X-Shooter spectroscopy of FU Tau A

0.92 MB·English

by  B. Stelzer

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, [email protected]
2 INAF-OsservatorioAstronomicodiCapodimonte,ViaMoiariello16,I-80131Napoli,Italy
3 SchoolofCosmicPhysics,DublinInstituteforAdvancedStudies,31FitzwilliamPlace,Dublin2,Ireland
4 INAF-OsservatorioAstrofisicodiArcetri,LargeE.Fermi5,I-50125Firenze,Italy
Received<date>/Accepted<date>
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(Alcala´etal.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<olym1intmhceoagmn)epaiasrt-iaIbRlsleoswpseiimcthtirlutahmretX(oA-oSVuhr=osobt2uemrtNtahgIeR)y.
o
l
−16 spectrumforwhichwederiveAV ∼0.75±0.5maginananal-
600 700 800 900 1000 ogous way as the one described for the VIS. Throughout this
λ[nm] paper we adopt the spectral type and extinction derived from
theVISspectrum.Weexpectthedeterminationofextinctionus-
Fig.2. X-Shooter VIS spectrum of FUTauA compared to two
ing zero-extinction spectral templates observed with the same
spectral templates from Manara et al. (2013). A reddening of
instrument to be preciser in the VIS than in the NIR because,
A = 0.5mag has been applied to the templates. The spectra
V generally,theeffectsofextinctionare largerin the VISthanin
arenormalizedat750nmandthetemplatesareverticallyoffset.
theNIR.
3.2. Stellarparameters
that we have used are consistent with the spectral sequence of
Manara et al. (2013). The flux ratios for the spectral indices Ournewmeasuresforthespectraltypeandtheextinctioncanbe
werecalculatedfromthespectrumofFUTauAaftermodifying usedtorevisethestellarparametersofFUTauA.Theradiusof
itbyvariousamountsofextinctionbetweenA =0and2mag. FUTauAcanbeobtainedfromStefan-Boltzmann’slaw.Rather
V
We confirmthatthe opticalspectralindicesfromRiddicketal. thanusingtheliteraturevalueforthebolometricluminositywe
(2007) are independent on extinction for the range of extinc- recompute it taking account of the difference in our adopted
tions tested here and derive a spectral type of M6.6 ±0.4 for A and the value assumed by Luhmanetal. (2009a). From
V
FUTaubuildingtheaverageandstandarddeviationoftheVO2, the J magnitude given by Luhmanetal. (2009a) and a bolo-
R1, R2, R3, TiO8465, and VO7445 indices. The H O index metric correction of B.C. = 2.05 (Dahnetal. 2002) we find
2 J
yields M6.7 ± 0.2 and the H2O-K2 index M7.1 for a range of Lbol = 0.13L⊙. We have measured a slightly earlier spectral
A = 0...1mag. Riddicketal. (2007) and Allersetal. (2007) typethanLuhmanetal.(2009a),yieldinganeffectivetempera-
V
provide calibrations specifically for young M dwarfs, i.e. their ture of T = 2940K for a temperaturescale intermediatebe-
eff
spectral indices are independenton gravity, and we find excel- tweendwarfsandgiants(Luhman1999).Forthesenumberswe
lent correspondence in the derived spectral types. The slightly derive R∗ = 1.4R⊙, substantially lower than the value result-
laterspectraltypeobtainedfromtheH O-K2indexmaybere- ingfromtheluminosityandtemperaturegivenbyLuhmanetal.
2
latedtothefactthatRojas-Ayalaetal.(2012)havecalibratedit (2009a),R∗ = 1.8R⊙. ExtrapolatingthepositionofFUTauA
fornearby(evolved)Mdwarfs. intheHRdiagramdowntotheyoungestisochroneoftheevolu-
4 B.Stelzeretal.:X-ShooterspectroscopyofFUTauA
Fig.3. Detail of the spectrum of FUTau (black solid lines) in Fig.4.LiIλ670.8nmabsorptioninFUTauA(top)andTiOab-
the wavelength range around the KI (upper panel) and NaI sorption band at 615nm, a diagnostic of veiling, compared to
(lowerpanel)absorptiondoublets.ThespectrumofFUTauhas the same spectralregionin the unveiledClassIII template Par-
beencorrectedfortelluric absorptionlines. The red dottedline Lup3-1(bottom).
andthegreendashedlinerepresentsyntheticspectrawithlogg
as labelled for T = 2900K and rotationally broadened to
eff
vsini=20km/s.
cosmicabundanceofA(Li)≈ 3.1(ZapateroOsorioetal.2002)
isexpected.
The LiI λ670.8nm absorption of FUTauA is shown in
the top panel of Fig. 4. We determined an equivalent width of
tionarymodelsby Chabrieretal. (2000), the mass is 0.08M⊙, 0.43 ± 0.02A˚. In practice, for accreting objects photospheric
placingFUTauArightatthehydrogenburningmasslimit.
absorption lines may be filled in by excess continuum emis-
We used the gravity- and temperature-sensitive absorption
sionleadingtoshallowerlinesandanunderestimateoftheline
doublets of NaI at λλ 818.33, 819.48nm and of KI at λλ
equivalentwidths.Theamountofthis‘veiling’canbeestimated
766.48, 769.89nm to determine logg by comparison of the
by comparison with the spectrum of an unveiled star of the
X-Shooter data to synthetic spectra. We retrieved BT-DUSTY
same spectral type. The only available template for FUTauA
modelspectra for a range of T around the expected value of
eff observedwith the same instrumentand the same spectralreso-
2940Kandarangeofloggvaluesfromthestar,browndwarf&
lution is Par-Lup3-1 (M6.5). We have thus measured the EWs
planetatmospherewebsimulator(Allardetal.2010),electroni-
of the TiO absorptionbandsin the regionbetween 660nm and
callyavailable2.InFig.3thesyntheticspectraforT =2900K
eff 680nm(Valentietal.1998)forboththetemplateandFUTauA.
withthreedifferentvaluesforloggareoverlaidonthespectrum
We calculated the ratio EW /EW for eight dif-
ofFUTauAintheregionoftheNaandKdoublets.Themodel template FUTauA
ferent bands and found that the ratio is at most 1.2. A more
spectra have been rotationally broadenedto vsini = 20km/s,
detailed evaluation is prohibited by the noise level of the tem-
therotationratedeterminedforFUTauAinSect.3.5.Thewidth
plate spectrum. If these differences in EWs are due to veiling
oftheobservedlinesinFig.3isingoodagreementwithagravity
thecorrectedEW(Li)valueforFUTauAwouldbe <516mA˚.
oflogg = 3.5±0.5.Theuncertaintyisduetothe0.5stepsfor ∼
We have repeatedthe same exerciseusing the spectral features
logg in the grid of synthetic spectra. The gravity derivedfrom
shown in the bottom panel of Fig. 4. The result is a correction
thespectrumagreeswiththevalueexpectedfromtheevolution-
factorof≤1.3whichisconsistentwithweakveiling.
arymodelsofBaraffeetal.(1998)adoptingthestellarparame-
In Fig. 5 we compare our measurement to other studies of
tersderivedaboveandanageof1Myr.
lithiuminYSOsoflate-Mspectraltypefromtheliterature:σOri
(ZapateroOsorioetal.2002),λOri(Bayoetal.2011),andthree
3.3. Lithiumabsorption very low-mass (VLM) objects in Taurus (BarradoyNavascue´s
2004).Acompilationoflithiumequivalentwidthmeasurements
Theoreticalmodelspredictthatthelithiumcontentoflow-mass in Taurus is given by Sestitoetal. (2008) for a larger range of
starsgetsrapidlydepletedthroughoutthefirst∼50Myrsofthe spectraltypes.Wedonotconsiderthissampleherebecauseitin-
pre-mainsequenceevolution(e.g.D’Antona&Mazzitelli1997; cludesanumberofstarswithdubiousmembership.Forthecase
Baraffeetal. 1998). In particular,the LiIλ670.8nm absorption of λOri we consider only the subsample with confirmed clus-
line is a well-known age indicator in young low-mass objects. ter membershipandwithspectroscopicallydeterminedspectral
This was shown, e.g., in studies of the line equivalent width types.
for clusters and associations with different HR diagram ages Accreting objects are highlighted with filled plotting sym-
(e.g. Mentuchetal. 2008). The age at which lithium depletes bols in Fig. 5 and have been selected as follows: In σOri we
increases with decreasing mass. For the fully convective very consider accreting all objects with W > 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 sig-
natures 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