Astronomy&Astrophysicsmanuscriptno.futauXS˙v2 (cid:13)c ESO2013 December11,2013 ⋆ X-Shooter spectroscopy of FUTauA B.Stelzer1,J.M.Alcala´2,A.Scholz3,A.Natta3,4,S.Randich4,andE.Covino2 1 INAF-OsservatorioAstronomicodiPalermo,PiazzadelParlamento1,I-90134Palermo,Italy e-mail:B. Stelzer, stelzer@astropa.inaf.it 2 INAF-OsservatorioAstronomicodiCapodimonte,ViaMoiariello16,I-80131Napoli,Italy 3 SchoolofCosmicPhysics,DublinInstituteforAdvancedStudies,31FitzwilliamPlace,Dublin2,Ireland 4 INAF-OsservatorioAstrofisicodiArcetri,LargeE.Fermi5,I-50125Firenze,Italy Received/Accepted 3 ABSTRACT 1 0 Wehaveanalyzedabroad-bandopticalandnear-infraredspectrumofFUTauA,apresumedyoungbrowndwarfintheTaurusstar 2 formingregionthathasintriguedboththeoristsandobserversbyitsoverluminosityintheHRdiagramwithrespecttostandardpre- n mainsequenceevolutionarymodels.FUTauAisbrighterthananyotherTaurusmemberofsameorsimilarspectraltype,andvarious a phenomena(accretion,activity,binarity)havebeenputforthasapossibleexplanation.Thenewdata,obtainedwiththeX-Shooter J spectrographattheVeryLargeTelescope,includeanunprecedentedwealthofinformationonstellarparametersandsimultaneously 3 observed accretion andoutflow indicatorsfor FUTauA.Wepresent thefirstmeasurements of gravity(logg = 3.5±0.5), radial velocity(RV = 22.5±2.9km/s),rotationalvelocity(vsini= 20±5km/s)andlithiumequivalentwidth(WLi = 430±20mA˚) ] for FUTauA. From the rotational velocity and the published period we infer a disk inclination of i >∼50◦. The lithium content R ismuch lower thantheoreticallyexpected forsuch ayoung verylow massobject, addinganother puzzling featuretothisobject’s S properties.WedeterminethemassaccretionrateofFUTauAfromcomparisonoftheluminositiesof24emissionlinestoempirical h. calibrationsfromtheliteratureandfindameanoflogM˙acc[M⊙/yr] = −9.9withstandarddeviationσ = 0.2.Theaccretionrate determinedindependentlyfrommodelingoftheexcessemissionintheBalmerandPaschencontinuaisconsistentwiththisvalue. p Thecorrespondingaccretionluminosityistoosmalltomakeasignificantcontributiontothebolometricluminosity.Strongmagnetic - o activityaffectingthestellarparametersorbinarityofFUTauA,bothcombinedwithextremeyouth,mayberesponsibleforitsposition r intheHRdiagram.TheexistenceofanoutflowinFUTauAisdemonstratedthroughthefirstdetectionofforbiddenemissionlines st fromwhichweobtainanestimateforthemasslossrate,logM˙out[M⊙/yr] < −10.4. Themassoutflow andinflowratescanbe a combinedtoyieldM˙out/M˙acc ∼0.3,avaluethatisinagreementwithjetlaunchingmodels. [ Keywords.stars:BrownDwarfs–stars:pre-mainsequence–accretion–stars:individual:FUTauA 1 v 0 1 1. Introduction minosity.Massesof0.05M⊙and0.015M⊙wereinferredcom- 4 paringthepositionofthetwoobjectsintheHRdiagramtothe FUTauA was discovered by Luhmanetal. (2009a) as the pri- 0 evolutionarypre-mainsequencemodelsofBaraffeetal.(1998) mary component of a young brown dwarf (BD) binary in the . andChabrieretal. (2000).However,asnotedby Luhmanetal. 1 Barnard215 cloud. This dark cloud is considered to be part of (2009a),intheHRdiagramFUTauAislocatedwellabovethe 0 theTaurusmolecularcloudcomplex,andadistanceof140pcis 3 youngest (1Myr) isochrone of these models. The secondary, assigned(e.g.Torresetal.2009).Accordingtomid-infraredim- 1 FUTauB, also seems younger than 1Myr albeit by a less agesfromSpitzer,onlyoneotheryoungstar,FTTau,ispresent : amount,contestingthegeneralnotionofcoevalityforthecom- v within 0.5◦ of FUTau (Luhmanetal. 2009b). Its isolated po- ponentinbinaries(Luhmanetal.2010). i sition makes FUTau a benchmark object for studies of brown X dwarf formation scenarios as most mechanisms, such as e.g. Inthisarticle,we aim atinvestigatingthe natureofthe pri- r a ejection from a proto-stellarcluster, photo-evaporation,or disk mary, FUTauA. Various signatures of youth are apparent in fragmentationrequirethepresenceofhigher-massstars,seee.g. the available observationsof thisobject. Thepresenceof a cir- Whitworthetal.(2007). cumstellar disk was established from an analysis of its spec- Luhmanetal. (2009a) obtained optical and near-IR low- tral energy distribution (SED), where excess emission over a resolution spectroscopy for the primary (FUTauA) and an op- photosphere of the same spectral type is observed in all four tical spectrum for the secondary. They derived spectral types Spitzer/IRAC bands, i.e. at 3.6µm, 4.5µm, 5.8µm and 8.0µm of M7.25 and M9.25 for the two components,respectively.By (Luhmanetal. 2009a). Similarly, an excess in the blue part of comparison to spectral templates, they extracted differing val- the SED indicates ongoing accretion. This is bolstered by the ues for the extinction from the optical (A ∼ 2mag) and the high equivalentwidth of Hα emission measured in Luhman et V near-IR (A < 1mag) spectra of the primary, FUTauA, and al.’slow-resolutionspectra(93A˚),aswellasthelargewidthof V adoptedthehighervalueforthecalculationofitsbolometriclu- the Hα profile in a medium-resolution spectrum from Gemini analysed by Stelzeretal. (2010). The full-width at 10% of the Sendoffprintrequeststo:B.Stelzer peakheightwasmeasuredtobe350km/s,distinctlyhigherthan ⋆ BasedonobservationscollectedattheVeryLargeTelescopeofthe thecanonicallimitof200km/sconsideredtorepresentthebor- EuropeanSouthernObservatoryunderprogram086.C-0173(A). derlinebetweenmagneticactivityandaccretion-dominatedHα 2 B.Stelzeretal.:X-ShooterspectroscopyofFUTauA emission in BDs (Jayawardhanaetal. 2003). Applyingthe cal- tion, wavelength calibration, sky subtraction, correction for at- ibration provided by Nattaetal. (2004) to the Hα 10% width mosphericextinctionandfluxcalibration.However,thepipeline yieldedamassaccretionrateof3.5·10−10M⊙/yr.Stelzeretal. flux-calibrated spectra are not corrected for telluric absorption (2010) caution that the Hα line is only marginally resolved in bands. The telluric correction was performed independently in theGeminispectrum.However,themassaccretionratederived theVISandNIRspectrainthefollowingway.FortheVISarm from the line flux of the HeI5876 line in the same spectrum, the spectrum of the telluric standard (HIP20789, spectral type usingthe calibrationof Herczeg&Hillenbrand(2008) is about B7V), normalized to the continuum, was used as input in the afactortwohigherthanthevalueobtainedfromHα,providing IRAF1 task ”telluric”. For the NIR spectrum, a response func- furtherevidenceforstrongaccretion. tion was first derived by dividing the non flux-calibrated spec- Recently, the FUTau binary was the target of a Chandra trum of the telluric by a black-bodyof the same effectivetem- X-ray observation with the aim to study magnetic activity in perature(T =13,000K)asthetelluricstandard.Thisresponse eff twocoevalBDsofslightlydifferenteffectivetemperatureand/or function,containingthetelluriclines,wasthenusedasinputin mass. Unexpectedly,while the secondarywas notdetected,the theIRAFtask”telluric”.Inthisway,thetelluriccorrectionand primary showed very strong and soft X-ray emission, reminis- the correction for the response function were done simultane- cent of the TTauri star TWHya where the bulk of X-rays is ously. Although the shape of the resulting spectrum after this producedinaccretionshocksratherthanthestellarcorona(e.g. procedure is correct, the flux calibration is only relative to the Kastneretal.2002).ConsideringtheuntypicallylowX-raytem- response function. To bring the NIR spectrum into an absolute perature and this analogy with TWHya, FUTauA may be the flux scale we multiplied with a factor that was estimated using firstBD whereX-rayemission fromaccretionshockshasbeen thepipelineflux-calibratedspectrum.Finally,wavelengthshifts detected(Stelzeretal.2010). due to instrumentalflexures were corrected using the flexcomp ThecurrentobservationalpictureofFUTaupresentsanum- package within the pipeline. The precision of the wavelength berofambiguities.First,theover-luminosityintheHRdiagram, calibrationis betterthan 0.01pixcorrespondingto0.002nm in especiallyfortheprimary,allowsfordifferentinterpretationsas theUVBandVISarmsandto0.006nmintheNIRarm. discussed by Stelzeretal. (2010);Scholzetal. (2012). It could The flux-calibrated spectrum of FUTauA for the full X- beduetoextremeyouth,totheprimarybeinganunresolvedbi- Shooterwavelengthrangeis shownin Fig.1. Photometryfrom nary,ortoastrongcontributionfromaccretiontotheluminosity the literature is overlaid, demonstrating the high quality of the ofFUTauA.Alternatively,theinhibitinginfluenceofmagnetic flux calibration.In particular,the match of the differentinstru- fieldsand/orrotationontoconvectionmightforgivenluminos- mental arms is very good. In fact, during the observation the ity lead to smaller effective temperature and, consequently, a seeingwasexcellent(≈0.85′′). wrong mass estimate (Chabrieretal. 2007). Indeed, the mod- ulations in photometric time series reveal the presence of both 3. Stellar properties hot spots, i.e. accretion, and cool spots, i.e. magnetic activity (Scholzetal. 2012). Secondly,the velocitiesderivedin the Hα 3.1. Spectraltypeandextinction profile are smaller than the infall speeds suggested by the ob- servedX-raytemperature,leavinga doubtontheinterpretation We estimated the spectral type and extinction of FUTauA by oftheoriginoftheX-rayemissioninaccretionshocks. comparingit to the spectral templatesdefinedby Manaraet al. Aiming at a better understanding of this puzzling BD, we (2013, A&A subm.) on the basis of X-Shooter spectra for 24 haveobtainedbroad-bandspectroscopyfromtheUVtothenear- non-accretingandunabsorbedYSOsinvariousstarformingre- IR and multi-color time-series photometry for FUTauA. The gions.Thesespectradefineacontinuousspectralsequencefrom motivationforcollectingthesedatawastoseekforabetterun- M0 to M6.5 with steps of 0.5 in spectral subclass. In addition derstandingofitsaccretionandactivitycharacteristicsaspossi- theycomprisetwoobjectsattheendoftheMsubclass.Attheir blecausefortheoverluminosityofFUTauAintheHRdiagram youngages(1−10Myr)theseobjectsareexpectedtohavesimi- throughthestudyofspectralsignatures.Thenewphotometryis largravitytoFUTauA.Wedeterminedsimultaneouslythespec- partofourstudyofFUTauA’slong-termvariability.Theobser- tral type and the extinction of FUTauA by artificially redden- vationsanddata analysisaredescribedin Sect. 2. Stellar prop- ingthetemplatesbetweenAV = 0...2maguntilthebestmatch erties are derived in Sect. 3. In Sects. 4 and 5 we examine the to FUTauA was found. For the reddeningwe used the extinc- outflowandaccretioncharacteristicsofFUTauA,andinSect.6 tion law of Weingartner&Draine (2001). In Fig. 2, the spec- wediscussourresults. trumofFUTauAiscomparedtothatofPar-Lup3-1(M6.5)and DENIS-PJ124514.1-442907(M9). The spectrum of Par-Lup3- 1 appears very similar to FUTauA and we conclude that the 2. Observationsanddatareduction spectral type of FUTauA is M6.5 or slightly later. The gap in the spectral type sequenceof Manara etal. (2013)doesnotal- FUTauA was observed on Jan 11, 2011 with the X-Shooter lowustoputstrongerconstraints.Weestimatedanextinctionof spectrograph at the VLT. The data were acquired within the A ∼0.5±0.5mag.ThetemplatesinFig.2arebothreddened INAF/GTOtime(Alcalaétal.2011).Withitsthreespectrograph V bythisamount. arms, X-Shooter provides simultaneous wavelength coverage from300−2480nm.Slit widthsof1.0′′/0.9′′/0.9′′ wereused In a second approach to determine the spectral type of FUTauAwehavecalculatedvariousspectralindicesfollowing intheUVB/VIS/NIRarms,respectively,yieldingspectralreso- Riddicketal. (2007) for optical wavelengths, and the H O in- lutionsof5100/8800/5600.Thetotalexposuretimeineachof 2 dexdefinedbyAllersetal.(2007)andtheH O-K2indexfrom thethreespectrographarmswas1800sec.Weobtainedasignal- 2 Rojas-Ayalaetal.(2012)forthenear-IRspectrum.Theindices to-noiseratioof1−8intheUVB,10−20intheVIS,and20−30 intheNIRarm.Thedatawereobtainedinnodmodeandwere 1 IRAF is distributed by the National Optical Astronomy reducedindependentlyforeacharmwiththeX-Shooterpipeline, Observatories, which are operated by the Association of Universities v1.3.7(Modiglianietal.2010).Followingthestandardstepsin- forResearchinAstronomy,Inc.,undercooperativeagreementwiththe cluding bias or dark subtraction, flat fielding, optimal extrac- NationalScienceFoundation. B.Stelzeretal.:X-ShooterspectroscopyofFUTauA 3 10−12 JJ H 10−13 zz KKss ii III nm] 10−14 rr RRR s/ 2cm/ 10−15 uu gg g/ er x [ 10−16 u fl 10−17 10−18 300 1000 2000 λ[nm] Fig.1. Broad-band flux-calibrated X-Shooter spectrum of FUTauA. Published photometry from Luhmanetal. (2009a), from Scholzetal.(2012)andunpublishedRIJK photometryobtainedinSeptember2012withAndicamatthe1.3mtelescopeonCerro TololoaspartoftheSMARTSprogramDUBLIN-12A-001isoverplotted. ThespectraltypeweobtainedfromtheX-Shooterspectrum with the two methods explained above is also consistent with st −12 M6.5 (AV = 0.5 mag) n our earlier results from low-resolution spectroscopy where we o + c foundvaluesbetweenM6.6andM6.8inaseriesoffivespectra m] −13 FU Tau A (Scholzetal. 2012). Using a differentset of spectral templates n s/ Luhmanetal. (2009a) found a similar, slightly cooler spectral 2m/ typeofM7.25±0.25. erg/c −14 Luhman et al.’s value for the extinction derived from their g (f/f) [750 −15 M9 (AV = 0.5 mag) Tofophuteincldaaltatsehpriegvchatlreuuremvias(lAucelVefarr 20A˚, in λOri we Hα low-mass(<0.35M⊙)objectsinstarformingregionsstandard rely on the ‘accretor flag’ given by Bayoetal. (2011) which evolutionarymodelspredictthattheoriginalabundancesarestill is based on Hα emission and in Taurus we consider accret- retaineduntilanageofatleast10Myrs(e.g.Jeffries2006)anda ing objectswith W > 300km/s. The choiceof the 10% Hα10% widthratherthantheequivalentwidthasaccretiondiagnosticfor 2 http://phoenix.ens-lyon.fr/simulator/index.faces Taurusismotivatedbythefactthatoneobject,KPNO-Tau5,has B.Stelzeretal.:X-ShooterspectroscopyofFUTauA 5 Table 1. Proper motion of FUTauA from the literature and 1000 galactic velocity calculated using our RV measurement and d=140pc. 800 Reference µα,µδ (U,V,W) 600 A(Li)=3.5 [mas/yr] [km/s] mÅ] A(Li)=3.1 Ducourantetal.(2005) 14.7,−26.0 (−22.09,−9.08,−10.36) W [Li Luhmanetal.(2009a) 7.2,−17.5 (−23.54,−16.55,−10.41) 400 A(Li)=1.9 λOri σOri 200 Taurus FU Tau A 0 0 2 4 6 8 SpT (M subclass) Fig.5. Lithium equivalent width versus spectral type for FUTauA (filled star symbol) and other measurements of M- type YSOs collected from the literature (see text in Sect.3.3). Accretors are characterized by filled symbols. Also plotted are the curvesof growth for logg = 4.0 and three differentabun- danceswithlabels,calculatedbyZapateroOsorioetal.(2002). W = 21.1A˚, attheborderlinetotheaccretionregimebutis Hα classifiedasnon-accretorduetothenarrowHαprofileandweak HeIλ667.8emission. Fig.6. The NaI absorption doublet λλ818.33,819.48nm of FUTauAisshownascontinuousblackline.Thesyntheticspec- Theabove-mentionedstarformingregionsallhaveanageof trum,broadenedatprojectedrotationalvelocitiesof15,20and 1−5MyrandthelithiumcontentofMstarsisexpectedtore- 30km/s, is represented with the green-dotted, red-dashed and flecttheinitialcosmicabundance,A(Li)=3.1.However,alarge blue-dottedlines,respectively. spreadofequivalentwidthsisobservedforagivenspectraltype. Observationsobtainedwithdifferentspectralresolutionmayre- sultinsystematicdifferencesofthelinemeasurementsbutthese errors are smaller than the observed spread of the equivalent width values. This spread also corresponds to an unexplained (1988).About∼5%ofthislisthaveRV ≥22km/s,i.e.theRV spread in abundances as evident by the over-plotted curves of ofFUTauAisonlymarginallycompatiblewiththatofTaurus. growth for logg = 4.0 calculated by ZapateroOsorioetal. We have combined our RV measurement with the proper (2002).FUTauApresentsthelowestLimeasurementobserved motion of FUTauA given in the literature to obtain the sofarforitsspectralclass.Takingintoaccountveiling,FUTauA UVW velocities. We use both the proper motion values of movesupinthediagrambutisstillwellbelowthetheoretically Ducourantetal.(2005)andthoseofLuhmanetal.(2009a).The expectedabundance. resulting galactic velocity componentsfor a distance of 140pc are listed in Table 1. The average space motion for Taurus for the same distance is (U,V,W) = (−16.5±4.6,−13.2± 3.4. RadialvelocityandUVWvelocity 2.5,−11.0±4.0)km/s (Bertout&Genova 2006). FUTauA is inroughagreementwiththesevalues. We have used the strongest absorption lines, the KI doublet at λλ 766.48, 769.89nm and the NaI doublet at λλ 818.33, 819.48nm,toestimatetheradialvelocityofFUTauA bycom- 3.5. Rotationalvelocity parison of the observed line centers to the rest wavelengths extracted from the NIST Atomic Spectra Database3. After ap- The projected rotational velocity, vsini, was estimated by plication of the barycentric correction (−20.6km/s) the mean comparing the profile of the NaI absorption doublet at Doppler shift of these lines yields RV = 22.5 ± 2.9km/s λλ818.33,819.48nmwiththatofa syntheticBT-DUSTYspec- where we have added to the standard deviation of the four trumofthesameeffectivetemperatureandgravityasFUTauA. measured lines the uncertainty of the wavelength calibration Thesyntheticspectrumwasgatheredfromthestar,browndwarf (seeSect.2).Thelocal-standard-of-restvelocityofFUTauAis & planet atmosphere web simulator (Allardetal. 2010) using 12.5±2.9km/s. a vsini = 0km/s.In orderto reproducethe profileof the NaI Bertout&Genova(2006)havecompiledalistof127starsin absorptiondoubletinFUTauA,thesyntheticspectrumwascon- theTaurusstarformingcomplexwithRV measurements.These volvedwithrotationalprofiles(seeGray1992)ofseveralvalues authors note that no systematic high-precision RV survey has of vsini. The best match is for vsini = 20km/s (see Fig. 6). been performed in Taurus. Their RV distribution is strongly Weestimateanerrorontheorder5km/s. peaked at 15 − 16km/s, in agreement with historic smaller We can combineour measurementsof the rotationalveloc- samples presented by Hartmannetal. (1986) and Walteretal. ity(vsini)withthephotometricallydeterminedrotationperiod fromScholzetal. (2012) (P = 4.0±0.2d)andfindRsini = 3 http://www.nist.gov/pml/data/asd.cfm/ 1.6±0.5R⊙.ThevalueobtainedforRsiniiscompatiblewith 6 B.Stelzeretal.:X-ShooterspectroscopyofFUTauA our new value for the stellar radius (R∗ = 1.4R⊙) within the minimum possible value on the basis of the rotational proper- errorsandimpliesadiskinclinationanglei≥53◦. tiesofFUTauA.Ifthediskinclinationishigherthanthatvalue, theupperlimitonthemasslossrateishigherandlessmeaning- ful, e.g. for i = 75◦ the mass loss rate obtained from [SII] is 4. Outflows < 10−10.0M⊙/yr for ne > 103cm−3 and < 10−11.0M⊙/yr forn ∼105cm−3. e We have searched for forbidden emission lines (FELs) as signatures of shocks forming in outflows. A list of the equivalent widths(EWs)andlinefluxesfortheFELsdetectedinFUTauA 5. Accretion is given in Cols. 3 and 4 of Table 2. The EWs and fluxes are obtained from an by-eye estimate of the local continuum. The Different methods have been described in the literature for es- uncertaintiesoftheEWsandthefluxesrepresentthemeanand timating the mass accretion rate (M˙ ) from the properties acc standarddeviationfromthreemeasurementscarriedoutoneach of emission lines or continuum emission. The various accre- line. In our approach of estimating these uncertainties we take tion diagnostics probe several physical regions, e.g. the ex- account of the fact that the uncertainties in the flux are domi- cess continuumthat veils the absorptionfeaturesof classical T natedbytheuncertaintyoftheextinctionwhilethemajorsource Tauri stars is ascribed to accretion shocks (Calvet&Gullbring of the uncertainties in the EWs are the statistical fluctuations. 1998) and the high-excitation HeI emission to post-shock re- Therefore,fortheerrorsoftheEWswehavemeasuredtheline gions (Beristainetal. 2001). Hydrogen lines from the Paschen inthespectrumdereddenedby0.5magatanothertwopositions andBalmerseriesareproducedinaccretionflows,withHα,hav- correspondingto our estimate of the upper and lower value of ingthelargestopticaldepth,originatingintheiroutermostparts theadjacentcontinuumemissionandcomputedthestandardde- (Hartmannetal.1994;Muzerolleetal.1998). viation of the three measurements. For the uncertainties of the Here,weusetwoapproachesformeasuringthemassaccre- linefluxeswehavemeasuredthelineintheobservedspectrum tionrateofFUTauA:We makeuseofempiricalrelationswith (corresponding to an assumption of AV = 0mag) and in the emissionlinefluxesandluminosities(Sect.5.2.1)andwemodel spectrumdereddenedforAV = 1mag,andcombinethosetwo thecontinuumexcess(Sect.5.2.2). valueswiththatfromthespectrumdereddenedby0.5magtoget thestandarddeviation. 5.1. Emissionlineanalysis Thenon-detectionofthe[SII]λ671.6nmlinedoesnotallow ustoapplythetechniqueofBacciotti&Eislo¨ffel(1999)foresti- UsingIRAFwehavemeasuredequivalentwidthsandlinefluxes matingplasmaparametersfromthelineratiosofFELs.Forade- ofthedominantemissionlinesinthespectruminthesameman- tailedanalysisofluminosities,kinematicsandmassoutflowrate nerasdescribedinSect.4forthecaseoftheFELs. (M˙ ) of FUTauA we consider only the two most prominent out The by far strongest line in the X-Shooter spectrum of lines,[OI]λ630.0nmand[SII]λ673.1nm,andwefollowthepre- FUTauA is Hα. We measurean equivalentwidthof93±5A˚. scription describedby Hartiganetal. (1995) who presentedre- Previous Hα measurementsof FUTauA reported in the litera- lationsofthetype ture,allbasedonlower-resolutionspectra,rangebetween93and 155A˚ (Luhmanetal. 2009b; Stelzeretal. 2010; Scholzetal. n V l L logM˙ =X·(1+ c)( T )( T )−1( line)(1) 2012).Consideringthatlow-resolutionobservationstendtoun- out ne 150kms−1 2·1015cm Lsun derestimate line equivalent widths, this points at only modest variability. The Balmer series can be reliably identified up to The numerical constant X is derived in Hartiganetal. (1995) n=16.NolinesofthePaschenorBrackettseriesaredetected. for the [SII]λ673.1nm and the [OI]λ630.0nm lines. Eq. 1 in- SomeoftheBalmerlineprofilesaredisplayedinFig.7.All volves the electron density (n ), the tangential velocity of the e BalmerlinesincludingHαaresingle-peaked.Theverticallines outflow(V )andtheprojectedsize oftheapertureintheplane T inFig.7representtheexpectedlinecentercorrectedforbarycen- ofthesky(l ).Fora1′′slitweobtainforthedistanceofTaurus T termotionandthesystemicRV ofFUTauAderivedinSect.3.4. l =2·1015cm.Forthereasonableassumptionthattheoutflow T WhiletheHαprofileisnearlysymmetrictheotherlow-nBalmer is perpendicularto the disk we use the minimumpossible disk linesshowsmallasymmetrieswith a fluxdeficiencyonthered inclinationangle(i ∼ 50◦;see Sect.3.5) andthemeasured disk sideoftheprofile.Thismightbeinterpretedasredabsorption,a RV of the FELs with respectto the stellar RV to estimate the typicalsignatureofinfall(Hartmannetal.1994). tangentialvelocities.ThederivedquantitiesaregiveninTable2. Theuncertaintyinthevelocitiesaregivenincol.7.Theyinclude the standard deviation of three wavelength measurements and 5.2. Massaccretionrates theuncertaintyofthestellarRV. 5.2.1. Emissionlinediagnostics Wehavenomeasurementoftheelectrondensityfortheout- flow of FUTauA. A lower limit to n is given by the fact that e The published calibrations between emission line strength and wedetect[SII]λ673.1nmbutnot[SII]λ617.7nmemission.The massaccretionratecomeintwoflavors.Thefirstconsistsinre- fluxratiobetweenthesetwolinesis,therefore,largerthanunity lationsbetweenlineluminosity(L )andaccretionluminosity andn >103cm−3(e.g.Osterbrock1989).Thisgivesanupper line e (L )ofthetype acc limit to the mass loss rate determined from [SII]λ673.1nm of tlohgeMe˙leocuttro[Mn⊙de/nysri]ty<, t−he10[O.4I.]Aλ6ss3u0m.0inngmthlienseagmiveelsowaemrluicmhitlefossr log(Lacc/L⊙)=a·log(Lline/L⊙)+b (2) stringent constraint on M˙out. We note that the mass loss rates Thiscanbeconvertedtoaccretionrateaccordingto computedfromthetwolinesagreeforn ∼105cm−3.Forthat e case, we find logM˙out[M⊙/yr] < −11.5.Recall also that the M˙ =1.25· Lacc·R∗ (3) valueoftheinclinationangleassumedin thiscalculationisthe acc G·M∗ B.Stelzeretal.:X-ShooterspectroscopyofFUTauA 7 Table 2.Forbiddenemissionlines:nominalwavelength(col.2),lineequivalentwidthandflux(cols.3and4),radialvelocitywith respecttothebarycenter-correctedstellarvelocity(col.5),tangentialvelocity(col.6)anduncertaintyofthevelocities. Element λ0 EW fline·1016 RV VT ∆V [nm] [nm] [erg/cm2/s] [km/s] [km/s] [km/s] SII 406.860 −0.305±0.102 2.7±1.5 −26.6 99.2 3.2 SII 407.635 −0.081±0.083 0.4±0.2 ... ... ... OI 630.030 −0.323±0.053 8.8±3.4 −12.6 47.2 3.2 OI 636.378 −0.067±0.011 2.7±1.0 ... ... ... NII 658.260 −0.009±0.007 0.7±0.4 ... ... ... SII 673.082 −0.030±0.010 1.2±0.4 −11.0 41.1 3.2 2.5•10−13 λ656.4 nm 4•10−14 λ486.2 nm 2.0•10−14 λ434.1 nm m] m] m] 2m/s/n2.0•10−13 2m/s/n3•10−14 2m/s/n1.5•10−14 −12Flux [10 erg/c511...005•••111000−−−111433 −12Flux [10 erg/c12••1100−−1144 −12Flux [10 erg/c51..00••1100−−1154 0 0 0 −400 −200 0 200 400 −400 −200 0 200 400 −400 −200 0 200 400 v [km/s] v [km/s] v [km/s] 1.5•10−14 λ410.2 nm 2.5•10−14 λ397.1 nm 1•10−14 λ388.9 nm m] m]2.0•10−14 m] −122Flux [10 erg/cm/s/n51..00••1100−−1154 −122Flux [10 erg/cm/s/n511...005•••111000−−−111544 −122Flux [10 erg/cm/s/n2468••••11110000−−−−11115555 0 0 0 −400 −200 0 200 400 −400 −200 0 200 400 −400 −200 0 200 400 v [km/s] v [km/s] v [km/s] 8•10−15 λ383.6 nm 6•10−15 λ379.8 nm 5•10−15 λ377.1 nm m] m] m] −122Flux [10 erg/cm/s/n246•••111000−−−111555 −122Flux [10 erg/cm/s/n12345•••••1111100000−−−−−1111155555 −122Flux [10 erg/cm/s/n1234••••11110000−−−−11115555 0 0 0 −400 −200 0 200 400 −400 −200 0 200 400 −400 −200 0 200 400 v [km/s] v [km/s] v [km/s] Fig.7.LineprofilesforthelowerBalmerseriesofFUTauA.Thedashedverticallinerepresentstheexpectedlinecentercorrecting forthebarycentermotionandthestellarRV computedfromabsorptionlines. wherethenumericalfactorderivesfromtheassumptionforthe Sect.3.2.Thederivedmassaccretionratesforalldetectedemis- inner disk radius, Rin = 5R∗ (Gullbringetal. 1998). The sec- sion linesin commonwith the relationspresentedin the litera- ondapproachrelatesthelinesurfaceflux(F )directlytothe ture are shownin Fig. 8. Each literaturesource is markedwith s,line accretionrateintheformof a different plotting symbol. Some values that deviate strongly fromthebulkofthemeasurementsarenotshownforclaritybut logM˙ =c·logF +d (4) acc s,line discussedbelow.Theerrorbarsrepresentagainthe uncertainty Thecoefficientsa,b,canddhavebeenderivedforanincreasing inthereddeningwithupperandlowerboundscorrespondingto sampleofYSOs,however,sofarwithoutconsensusbetweenthe AV =0andAV =1mag,respectively. variousstudies. To estimate M˙ for FUTauA we have used the pub- We havecalculatedthe massaccretionrate forallemission acc lished relations presented by Herczeg&Hillenbrand (2008), linesdetectedintheX-ShooterspectrumofFUTauAthathave Mohantyetal.(2005),Rigliacoetal.(2012,A&Ainpress;here- been calibrated in the literature. However, we have computed after RNT12) as well as new calibrations by Alcala´ et al.,(in theaverageofM˙ onlyfromthoselinesthatwe considerthe acc prep). The latter ones are based on an X-Shooter sample of most reliable accretion diagnostics. In particular, we excluded 36YSOsfromtheLupusstarformingregionandrepresentthe theCaIIIRTlinesandNaID,forwhichstronglydiscrepantac- largesthomogeneousanalysisofthiskindperformedsofar.The cretion rates are found with differentcalibrations. Some of the distance used to convert line fluxes to luminosities is that of valuesforM˙ thatwehavederivedfortheselinesareoutside acc the Taurus star forming complex, 140pc. The mass and radius the range of values shown in Fig. 8. For a detailed investiga- of FUTauA required for evaluating Eqs. 3 and 4 are given in tion of the reliability of individual emission lines as accretion 8 B.Stelzeretal.:X-ShooterspectroscopyofFUTauA diagnosticsee e.g.Alcalaetal.(inprep.).We also didnottake into account [OI]λ630nm and Hα that are often affected by winds in YSOs (see Sect. 4). CaIIK and Hǫ are also excluded because these lines are partially blended in our spectrum. The average obtained from the remaining lines shown in Fig. 8 is hlogM˙acci[M⊙/yr] = −9.9±0.2 for 24 lines from Alcala et al.,hlogM˙acci[M⊙/yr] = −9.8±0.2for6linesfromRNT12, andhlogM˙acci[M⊙/yr]=−10.0±0.3for11linesfromHH08. Here the uncertainties represent the standard deviation of all measurements.Themeanandstandarddeviationofthecalibra- tionsbyAlcalaetal.areoverplottedinFig.8.Theinfluenceof theuncertaintiesofmassanddistanceonthisresultisexamined in Sect. 6. We anticipate here that they make logM˙ change acc onlymarginally.Similarly,asmallervaluefortheinnerdiskra- dius,e.g.Rin = 2R⊙,wouldyieldamassaccretionratethatis marginallycompatiblewiththevaluegivenabove. We have also derived the mass accretion rate from the Hα Fig.9.Balmer jumpregionin thespectrumofFUTauA, heav- 10% width. The measured full width half maximum at 10% ily smoothed to reduce the errors on the continuum (red), the of the peak heightis 228km/s. For the calibration providedby ClassIIItemplateSO925(black),theexcessemissionfromthe Nattaetal. (2004) this correspondsto a mass accretion rate of logM˙acc[M⊙/yr] = −10.7± 0.5. This value, also shown in eamccirsestiioonn,sil.aeb. tmemodpelalt(edpasluhsedexgcreesesn)emanidsstihoen,reinsubltliuneg.mAloldseplelecd- Fig. 8, is almost one order of magnitude lower than the mean tra are normalized in the region 700 − 720nm. Note that the valuederivedfromtherelationspresentedbyAlcalaetal. observedemissionintherange346.6nmto∼ 370nmisdomi- natedbyunresolvedBalmerlines,whicharenotincludedinthe 5.2.2. Continuumexcess slabmodel. Theaccretionluminositycanbeestimateddirectlybymeasuring thecontinuumemissioninexcessofthephotosphericone.This Table3.Equivalentwidthsandfluxesforemissionlines. is done by comparison to a non-accreting template star of the samespectraltype.Unfortunately,theClassIIIPar-Lup3-1that Line λ0 EW logfline wehaveusedasspectraltemplateinSect.3.1istoonoisytobe [nm] [nm] [erg/cm2/s/nm] usedforthispurpose.Fig.9showsthebluepartofthespectrum HeI1083 1083.038 −0.150±0.026 −13.73+0.06 −0.07 ofFUTauAcomparedtothatofanotheryoungClassIIIstarof CaII866 866.381 −0.017±0.003 −15.11+0.10 −0.11 similar(albeitslightlyearlier)spectraltype,SO925(M5.5;see CaII854 854.347 −0.076±0.015 −14.61+0.10 −0.12 RNT12andManaraetal.2013)normalizedintheregion700− CaII849 849.936 −0.042±0.010 −14.86+0.10 −0.12 720nm.FUTauAshowsaclearexcessemissionintheBalmer HeI667 667.928 −0.089±0.014 −15.40+0.18 −0.14 continuum and a relatively small Balmer jump, defined as the Hα 656.374 −9.766±0.709 −13.18+0.17 −0.16 ratio of the fluxes observedin the continuumat 360nm and at [OI]630 630.093 −0.314±0.039 −15.09+0.15 −0.19 420nm,of∼1.2.TheBalmerlimitisat346.6nmbutduetothe NaID1 589.697 −0.221±0.044 −15.51+0.18 −0.21 blendingofthehigherBalmerlinesitappearsshiftedto370nm. NaID2 589.096 −0.372±0.067 −15.27+0.18 −0.20 We model the excess emission as due to a slab of hydro- HeI587 587.670 −0.581±0.065 −15.01+0.20 −0.18 genofgivenelectrondensity,temperatureandlength,(seee.g., HeI501 501.625 −0.071±0.016 −16.07+0.18 −0.24 Valentietal. 1993); HH08; RNT12. and obtain an estimate of Hβ 486.184 −6.360±1.765 −14.14+0.22 the excess continuum luminosity of Lacc ∼ 1.3 × 10−4L⊙. HeI471 471.397 −0.031±0.009 −16.50−+−000...122534 Fig. 9 shows the best-fitting modelin blue. It reproducesquite HeII468 468.633 −0.048±0.016 −16.30+0.20 −0.23 welltheobservedBalmercontinumm,theBalmerjumpandthe HeI447 447.216 −0.470±0.164 −15.47+0.24 −0.23 profileoftheCaII lineat420nm.Veilingofphotosphericlines Hγ 434.096 −6.334±2.929 −14.44+0.24 −0.23 at longer wavelengthsis very weak as shown in Sect. 3.3. The Hδ 410.221 −4.151±2.327 −14.64+0.22 −0.26 uncertainties of Lacc are quite large (50% at least), due to the HeI402 402.687 −0.238±0.120 −15.87+−00..2256 noiseofthespectraofbothFUTauAandtheClassIIItemplate Hǫ 397.068 −1.767±1.078 −14.89+0.25 −0.26 andduetothemismatchbetweenthespectraltypeofFUTauA CaIIH 396.898 −5.297±3.764 −14.63+0.28 −0.20 and that of the template. For M = 0.08M⊙, R∗ = 1.38R⊙ CaIIK 393.419 −5.981±2.717 −14.52+0.25 −0.25 asdeterminedinSect.3.2thecorrespondingmassaccretionrate H8 388.944 −4.875±3.823 −14.78+0.25 −0.25 from Eq. 3 is logM˙acc[M⊙/yr] ∼ −10.1,in reasonablygood H9 383.588 −2.392±1.852 −14.98+−00..2264 agreementwiththeestimatesfromtheemissionlinespresented H10 379.845 −0.975±0.447 −15.15+0.24 −0.26 inSect.5.2.1. H11 377.113 −0.855±0.512 −15.25+0.22 −0.27 H12 375.071 −1.230±0.684 −15.28+0.24 −0.26 H13 373.484 −0.658±0.422 −15.44+0.18 6. Discussion H14 372.242 −0.384±0.250 −15.66−+00..1295 −0.27 We have analysed a broad-band (300 − 2480nm) medium- H15 371.232 −0.377±0.276 −15.64+−00..2258 resolution(R ∼ 5000−9000)spectrumof FUTauA obtained H16 370.432 −0.105±0.057 −16.04+−00..2225 withX-Shooter.Onthebasisofthesedatawehavepresentedthe first measurements of gravity, radial velocity, rotational veloc- B.Stelzeretal.:X-ShooterspectroscopyofFUTauA 9 −8.5 −9.0 yr] −9.5 M /sun ot [ −10.0 d M g −10.5 o l Alcala et al. 2013 relations HH08 relations −11.0 RNT12 relations Hα10% width −11.5 H15 H14 H13 H12 H11 H10 H9 H8 CaIIK CaIIH εH HeI402 δH γH HeI447 HeII468 HeI471 βH HeI501 HeI587 NaID OI630 αH HeI667 CaII849 CaII854 CaII866 HeI1083 Fig.8. Mass accretion rate for FUTauA derived from variousemission line fluxes and luminosities and from the 10% width of Hαusingdifferentcalibrationsprovidedintheliteratureaslabeledinthe figure.Dashedanddottedlinesrepresentthemeanand standarddeviationfortherelationsfromA13using24emissionlines. ity and lithium contentas well as the first detection of outflow emittingplasmaobservedbyStelzeretal.(2010)asoriginating activity in FUTauA. Moreover,this spectrum providesa large inanaccretionshock. numberofaccretiondiagnosticswhichwehaveemployedfora We have presentedthe first evidencefor outflow activity in detailedevaluationofthemassaccretionrate. FUTauA in the form of several forbidden emission lines. The The motivation for our spectroscopic study was the search outflowofFUTauAhasrecentlyalsobeendetectedatmillime- foranexplanationofthe(apparent)overluminosityofFUTauA ter wavelengths (Monin et al., A&A subm.). In this paper, we with respect to the predictions of evolutionary models for its have used the [OI] λ630.0nm and [SII] λ673.1nm lines de- age and mass. This requires precise measurements of the fun- tected in the X-Shooter spectrum to derive the mass loss rate damental stellar parameters (Teff, Lbol, R∗, logg) as well as a of the outflow using the approach presented by Hartiganetal. studyofotherphenomenasuchasaccretionandmagneticactiv- (1995). We trust the result from [SII] λ673.1nm more than ity that may lead to a wrong estimate of those parameters. We that for [OI] λ630.0nm because the former has a weaker de- have discussed several possible factors that may influence the pendence on the unknown electron density. An additional un- positionofFUTauAintheHRdiagraminourpreviouspapers certainty is introduced by the fact that we have only a lower (Stelzeretal.2010;Scholzetal.2012)andweresumethisdis- limitonthediskinclinationanglethatdeterminesthetransverse cussionhere,addingthewealthofinformationprovidedbythe outflow velocity. Assuming i = 53◦ and n ∼ 10−3cm−2 e X-Shooterspectrum. we derive from the [SII] λ673.1nm flux a mass outflow rate of logM˙out[M⊙/yr] ∼ −10.4. Combining this with the con- temporaneousmeasurementof the mass accretion rate we find 6.1. AccretionandoutflowinFUTauA an outflow-to-inflow rate M˙ /M˙ ∼ 0.3. This value for out acc the mass outflow rate of FUTauA could be both a lower limit In our detailed investigationof mass accretion in FUTauA we (given that n is an upper limit) or an upper limit (given that made use of an unprecedentedlarge number of empirical rela- e i is a lower limit); see Sect. 4 for details. Very few measure- tionsbetweentheluminosityofindividualemissionlinesandthe accretion luminosity derived from a comprehensive X-Shooter ments of M˙out/M˙acc have been presented in the literature for study of accretors in the Lupusclouds (Alcala et al., in prep.). VLM objects. While some of these observations suggest that mass inflow and outflow rate are similar in the VLM regime These relations comprise emission features collected with all (Bacciottietal. 2011; Whelanetal. 2009), others point at re- threearmsoftheX-Shooterspectrograph.Ascomparedtosim- ilar relations presented in the literature, we consider them the sults close to the ‘canonical’ value for higher-mass cTTS of mostreliablecalibrationsforourobservationofFUTauAgiven M˙out/M˙acc ∼ 0.1...0.01(Joergensetal.2012).Afinalconclu- thattheywerederivedwiththesameinstrument.Ameanvalue sion on the comparisonof the mass outflow to inflow ratio be- of hlogM˙acci[M⊙/yr] = −9.9±0.2 is found from a total of tweenBDsandcTTSsandthecomparisonofobservationstothe predictionsof jet-launchingmodels (Cabrit2009) is premature 24emissionlines.TheaccretionrateobtainedfromtheHα10% width(logM˙acc,10%[M⊙/yr]=−10.7±0.5)islowerbyalmost consideringthelargeuncertaintiesassociatedwith theobserva- one orderof magnitude.The discrepancyof logM˙ with tionsofM˙outandM˙accinVLMobjectsandtheinhomogeneous acc,10% approachbydifferentauthorsforcalculatingthesenumbers. respecttootheraccretiondiagnosticssupportspreviousnotions of the poor reliability of this tracer (e.g., Costigan et al. 2012, Signsforin-andoutflowsofFUTauAarealsoevidentfrom MNRAS in press). On the other hand, the accretion rate from thelineprofilesandlineshiftsobservedintheX-Shooterspec- the Hα10% width as measured in the X-Shooter spectrum is trum.Themodestdeficiencyoffluxontheredsideofthelow- alsomuchlowerthanthevaluemeasuredwiththesamediagnos- n Balmer lines is in qualitative agreementwith the outcomeof ticsfromapreviousGeminispectrum(logM˙acc,10%[M⊙/yr]= magnetosphericaccretionmodelsforhigh inclination.The cal- −9.5;Stelzeretal.2010).ThelowvelocitiesinthenewHαpro- culationsbyHartmannetal.(1994)predictalackoffluxinthe file are not sufficient to explain the temperature of the X-ray redwingofopticallythinemissionlinesthatincreaseswithde- 10 B.Stelzeretal.:X-ShooterspectroscopyofFUTauA creasing inclination angle due to occultation of the receeding etersgivenbyLuhmanetal. (2009a) and we hadestimated the part of the accretion flow by the disk. The strongest forbidden rotation period and the magnetic field strength using standard linesshowasimilarblueshiftof≈−10km/swithrespecttothe magnetosphericaccretion models. These estimates can now be stellar motion. This emission likely representsthe approaching updatedmakinguseofthenewvaluesforthestellarparameters lobeofthe jet, while thereceeding(redshifted)partofthe out- derivedfromtheX-Shooterspectrum. flowmaybeoccultedbytheaccretiondisk.Theseareplausible First, the new value for the bolometric luminosity corre- interpretationswhichare consistentwith ourrangeforthe disk sponds to a mass of 0.15M⊙ and a temperature of 3090K on inclinationangle(derivedfromvsini,rotationperiodandstellar the 1Myr isochrone of Baraffeetal. (1998). Our new value radius)thatexcludesapole-onview. for the mass accretion rate (see Sect. 5.2.1) was based on a mass of M = 0.08M⊙. This would be corrected downward 6.2. FUTauAintheHRdiagram to loghM˙acci[M⊙/yr] = −10.1 ± 0.2. Using these new pa- rametersand an innerdisk truncationradiusof Rin ∼ 2...5R∗ 6.2.1. Extinction themagneticfieldstrengthpredictedbyKoenigl(1991)isB ≈ 40...180G.Nomagneticfieldmeasurementhasbeenperformed A wrong extinction estimate could, in principle, misplace the yetforFUTauAthatwouldallowtotestthesenumbers.Inany object in the HR diagram in the vertical direction. We have case, this estimate shows that the combination of stellar pa- determined the spectral type of FUTauA to M6.5 ± 0.4 and rameters and mass accretion rate observed for FUTauA sup- the optical extinction to A = 0.5±0.5mag. This is slightly V port the presence of a strong surface field. The X-wind model hotter and less absorbed than found by Luhmanetal. (2009a), (e.g., Mohanty&Shu 2008), where the Keplerian angular ve- and results in a higher effective temperature (T = 2940K) eff locityis equalto thestellar rotationalvelocityattheinnerdisk andsmallerbolometricluminosity(Lbol = 0.13L⊙)andradius truncation radius, predicts a stellar rotation period of P ≈ (R∗ = 1.38R⊙). The smaller extinctionmovesthe objectver- 1.4...5.5d for the range of R given above. The valureotob- tically downin the HR diagramby less than a factor two. This in served by Scholzetal. (2012) for the rotation period (P = is by far not sufficient for resolving the luminosity problem of rot 4.0±0.2d)isconsistentwiththisprediction.Aspointedoutin FUTauA. ourpreviousworkthisperiodisratherlargeforsubstellarmass accordingtotheempiricaltrendbetweenperiodandmass(e.g., 6.2.2. Accretion Scholz&Eislo¨ffel2005). Theinfluenceofmagneticactivityonthestellarparameters ApossibleexplanationfortheoverluminosityofFUTauAwith isalsoexpectedtohaveaneffectonthelithiumdepletionoflate- respecttotheyoungestisochronesofevolutionarymodelscould typestarsbutthisisunlikelytoberelevantforFUTauAaswe beanoverestimateofthebolometricluminosityduetoasubstan- describeinSect.6.3. tialcontributionofaccretionluminosity.InScholzetal.(2012) wehavearguedthatthisscenarioisunlikelybecauseoftherela- tivelylowtemperaturederivedfortheaccretionhotspots.Itwas 6.2.4. Age also pointedoutthata significantlylowervalueforL , when bol combinedwiththeX-raydatapresentedbyStelzeretal.(2010), Both FUTauA and its browndwarf companionFUTauB have wouldyieldanexceptionallyhighX-raytobolometricfluxratio. the highest absolute J band magnitude of all Taurus members TheX-Shooterspectrumhasnowallowedustodirectlycompute withwhichtheysharetheeffectivetemperature,Teff ∼ 2800K theaccretionluminositybymodelingoftheBalmercontinuum. and Teff ∼ 2400K respectively (Scholzetal. 2012). This sug- Wefoundthataccretionmakesupforonly0.1%ofthebolomet- geststhatthepairiscoevalbutyoungerthantheotherVLMob- ricluminosityofFUTauA.Therefore,accretionluminositycan jectsinTaurus.Thelowgravityoflogg = 3.5±0.5measured notexplaintheoverluminosityofFUTauA.Thisisalsocorrob- fromtheX-Shooterspectrumis aconfirmationofthe pre-main orated by the absence of strong veiling in the X-Shooter spec- sequence nature of FUTauA. However,in the standard picture trum. extremeyouthisnotcompatiblewiththeweaklithiumsignature. Accretion possibly plays a role in another puzzling prop- Furthermore,therearenosignsforanenvelopewhichwouldbe ertyofFUTauA,itsweaklithiumabsorption(seediscussionin expectediftheobjectwerestillinaprotostellarphase. Sect.6.3). 6.2.5. Binarity 6.2.3. Magneticactivity The picture could be further complicated if FUTauA was a An alternative possibility to bring FUTauA in better agree- close binary. The maximum decrease in L obtained in this bol ment with evolutionary models would be to shift the object scenarioisafactortwowhichwouldmakeFUTauAsimilarto horizontally towards higher effective temperature and mass. It some other VLM objects in Taurus that are overluminouswith wasshownbyChabrieretal.(2007)andMacDonald&Mullan respect to the Baraffeetal. (1998) 1Myr isochrone. An equal (2009) that the influence of a strong magnetic field onto con- mass binary with 140AU separation (≈ 1′′ at the distance of vection may provide such an effect. In this scenario FUTauA Taurus) would have an RV amplitude of less than ∼ 1km/s appearscoolerandwithlargerradiusthanexpectedforitsmass. but a sub-arcsecond binary would produce a much larger RV AsT isobtainedfromobservationswithoutresortingtoevolu- signal.Binaritycouldberesponsibleforsomeofthedifference eff tionarymodelswhile massis model-dependent,thiswouldim- between the RV we measured for FUTauA and most Taurus plyahighermassforFUTauAthanthe0.08M⊙obtainedwhen members. One of the most obvious effects of binarity, besides extrapolatingtheobjectverticallydownto the1Myrisochrone itsinfluenceontheRV,wouldbethattherotationalvelocityhas ofBaraffeetal.(1998). an unknowncontributionfromthe orbitalmotionandthe mea- InStelzeretal.(2010)wehadderivedamassof∼ 0.2M⊙ suredvsiniisanupperlimit.TheRsiniestimatederivedfrom forFUTauAassumingthisscenariobasedonthestellarparam- therotationalvelocityandtherotationperiodis,infact,slightly

X-Shooter spectroscopy of FU Tau A

Similar X-Shooter spectroscopy of FU Tau A