The X-ray activity-rotation relation of T Tauri stars in Taurus-Auriga

0.32 MB·English

by  K. R. Briggs

Astronomy&Astrophysicsmanuscriptno.6823 c ESO2008
(cid:13)
February5,2008
The X-ray activity–rotation relation of T Tauri stars in Taurus-Auriga
K.R.Briggs1,M.Gu¨del1,A.Telleschi1,T.Preibisch2,B.Stelzer3,J.Bouvier4,L.Rebull5,M.Audard6,7,8,L.Scelsi9,
G.Micela3,N.Grosso4,andF.Palla10
1 PaulScherrerInstitut,VilligenundWu¨renlingen,CH-5232,Switzerland
e-mail:[email protected]
2 Max-Planck-Institutfu¨rRadioastronomie,AufdemHu¨gel69,53121Bonn,Germany
7 3 INAF,OsservatorioAstronomicodiPalermo,PiazzadelParlamento1,90134Palermo,Italy
0 4 Laboratoired’AstrophysiquedeGrenoble,Universite´JosephFourier,BP53,38041,GrenobleCedex9,France
0 5 SpitzerScienceCenter,CaltechM/S220-6,1200EastCaliforniaBoulevard,Pasadena,CA91125,USA
2 6 ColumbiaAstrophysicsLaboratory,MailCode5247,550West120thSteet,NewYork,NY10027,USA
7 IntegralScienceDataCenter,Ch.d’Ecogia16,CH-1290Versoix,Switzerland
n
8 GenevaObservatory,UniversityofGeneva,Ch.desMaillettes51,1290Sauverny,Switzerland
a
J 9 DipartimentodiScienzeFisicheedAstronomiche,SezionediAstronomia,Universita`diPalermo,PiazzadelParlamento1,90134
Palermo,Italy
5
10 INAF,OsservatorioAstrofisicadiArcetri,LargoE.Fermi5,50125Florence,Italy
1
Received28thNovember,2006;accepted10thJanuary,2007
1
v
ABSTRACT
2
2 Context.TheTaurus-Aurigastar-formingcomplex hosts theonlypopulation of TTauri starsinwhichan anticorrelationof X-ray
4 activityandrotationperiodhasbeenobserved.
1 Aims.WeaimtoexplaintheoriginoftheX-rayactivity–rotationrelationinTaurus-Auriga.WealsoaimtoputtheX-rayactivityof
0 thesestarsintothecontextoftheactivityoflate-typemain-sequencestarsandTTauristarsintheOrionNebulaCluster.
7 Methods.We have used XMM-Newton’s European Photon Imaging Cameras to perform the most sensitive survey to date of X-
0 rayemission(0.3-10keV)fromyoungstarsinTaurus-Auriga.WeinvestigatedthedependencesofX-rayactivitymeasures–X-ray
/ luminosity,L ,itsratiowiththestellarluminosity,L /L ,andthesurface-averagedX-rayflux,F –onrotationperiodandcompared
h X X ⋆ XS
themwithpredictionsbasedsolelyontheobserveddependenceofL onastar’sL andwhetheritisaccretingornot.Wetestedfor
p X ⋆
differencesinthedistributionsofL /L offastandslowrotators,accretorsandnon-accretors,andcomparedthedependenceofL /L
- X ⋆ X ⋆
o ontheratiooftherotationperiodandtheconvectiveturnovertimescale,theRossbynumber,withthatoflate-typemain-sequence
r stars.
t
s Results.WefoundsignificantanticorrelationsofLXandFXSwithrotationperiod,butthesecouldbeexplainedbythetypicallyhigher
a stellarluminosityandeffectivetemperatureoffast-rotatorsinTaurus-Aurigaandanear-lineardependence of L on L .Wefound
X ⋆
: noevidenceforadependenceof L /L onrotationperiod,butforaccretorstohavelowerL /L thannon-accretorsatallrotation
v X ⋆ X ⋆
periods. TheRossby numbers of accretorsandnon-accretors werefound tobethesameasthose of late-typemain-sequence stars
i
X showingsaturatedX-rayemission.
Conclusions.Non-accretingTTauristarsshowX-rayactivityentirelyconsistentwiththesaturatedactivityoffast-rotatinglate-type
r
a main-sequencestars.AccretingTTauristarsshowlowerX-rayactivity,butthiscannotbeattributedtotheirslowerrotation.
Keywords.stars:pre-mainsequence–stars:activity–stars:rotation–X-rays:stars–Openclustersandassociations:Individual:
Taurus-Auriga
1. Introduction is believed to be predominantlygenerated by a dynamo that is
located in a shell at the interface of the radiative and convec-
T Tauri stars are young stars contracting toward the main se-
tive zonesandis drivenby(differential)rotationalandconvec-
quence,a subclassof which,the ‘classical’T Tauristars, show
tive motions (an αΩ dynamo:Parker, 1955). This is supported
signaturesthattheyareactivelyaccretingmaterialfromcircum-
byobservationsofsignaturesofmagneticactivity,suchaschro-
stellaraccretiondisks.Bothaccretingandnon-accretingTTauri
mospheric Hα or Ca H and K line emission or coronal X-ray
starsexhibitstrong,variable,X-rayemissionwhichprovidesev-
emission, in stars with convective envelopes. The strengths of
idenceforhotcoronaegeneratedbymagneticactivity.Keyopen
these activity signatures correlate directly with the projected
questionsconcerningthis emission are the extentto whichthis
rotation velocity, vsini, and inversely with the rotation period
magneticactivityis analogousto thatexhibitedbytheSunand
of the star, P (e.g. Pallavicinietal., 1981). They furthermore
similar stars, and what effect interaction with the circumstellar rot
show a tighterinversecorrelationwith the Rossby number,R ,
0
material,particularlythroughaccretion,hasonthemagneticac-
whichistheratiooftherotationperiodtotheconvectiveturnover
tivity.
timescale at the base of the convective envelope, τ (e.g.
conv
In the Sun, and all stars whose structure is composed of a Noyesetal.,1984).
radiative interior and a convective envelope, magnetic activity
Thestrengthsof the activity signaturesare observedto sat-
Sendoffprintrequeststo:K.R.Briggs urate at fast rotation velocities, short rotation periods and low
2 K.R.Briggs:TheX-rayactivity–rotationrelationinTaurus-Auriga
Rossby numbers(Vilhu, 1984; Vilhu&Walter, 1987). It is not ourfindingsandidentifyingoutstandingquestionsandsuggest-
yetknownwhetherthisisduetoa saturationofthedynamoit- ingprospectiveobservingstrategieswhichcouldaddressthem.
selforconstraintsontheavailablevolumeofthecorona(seee.g.
Jardine&Unruh,1999).
2. TheXMM-NewtonExtendedSurveyoftheTaurus
Pizzolatoetal. (2003) have shown that the ratio of the X-
ray luminosity to the stellar bolometric luminosity, L /L , for MolecularCloud(XEST)
X ⋆
main-sequence stars of spectral types from G to early M is
The XEST comprises 28 observations with exposure times of
well-determinedbytheRossbynumber,R ,suchthat L /L
10 3.2(R /R ) 2 for R > R and L0 /L 10X3.2⋆fo≈r at least 30 ks of regions of the Taurus-Auriga complex that
− 0 0,sat − 0 0,sat X ⋆ ≈ − are most densely-populated by T Tauri stars. XMM-Newton’s
R < R , where R 0.2 is the Rossby number at which
0 0,sat 0,sat ≈ coalignedEuropeanPhotonImagingCamera(EPIC)PN,MOS1
saturationsetsin.
andMOS2detectorseachhaveafullfieldofviewapproximately
TTauristarswithmassesgreaterthanapproximately0.3M 30arcminindiameterandperformsimultaneousimaging,tim-
are expected to be initially fully-convective and to develop ⊙a ingandspectroscopyofX-raysintheenergyrange0.2–12keV,
solar-likestructureafternuclearfusionbeginsinthecore.Stars withrespectiveresolutionsof6arcsec(FWHM),lessthan2.5s,
withmasseslessthanapproximately0.3 M areexpectedtore- and50–200eV(E/∆E 20–50).Thesurveyobservationsand
main fully-convective. The majority of T⊙Tauri stars are thus datareductionprocedure≈saredescribedindetailinGu¨deletal.
expected to have long convective turnover timescales of 100– (2007a).
300 d. Despite the fact that an αΩ dynamo cannot operate in XEST makes key improvements over previous surveys
fully-convectivemain-sequence stars, there is no empirical ev- of Taurus-Auriga using the Einstein Imaging Proportional
idence for different characteristics in the magnetic activity of Camera (IPC) and ROSAT Position Sensitive Proportional
thesestars:fastrotatorsshowactivityatthesaturatedleveland Camera (PSPC) (Bouvier, 1990; Damiani&Micela, 1995;
only slow rotatorsshow activity well below the saturated level Neuha¨useretal., 1995; Stelzer&Neuha¨user, 2001) thanks pri-
(Delfosseetal.,1998;Mohanty&Basri,2003). marilytothegreatersensitivity,broaderenergybandandhigher
The T Tauri stars in Taurus-Auriga stand out as the only spectralresolutionofthe EPICdetectors,augmentedbylonger
populationof pre-mainsequence stars in which an anticorrela- exposuretimes.XESTreachesX-rayluminositiesof1028ergs 1
−
tion of X-ray activity with rotation period has been observed at the 140 pc distance of Taurus-Auriga (Loinardetal., 2005),
(Bouvier, 1990; Damiani&Micela, 1995; Neuha¨useretal., approximately an order of magnitude lower than the ROSAT
1995;Stelzer&Neuha¨user,2001).Thelargestsurveytodateof PSPC pointing survey (Stelzer&Neuha¨user, 2001), enabling
the X-ray emission from T Tauri stars, the Chandra Ultradeep thedetectionofalmostallknownTTauristarsinthesurveyarea
OrionProject(COUP)observationofalmost600TTauristarsin downtothesubstellarmasslimitforthefirsttime.Thisisvital
theOrionNebulaCluster(ONC),foundnosuchanticorrelation for accurately quantifyingthe dependenceof X-ray activity on
(Preibischetal., 2005). The Rossby numbers of all the COUP parameterssuchasstellarluminosity.TTauristars,particularly
starswithmeasuredrotationperiodsplacedtheminthesaturated accretingTTauristars,aretypicallysurroundedbycircumstellar
regimeofmain-sequencestars,andtheirL /L waslargelycon- materialor observedthroughmolecularcloudmaterialthatab-
X ⋆
sistentwiththesaturationlevelofmain-sequencestarsbutwith sorbssoftX-rayemissionfromthestar.Thebroadbandpassof
a very large scatter. A strong dependence of X-ray luminosity EPICallowsittodetectthelessabsorbedharderemission.The
on the stellar bolometric luminosity was found, as had been EPICspectraenabledtheabsorptiontobemeasuredforeachT
seen in previous surveys of X-ray emission from T Tauri stars Tauri star and accounted for in calculating its X-ray luminos-
(e.g.Preibisch&Zinnecker,2002).AccretingTTauristarswere ity.ThisiscrucialforaccuratedeterminationofX-rayluminosi-
found to have generally lower X-ray activity levels than non- tiesandwasnotgenerallypossiblewiththedataobtainedbythe
accretors. EinsteinandROSATsurveys.Additionally,sincetheROSATsur-
The lower X-ray activity of accreting T Tauri stars has veysmanymorelow-massmembersofTaurus-Aurigahavebeen
alsobeenobservedinTaurus-Auriga(Damiani&Micela,1995; identifiedbyopticalandnear-infraredphotometricsurveys(e.g.
Neuha¨useretal., 1995; Stelzer&Neuha¨user, 2001), and has Luhmanetal., 2003), makingthe knownpopulationof T Tauri
beenusuallyattributedto theirtypicallyslowerrotationandan starsmuchmorecomplete.
anticorrelationofX-rayactivitywithrotationperiod.
We usetheresultsofanewX-raysurveyofTaurus-Auriga,
3. Sampleselection
the XMM-Newton Extended Survey of the Taurus Molecular
Cloud(XEST),describedinSect.2,toreinvestigatethedepen- Weperformedaliteraturestudytocompileacatalogueofrecog-
denceofX-rayemissiononrotationinthisstar-formingregion. nised members of Taurus-Auriga and their relevant data, such
Our sample selection and data analysis methods are described asstellarluminosity,L ,effectivetemperature,T ,multiplicity,
⋆ eff
in Sects. 3 and 4, respectively. In Sect. 5 we present, and pro- photometric rotation period, P , projected equatorial rotation
rot
poseanexplanationfor,theobserveddependencesofX-rayac- velocity,vsini, and signatures of accretion such as the equiva-
tivitymeasuresonrotationperiod.InSect.6,weexploretherole lentwidthoftheHαline.Thiscatalogueandthemanysources
of rotation in the differentactivity levels of accreting and non- ofinformationusedinitscompilationarefound,alongwiththe
accretingstars.WeinvestigatethedependenceofX-rayactivity XESTX-raydataforthesestars,inGu¨deletal.(2007a).
on Rossby numberin Sect. 7, to examinethe observedactivity TheXEST dataprovidean almostdetection-completesam-
in the context of the activity of solar-like main-sequence stars pleofmorethan100TTauristars(excludingClassIprotostars,
andofTTauristarsintheONCstudiedbyCOUP.InSect.8,we HerbigAe starsandsubstellarobjects)evenlydividedbetween
makeacarefulcomparisonwith,andreexaminationof,themost accretorsandnon-accretorsandwell-spreadinmassesfrom0.1
comprehensivepreviousX-raystudyofTaurus-Auriga,madeby to2.5M .WehavechosentorestrictourstudytoTTauristars
Stelzer&Neuha¨user(2001)withtheROSATPositionSensitive ofspectr⊙altypesM3andearlier(T > 3400K).Nostarinthe
eff
ProportionalCounter. In Sect. 9, we conclude by summarizing XESTsampleoflaterspectraltypehasameasuredphotometric
K.R.Briggs:TheX-rayactivity–rotationrelationinTaurus-Auriga 3
period, and few such stars have measured vsini. Stars of later
spectraltypearetypicallyless-wellstudiedandarenotexpected
todevelopasolar-likestructure.
We have excluded four undetected accreting T Tauri stars
for which L upper limits could not be calculated due to lack
X
ofknowledgeofabsorption(Coku-Tau1,HH30,ITG33Aand
IRAS S04301+261),two T Tauri stars whose characteristic X-
rayactivitycouldnotbeassessedbecauseadecayfromalarge
flarepersistedthroughouttheirXESTobservation(DHTauand
V830Tau), andall T Tauristars forwhich T and/or L were
eff ⋆
notavailable,orwhoseT andL placedthemoutsidethestel-
eff ⋆
larevolutionarymodelgridsofSiessetal.(2000).Thedecayof
alargeflarealsodominatedtheXESTobservationofFSTau,but
wehaveusedadditionalChandradatatoassessitscharacteristic
X-rayactivity.
Oursamplecontainsatotalof74TTauristars,47accretors
and 27 non-accretors. These include 23 stars with a measured
photometricrotationperiod,ofwhich13areaccretorsand10are
non-accretors.Thereare47starsthathaveameasuredvsini,of Fig.1. The dependence of X-ray luminosity upon stellar lu-
which30areaccretorsand17arenon-accretors,andthissample minosity for the accreting (black triangles) and non-accreting
includes all stars with a measured photometric rotation period (greycircles)TTauristarsinTaurus-AurigaobservedbyXMM-
exceptfortheaccretorXZTau. Newton. The black and grey lines show the best-fitting corre-
lations for accretors and non-accretors, respectively, found us-
ing the EM algorithmin ASURV (Telleschietal., 2007). Open
4. DataAnalysis symbolsanddownward-pointingarrowsmarkupperlimits.The
crossshowsKPNO-Tau8,whichwasexcludedfromthefitting.
Ourstudycomprisesfourinvestigations.Thedataanalysisper-
formedintheseinvestigationsisdescribedinthissection,while
theresultsofeachinvestigationarepresentedanddiscussedina
ing the ROSAT PSPC. The results of this comparison are pre-
dedicatedsectioninSects.5–8.
sentedanddiscussedinSect.8.
Firstly, we examined the correlations with rotation period
The X-ray activity measures and rotation periods we used
of the three most commonly-used measures of X-ray activ-
are described in Sects. 4.1 and 4.2, respectively. The criterion
ity: the X-ray luminosity, L , its ratio with the stellar bolo-
X we used to distinguish accretors and non-accretors is given in
metricluminosity,L /L ,andthesurface-averagedX-rayflux,
X ⋆ Sect. 4.3. The estimation of the convective turnover timescale
F = L /4πR2. In order to compare to previous studies we
XS X ⋆ foreachofthestarsinoursampleisdescribedinSect.4.4.The
havetreatedaccretingandnon-accretingstarsasasinglesample.
statistical methods used in our investigations are described in
Wetriedtounderstandtheobservedactivity–rotationrelationsin
Sect.4.5.
termsofthe near-lineardependencyof L on L , andthe typi-
X ⋆
callylower L ofaccretingstars,whichhavebeenconsistently
X
observedinpopulationsofTTauristarsthatdonotshowanan- 4.1.DeterminationoftheX-rayactivitymeasures
ticorrelation of activity on rotation (e.g. Preibischetal., 2005;
X-ray activity measures, L , L /L and F , were calculated
Preibisch&Zinnecker, 2002). We used the correlations of L X X ⋆ XS
X foreachstarusingthedatatabulatedinGu¨deletal.(2007a).The
on L derivedseparatelyfor accretorsand non-accretorsin the
⋆ L of eachobservedT Tauristar wascalculatedinGu¨deletal.
XEST study by Telleschietal. (2007, see Fig. 1) to calculate X
(2007a) as follows. A spectral fit of each source was made
theexpected L ,andhencethe L /L and F , ofeachstar in
X X ⋆ XS in XSPEC (Arnaud, 1996) using a model composed of an
oursamplebasedonitsL andwhetheritwasaccretingornot.
⋆ optically-thin collisionally-ionized,so-called ‘coronal’, plasma
We comparedthe resulting correlationswith rotation period of
(APEC;Smithetal.,2001),havingatwo-power-lawdistribution
theexpectedandtheobservedactivitymeasures.Theresultsare
ofemissionmeasurewithtemperature,multipliedwithaphoto-
presentedanddiscussedinSect.5.
electricabsorptionmodel(WABS).Theemissionmeasuredistri-
Secondly,wetested thehypothesisthatthelower L /L of
X ⋆ butionwascutoffat106 and108 K.Theelementalabundances
accretors compared to non-accretors is due to their slower ro-
oftheplasmawerefixedtovaluesderivedfromhigh-resolution
tation period. In this analysis we have divided the full sample
X-rayspectraofyoungstars1.Thepower-lawindexatlowener-
described in Sect. 3, into subsamples based on rotation period
gies,α,isunconstrainedinEPICspectraandwasfixedto+2,as
calculatedfromvsiniandonwhetherthestar wasaccretingor
observedin young solar analogues(Telleschietal., 2005). The
notandstatisticallycomparedthedistributionsof L /L ofthe
X ⋆ free parameters were the break temperature of the two power
subsamples.TheresultsarepresentedanddiscussedinSect.6.
laws,6.3 logT 7.5,thepower-lawindexathighenergies,
Thirdly,wecomparedtheX-rayactivityoftheTTauristars ≥ 0 ≥
3 β +1, the normalization, and the equivalenthydrogen
inoursamplewiththeactivityofsolar-likemain-sequencestars − ≥ ≥
columndensityoftheabsorbinggas,N .Weintegratedtheflux
andTTauristarsintheONCbasedonthedependenceofL /L H
X ⋆ under the best-fitting modelbetween 0.3 and 10.0 keV and as-
on Rossby number. The results are presented and discussed in
Sect.7.
1 The abundance values used were, with respect to the solar pho-
Finally we compared our activity–rotation relations with tosphericabundancesofAnders&Grevesse(1989):C=0.45,N=0.788,
those obtainedby the previousmost-comprehensiveX-ray sur- O=0.426,Ne=0.832,Mg=0.263,Al=0.5,Si=0.309,S=0.417,Ar=0.55,
veyofTaurus-Auriga,madebyStelzer&Neuha¨user(2001)us- Ca=0.195,Fe=0.195,Ni=0.195.
4 K.R.Briggs:TheX-rayactivity–rotationrelationinTaurus-Auriga
sumed a distance of 140 pc to calculate the X-ray luminosity. thatthe L ofthecomponentsscaleslinearlywith L , asmoti-
X ⋆
Timeintervalsinwhichastarwasclearlyflaringwereremoved vated by Fig. 1. Thus, each componentof a multiple system is
beforespectralanalysisifpossible. assumed to have the same L /L , which is the ratio of the ob-
X ⋆
Weexcludedobservationsofstarsinwhichtheemissionwas served L and the total stellar luminosity of the system. The
X
clearlydominatedbyaflarethroughouttheobservation.Ifthere multiplicity-corrected L of the primary is therefore the ob-
X
were two or more acceptable observationsof the same star we served L multiplied by the fraction of the summed stellar lu-
X
calculated the X-ray luminosity as the logarithmic average of minositiesthatis contributedby the primary.Thisfractionwas
theindividualL determinations. estimatedusingtheluminositiesoftheindividualcomponentsif
X
Foreachobservationofastarweestimatedtheuncertaintyin thesehadbeendetermined,elsethemagnitudedifferencesinthe
LX byfindingthe68percentconfidenceintervalinNH andcal- K-band or I-band, as available2. LX/L⋆ was always calculated
culatingLXforthebest-fittingmodelswithNHfixedtothelower usingtheuncorrectedobservedLX andthetotalstellarluminos-
andupperlimitsofthisconfidenceinterval.LX waspoorlycon- ityofthesystem. FXS = LX/4πR2⋆ = (LX/L⋆)σTe4ff wasalways
strainedwhentherewerefewcounts,particularlywhenNH was calculatedusingLX/L⋆andtheeffectivetemperature,Teff ofthe
appreciable(> few 1021 cm 2) and the slope of the spectrum primary.
−
×
atenergiesabove2keVwasnotwell-defined.Adegeneracybe-
tweenlogT andN sometimesdeveloped,thatintheworstcase
0 H
led to well-fitting models with very low logT and very high 4.2.Rotationperiods
0
N , hence high L . Such cool temperatureswere not observed
H X
ingood-qualityspectrashowinglowN anditisthereforelikely Rotationaldataintheformofrotationperiodsandspectroscopic
H
that such modelshighly overestimate L . In such cases the er- vsini have been taken fromRebulletal. (2004), where a com-
X
rorbarsplottedthroughoutthisworkextendfromtheseprobably prehensive list of references can be found, with a small num-
unrealisticallyhighvaluesof L tolowervaluescorresponding berofadditionsfromL.Rebull(priv.comm.),andwerealways
X
to more realistic hotter models with lower N . In some poorly assignedtotheprimaryinthecasesofunresolvedmultiplesys-
H
constrained fits we fixed β, usually the least well-constrained tems.ForthelargersampleofTTauristarswithmeasuredvsini,
parameter, to a typical value of 1. In Gu¨deletal. (2007a) we weestimatedrotationperiodsasProt/sini=2πR⋆/vsini.These
also fixed N in several particu−larly poorly-constrained cases are essentially upper limits to the rotation period3 but we note
H
to the value indicated by the visual or near-infraredextinction. thatfor a randomlyorientedsample sini = π/4 0.785,the
h i ≈
However, this results in unrepresentative uncertainies and we 1σ lower limit to sini is 0.73 and there is just a 10 per cent
have reanalysed those four cases here with just β fixed to 1. chancethatsiniislessthan0.44.RYTau,LkCa21andCWTau
TheresultsofthisadditionalanalysisaregiveninTable1. − have tabulated photometric periods in Gu¨deletal. (2007a) but
We have also performed additional spectral analysis of their high measured vsini indicate that the photometric peri-
four jet-driving accreting T Tauri stars, DG Tau A, GV Tau, odsaretoolongtobetherotationperiods(Bouvieretal.,1993,
DP Tau and CW Tau, whose X-ray spectra could not be well- 1995) and we use onlythe vsini valuesfor these three stars in
described by a model with just a single absorbing column. thiswork.
Gu¨deletal. (2007b) found instead a little-absorbed unvarying
low-temperaturecomponent,which they proposedto be due to
4.3.Accretioncriterion
shocksinthejets,andahighly-absorbedandvariablehotcom-
ponent, which they attributed to a corona. We have modelled
Accretingpre-mainsequencestars,classicalTTauristars,have
the hot component with the spectral model described above –
been traditionally identified through their strong Hα emission,
ratherthantheabsorbedisothermalplasmausedbyGu¨deletal.
believedtoarisefromheatedaccretingmaterial.Hαemissionis
(2007b) – and, the cool component with an isothermal model
alsoproducedbychromosphericactivity,but,outsideofexcep-
withphotoelectricabsorption.ThehotcomponentsofDGTauA
tionalflares, is restricted by the saturation of magneticactivity
andGVTauwerestronglyvariableintheirXESTobservations.
and one can use the criterion log(L /L ) > 3.3 to identify
Inthesecaseswehavefirstusedtheaveragespectrumfromthe Hα ⋆ −
accretingstars(BarradoyNavascue´s&Mart´ın,2003).Itissim-
wholeobservationtodeterminethebest-fittingparametersofthe
plertomeasuretheequivalentwidthofthe Hαline, EW ,but
unvaryingcoolcomponent.Theparametersofthecoolcompo- Hα
cooler stars have less continuum emission at the Hα line and
nentwerethenfixedtothesevalueswhenwefittedthespectrum
so the criterion becomes a function of spectral type. We have
of the time intervals when the star appeared to be in a low or
adoptedthedependenceonspectraltypeofthe EW criterion
‘quiescent’X-rayemissionstatetoderivecharacteristicparame- Hα
that was proposed by BarradoyNavascue´s&Mart´ın (2003)4,
tersforthehotcomponent.CWTaurequirednocoolcomponent
directly based on the criterion log(L /L ) > 3.3. The clas-
whenweexcludedenergiesbelow0.5keV.Wefixedβto 1in Hα ⋆ −
− sification of the primarywas used in cases of multiplesystems
thesefits.Nevertheless,logT wasunconstrainedinallcases,so
0 notresolvedbyXMM-Newton.
we havecalculated L forlogT = 7.0andhaveexpressedthe
X 0
range of uncertaintiesfound when logT was a free parameter
0
(seeTable1).
Just one star in our sample, FV Tau/c, was not detected in 2 TheK-bandisnotidealasaccretiondiskscanemitsubstantiallyin
thisband,andlower-masscompanionsemitahigherproportionoftheir
the XEST observation.FV Tau/c hasno measuredphotometric
bolometricfluxinthisbandrelativetohigher-massprimaries.Forthis
rotation period or vsini. As described in Gu¨deletal. (2007a),
latterreason,however,manymultiplicitystudieshavebeenperformed
we calculated a 95 per cent confidenceupper limit to its EPIC
inthisband.
count-rateand used a spectrumwith logT = 7.0 andβ = 1,
0 3 Twooftheslowly-rotatingnon-accretors,HBC358andHBC359,
−
with NH estimated from its optical extinction, to calculate an haveonlyupperlimitstovsiniof10kms−1 andhencelowerlimitsto
upperlimittoitsX-rayluminosity. P /siniof>6.98and>6.72d,respectively.
rot
For recognisedmultiple systems not spatially resolved into 4 ThecriterionproposedbyMart´ın(1997)givesidenticalclassifica-
theirindividualcomponentsbyXMM-Newton,wehaveassumed tionsforthepresentsample.
K.R.Briggs:TheX-rayactivity–rotationrelationinTaurus-Auriga 5
Table1.TheresultsofspectralfitsadditionaltothoseperformedinGu¨deletal.(2007a).
XEST Name N (1σrange) logT (1σrange) L (1σrange) log(L /L ) χ2 dof
H 0 X X ⋆ red
1022cm 2 K 1030ergs 1
− −
04-010 GHTauAB 0.48(0.33,0.63) 6.4(6.3,7.3) 0.39(0.29,0.50) 3.91 0.89 4
−
09-010 HOTauAB 0.27(0.09,0.44) 6.6(6.3,6.9) 0.064(0.034,0.13) 4.00 1.11 3
−
07-011 JH223 0.19(0.14,0.31) 6.6(6.3,6.7) 0.099(0.078,0.18) 3.84 0.89 4
−
11-079 CFHT-Tau21 1.72(1.20,2.20) 7.2(6.3,7.5) 0.22(0.15,0.36) 3.82 0.68 7
−
02-022 DGTauA 4.50(2.07,7.61) 7.0(6.3,7.5) 0.55(0.17,2.39) 4.08 0.77 13
−
20-046 CWTau 2.93(1.33,4.72) 7.0(6.3,7.5) 0.11(0.038,0.31) 4.60 0.94 9
−
10-045 DPTau 6.31(2.98,9.02) 7.0(6.3,7.5) 0.33(0.089,0.74) 3.37 1.16 8
−
13-004 GVTauA 3.02(1.31,5.16) 7.0(6.3,7.5) 0.55(0.19,2.86) 4.11 0.82 6
−
4.5.Statisticalanalysis
We used assumed power-law correlations between the X-ray
activity measures and rotation period, and we have used the
ASURVsurvivalanalysispackage(Lavalleyetal.,1992)toper-
form the regression. Although ASURV is designed to analyse
data containing upper and/or lower limits (Isobeetal., 1986)
while our sample of T Tauri stars with measured photomet-
ric rotation periods contained no non-detections, this enabled
us to compare our results directly with those of previousstud-
ies(e.g.Stelzer&Neuha¨user, 2001;Preibischetal.,2005).We
usedtheparametricEstimationMaximization(EM)methodfor
the regression and have quoted the full range of probabilities
of no correlation calculated by the three tests in ASURV (Cox
Proportional Hazard, Generalized Kendall’s τ and Spearman’s
ρ).
Weusedthetwo-sampletestsinASURVtoassesstheproba-
bilitiesthatpairsofobserveddistributionsofL /L weredrawn
X ⋆
fromthesamedistribution(oneofthesubsamplesofstarswith
Fig.2.ConvectiveturnovertimescalesofTTauristarsinTaurus-
no rotational information includes an upper limit). We have
AurigaobservedbyXMM-Newton,plottedasafunctionofeffec-
quotedthefullrangeofprobabilitiescalculatedbythefivetests
tivetemperature.Thevalueshavebeeninterpolatedfromvalues
inASURV (Gehan’sGeneralizedWilcoxonTestsusingpermu-
calculatedforTTauristarsintheOrionNebulaCluster(shown
tationvarianceandhypergeometricvariance,LogrankTest,Peto
bygreysquares, Preibischetal.,2005,Preibisch,priv.comm.),
and Peto Generalized Wilcoxon Test, and Peto and Prentice
basedonstellarluminosityandeffectivetemperature.
GeneralizedWilcoxonTest).
5. Theobservedactivity–rotationrelationsandtheir
origins
4.4.Determinationofconvectiveturnovertimescales Fig. 3 (left) presents the relationship of the observed activity
measures L , L /L and F with rotation period for the 23
X X ⋆ XS
Convectiveturnovertimescalesmustbedeterminedempirically starsinoursamplewithmeasuredphotometricrotationperiods.
orderivedfromstellarmodels.InPreibischetal.(2005),acon- If we consider accretors and non-accretorstogether as a single
vectiveturnovertimescalewasderivedfromafullstellarmodel sample,as hasbeendonein previousstudies,we observeclear
foreach of 596T Tauristars in the ONC. We have determined anticorrelations of LX and FXS with rotation period. The tests
the value of τ for each T Tauri star in the XEST sample performed in ASURV give probabilities of less than 0.01 that
conv
fromthevaluescalculatedfortheONCstars(Th.Preibisch,priv. nocorrelationexists.Theslopesofthecorrelationsareapproxi-
comm.),basedonitsstellarluminosityandeffectivetemperature mately 1(Table2).However,weseenoconvincinganticorre-
−
(see Fig. 2). For multiple systems, the L⋆ and Teff of the pri- lation of LX/L⋆ with rotation period; the tests in ASURV give
marywere used whereavailable. Thestars with T < 4100K probabilities of 0.14–0.26 that there is no correlation and the
eff
are essentially all fully-convective according to the evolution- best-fitslopeofapproximately 0.4islessthan1.5standardde-
−
ary modelsof Siessetal. (2000) and have verysimilar τ of viationsfromzero(Table2).
conv
200–250d.ThestarsinoursamplewithT intherange4100– Telleschietal. (2007) found a near-linearcorrelationof L
eff X
5400 K have radiative interiors and shorter τ , but these are with L in the XEST sample which they proposed was due to
conv ⋆
stilllongerthan100d.τ fallssteeplywithfurtherincreases saturatedactivity.Whentheyanalysedthecorrelationsofaccre-
conv
inT .AsthethreeG-typestarsintheXESTsample(SUAur, tors and non-accretorsseparately, using the parametric EM al-
eff
HD283572andHPTau/G2)aremuchlessluminousthanstars gorithminASURV,theyfoundsimilarslopesofL withL for
X ⋆
with similar T in the ONC sample, all we can say is that the the two samplesbut that L was systematically 0.4 dex(a fac-
eff X
τ ofthesestarsareshorterthantheshortestvaluecalculated tor2.5)lowerforaccretors(seeFig.1).Thebest-fittingrelations
conv
forONCstars,11.6d. were:logL =(30.24 0.06)+(1.17 0.09)log(L /L )fornon-
X ⋆
± ± ⊙
6 K.R.Briggs:TheX-rayactivity–rotationrelationinTaurus-Auriga
Table2.RegressionparametersandtheirstandarddeviationsforlinearfitsofobservedandexpectedvaluesoflogL ,log(L /L )
X X ⋆
and logF as a function of logP . A is the intercept and B is the slope. The expected activity measures were calculated for
XS rot
eachstarbasedonitsstellarluminosityandwhetheritwasanaccretorornot.TheregressionwasperformedusingtheParametric
EstimationMaximizationmethodintheASURVanalysispackage(Lavalleyetal.,1992).
Values logL (ergs 1) log(L /L ) logF (ergcm 2s 1)
X − X ⋆ XS − −
A B A B A B
Observed 31.04 0.21 1.20 0.30 3.20 0.16 0.37 0.24 7.53 0.17 1.04 0.25
± − ± − ± − ± ± − ±
Expected 30.98 0.25 1.27 0.36 3.27 0.10 0.44 0.15 7.46 0.17 1.11 0.24
± − ± − ± − ± ± − ±
accretorsandlogL =(29.83 0.06)+(1.16 0.09)log(L /L ) sothedependenceofrotationonL andT ,andhencetheob-
X ⋆ ⋆ eff
foraccretors. ± ± ⊙ servedcorrelationsofL ,L /L andF cannotbeexpectedto
X X ⋆ XS
There is the impression in Fig. 3 (left middle) that the bethesameinallpopulationsofTTauristars.
accretors have consistently lower L /L at all rotation peri- AnintrinsicdependenceofX-rayactivityonrotationperiod
X ⋆
ods,whiletheabsenceofasignificantanticorrelationof LX/L⋆ isnotrequiredtoproducetheanticorrelationsofLX andFXSon
with rotation period supports the concept of saturated activity. rotationperiodobservedinTaurus-Auriga,andassuchanticor-
Nevertheless,thesignificantanticorrelationsofL andF with relationshavenotbeenreportedinotherpopulationssuchasthe
X XS
rotationperiodstillsuggestsomekindofX-rayactivity–rotation ONC,weconsidertheretobenoconvincingevidencethatthey
relation. areintrinsicpropertiesofthemagneticactivityofTTauristars.
Let us assume that the X-ray activity of T Tauri stars in
Taurus-AurigaissaturatedinthesensethatL , L /L andF
X X ⋆ XS
have no intrinsic dependence on rotation period, but LX is in- 6. Originofthelower LX/L⋆ ofaccretors
steadcharacterizedsolelybythedependencesonL andaccre-
⋆ The lower L /L of accretors compared to non-accretors in
tionreportedbyTelleschietal.(2007).Wehavethuscalculated X ⋆
Taurus-Auriga has been reported before (Damiani&Micela,
the expected value of L , and hence L /L and F , for each
X X ⋆ XS 1995; Neuha¨useretal., 1995; Stelzer&Neuha¨user, 2001), and
star in our sample based on its stellar luminosity and whether
hasusuallybeenattributedtotheirslowerrotationandtheaction
itisanaccretorornon-accretor.Thenweanalysedtheresulting
ofanactivity–rotationrelationship.However,theassumptionwe
correlationbetweeneachactivitymeasureandrotationperiodas
madeintheprevioussectionwasthataccretorshadanintrinsi-
wedidfortheobservedactivitymeasures(Fig.3,right).Wefind
callylowerL /L thannon-accretorsatallrotationperiods.
that significant anticorrelations of L and F , with probabili- X ⋆
X XS Ifwe analysethe correlationof L /L with rotationperiod
tiesoflessthan0.01thatnocorrelationexists,result,alongwith X ⋆
separatelyfortheaccretorsandnon-accretors,thesesamples,13
a shallow anticorrelation of L /L . The intercepts and slopes
X ⋆ accretorsand10non-accretors,areverysmallandtheresultant
of these anticorrelationsare consistent with those found in the
fitsinASURVhavelargeuncertaintiesandlowsignificancesand
observeddata(seeTable2).
canbestronglyinfluencedbyasingledatapoint.Theprobability
The anticorrelation of L with rotation period can be ex-
X thatnocorrelationexistsisapproximately0.5forbothsamples.
plained by the the dependence of L on L and the observa-
X ⋆ The slope of +0.16 0.19 for the non-accreting sample sug-
tion that the fast rotators in Taurus-Auriga are typically more ±
gests that L /L is not anticorrelated with rotation period, but
luminous, while the slow rotators are typically less luminous X ⋆
the slope of 0.38 0.34for the accretingsample leavesopen
(Fig.4,top).TheshallowanticorrelationofLX/L⋆withrotation thepossibilit−ythatr±otationcouldplayaroleinthelowerL /L
periodcanbeattributedtothelowerL ofaccretorsatanygiven X ⋆
X ofaccretors.TheexclusionofXZTau5reducestheslopeforthe
L and the observation that the fast rotators in Taurus-Auriga
⋆ accretorsto 0.11 0.34.
aremainlynon-accretors,whiletheslow-rotatorsaremainlyac- − ±
We have used the larger sample of T Tauri stars with mea-
cretors(Fig.4).Theanticorrelationof F withrotationperiod
XS sured projected equatorial rotational velocities, vsini, to study
thenarisesfromtheshallowL /L relationduetotheobserva-
X ⋆ in more detail the dependences of L /L on rotation for ac-
tionthatthefastrotatorsinTaurus-Aurigahavegenerallyearlier X ⋆
cretors and non-accretors. This sample contains 30 accretors
spectraltypes,hencehigherT (Fig.4,bottom),thantheslow-
eff and 17 non-accretors. We have calculated rotation periods as
rotators;recallF =(L /L )σT4 .
XS X ⋆ eff P /sini = 2πR /vsini. Fig. 5 shows L /L plotted against
rot ⋆ X ⋆
A plausible physical explanation for the dependence of ro-
P /sini.
tation period on accretion and stellar luminosities and effec- rot
We have divided both the accretors and non-accretors into
tive temperatures has been described by Bouvieretal. (1993).
threesubsamples:fast-rotators,withP /sini < 6d,slowrota-
T Tauri stars spin up if they conserve angular momentum as rot
tors,withP /sini>6d,andstarswithnomeasuredvsini.The
theycontracttowardthemain-sequence.Accretingstarsmaybe rot
cumulativefrequencydistributionsoflog(L /L )forthesesub-
preventedfromspinningupeitherbylosingangularmomentum X ⋆
samplesarecomparedinFig.6.Wecanimmediatelyseethatthe
throughtheirstrongoutflowsorbybeingmagneticallycoupled
distributions for the three subsamples of accretorsare at lower
to the rotation of their accretion disks, whereas non-accreting
log(L /L ) than the distributions for the three subsamples of
stars have been allowed to spin up as expected. More massive X ⋆
non-accretors,andthatthedistributionforfast-rotatingaccretors
stars, which have higher L and T , may originate with faster
⋆ eff
rotation or may spin up more quickly due to their faster evo-
5 XZTauhasthehighestL /L oftheaccretors,andhasthelowest
lution toward the main-sequence. We note, however, that the X ⋆
massandstellarluminosityofanystarinoursamplewithameasured
lowest-massTTauristars(spectraltypeslaterthanM2),which
rotation period. It is moreover a binary system whose secondary was
arelargelyexcludedfromourstudy,alsoappeartobetypically known tobe undergoing an optical outburst that influenced theX-ray
fastrotators(Herbstetal.,2006),andotherstar-formingregions emission of the system at the time of the observation used in XEST
have different mass and age distributions from Taurus-Auriga, (Giardinoetal.,2006).
K.R.Briggs:TheX-rayactivity–rotationrelationinTaurus-Auriga 7
Fig.5. The ratio of X-ray and stellar luminosities plotted as a
function of rotation period for T Tauri stars in Taurus-Auriga
observedbyXMM-Newtonwithmeasuredvsini.Rotationperi-
ods have been calculated from vsini and the stellar radius and
theplottedvaluesincludeanunknownfactorsiniandarethere-
fore upper limits. Triangles denote accretors and circles mark
non-accretors.
However,foreachofthethreesubsamples,thedistributions
of L /L of accretors and non-accretors are significantly dif-
X ⋆
ferent (P = 0.01–0.03 for fast-rotators, < 0.002 for slow-
same
rotators,and0.005–0.03forstarswithnomeasuredvsini).We
therefore find good evidence for a property of accretors other
thantheirslowrotationtobethereasonfortheirlowerL /L .
X ⋆
IfweseparatethesamplesatP /sini= 5dinsteadof6d,
rot
Fig.4. The stellar luminosities (top) and effective tempera- the distributions of L /L of fast-rotating accretors and non-
X ⋆
tures (bottom) of T Tauri stars in Taurus-Auriga with mea- accretorsarelesssignificantlydifferent(P = 0.03–0.10)but
same
suredphotometricrotationperiods.Black trianglesindicateac- thedistributionsofL /L ofaccretingfastandslowrotatorsare
X ⋆
cretorsandgreycirclesmarknon-accretors.Filledsymbolsde- evenlesssignificantlydifferent(P =0.18–0.49).Thereisno
same
note stars observed by XMM-Newton in the present survey; supportforadependenceofL /L onrotation.
X ⋆
unfilled symbols denote additional stars observed by ROSAT The slightly higher log(L /L ) of fast-rotating accretorsin
X ⋆
(Stelzer&Neuha¨user,2001). ouranalysismaycomeaboutfromanunderestimationofL for
⋆
some stars. L /L and P /sini are not entirely independent
X ⋆ rot
variables as P sini R and R √L . If L is underesti-
rot ⋆ ⋆ ⋆ ⋆
∝ ∝
is at higher log(L /L ) than those for the slow-rotating accre- mated, a data pointmovesto the upperleft in Fig. 5, to higher
X ⋆
tors. The meanlog(LX/L⋆) of each sample is givenin Table 3. log(LX/L⋆)andlowerProt/sini.TheL⋆ofaccretorscanbedif-
The means of the three subsamples of non-accretors are well ficult to determine due to veiling and absorption. Three of the
within 1σ of each other, while that of the fast-rotating accre- fouraccretorswithProt/sini<5dandlog(LX/L⋆)> 3.4have
−
tors is approximately 2σ higher than those of the other accre- suspiciouslyhighagesof > 10 Myrin the table ofGu¨deletal.
tors.Foreachofthethreesubsamples,theaccretorshaveamean (2007a)whichsuggestunderestimatedL⋆.
log(L /L )atleast3σlowerthanthenon-accretors. Theaccretingstarshavenoticeablylargererrorbarsthanthe
X ⋆
We haveusedthetwo-sampletestsinASURVtoassessthe non-accretors.The accretors had typically lower-quality XEST
probability, P , that the distributions of L /L in pairs of X-rayspectra,partlyduetotheirlowerL ,whichgivesalower
same X ⋆ X
these subsamples were drawn from the same distribution. The X-ray flux from the star, and partly due to higher N , which
H
distributionsof L /L for the subsamplesof non-accretorsare absorbs more of this X-ray flux. However, the upper limits of
X ⋆
indistinguishable from one another (P > 0.5). The distri- thelargererrorbarsareproducedbymodelswithlargeamounts
same
butionsof L /L for the accretorswhich are slow rotatorsand of very cool plasma (T 2 MK), absorbed by very high N .
X ⋆ 0 H
≈
those with no measured vsini are also indistinguishable from Asmodelswithsuchcooltemperatureswerenotfoundtofitthe
oneanother(P > 0.75),andnotsignificantlydifferentfrom spectraofeitheraccretorsornon-accretorswithlowN ,wecon-
same H
that of the fast-rotating accretors(P = 0.09–0.35for slow- sider these unlikely to be realistic models. Such models would
same
rotators and 0.16–0.24 for stars with no measured vsini). We anywaypointtofundamentaldifferencesinthecoronaeofthese
findno strongevidencefora dependenceof L /L onrotation accretorsandthenon-accretors,intemperatureratherthanlumi-
X ⋆
foraccretingornon-accretingTTauristars. nosity.
8 K.R.Briggs:TheX-rayactivity–rotationrelationinTaurus-Auriga
Table 3. A comparisonof the mean log(L /L ) of subsamples
X ⋆
of T Tauri stars in Taurus-Auriga observed by XMM-Newton.
StarsinthefastsampleshaveP /sini< 6d,thoseintheslow
rot
samplehaveP /sini>6d,andthoseinthenonesamplehave
rot
nomeasuredvsini.
Sample Non-accretors Accretors
N log(L /L ) N log(L /L )
h X ⋆ i h X ⋆ i
Fast 9 3.27 0.06 17 3.61 0.09
− ± − ±
Slow 8 3.28 0.08 13 3.81 0.09
− ± − ±
None 10 3.31 0.13 17 3.79 0.10
− ± − ±
7. Comparisonwiththeactivityofmain-sequence
starsandthatofTTauristarsintheONC
Fig.7showsL /L plottedasafunctionofRossbynumberfor
X ⋆
T Tauri stars in the XEST sample with measured photometric
rotation periods, or for which rotation periods could be esti-
Fig.6. Cumulative frequency distributions of L /L for sub- matedfromvsiniasinSect.6.Fig.7alsocomparesthissample
X ⋆
samples of T Tauri stars in Taurus-Auriga observed by XMM- withmain-sequencestarsstudiedbyPizzolatoetal.(2003)and
Newton.Greylinesshownon-accretorsandblacklinesshowac- T Tauri stars in the COUP study of the ONC (Preibischetal.,
cretors. Unbroken lines mark fast rotators (P /sini < 6 d), 2005).
rot
dashed lines mark slow rotators (P /sini > 6 d), and dot- TheK-andM-typeTTauristarsinoursampleallhaveval-
rot
dashedlinesmarkstarswithnomeasuredvsini. uesofR0 thatplacethemdeepinsidethesaturatedregime.The
non-accretorswith measuredphotometricrotationperiodshave
L /L andR consistentwiththemainbodyofsaturatedmain-
X ⋆ 0
sequencestars. Thenon-accretorswithrotationperiodsderived
from vsini have a little lower L /L but still within the range
X ⋆
of values observed for saturated main-sequence stars. We con-
clude that the X-ray activity of non-accreting T Tauri stars in
We conclude that there is no demonstrable dependence of
Taurus-Aurigaisentirelyconsistentwiththatofmain-sequence
L /L on rotation among T Tauri stars in Taurus-Auriga. The
X ⋆ starsshowingsaturatedactivity.
activity is consistent with being saturated at log(L /L )
X ⋆ ≈ TheaccretorshaveRossbynumbersconcentratedtowardthe
3.3 for accretors and 3.7 for non-accretors. These values
− ≈ − higher end of the range of values of the non-accretors, due to
are consistent with those found for T Tauri stars in the ONC
theirtypicallylongerrotationperiods,butwellwithintherange
(Preibischetal., 2005). The lower L /L of accretors com-
X ⋆ of R that defines saturated main-sequence stars. However, as
paredtonon-accretorsisnotcausedbytheirslowerrotation,but 0
wesawinSect.6,the L /L ofaccretorsissignificantlylower
to some other property that distinguishes accretors from non- X ⋆
than that of the non-accretors and a significant fraction have
accretors.Themostnaturalinterpretationisthatitisadirector
log(L /L )< 4.Suchanactivitylevelwouldbeclearlyrecog-
indirecteffectoftheaccretionprocessitself.Wedirectthereader X ⋆ −
nised as unsaturated for a late-type main-sequencestar and in-
to Telleschietal. (2007) and Preibischetal. (2005) for discus-
terpreted as evidence for a less efficient magnetic dynamo due
sionsofmechanismsbywhichaccretioncouldreducetheX-ray
to slow rotation. If T Tauri stars harbour the same kind of dy-
outputofaccretingstars.
namo,theτ ofthese starswouldneedto havebeensystem-
conv
Additional to these discussions, Jardineetal. (2006) have atically overestimatedby approximatelyan orderof magnitude
proposedthatthecoronaeofTTauristars,andhencetheirX-ray forthisinterpretationtoexplaintheloweractivityoftheaccre-
emission, are limited by pressure stripping (resulting in lower- tors. Wuchterl&Tscharnuter (2003) have presented pre-main
mass stars having lower L , as observede.g. in the ONC), but sequenceevolutionarymodelsinwhichTTauristarsthatwould
X
those of accreting T Tauri stars may be further limited by the be expected to be fully convectiveinstead have radiative cores
encroachment of the accretion disk. As a reasonable estimate anda solar-likestructuredueto ongoingaccretion.Thiswould
forthe locationofthe inneredgeof theaccretiondisk is atthe resultinshorterconvectiveturnovertimescalesthanwehavede-
corotationradius,whereorbitingmaterialrotatesaroundthestar terminedhere,butdetailedmodellingwouldberequiredto de-
withthesameperiodasthestaritselfrotates,Jardineetal.fur- termine how much shorter. Even if this would be the case, it
ther suggested that the wide range of L and L /L observed would be the effect of accretion on the τ that would cause
X X ⋆ conv
amongtheaccretingTTauristarsintheONCcouldresultfrom thelowerX-rayactivityofaccretorscomparedtonon-accretors,
thisrotationdependence.Wehaveobservedneitherawiderange andnottheirslowerrotation.Ourresults,however,pointlessto
in L /L among accretors nor a positive correlation of L or areducedmagneticdynamoefficiencyandmoretotheeffectsof
X ⋆ X
L /L withrotationrate.However,adirectlyobservabledepen- circumstellar material on the stellar surface and atmosphere as
X ⋆
denceonrotationcouldbeswampedbyotherinfluentialparam- anexplanationofthelowerX-rayemissionofaccretors(seee.g.
eters in the model (e.g. stellar mass or coronal temperature), Telleschietal.,2007;Preibischetal.,2005).
and the mass-dependence of L and the lower L /L of ac- AlsostrikinginFig.7arethepositionsoftheG-typeTTauri
X X ⋆
cretorsin the XEST sample reflect those observedin the ONC starsSU Aur,HD 283572andHPTau/G2,forwhichwe could
(Telleschietal., 2007). Therefore, the Jardine et al. model ap- estimateonlyupperlimitstoτ .Althoughtheyareamongthe
conv
pears to be a viable description of the X-ray characteristics of fastestrotatorsinoursample,theirshortτ movesthemtothe
conv
TTauristarsthatmeritsfurtherobservationaltesting. highest R , and they are almost outside the expected saturated
0
K.R.Briggs:TheX-rayactivity–rotationrelationinTaurus-Auriga 9
regime.Onemightexpectsimilarstarswithlongerrotationpe- soft energy bandpassof 0.1–2.4keV and low sensitivity, espe-
riods, perhaps > 5 d instead of 1–2 d, to fall outside the satu- ciallyatharderenergies,combinedwithmodestexposuretimes,
rated regime and to show significantly lower L /L . This may providedspectraofinsufficientqualityforStelzer&Neuha¨user
X ⋆
havebeenobservedamongTTauristarswithradiativezonesin (2001)toperformspectralfittingforeachstar.Theyinsteadcon-
NGC2264(Rebulletal., 2006):while manyhad L /L atsat- verted observed count-ratesto X-ray fluxes using a conversion
X ⋆
urated levels, a significant fraction also had log(L /L ) < 5. factorthataimedtoaccountforabsorptionthroughadependance
X ⋆
−
Themeanlog(L /L )ofthestarswithradiativezoneswassig- onhardnessratio.OurcalculationsusingthePSPCon-axisspec-
X ⋆
nificantlylowerthanthatofthosewithout.Apopulationofstars tral response in XSPEC have found that the maximum value
with radiative zones showing such low activity is not found in ofthisconversionfactorwouldaccountforabsorptiononlyfor
the XEST sample.A two-sampletest of starswith and without N < 2.5 1020 cm 2,whilealmostallTTauristarsinTaurus-
H −
×
radiative zones finds no significant difference in the logL /L Aurigaare observedthroughgreaterabsorbingcolumns.Fig. 8
X ⋆
distributions of non-accretors or accretors (P = 0.15–0.44 shows that 42 of the 45 stars detected in both the XEST and
same
and0.17–0.21,respectively).Weproposethatasignificantpro- theROSATPSPCstudyhavehigherL intheXESTstudy6,and
X
portion of the stars with radiative zones in NGC 2264 may be thatthefactorofunderestimationinStelzer&Neuha¨user(2001)
rotatingslowlyenoughthattheiractivity,producedinthesame is dependenton N in the mannerexpectedand can exceedan
H
wayassolar-likemain-sequencestars,isnolongersaturated. orderofmagnitude.Asaccretingstarsaregenerallysurrounded
TheTTauristarsinTaurus-Aurigaareconcentratedtohigher bymorecircumstellarmaterialthannon-accretors,theyaretyp-
Rossby numbersthan those in the ONC. This is mainly due to ically observed through higher N and therefore their L was
H X
lower-massTTauristars,whoserotationalpropertiesarepoorly typicallyunderestimatedbyalargerfactor.
studied in Taurus-Auriga, but which form a large population
The overestimation of L in Stelzer&Neuha¨user (2001)
⋆
in the ONC and are mostly fast-rotating. The ONC stars have
came about from the use of the bolometric luminosities listed
a larger scatter in L /L (Fig. 7) and the ONC accretors and
X ⋆ byKenyon&Hartmann(1995),astheseincludeemissionfrom
non-accretors have much more overlap in L /L (not shown
X ⋆ circumstellarmaterial(mostlyintheinfrared)aswellastheout-
in Fig. 7) than thosein Taurus-Auriga.Thiscouldindicatethat
putfromthestaritself.Whilethegreateramountofcircumstellar
youngerTTauristarsexperiencegreaterX-rayvariability,orit
materialaroundaccretorsledtoagreaterunderestimationofL
couldreflectdifficultiesin estimatingthe L and L duetoab- X
X ⋆ withrespecttonon-accretors,soitalsoledtoagreateroveresti-
sorption.Thereareanumberofnon-accretorsintheCOUPdata
mationoftheirL .
⋆
with log(L /L ) < 4 whose Rossby numbers indicate they
X ⋆
− A secondary reason for the underestimation of L /L in
shouldshowsaturatedemissionbyanalogywithsolar-likemain- X ⋆
Stelzer&Neuha¨user(2001)wastheirtreatmentofmultiplesys-
sequencestars. Accretioncannotbe the cause of loweractivity
tems. The observed L was divided equally between the com-
innon-accretingTTauristars,sosuchstarsarepotentiallyvery X
interesting for investigating the differences between the X-ray ponents, whereas we now know that LX is approximately pro-
portionaltoL sothisapproachtypicallyunderestimatestheL
activityofTTauristarsandlate-typemain-sequencestars. ⋆ X
oftheprimarycomponent.Atthesametime,thebolometriclu-
minositiesfromKenyon&Hartmann(1995)includedemission
8. Comparisonwithpreviousresults fromallcomponentsofmultiplesystems,andsooverestimated
theL oftheprimary.
The anticorrelations of L , F and L /L with rotation ⋆
X XS X ⋆
period that we have found for the combined sample of Thesedifficultiesdemonstratesomeofthechallengesfaced
accreting and non-accreting stars differ from those found bystudiesoftheX-rayactivityofTTauristars.Whilethehigher-
by Stelzer&Neuha¨user (2001) using ROSAT PSPC data. energy bandpasses and higher sensitivities of XMM-Newton’s
Stelzer&Neuha¨user(2001)foundsteeperpower-lawslopesfor EPICandChandra’sACISdetectorshavemadeiteasierto ac-
all three activity measures, approximately 1.5 for L and count for absorption than was the case for the ROSAT PSPC,
X
LX/L⋆ and approximately 2 for FXS, and a−highly significant there are still cases where NH, and hence LX are very poorly
anticorrelationofLX/L⋆wi−throtationperiod. constrained. Uncertainties in LX that account for uncertainties
We have made a careful reexaminationof the data used by in the spectral fitting are rarely shown in the literature. Could
Stelzer&Neuha¨user (2001, B. Stelzer, priv. comm.) to try to the surprisinglylargescatter in the COUP data be attributed to
understand the differences between the two results. This has such uncertainties? The determination of the spectral type and
revealed systematic underestimation of L and overestimation extinction of accretors is complicated by veiling, the filling in
X
of L , both of which were stronger for accretors than for non- of photospheric absorption lines by continuum emission from
⋆
accretors. This caused a particularly strong underestimation of heatedaccretingmaterial,whichcanmakeL⋆ alsoquiteuncer-
L /L , and hence F , for some accretors with respect to non- tain. The division of the observed X-ray flux among the com-
X ⋆ X
accretors. Because the accretors have generally longer rotation ponentsofunresolvedmultiplesystemsisalwayssomewhatar-
periodsthanthenon-accretors,thisledtoexaggeratedslopesin bitrary,andevenstarswhicharenotrecognisedtobemultiples
the anticorrelations of the activity measures with rotation pe- mayyethaveundiscoveredcompanions.Thereremainsroomfor
riod. Significant correlations of L and F , and perhaps even improvementin studies of the X-ray emission of T Tauristars.
X XS
L /L , with rotation period are nonetheless to be expected as While the usefulnessof precisionmeasurementsis limited, be-
X ⋆
thetendencyforfasterrotatorstohavehigherL⋆andTeff andto causetheLXofanyindividualTTauristarislikelytobevariable
be non-accretingis also true in the larger Stelzer&Neuha¨user byfactorsof2–3ontimescalesofweeks–yearsandbylargerfac-
(2001)sample(Fig.4). torsduringflaringonshortertimescales,itisimportanttoavoid
TheunderestimationofL wasprimarilyduetoinsufficient systematic problems, for example between accreting and non-
X
accounting of the absorption of X-ray flux by interstellar and accretingstars.
circumstellarmaterialinthelineofsighttothestar.TheROSAT
PSPChad,inprinciple,thespectroscopiccapabilitytomeasure
andaccountfortheabsorbingcolumndensity,NH.However,its 6 ThisincludesalsostarswithspectraltypeslaterthanM3.
10 K.R.Briggs:TheX-rayactivity–rotationrelationinTaurus-Auriga
with rotationperiodthencomesaboutfromthe shallow L /L
X ⋆
relation due to the observation that the fast rotators in Taurus-
AurigahavetypicallyhigherT thantheslowrotators.
eff
PreviousstudieswhichhavereportedthelowerL /L ofac-
X ⋆
cretors in Taurus-Auriga have usually attributed it to the typi-
callyslowerrotationperiodsofaccretorsandananticorrelation
of L /L with rotation period. Using a larger sample of stars
X ⋆
fromtheXESTsamplewithspectraltypesM3orearlier,whose
rotation periods were derived from vsini, we find no evidence
thatslow-rotatingaccretorshavesignificantlylowerL /L than
X ⋆
faster-rotatingaccretors,orthatslow-rotatingnon-accretorshave
lower L /L than fast-rotating non-accretors. This is consis-
X ⋆
tent with saturated emission. However accretorswere found to
havesignificantlylowerL /L thannon-accretors,whetherthey
X ⋆
werefast-rotating,slow-rotating,orhadnomeasuredvsini.The
lowerL /L ofaccretorscomparedtonon-accretorsistherefore
X ⋆
notduetotheirslowerrotation.
Pizzolatoetal. (2003) have shown that the L /L of solar-
X ⋆
Fig.8.LogarithmoftheratioofX-rayluminositiesintheROSAT like main-sequence stars of spectral types G to early M is de-
studyofStelzer&Neuha¨user(2001)andmultiplicity-corrected termined by the Rossby number,the ratio of rotation period to
X-ray luminosities in the present (XEST) study, plotted as a convective turnover timescale. We have estimated the convec-
functionofabsorptioncolumndensity, N . Black trianglesand tive turnover timescale of each star in the XEST sample by
H
greycirclesmarkaccretingandnon-accretingTTauristars,re- interpolating values calculated for T Tauri stars in the ONC
spectively.LinesshowtheexpectedratioasafunctionofN for (Preibischetal.,2005)andhencederivedtheRossbynumberof
H
threespectralmodelscoveringtherangeoftemperatureconsid- each T Tauri star in the XEST sample with rotationalinforma-
ered in the XEST survey (logT = 6.3, dashed; 7.0, full; 7.5, tion.
0
dot-dashed, with β = 1 in each case), assuming a count-to- We have found that all T Tauri stars of spectral types K
flux conversion factor −of 1.0 10 11ergcm 2s 1 per ROSAT and M in the XEST sample haveRossby numbersthatlie well
− − −
PSPC counts 1 was used to×calculate the ROSAT luminosi- within the saturated regime of main-sequence stars. The non-
−
ties.Stelzer&Neuha¨user(2001)usedthisfactorincaseswhere accretorshaveLX/L⋆ entirelyconsistentwiththoseofsaturated
the hardness ratio could not be measured, for example, for the main-sequence stars. The accretors have, however, generally
most-heavilyabsorbedstarswhichwerenotdetectedinthe0.1– lower LX/L⋆,andasignificantfractionhavelog(LX/L⋆) < 4.
−
0.4keVband,whilethemaximumvalueusedwas1.36 10 11. Althoughthiswouldbeinterpretedasunsaturatedemissionifex-
−
× hibitedbyamain-sequencestar,thedeterminedRossbynumbers
oftheseaccretorsareapproximatelyanorderofmagnitudetoo
9. SummaryandOutlook lowtobeintheconventionalunsaturatedregime.Thispointsto-
wardsomeeffectofthecouplingofthestellarsurfaceandatmo-
WehaveusedtheXMM-NewtonExtendedSurveyoftheTaurus sphere to circumstellar material, rather than a reduceddynamo
MolecularCloud(XEST)toinvestigatethedependenceofX-ray efficiency, as the cause of the lower X-ray output of accreting
activitymeasures,LX,LX/L⋆andFXS,onrotationperiod. TTauristars(seeTelleschietal.,2007;Preibischetal.,2005).
Using only those stars with measured photometric rotation Identifyingtheprocesswhichcausesthisgeneralreductionis
periods, we found power-law anticorrelations of L and F animportantmotivationforfuturestudies,butjustoneelement
X XS
withrotationperiodsthataresignificantatthe99percentlevel, in understanding the complex interaction of magnetic activity
and have slopes of approximately 1. However, we found no and the circumstellar environment. Recent studies have found,
−
significant anticorrelation of L /L with rotation period, with for instance, that dramatic increases in accretion rate can have
X ⋆
a best-fitting slope of approximately 0.3, which is consistent quite differing effects on the observed X-ray emission in dif-
−
withsaturation. ferentcases(Kastneretal.,2004;Audardetal.,2005),andthat
AlthoughStelzer&Neuha¨user(2001)foundsteeperanticor- X-ray emission may also arise in shocks, in accretion streams
relationsforallthreeof these activitymeasures,reexamination onto the star (e.g. Kastneretal., 2002) or in jet outflows (e.g.
ofthedatausedbyStelzer&Neuha¨user(2001)hasrevealedthat Gu¨deletal.,2007b).
thiscanbeexplainedbyasystematicunderestimationofL and Non-accreting stars offer simpler means for understanding
X
overestimationofL thatpreferentiallyaffectedaccretingstars, the magnetic activity and dynamo processes in pre-main se-
⋆
whicharetypicallyslowrotators. quencestars.Theidentificationofnon-accretorswithL /L sig-
X ⋆
Telleschietal.(2007)haveshownthattheL ofTTauristars nificantly below the conventionalsaturation level and determi-
X
inTaurus-Aurigahasanear-linearcorrelationwithstellarlumi- nationoftheirRossbynumbersisapotentiallyimportantprobe
nosity, suggesting saturation, but that accreting stars have sys- of their dynamomechanism. In particular, do there exist slow-
tematically lower L than non-accretors by a factor of 2.5 at rotating non-accreting T Tauri stars with low activity that are
X
anygivenL .TheanticorrelationofL withrotationperiodcan analogoustothelow-activityslow-rotatingsolar-likeandfully-
⋆ X
beexplainedbytheobservationthatthefastrotatorsinTaurus- convective main-sequence stars and which would provide evi-
Auriga have typically higher stellar luminosities than the slow denceforlowdynamoefficiencyduetoslowrotation?
rotators.TheshallowanticorrelationofL /L withrotationpe- Investigations of the influence of stellar or circumstellar
X ⋆
riodcanbeattributedtotheobservationthatthefastrotatorsin properties on the X-ray emission depend on a good character-
Taurus-Aurigaaremainlynon-accretorswhiletheslow-rotators ization of those properties. The T Tauri stars in Taurus-Auriga
aremainlyaccretors.TheanticorrelationofF =(L /L )σT4 form a relatively small but very important population for such
XS X ⋆ eff