X-ray emission from the M9 dwarf 1RXS J115928.5-524717 - Quasi-quiescent coronal activity at the end of the main sequence

0.12 MB·English

by  J. Robrade

Astronomy&Astrophysicsmanuscriptno.11224 (cid:13)c ESO2009
January20,2009
X-ray emission from the M9 dwarf 1RXS J115928.5-524717
Quasi-quiescent coronal activity at the end of the main sequence
J.RobradeandJ.H.M.M.Schmitt
9
0 Universita¨tHamburg,HamburgerSternwarte,Gojenbergsweg112,D-21029Hamburg,Germany
0 e-mail:[email protected]
2
Received27October2008;Accepted19December2008

n
a ABSTRACT
J

0 Aims.X-rayemissionisanimportantdiagnostictostudymagneticactivityinpresumablyfullyconvective,verylow-massstarswith
2 aneffectivelyneutralphotosphere.
Methods.We analyse an XMM-Newton observation of 1RXS J115928.5-524717, an ultracool dwarf with spectral type M9 and

] compareitsX-raypropertiestothoseofothersimilarverylate-typestars.
R Results.Weclearlydetected1RXSJ115928.5-524717atsoftX-rayenergiesinallEPICdetectors.Onlyminorvariabilitywaspresent
S duringtheobservationandweattributetheX-rayemissiontoquasi-quiescentactivity.Thecoronalplasmaisdescribedwellbyatwo-
. temperaturemodelatsolarmetallicitywithtemperaturesof2MKand6MKandanX-rayluminosityofaboutLX =1.0×1026erg/s
h inthe0.2–2.0keVband.ThecorrespondingactivityleveloflogL /L ≈−4.1pointstoamoderatelyactivestar.Altogether,X-ray
X bol
p activityfromverylow-massstarsshowssimilartrendstomoremassivestars,despitetheirdifferentinteriorstructure.
-
Conclusions.1RXSJ115928.5-524717 is,afterLHS2065,thesecondultracoolM9dwarfthatemitsX-raysatdetectablelevelsin
o
quasi-quiescence.Whilefaintinabsolutenumbers,bothstarsareratherX-rayactive,implyingtheexistenceofanefficientdynamo
r
t mechanismsthatiscapableofcreatingmagneticactivityandcoronalX-rayemission.
s
a Keywords.Stars:activity–Stars:coronae–Stars:individual1RXSJ115928.5-524717–Stars:low-mass,browndwarfs–X-rays:
[
stars

1
v
7 1. Introduction straints on the possible activity generating mechanisms. Very
2 low-mass stars are generally assumed to be fully convective
0 The ultracool dwarf 1RXS J115928.5-524717 with a spec- and hence a solar-type dynamo is not expected to operate. In
3 tral type of M9V is a sparsely studied, very low-mass star moremassive,solar-typestarsmagneticactivityisgeneratedat
.
1 in the solar vicinity. The variable RASS (ROSAT All Sky theinterfacelayerbetweentheradiativecoreandouterconvec-
0 Survey) source 1RXS J115928.5-524717 was first investi- tion zone (αΩ dynamo). However, around spectral type mid-
9 gated by Greineretal. (2000) in a search for GRB X-ray M, the stellar interior becomes fully convective and the pres-
0 afterglow candidates. Later, Hambaryanetal. (2004) associ- ence of magnetic activity in these objects requires another dy-
v: ated 1RXS J115928.5-524717 with the nearby late-type star namo mechanisms, e.g. an α2 or turbulentdynamo,to operate.
i 2MASSJ11592743-5247188.Duringastrongflareobservedin Furthermore,thecoolphotosphereswithtemperaturesofTeff .
X 1991 with ROSAT, its X-ray activity level reached a value of 2500K should be effectively neutral, leading to a high electric
r log L /L = −1, one of the highest levels observed so far in resistivity, inhibiting the transport of magnetic energy through
a X bol
anystar.Hambaryanetal.(2004)initiatedopticalfollow-upob- the photosphere and consequently inhibiting magnetic activity
servations,investigatedarchivaldataintheoptical/IRband,and intheouterlayersofthestar.Acomprehensiveoverviewofthe
concludedthat1RXSJ115928.5-524717isahighproper-motion theoryofverylow-massstarsisgivene.g.inChabrier&Baraffe
star with spectraltype M9±0.5at a distance of only 11±2 pc (2000).
and a total luminosity of Lbol = 1.2×1030 erg/s.Observations Evidence for magnetic activity in the X-ray regime comes
withVLT/UVESdidnotshowthepresenceofLiandindicated fromlargeflaresdetectedfromseveralverylow-massstars,but
a relatively fast rotation of Vsini ≈ 25 km/s (Hambaryanetal. alsofromquiescentemissionthatpointstotheexistenceofsta-
2005).Recently,Phan-Baoetal.(2008)describedthesamestar ble coronae as observed in solar-like stars. Other indicators of
as DENIS-P J115927.4-524718 with a spectral type again of magnetic activity are most prominently their often strong and
M9 and derived a distance of 10.2±1.7 pc. Obviously in the variableHαemission(Gizisetal.2000;Mohanty&Basri2003;
solar vicinitythepopulationsof older,verylow-massstarsand Schmidtetal. 2007), suggesting the ubiquitous existence of a
youngbrowndwarfsoverlapintheregimeoflateMdwarfs(see chromosphereandofradiogyrosynchrotronemission,whichre-
e.g.Gizisetal.2000).Thereforeadefiniteassignmenttoaspe- quiresthepresenceoffastelectronsandmagneticfields(Berger
cific populationfor an individualobject like 1RXS J115928.5- 2002).Recently,strongmagneticfieldswithBf-valuesofuptoa
524717,basedonIR-magnitudesalone,isdifficult. fewkGhavebeendetectedonseveralultracooldwarfs,utilizing
theprofilesoftheirFeHlines(Reiners&Basri2007).
The presence of magnetic activity phenomena in the outer
atmospheric layers of these ultracool stars is remarkable, and The X-ray source 1RXS J115928.5-524717 has not been
amongotherdiagnostics,X-rayemissioncanputimportantcon- studied in detail up to now. Only the ROSAT X-ray detection
2 J.RobradeandJ.H.M.M.Schmitt:X-rayemissionfromtheM9dwarf1RXSJ115928.5-524717
is mentioned in the literature, and the X-ray signal was rather We applied standard selection criteria, but due to the over-
weak during the observation, despite the observed large flare. allhighbackgroundlevels,weusednotimefilteringforsource
Intotal52(source+background)photonsweredetectedin356s detection purposes. Instead, we suppressed the background by
observationtimedividedinto17exposures,whereasroughly24 restrictingtheenergyrangeoftheconsideredphotonssincewe
background photons were expected. Nearly all source photons expectapossiblesourcesignaltobepredominantlyatsoftener-
weredetectedintwosubsequentexposuresseparatedby1.6hin gies;specificallyweusedthe0.2–1.0keVbandwhereanade-
a total observation time of less than a minute. The X-ray flare quateS/Nispresentinalldetectors.Fromthesephotonswecre-
decayedratherfastwithadecaytime(e-folding)ofabout1.5h. atedimagesforeachdetectorandthenranthesourcedetection
Fromthespectralmodellingofthisquitelimitedsignaltheyde- algorithm’edetect chain’,requiringamaximumlikelihoodofat
ducedaplasmatemperatureofabout2.5MK,albeitwithalarge least 10. The source is clearly detected in all EPIC detectors.
error. A peak X-ray luminosity of L ≈ 1.3 × 1029 erg/s and Figure1 shows a partial image obtained in the MOS1 detector
X
an upper limit of the quiescent flux of L . 1.8 × 1027 erg/s thatprovidesthehighestspatialresolution,coveringthedetected
X
were derived. Little is known about the magnetic activity on X-ray source and the expected position of 1RXS J115928.5-
1RXSJ115928.5-524717;however,togetherwiththeM9dwarf 524717.
LHS 2065, whose quiescent X-ray emission was recently con- To optimize the SNR we constrained our further analysis
firmedbyXMM-Newton(Robrade&Schmitt2008),itisamong to the core of the PSF. For the generation of light curves we
thelateststarsdetectedinX-rays. merged all EPIC data and extracted the source photons from
The X-ray observations of ultracool dwarfs are quite rare, a 10′′circular region around the position of 1RXS J115928.5-
and we examined a previously unpublished XMM-Newton ob- 524717. The backgroundwas taken from a nearby region. For
servation of the M9 dwarf 1RXS J115928.5-524717.Here we a spectral analysis the spectra from the individual instruments
report its X-ray detection in the quasi-quiescent state with the wereconsidered,butsincethesignalisratherweakintheMOS
EPICdetectorsatsoftX-rayenergiesdespiteratherunfavourable detectors, we only used the data from the PN detector. The
background conditions. In Sect.2 we describe the observation PN detector has by far the highest sensitivity and the energy
range with sufficient S/N is widest. To obtain a source spec-
anddataanalysis,inSect.3 wepresentourresults,putthemin
trum, we again used only photons from a 10′′circular region
thecontextofsimilarobjectsinSect.4,andsummariseourfind-
aroundthesourcepositionanddeterminedthebackgroundfrom
ingsinSect.5.
a nearby region on the same CCD. Spectral analysis was per-
formedwithXSPECV12.3(Arnaud1996),andweusedmulti-
temperature APEC models with solar abundances as given by
Grevesse&Sauval(1998). We notethattheappliedmetallicity
2. Observationsanddataanalysis
isinterdependentwiththeemissionmeasureanddifferentcom-
binationsofbothparametersleadtoverysimilarresults.Dueto
1RXS J115928.5-524717 was observed by XMM-Newton in
the proximity of the target, interstellar absorption is negligible
January 2006 for approximately 38ks (Obs.-ID 0301430101).
andnotrequiredinthemodellingoftheX-raydata.
We only consider data taken with the EPIC (European Photon
Tostudyapossibleremainingbackgroundcontaminationof
Imaging Camera) detector, i.e., the PN and MOS, which were
the analysed data, we also extracted photons from a reduced
both operated in ’Full Frame’ mode with the thin filter in-
datasetthatexcludesperiodsofveryhighbackground(PNrate
serted. No useful source signal is present in the RGS data and
above10keV>3cts/s;duration28.8ks)oruseddifferentsource
the OM operated in the imaging mode with the UVW1 fil-
and background extraction regions and crosschecked our find-
ter (λ =2910Å), but we did not detect 1RXS J115928.5-
eff ings.Wefindoverallconsistentresultsthatonlymarginallyde-
524717.The XMM-Newton data analysis was carriedout with
pendonthespecificcombinationofdatasets.
the Science Analysis System (SAS) version 8.0 (Loiseauetal.
2007).Thebackgroundconditionsofthisdataareunfavourably
high; however, only a few minor data gaps (especially around
3. Results
theobservationtimeof12ks)arepresent.
3.1.Sourcedetectionandlightcurves
A faint X-ray source is clearly detected close to the expected
positionof1RXSJ115928.5-524717inallEPICdetectors.The
05.0
detectionatsoftX-rayenergiesisveryrobust,inthemostsensi-
tiveinstrumentPN alonethedetectedexcesshasa significance
10.0
ofmorethan8σ.Wenotethatthesourceisnotsignificantlyde-
tectedathigherenergies,e.g.inthe1.0–3.0keVband,evenwith
15.0 afactorofthreereducedminimumlikelihood.Thesourcedetec-
tionparameterfortheindividualinstrumentsaresummarisedin
-52:47:20.0 X Table1,denotederrorsarePoissonianerrors(1σ).
ThederivedsourcepositionsfromtheEPICdetectorsagree
with each other within errors and are within 2′′ of the ex-
25.0
pected position of 1RXS J115928.5-524717for epoch 2006.0;
we derivedthe expectedpositionfromthe 2MASS coordinates
30.0 (11 59 27.43 -52 47 18.8 at epoch 1999.36) - which are more
accurate than the ROSAT coordinates - and the proper mo-
29.0 28.0 11:59:27.0 26.0 25.0
tion (-1077mas/yr, -131mas/yr) as given by Hambaryanetal.
Fig.1.ImageofthedetectedX-raysourceandexpectedposition (2004), leading to an expected position of RA: 11 59 26.64
of1RXSJ115928.5-524717asmarkedbyX(seetext). and DEC: -52 47 19.7. The detected source is the only X-ray
J.RobradeandJ.H.M.M.Schmitt:X-rayemissionfromtheM9dwarf1RXSJ115928.5-524717 3
Table 1. X-raydetectionof 1RXSJ115928.5-524717with the WesearchedforspectralvariationsrelatedtochangesinX-
SAS-tool ’edetect chain’, optical pos: RA: 11 59 26.64 DEC: ray brightness for the 1h time bins by studying the respective
-524719.7. hardness ratio HR=(H-S)/(H+S) with S=0.2–0.6 keV and
H=0.6–1.0 keV being the photon energy bands. The SNR is
Par. PN MOS1 MOS2 rather poor and we find no clear correlation between HR and
OnTime(ks) 36.0 38.1 38.1 countrate,linearregressionresultedinaslopeof0.035±0.044.
RA 1159 26.84(±1.0′′) 26.83(±1.7′′) 26.73(±1.7′′)
Thereforeweperformedthespectralanalysisforthetotalobser-
DEC -5247 20.25(±1.0′′) 18.79(±1.7′′) 19.01(±1.7′′)
vation.
Counts 177±20 51±11 43±9
Det.likelihood 78 29 23
3.2.Spectralanalysis
To determine the spectral properties of 1RXS J115928.5-
sourcewithin5′ fromtheon-axisposition.NootherknownX-
524717, we fitted the PN spectrum with spectral models con-
ray source of comparable strength is located in the vicinity of
sisting of one and two temperature components. The elemen-
1RXS J115928.5-524717,therefore the identification is unam-
talabundancescannotbeconstrainedwiththeexistingdataand
biguous,andtheobservedsoftX-rayspectramakeanunknown
weresettosolarvalues.We showtheX-rayspectrumandboth
extragalacticsourceextremelyunlikely.
respectivebestfit modelsin Fig.3, thederivedspectralproper-
tiesaresummarisedinTable2.Themodelwithonetemperature
componentistechnicallyacceptablegiventheerrors,butresults
inansomewhatpoorerfit.AsvisibleinFig.3,greaterdiscrepan-
ciesarepresentforthismodelespeciallyaroundthepeakofthe
spectrum,indicatingthatitisaoversimplification.Giventhefact
thatatwo-temperaturecomponentmodelisalsophysicallymore
realistic,sinceX-rayspectrawithhigherSNRgenerallyrequire
multiplecomponents,weadoptitsresultsforfurtherdiscussion.
0−3
1
V−1 6×
e
d counts s k−1 4×10−3
e
aliz
Fig.2.Lightcurveof1RXSJ115928.5-524717derivedfromthe norm ×10−3
2
combined EPIC data with a binning of 1h (top) and 1ks (bot-
tom).
0
0.4 0.6 0.8 1
Energy (keV)
ToinvestigateX-rayvariabilityof1RXSJ115928.5-524717
we created light curves with 1ks and 1h binning respectively Fig.3.PNspectrumof1RXSJ115928.5-524717withabinning
from the photons detected in a 10′′region around the source ofatleast15cts/binandtheappliedspectralmodels(black:2-T,
position.InFig.2 we show thethusobtained,backgroundsub- red:1-T).
tracted light curve derived from the merged EPIC data in the
0.2–1.0keVbandfortwodifferenttimeresolutions.
ThelightcurvesshowninFig.2clearlyconfirmthatthede- Thebestfittwo-temperaturemodelcorrespondstoasource
tection of 1RXS J115928.5-524717 with XMM-Newton is not flux of 6.9×10−15 ergcm−2s−1 in the 0.2–2.0 keV band; for
caused by a single flare event, ratherpersistent X-rayemission theROSAT0.1–2.4keVbandwederivearoughly15%higher
is detected during the total observation, however, variability is flux of 8.0× 10−15 ergcm−2s−1. Adopting a distance of 11pc
alsopresentatasignificantlevel.Therefore,insteadofthecom- for1RXSJ115928.5-524717,weobtainanX-rayluminosityof
monly used term quiescence, we attribute the X-ray emission L =1.0×1026erg/sinthe0.2–2.0keVband,correspondingto
X
to a quasi-quiescence flux level. Some enhanced activity or a amoderateactivitylevelofaroundlogL /L =−4.1.Ourspec-
X bol
smaller flare might be present at the beginningof the observa- tralmodellingresultsinacoolplasmacomponentwithatemper-
tion,butthe overallvariationsinX-raybrightnessappearquite atureof2MKandamoderatelyhottercomponentwithtempera-
smooth,atleastwhenthe1haveragedlightcurveshowninthe turesaround6MK.Thecoolerplasmaapparentlyslightlydomi-
upper panel is considered. Otherwise, the light curve in lower natesthecoronalemissionmeasuredistributionduringtheseob-
panelwith 1ks time bins indicatesstronger variability,i.e. fre- servation.Asignificant,hot(&10MK)componentisnotpresent
quentsmaller flares of differentamplitude and duration. Given asexpectedforquasi-quiescentemissionofamoderatelyactive
theerrors,bothscenariosarepossibleandthe maximumX-ray star. On the other hand, an inactive corona composed only of
variabilitymaybearoundorevenlessthana factoroftwo,but verycool(2MK)plasmaasseenontheSunduringactivitymin-
couldalsobeeasilyoftheorderofafewassuggestedfromlight imumcan also be ruledout. We note thatthe coronaltempera-
curveswithshortertimebins. turesof1RXSJ115928.5-524717arecomparabletothetypical
4 J.RobradeandJ.H.M.M.Schmitt:X-rayemissionfromtheM9dwarf1RXSJ115928.5-524717
coronaltemperaturesderivedforhighermasswithasimilarac- (Schmitt&Liefke 2002). More recently, two M9 dwarfs
tivitylevel. (LHS 2065, 1RXS J115928.5-524717) and three M8 dwarfs
(VB 10, LP 412-31, TVLM 513-46546)have been clearly de-
tected in X-rays in quasi-quiescence by Chandra and XMM-
Table2. SpectralresultsderivedfromthePNdata.
Newton as summarised in Table3. Besides emitting quiescent
X-rayemission,allofthemarealsoknowntoshowX-rayflares
Par. 2-T 1-T unit
(andapossibleflarefromTVLM513-46546)withfluxincreases
T1 0.18±0.05 0.30±0.03 keV
uptoafactor100.WenotethatAudardetal.(2007)reportanX-
EM1 3.1±1.3×1048 5.1±0.7×1048 cm−3
raydetectionoftheearlyL-dwarfbinaryKelu-1withChandra,
T2 0.56±0.19 - keV
EM2 2.3±1.4×1048 - cm−3 but a more quantitative analysis of these data was not possible
χ2 (d.o.f.) 0.70(6) 0.83(8) since only four photons were detected in roughly six hours of
Lred(0.2–2.0keV) 1.0×1026 0.9×1026 erg/s observationtime.
X
Giventhe factthatfewerthanten photonswere detectedin
the total observation of TVLM 513-46546 and that the qual-
ity of the LHS 2065 data is rather poor, only three stars re-
The spectra of magnetically active stars are usually domi- main for a spectral comparison. The quasi-quiescent state of
nated by emission lines, which remain unresolved in the PN VB 10 was modelled by Flemingetal. (2003) with a 2.8MK
spectra. For moderately active stars like 1RXS J115928.5- plasmacomponent(basedon26photons).Theobservationper-
524717, we expect a cool plasma component emitting strong formed in 2007 was better described by a significantly hotter
lines of Ovii with a peak formation temperature of 2MK and model; however, most of the photons in this observation were
of Oviii, whichis predominantlyformedat3–5MK. TheHe- from a flare. The quasi-quiescent emission from LP 412-31 is
liketripletlinesofOviiatenergiesaround570eVandtheOviii notdiscussedindetailinStelzeretal.(2006).Toobtainamore
Lyαline at650eV thennaturallyexplainthe peakof the spec- complete picture from all available data, we derived its coro-
trum around 0.6keV, whereas the emission above 0.7keV re- nal properties in analogy to 1RXS J115928.5-524717, using
quiresthepresenceofhotterplasmathatemitse.g.severalstrong a two-temperature model with solar metallicity. We find best-
Fexviilinesinthisenergyrange.Thus,thetheoreticalexpecta- fit values around T1=3.5MK and T2=15MK with EM1/2=
tionscombinedwiththeobservedspectralshapealsofavourthe 4.8/3.3×1048cm−3,thoughwithsubstantialerror,especiallyon
two-temperature model and indicate that residuals in the one- the hotter component. These values lead to X-ray luminosities
temperaturemodelareduetoanunderpredictionofOviiandan of L = 1.3×1027 erg/sin the 0.2–2.0 keV band,confirming
X
overpredictionofOviii,insteadofnoiseorbackgroundcontam- thatLP412-31isanorderofmagnitudeX-raybrighterand,cor-
ination. respondingly,roughlyafactortenmoreactive.Itscoronaltem-
Asanadditionalcrosscheckweconvertedtheobservedcount peraturesarealsohigherthanthoseof1RXSJ115928.5-524717,
ratesfromeachMOSandPNdatasettoanenergyfluxbyusing andlargeramountsofflaring(& 10MK)plasmaareapparently
ECF’s (EnergyConversionFactors)in the ’edetect chain’task. also present in quasi-quiescence. In contrast, the corona of the
Forthe0.2–1.0keVbandusedinsourcedetection,werespec- lessactivestarVB10seemstobecoolerthanthoseoftheother
tivelyobtainasourcefluxof6.3±0.8×10−15ergcm−2s−1(PN) stars. ThisindicatesthatcoronaltemperaturesandX-rayactiv-
andof4.6(5.1)±1,3×10−15ergcm−2s−1(MOS1/2).TheMOS ityalsocorrelateinfullyconvectiveobjects.Acounter-example
predictsaslightlylowerflux;howevergiventheerrors,allmeth- isthecorrelationbetweenradioandX-rayluminosity,knownas
odsagreeratherwell.Thevaluesarealsoconsistentwiththecor- the Gu¨del-Benz relation (Guedel&Benz 1993), which is valid
respondingsourcefluxderivedfromthespectralmodelling,e.g. for a variety of magnetic activity phenomena in the F-type to
6.1× 10−15 ergcm−2s−1 for the two-temperature model in the mid-Mtypestars,butisstronglyviolatedintheregimeofultra-
0.2–1.0 keV band. We note that the given errors are pure sta- cooldwarfs(Bergeretal.2008a).Whilespectralinformationon
tistical errorsand furthererrors,e.g. in the distance of the star, ultracooldwarfsis still sparse in the X-ray regime, at least the
arepresentandmayaffectderivedvaluesbyalmostupto50%. studied objects show the same trend as observed for solar-like
Thequiescentfluxfrom1RXSJ115928.5-524717isanorderof stars,i.e.hottercoronaeinmoreactivestars.
magnitude below and therefore fully consistent with the upper TheX-rayactivitylevelsforhighermassstarswithspectral
limitgivenbyHambaryanetal.(2004)fortheROSATdata. types F to mid-M span the range of log L /L ≈ −3... − 7,
X bol
with the saturation limit around log L /L = −3 (see e.g.
X bol
Pizzolatoetal. 2003). This trend is most likely to continue all
4. X-rayactivityinultracooldwarfs
the way down the main-sequence as indicated by the activity
In the following we put our results from 1RXS J115928.5- levelsseeninverylow-massstars.TheX-raydetectedverylow-
524717intothecontextof(quasi-)quiescentX-raypropertiesof massstarsalreadyspantwoordersofmagnitudeinactivitylevel,
otherultracoolMdwarfsinthesolarvicinity.Asalreadynoted, whereasthehighestactivitylevelsintheseobjectsaresimilarto
thedetectionstatisticsarerathersparseforstarsintheregimeof thoseofhighermassstars.Theabsenceofultracooldwarfswith
spectral type beyond M7 and, additionally,we do not consider verylowactivitylevelsisnotsurprising,since itwouldrequire
stars that are only detected during flares. As mentioned above, amuchhighersensitivitythanavailableintheperformedX-ray
ageandmassarenotknownforallobjectsandthereforeaclear observation.
distinction between stars and hot brown dwarfs is not possible InHα,highactivitylevelsarecommonlyobservedinmidM
(see e.g. Gizisetal. 2000); however, most of the objects dis- dwarfs,evenforonlymoderaterotatingstars;however,Hαactiv-
cussed here are likely to be old (&1.0 Gyr) and therefore real itydeclinesbeyondspectraltypeM7followedbyastrongdrop
stars,asdeducede.g.fromtheabsenceoflithiumabsorption. beyond spectral type M9 (Gizisetal. 2000; Mohanty&Basri
The latest stars detected by ROSAT in quasi-quiescence 2003).NoneoftheseverylateobjectsbeyondM7isseenathigh
are the M7 dwarf VB 8 (Flemingetal. 1993) and - at Hαactivitylevels,i.e.,abovelog L /L = −3.9,whichisthe
Hα bol
least at selected time intervals - the M9 dwarf LHS 2065 meanlevelin earliertype M dwarfs,especiallywhenconsider-
J.RobradeandJ.H.M.M.Schmitt:X-rayemissionfromtheM9dwarf1RXSJ115928.5-524717 5
Table3.UltracoolMdwarfswithdetectedquasi-quiescentX-rayemission.
Star Sp.type Vsini Bf logL /L logL logL /L Ref.(X-ray)
Hα bol X X bol
VB10a M8 6 1.3 -4.4/-5.0 25.4 -4.9 Flemingetal.(2003)1
LP412-31a M8 9 >3.9 -3.9/-4.4 27.2 -3.1 Stelzeretal.(2006)
TVLM513-46546b M8.5 60 - -4.8/-5.0 24.9 -5.1 Bergeretal.(2008b)
LHS2065a M9 12 >3.9 -4.0/-4.3 26.3 -3.7 Robrade&Schmitt(2008)2
1RXSJ115928.5-524717c M9 25 - - 26.0 -4.1 thiswork
aadditionalvaluesfromReiners&Basri(2007),bfromMohanty&Basri(2003),cfromHambaryanetal.(2005)
1Re-observedbyBergeretal.(2008a),2DetectedatspecifictimesalsobySchmitt&Liefke(2002)
ingvariabilityandtakingthelowervaluespresentedintheliter- inmany,ifnotall,late-typestarsdowntothehydrogenburning
ature. While 1RXS J115928.5-524717clearly shows Hα emis- mass limit. The detected stars are moderately to highly active,
sion(seeFig.3inHambaryanetal.2004),noquantitativeanal- and their highest X-ray activity levels are comparable to those
ysis of its Hα line is presented in their work. Sample studies observedfor more massive stars. Furthermore,the well known
indeedindicatethattheHαactivitysaturationlimitinverylate trend of higher coronaltemperaturestowards more active stars
M dwarfs is also significantly lower than those in earlier type is apparently also present in ultracool dwarfs and objects with
M dwarfs. However, this decline does not seem to be present thestrongestmagneticfieldarealsothemostX-rayactiveones.
for the observed X-ray activity, and the opposite trend is seen Altogether,thesefindingssupportthehypothesisthatfullycon-
in the radio emission from these objects (Bergeretal. 2008a). vectivelow-massstars andsolar-like stars exhibitsimilar over-
Even the ultracool dwarfs show the same high activity levels allcoronalproperties,regardlessoftheirdifferentinteriorstruc-
up to the saturation level as observed for more massive stars tures, and that X-ray activity rather depends on the amount of
(log L /L ≈ −3). This could indicate that coronaland chro- generated magnetic flux but not on the mechanism responsible
X bol
mosphericactivitylevelsdonotstrictlyco-evolveintheregime for its creation. Future X-ray observations of ultracool dwarfs
of ultracool dwarfs. On the other hand, strong magnetic fields arehighlydesirabletoextendanddeepenourunderstandingof
(Bf>1kG)arefoundinallinvestigatedobjectsofspectraltype magneticactivityandcoronaeintheregimeofthecooleststars
M7andbeyond(Reiners&Basri2007),andthestrongestmag- attheendofthemainsequence.
netic fields (Bf>2kG) are also associated with the most X-ray
activestars.Thissuggestsacorrelation-asnaturallyexpected-
betweenmagneticfieldstrengthandX-rayactivity.
5. Summaryandconclusions
Concerning dynamo mechanisms and their efficiencies and
considering that the stellar radii are in a narrow range around 1. We have clearly detected quasi-quiescent X-ray emission
0.1 R⊙ for our sample stars (Chabrieretal. 2000), the mea- fromtheultracooldwarf1RXSJ115928.5-524717(spectral
sured Vsini values lead to maximum periods of a few hours typeM9)atsoftX-rayenergies.ThederivedX-rayluminos-
for the faster rotators and to less than one day even for the ityofaboutL = 1.0×1026 erg/sinthe0.2–1.0keVband
X
slowest rotators. This is significantly shorter than the convec- leadsto an activitylevelof log L /L ≈ −4.1,pointingto
X bol
tiveturnovertimesexpectedtobeoftheorderofseveraltenup a moderatelyactive star. It is still relativelypoorly studied,
to hundreddays.Consequently,the Rossby number(Ro=P/τc), however,itisapromisingtargettobeinvestigatedingreater
whose inverse describes dynamo efficiency, is small in all ul- detailatotherwavelengths.
tracool dwarfs and thus all stars should be in the saturated (or 2. The X-ray emitting coronal plasma is best described by
evensuper-saturatedforthefastestrotators)regimeofmagnetic at least two temperature components, one relatively cool
activity,anexpectationobviouslycontradictedbytheX-rayob- (2MK)andaslightlyhottercomponent(6MK).Thesetem-
servations.Furthermore,rotationappearstobevirtuallyuncorre- peraturesarealsotypicalforstarsofhighermasswithasim-
latedwithHαactivityinultracooldwarfs(Reidetal.2002),and ilar activity level. We find that the correlation between X-
it does not seem to be the dominantfactor determining the X- rayactivitylevelandaveragecoronaltemperatureapparently
rayactivitylevel,againincontrasttomoremassivestars.Since alsoholdsforultracooldwarfs.
these correlations refer to an αΩ dynamo, the X-ray observa- 3. The repeated X-ray detection of very low mass stars sug-
tionssupportthepresenceofanalternativedynamomechanism gests that magnetic activity and a stable coronae are quite
(e.g.α2, turbulent),which operatesin a fully convectivestellar commonphenomenadownto the ultracoolendofthemain
interior,butfavourascenariowhereactivitydoesnotsolelyde- sequence.Remarkably,theseobjectsexhibittrendssimilarto
pend on rotation. On the other hand, the saturation level, i.e., moremassivestarsintheircoronalX-rayproperties,despite
the maximum activity level in log LX/Lbol, appears to be simi- theirdifferentinteriorstructure.Thedetectedstarsarequite
lar inverylow-massandin highermassstars. Thiswouldthen active (in L /L ) but overallX-ray faint (in L ) and were
X bol X
requiresaturationtobebasicallyindependentoftheunderlying mainly detected since they are nearby,thus they are proba-
interiorstructureordynamomechanismandtodependonlyon blyonlythe’tipoftheiceberg’concerningX-rayactivityin
the amount of magnetic flux generated and propagating to the ultracooldwarfs.
stellar surface where it then leadsto the observedactivity phe-
nomena.
Acknowledgements. Thisworkisbasedonobservations obtainedwithXMM-
Whilethesamplesizeistoosmalltoderivedefiniteconclu-
Newton, an ESA science mission with instruments and contributions directly
sionsontheoverallcoronalpropertiesinverylow-massstars,the fundedbyESAMemberStatesandtheUSA(NASA).J.R.acknowledgessup-
availabledataclearlyconfirmthatquasi-quiescentcoronaeexists portfromDLRunder50QR0803.
6 J.RobradeandJ.H.M.M.Schmitt:X-rayemissionfromtheM9dwarf1RXSJ115928.5-524717
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