofvariabilityonatimescaleofhours
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)wereabletodetecttheFefor-
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-raytoFelineratioforCN 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
wecovertheCaHandKlines.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 229ATiappearstobepresentinabsorption.
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 sphericTiemissionlinescannotbedetectedanymore.
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.TheTiemissionlinesaremarkedinthespectrumofAdLeo.
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 Tilines 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 Tilineunfortunatelyhasalargeerror
cussedindetailbelow.
initscentralwavelength.Giventhissituation,wecannotdraw
any meaningful conclusions about possible wavelength shifts
5.1.CNLeo andremarkthatduringaflare lineshiftsfrequentlyoccurdue
tomassmotions.
WereportacleardetectionoftheFelineforCNLeo,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
AcleardetectionoftheFe3388.1Å 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
OutsidetheflareintervalnosignificantTiemissionisde- causedbyabsorptionattheredwingoftheFelineduetoa
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.TheotherTilineswerefitted
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
fourthTilineinthemultiplet,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
Feline;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 Feemissioninthisspectrum.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.PropertiesoftheTiandtheFelineduringtheflareonLHS2076.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
ortoohotforproducingadetectableFelineflux.
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 noiseratioclearlytheTiandFelinescanberecognized.
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,buttheTilineshouldalwaysbefound totheseeing).Thereforethespectrumshouldbedominatedby
in lower SNR spectra.Thusthe levelof activityin these stars thebrighterA-component.Theemissionlinesseeninthespec-
mustbelowermakingthepersistentpresenceoftheFeline 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.
SearchingfortheFelinewecomparedthebroadcom-
ponentofthespectrumofUVCetwithanartificiallybroadened
spectrumofCNLeo(seeFig.4).TheFelinedisappearsin
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
Feandthefitcannotdisentanglethetwolineswell.Bottom
there is less similarity. Second, we know from CN Leo and
panel:Logarithmofthecombinedfluxofthe3387Å andthe
atomicdatathatthefluxintheTilineat3387.84Å shouldbe
Felineplottedversusthe3380Å 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
regimeofstarswithadditionalFeflux(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
foracomparisonwiththeTilineat3380.28Å forwhichone a width of about0.05 Å whereasthe line at 3387Å shows a
expectsequalfluxratios.Ifoneplotsthelineratiosforthetime widthof0.1Å.ThiscanbeduetotheFelineblendedbythe
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 uummayalsobeanexplanationforthebroaderTilinesince
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
emissiontotheFeline,butgiventhecomplicatedsituation MdwarfGJ229asatemplateforthephotosphericpartofthe
withthenarrowandbroadsetofemissionlinesweconsiderthe spectrumapplyingthespunupandscaledspectrumasa“back-
detectionofFeXIIIinUVCetastentative. ground”fortheemissionlinefitwithCORA.Forthethreestars
ATMic,YZCMiandADLeothismethodyieldsabestfitwith
anarrowTilineofabout0.05Å halfwidthσ,consistentwith
5.7.ADLeo,ATMic,YZCMi,andFNVir
thehalfwidthfoundfortheotherTilinesinthesestars.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- tinuumandthereisnoneedtoinvokeFeemissioninthese
trum.ThissituationappliesforthethreestarsATMic,YZCMi threestars.
B.Fuhrmeister,J.H.M.M.SchmittandR.Wichmann:FeXIIIincoolMstars 9
Table4.PropertiesoftheTiofUVCetmeasuredwithCORAusingabroadandanarrowcomponentsimultaneously.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
ForProximaCentauritheTilinesareclearlyseeninemission,
butagainthetemplateGJ229deviatessignificantlyifoverlaid
tothespectrum.Inadditiontothis,thecontinuumisilldefined
andthereforeafitofthelineverydifficultalthoughespecially
theTilineat3387Å isclearlyseen.Althoughthebumpon
theredsideoftheTilineat3387.84Å issomewhatsugges-
tiveofaFeemissionlineblendedwiththeabsorptionline
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. TimingbehavioroftheFelineinCNLeo
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.
Tilineat3372Å 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
needtoinvokeanyFeemissionforthisstar,either. listingofourfitresultscanbefoundinTable5.TheFeline
10 B.Fuhrmeister,J.H.M.M.SchmittandR.Wichmann:FeXIIIincoolMstars
was detected in all spectra. However, for the spectrum taken trumon2002-03-15.FortheFelinefluxofGLVirwecom-
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,
Felineisthecorrectone,sincethereisdefinitelyasecond 1.9·1025ergs−1forGLVirand3.2·1025ergs−1forLHS2076.
lineandaphysicalinterpretationwithtwonarrowlinesoneof Comparing these numbers with the non-simultaneously mea-
whichisascribedtoFewithahalfwidthσofonly0.1Å 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,whentheFeisionizedtohigherion- 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 forLHS2076theFewasmeasuredduringaflare,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
Feline.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.AblueshiftedFelineduringtheflarelinesinwith
theblueshiftoftheFelineweobservedinLHS2076during
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.TheFeline 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 Caat3327Å andFeat5303Å.
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
theFelineneednottobeapersistentfeature,andthehigh aboutthecoronalabundancesofverylowmassstars.Theion-
variabilityduringquiescenceissuggestiveofthemicroflaring isationbalancewascomputedusingtheionisationratiosfound
as proposed for the heating of stellar coronae (Kashyapetal. byMazzottaetal.(1998).Thecomputedfluxratiosprovedto
2002).AlsothechromosphericTilineshowsmajorvariabil- 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.AnalysisoftheCaline
7. X-rayandFelinefluxes 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 Cabecausethelinewidthistoonarrowforalineformedat
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 measured200countsintheFelineat3388Å 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.