On the kinematic age of brown dwarfs: Radial velocities and space motions of 43 nearby L dwarfs

0.19 MB·English

by  Andreas Seifahrt

Astronomy&Astrophysicsmanuscriptno.UVW˙Ldwarfs (cid:13)c ESO2010
January12,2010
On the kinematic age of brown dwarfs:
Radial velocities and space motions of 43 nearby L dwarfs
A.Seifahrt1,2,A.Reiners1,K.A.M.Almaghrbi1,andG.Basri3
1 Universita¨tGo¨ttingen,Institutfu¨rAstrophysik,Friedrich-Hund-Platz1,D-37077Go¨ttingen,Germany
2 PhysicsDepartment,UniversityofCalifornia,Davis,CA95616,USA
e-mail:[email protected]
3 AstronomyDepartment,UniversityofCalifornia,Berkeley,CA94720,USA
0 asofJanuary12,2010
1
0 ABSTRACT
2
Wepresent radial velocitymeasurements of asampleof L0–L8dwarfsobserved withVLT/UVESandKeck/HIRES.Wecombine
n
thesemeasurementswithdistanceandpropermotionfromtheliteraturetodeterminespacemotionsfor43ofourtargets.Weidentify
a
J ninecandidatemembersofyoungmovinggroups,whichhaveagesof50–600Myraccordingtotheirspacemotion.Fromthetotal
velocity dispersion of the 43 L dwarfs, we calculate a kinematic age of ∼ 5 Gyr for our sample. This age is significantly higher
2
thanthe∼ 3Gyr ageknownfor lateMdwarfsinthesolarneighbourhood. WefindthatthedistributionsoftheU andV velocity
1
componentsofoursampleareclearlynon-Gaussian,placingtheageestimateinferredfromthefullspacemotionvectorintoquestion.

TheW-componentexhibitsadistributionmoreconsistentwithanormaldistribution,andfromW alonewederiveanageof∼3Gyr,
]
R which is the same age found for late-M dwarf samples. Our brightness-limited sample is probably contaminated by a number of
outliersthatpredominantlybiastheUandVvelocitycomponents.Theoriginoftheoutliersremainunclear,butwesuggestthatthese
S
browndwarfsmayhavegainedtheirhighvelocitiesbymeansofejectionfrommultiplesystemsduringtheirformation.
.
h
Keywords.Stars:low-mass,browndwarfs–Techniques:radialvelocities
p
-
o
r 1. Introduction 2. Sampleandanalysis
t
s
a Early-andmid-Ldwarfsencompassamassrangethatincludes Reiners&Basri(2008,hereafterRB08)presenthighresolution
[ very low mass stars as well as more massive brown dwarfs (R >∼ 30,000) optical spectra of 45 L dwarfs1 obtained with
1 around the hydrogen-burningminimum mass (HBMM≈ 0.07– VLT/UVES and Keck/HIRES. They measure rotational veloc-
v 0.08 M⊙, Chabrier&Baraffe 1997). The kinematics of these itiesandchromosphericactivitytoinvestigaterotationalbraking
0 stars, especially within the solar neighbourhood, is important in M and L dwarfs. The sample selection and observationsare
8 because the intrinsic velocity dispersion of the stars can be presented in Sect. 2 of RB08, the targets being essentially the
7 used to determine their age (Wielen 1977; Fuchsetal. 2001). brightestL dwarfsdistributedoverthe wholesky (Mart´ınetal.
1 BrowndwarfsmaythenserveinturnasGalacticchronometers 1999; Kirkpatricketal. 2000; Cruzetal. 2003; Kendalletal.
1. (Burgasser2009),giventhattheyconstantlycoolovertimeand 2004;Reidetal.2008).Nopreselectionofanyotherparameter
0 neverreachastablestateonthemainsequence. wasmade.
0 While an age of ∼ 3 Gyr for the nearby late-M dwarfs WeuseGl406asourprimaryradialvelocitystandard(M6V,
1 is now well established (Reidetal. 2002; Reiners&Basri v =19±1kms−1,Mohanty&Basri2003)andemployacross-
rad
v: 2009), kinematic ages of L and T dwarfs remain preliminary. correlationtechniquetomeasuretheradialvelocityofeachstar
i ZapateroOsorioetal. (2007) derive an age ∼ 1 Gyr for 21 L forvariousspectralorders(see,e.g.,Mohanty&Basri2003and
X and T dwarfs based on space motions of these objects, while Basrietal. 2000 for a detailed description of the method). We
r Fahertyetal.(2009)deriveanageof3–6Gyrsforalargesam- focused on the resonance lines of CsI and RbI as well as the
a
pleofLandTdwarfs,basedontheirdistanceandpropermotion. prominent features of TiO, VO and, most importantly, FeH.
Thediscrepancybetweentheseresultsisprobablyduetodiffer- TelluricabsorptionlinesoftheO A-bandaswellasstrongH O
2 2
encesbetweentheagecomputationsbutmayalso beattributed lines around λλ 930–960nm were used to correct for shifts in
to thesmall numberof objectswith measuredradialvelocities, thewavelengthsolutionofeachspectrumwithrespecttoourra-
andthus,3Dspacemotions. dialvelocitystandard.Wefollowedtheconservativeapproachof
Kinematic studies of brown dwarfs have been predom- adoptingthestandarddeviationoftheradialvelocitiesobtained
inantly based on proper motion measurements (see, e.g., in differentspectralordersas the intrinsic error of our final ra-
Schmidtetal. 2007; Jamesonetal. 2008; Casewelletal. 2008; dialvelocities.Theintrinsicuncertaintiesrangefrom0.2kms−1
Fahertyetal. 2009). Absolute radial velocities of brown
dwarfs have been obtained for small samples by, e.g.,
1 Oneofthetargets(2MASSJ1159+00)hastoolowasignal-to-noise
Basrietal. (2000), Bailer-Jones (2004), Blakeetal. (2007), or
ratiotocomputearobustradialvelocity,reducingtheinitialsamplesize
ZapateroOsorioetal. (2007). In this paper, we measure radial to 44. One more target (2MASS J1854+84) has no published proper
velocitiesandprovidespacemotionsfor43L0–L8dwarfs,thus motionanddistance,reducingthenumberoffinaltargetsinoursample
foramuchlargersamplethanobtainedbefore. to43.
2 A.Seifahrtetal.:Spacemotionsofbrowndwarfs
5 3 3.2.Spacemotions
4 To calculate the space motionsof our sample, we combineour
s s
der der 2 radial velocity measurements with distance and proper motion
or 3 or data collected from the literature, mainly from the extensive
mber of 2 mber of 1 dspaetacbtraospehooftoFmaheetrritcyeesttaiml.a(2te0s0a9n).dDhiasvtaencaetsyapriecablausendcemrtoaisntltyyoonf
Nu Nu 1pc.AfewLdwarfsinoursamplehavemeasuredtrigonometric
1
parallaxes,whichnotablyreducestheirdistanceerrors.Wecom-
paredthereportedpropermotionsanddistancesinFahertyetal.
0 0
26.0 26.2 26.4 26.6 26.8 27.0 27.2 −0.2 0.0 0.2 0.4 0.6 (2009)withothersourcesandusedthevalueswiththesmallest
RV of stellar lines (km/s) RV of telluric lines (km/s) uncertainties.DetailsareprovidedinTable2.
Fig.1.Exampleof the distributionofradialvelocitiesobtained Space motions were computed using the IDL routine
in different spectral orders from stellar lines (left) and telluric gal uvw2. All space motions are relative to the sun. We adopt
lines(right)inthespectrumof2M1645-1319(L1.5V).Seetext a sign convention for U that is positive towards the Galactic
fordetails. Center.Wedetermineerrorsindistance,propermotion,andra-
dialvelocityinaMonteCarlofashiontocomputeerrorestimates
inU,V,andW.Inthiscomputation,wederivespacemotionsfor
for the earliest L dwarfs up to 4 kms−1 for the latest and most all possible combinationsof inputparametersand their 1σ un-
rapidly rotating targets. The final uncertainty in most cases is certainties.WethencalculatethestandarddeviationinU,V,and
dominated by the uncertainty in the absolute radial velocity of W for each targetfrom this distribution and adoptthese values
Gl406,whichweaddedinquadraturetotheintrinsicuncertainty asthe1σuncertaintiesinU,V,andW.Thecalculatedspacemo-
of each target. All radial velocities were finally corrected for tionsandtheirrespectiveerrorsaregiveninTable2alongwith
barycentricmotion. thedistanceandpropermotiondataobtainedfromtheliterature.
Asanexample,weshowinFig.1theresultsobtainedforthe
L1.5 dwarf 2M1645-1319.The radial velocities obtained from
stellarlinesin11differentordersshowanearlyGaussiandistri- 3.3.Membershipinyoungmovinggroups
bution with a mean of 26.6 kms−1 and a standard deviation of
0.21kms−1.Thetelluriclinesin4differentordersshowanoff- Youngmovinggroupsareassociationsofcoevalandcomoving
setof0.2±0.16kms−1.Hence,wedeterminedaradialvelocity stars.Membersofmovinggroupscanbeidentifiedeitheronthe
of26.4±1.0kms−1. basisof propermotionandpositiononthe sky,using themov-
ingclustermethod(see,e.g.,Bannister&Jameson2007)orby
For all L dwarfs later than L2, we also used the spectra
comparingthe 3Dspace motionsofindividualobjectswith the
of 2MASS1645-1319 (L1.5) and 2MASS1045-0149 (L1.0) as
mean space motion of the moving group. We found members
secondary radial velocity standards. Absolute radial velocities
offouryoungmovinggroups(MG),namelyoftheHyades,the
for both objects were determined against Gl406 with a preci-
sion of ∼ 0.3 kms−1. Both targets are relatively slow rotators Pleiades MG, the AB Dor MG, and the UMa MG, with ages
ranging from 50–600Myr. All these movinggroups are close-
(vsini = 9 kms−1 and <3 kms−1, respectively, RB08) and
by and located in or near the velocity ellipsoid of the young
their respective VLT/UVES and Keck/HIRES spectra have a
(thin)disk(−50 < U < +20kms−1,−30 < V < 0kms−1, and
high signal-to-noise ratio, making them well suited as cross-
−25< W < +10kms−1;see,e.g.,Leggett1992,andreferences
correlation templates. Radial velocities of all targets measured
therein). No members were found for other young and close
relative to Gl406 and our L dwarf templates are always identi-
movinggroupsconsideredin ouranalysis, suchas TWHydrae
cal,butthelattercorrelationproducedmorerobustresultswith
Association, the β Pictoris MG, and the Tucana/Horologium
smaller uncertainties since the L dwarf templates are a closer
Association (see, e.g., Zuckerman&Song 2004). While their
match to the spectral features of the targets. Thus, we adopted
UVW velocityellipsoidsarecloseto thoseofthePleiadesMG
theradialvelocitiesobtainedfromthecross-correlationwithour
and the AB Dor MG, we excludedmembershipsbased on dis-
secondaryvelocitystandardsforalltargetslaterthanL2asour
tanceorpositionontheskyinallcaseswhereconfusioninthe
finalresults.
UVW spacelimitedourabilitytoassignmembershipsbasedon
spacemotionsonly.
3. Results ThespacemotionofourtargetsareshowninFig.2.Weover-
plot the velocity ellipsoids of the young moving groups in the
3.1.Radialvelocities same figure, adopting a typical dispersion of 10 kms−1 for the
Hyadesand7kms−1 forthePleiades,ABDor,andUMainac-
We present the final results from our radial velocity measure-
cordancewith Chenetal. (1997) and Chereuletal. (1999). We
mentsinTable2.TwelvetargetswereobservedwithVLT/UVES
comparethe computedspace motion of our targetsto the typi-
andKeck/HIRESatdifferentepochs.Inallcases,theradialve-
calspacemotionofboththeyoungdiskandtheyoungmoving
locities obtained from both runs were fully consistent within
groupslistedinZuckerman&Song(2004).Wefoundatotalof
their individual uncertainties, showing no sign of significant
radial velocity variability on a kms−1 level. We also com- 11objectsthatbelongtothegalacticyoungdiskpopulation(see
Table2)andanother9objectsthatbelongtotheyoungmoving
pared our results to radial velocities reported in the litera-
groups(seeTable1).Commentsonindividualtargetsaregiven
ture(Basrietal.2000;Bailer-Jones2004;ZapateroOsorioetal.
inSect.3.5.
2007;Blakeetal.2007).Wefoundgoodagreementwithourval-
uesin16cases.However,wefoundtwocases,namely2M0255-
47and2M1305-25,whereourradialvelocitiesareinconsistent 2 Written by W. Landsman and S. Koposov, based on
withothermeasurements.WediscusstheseobjectsinSect.3.5. Johnson&Soderblom(1987).
A.Seifahrtetal.:Spacemotionsofbrowndwarfs 3
Table1.Candidatemembersofyoungmovinggroupsbasedon must apply a weight of |W| to all data points when calculating
ourUVW analysis thedispersionsineachdimension.Theweightsaccountforthe
fact that we are limited to objects within a small volume close
Movinggroupa UVW(kms−1) Age(Myr) tothemidplaneoftheGalacticdisk,andenableustoinferages
fromthekinematicalheatingofthewholedisk.
HyadesCluster −40,−17,−3 ∼600
We briefly summarise the results from three papers deal-
2MASSJ02550357-4700509
ing with the kinematic age of low-mass objects. Schmidtetal.
2MASSJ10452400-0149576b
(2007) infer an age estimate of 3.1 Gyr for their sample of
2MASSJ10473109-1815574b
M7–L8 L dwarfs, which agrees with the value reported by
2MASSJ14413716-0945590
2MASSJ22000201-3038327 Fahertyetal.(2009)andappearsconsistentwithresultsforlate-
M samples (Reidetal. 2002; Reiners&Basri 2009). On the
UrsaMajorisMG +14,+1,−9 ∼300 other hand, ZapateroOsorioetal. (2007) report an age of only
2MASSJ03140344+1603056 ∼1GyrfortheirsampleofLandTdwarfs.Wenotethatanim-
2MASSJ17054834-0516462 portantdifferencewithrespecttoourworkisthatSchmidtetal.
(2007), ZapateroOsorioetal. (2007), and Fahertyetal. (2009)
PleiadesMG −12,−21,−11 ∼100
calculatethetotalvelocitydispersionasthedispersionofv in-
2MASSJ06023045+3910592 tot
1/2
ABDoradusMG −8,−27,−14 ∼50 stead of σtot = (cid:16)σ2U +σ2V +σ2W(cid:17) , which may underestimate
the dispersion and thus the age. Unfortunately, an error intro-
2MASSJ17312974+2721233 duced in the formula computing age from velocity spread had
theeffectofalmosteliminatingthisoffset,sothatintwoofthe
a UVWandagefromZuckerman&Song(2004). papers,anageof3Gyriscomputed.3Amoredetaileddiscussion
b Membership already suggested by Jamesonetal. (2008), see also
of the computationsof kinematic age by Schmidtetal. (2007),
Sect.3.5
ZapateroOsorioetal.(2007),andFahertyetal.(2009)isgiven
inReiners&Basri(2009).
We reanalysethe space motionsofthe sample of 21brown
We note that only five out of 43 targets (∼ 12%) are can- dwarfs of spectral type L4V–T8V of ZapateroOsorioetal.
didate members of the Hyades. This is in notable contrast to (2007) and calculate |W|-weighted velocity dispersions of
the ∼ 40% (8 out of 21 targets) of L and T dwarfs reported (σ ,σ ,σ ) = (35.0,15.1,15.3)kms−1 fromthe valuesgiven
U V W
by ZapateroOsorioetal. (2007) that are within or close to the in Tables 3 and 4 of their paper. The |W|-weighted total veloc-
2σellipsoidoftheHyades.Whilethismembershipcriterionis ity dispersion is σ = 41.1 kms−1. This dispersion is an in-
tot
weakerthan thatappliedby ourselves,the space motionof our dicative of an age of ∼ 3.0 Gyr, which is in good agreement
samplehaveamuchwiderdispersioninUVW thanthesample withagesderivedforlate-Mdwarfs.Reidetal.(2002)inferan
of ZapateroOsorioetal. (2007) and significantlyfewer objects age of 3 Gyr from the velocity distribution of a sample of 31
fallinornearthevelocityellipsoidoftheHyadesinourcase. M7.5–M9dwarfs,andReiners&Basri(2009)findthesameage
foravolume-limitedsampleof63M7.5–M9dwarfsinthesolar
neighbourhood.
3.4.Kinematicageofthesample
3.4.1. Generalconsiderations
3.4.2. Kinematicageofoursample
Thecomputationofkinematicagesisbasedonthedispersionof
We now turn to our new sample of L dwarfs; we find that
truespacemotionsandrequiresacompletesetof3Dvelocityin-
the velocity dispersion of our 43 targets is (σ ,σ ,σ ) =
formation.Thetranslationofthetotalvelocitydispersion(σ ) U V W
tot (33.8,28.0,16.3)kms−1andcalculateatotalvelocitydispersion
totangentialvelocitiesbymeansofσ = (3/2)1/2σ assumes
tot tan ofσ =46.8kms−1.Our|W|-weightedvelocitydispersionsare
that the dispersions in v are spread equally between UVW. tot
tan (σ ,σ ,σ ) = (39.8,29.7,17.4)kms−1 and the |W|-weighted
Evenforalargesampleofobjects,thisassumptionmightintro- U V W
totalvelocity dispersionis σ = 52.7kms−1. From this value
duce a bias in the age determinationfrom tangential velocities tot
and Eq. (16) of Wielen (1977), we derive an age of ∼ 5.1 Gyr
alone,andradialvelocitymeasurementsareneededtocomplete
foroursample.
thefullsetofvelocityvectors.
Our result of ∼ 5.1 Gyr is much higher than the ∼ 3 Gyr
The velocity dispersion of a statistically significant sam-
we derived for the 21 L and T dwarfs in the sample of
ple of stars allows an estimate of the age of the sample, given
ZapateroOsorioetal. (2007). This valuealso seemscounterin-
the dynamical evolution of the Galaxy (see, e.g., Wielen 1977
tuitivecomparedtotheageof∼3Gyrforlate-Mdwarfs.Ifany-
or Fuchsetal. 2001). Examples of age estimations for sam-
thing,onewouldexpectasampleofL0–L8dwarfstobeyounger
ples including L dwarfs are given in Schmidtetal. (2007),
thana sampleof M7.5–M9dwarfs:the hydrogen-burningmin-
ZapateroOsorioetal. (2007), and Fahertyetal. (2009). While
imum mass (HBMM) divides late-type objects into low mass
the results of Schmidtetal. (2007) and Fahertyetal. (2009)
stars and brown dwarfs. Comparing the effective temperatures
are based on large samples of tangential velocities only,
ofthelowestmassstarsat1–10GyrgiveninChabrier&Baraffe
ZapateroOsorioetal. (2007) present radial velocities and thus
(1997)withthetemperaturescaleofVrbaetal.(2004)forLand
true space motionsof 21 L and T dwarfs. We revisit the popu-
lation of early- to mid-L dwarfs and estimate its age based on
3 Thiscanbeillustratedwhenconsidering,e.g.,thetotalvelocitydis-
the total velocity dispersion of the space motions in our larger
persionsinTable5of ZapateroOsorioetal.(2007).ForG-typestars,
sample. the dispersion peaks at σ = 35.9 kms−1, which should yield an
tot
To computethe ageof a sample ofstars basedon the work age lower than ∼ 2.2 Gyr (see Fig. 1 in Fuchsetal. (2001) and ta-
ofWielen(1977),onehastocalculatethevelocitydispersionin bles in Wielen (1977)). An age of 9Gyr is reported in Table 5 of
U, V, and W accordingto Eqs. 1–3 in Wielen (1977), i.e., one ZapateroOsorioetal.(2007).
4 A.Seifahrtetal.:Spacemotionsofbrowndwarfs
20 20
UMa
UMa
0 0
Young disk
Pleiades
−20 −20 Pleiades
Hyades Young disk
Hyades
/s) −40 AB Dor /s) −40 AB Dor
m m
k k
( (
V V
−60 −60
−80 −80
−100 −100
−100 −50 0 50 −60 −40 −20 0 20 40
U (km/s) W (km/s)
Fig.2.SpacevelocitiesintheU–V (left)andW–V (right)plane.Thevelocityellipsoidsfortheyoungdiskaswellforsomeyoung
movinggroupsareshown.Movinggroupsconsideredintheanalysisbutwithoutmembersinoursamplearenotshownforclarity.
Objectsincolourarecandidatemembersoftherespectivemovinggroups,asindicatedinTable2.
T dwarfs, this boundaryoccurs at spectral type L1–L2. Brown marginally consistent with our data. The closest fit is obtained
dwarfsbelowtheHBMMcoolovertimeandmovetomuchlater forthedistributionoftheWcomponent,whiletheUandVcom-
spectraltypesthanearly-L.Thiscausesadepletionofoldbrown ponents show notable deviations from a Gaussian distribution.
dwarfswithmassesjustbelowtheHBMMinasampleofearly- Usingourmeasuredvelocitydispersionσ andtherelationbe-
W
to mid-L dwarfs (see Burgasser 2004). Given this depletion of tweenσ andσ giveninWielen(1977),wecomputeanage
W tot
oldbrowndwarfswithspectraltypesofearly-tomid-L,theav- of 3.2 Gyr fromσ alone, which is indeedconsistentwith the
W
erageageofasampleofL0–L8dwarfsaspresentedheremight ageoflate-Mdwarfsamples.
beyounger,notolder,thanasampleoflate-Mdwarfs. Thenon-Gaussiandistributionofthespacemotionisproba-
Sincewecanexpectthistoaffectearly-Ldwarfsclosetothe bly not due to the limited size of our sample. Reiners&Basri
HBMM differently from the mid to late-L dwarfs, we divided (2009) show probability plots for their sample of 63 late-M
oursampleinhalfandcomputedthevelocitydispersionandage dwarfs,whichareverywellreproducedbyasingleGaussiandis-
for objects for spectral types L0V-L1.5V and L2V-L8V sepa- tribution.Oursampleisonly∼ 30%smallerbutexhibitsstrong
rately. We found no statistically significant difference between deviationsfromaGaussiandistribution.Theparticularchoiceof
thedispersionsandagesofthesetwogroups,nordidwenotice the targets for the sample might instead produce a bias. Since
any velocity outliers related to only one of the groups. This is oursamplecomprisesthebrightestobjectsperspectralbin,the
consistentwithasimilaranalysisbySchmidtetal.(2007),who sampleshouldbebiasedtowardsyoungerobjects.Thisis,how-
analyse the tangential velocity of a sample of M7–L8 dwarfs ever, not what we found. From σ , the sample appears much
tot
andfindnosignificantdifferencesinthevelocitydispersionand older.Ontheotherhand,thederivedagefromσ aloneissub-
W
numberofoutliersbetweensubsamplesofM7V–L2VandL3V– stantially younger than the one inferred from σ . In fact, the
tot
L8V. 3Gyrderivedfromσ issimilartotheageofthelate-Msam-
W
ples,whichmaybeexpectedtobeofcomparableage.Because
thedispersionsinU andV donotfollowaGaussiandistribution
3.4.3. Potentialexplanationsoftheoldage
while the onein W does,onemightarguethatthe agefromW
Wecanspeculateaboutthereasonforthisresult: aloneis a morereliableestimate ofthe trueage ofoursample.
(1) The velocity dispersion of our sample might not al- Unfortunately,at this pointwe have noexplanationof whythe
low us to determine an age of our sample because of the non- distributionsinU andV differfromtheoneinW.
Gaussianity of the distribution. While a dispersion can be for- (2) The lowest mass stars and brown dwarfs might have a
mallyobtainedfor anydistributionofvelocities,the dispersion differentinitialvelocitydispersionatthetimeofformationfrom
canonlybeusedasanageindicatorifthedistributionisnormal. highermassstars(v =10kms−1 inWielen1977).Bihainetal.
0
WeshowthedistributionsofU,V,andW velocitiesinoursam- (2006)findthatthevelocitydispersionofthebrowndwarfmem-
pleinthe toprowofFig.3.Inthebottomrowofthesamefig- bersin the Pleiades is aboutfour timesthe dispersionof solar-
ure, we show probabilityplots(see, e.g., Lutz&Upgren1980; typemembersofthecluster.Ahigherv wouldnaturallyleadto
0
Reidetal. 2002) to help assess how closely a single Gaussian a differentvelocitydispersionas a functionoftime withoutin-
distributionfits ourdata. Ascan beseen in the plots,a straight vokingadifferentfeedbackbythelowestmassstarsandbrown
line, corresponding to a single Gaussian distribution, is only dwarfs on the Galactic potential. This effect, however, should
A.Seifahrtetal.:Spacemotionsofbrowndwarfs 5
haveidenticalinfluencesonU,V,andW.Weseenoreasonwhy the high rotational velocity of the components in the sys-
onlyU andV shouldbeaffectedbysuchamechanism. tem (vsini ≈ 76 kms−1, RB08), we are unable to iden-
(3)Individualkinematicaloutlierscansignificantlybiasthe tifytheindividualcomponentsinthespectrum.Wenotethat
velocitydispersionsofsmallsamples. Thiseffectwouldbeen- Basrietal.(2000)reportaradialvelocityofv ∼17kms−1
rad
hancedbythe|W|-weighting,asdiscussedinReidetal.(2002). measuredinJune1997.Thisvaluedifferssignificantlyfrom
However,we find no clear correlationof |W| with either v or the v = 5±2.2 kms−1 that we measure in our spectrum
tot rad
v inoursample,andtheremovaloffiveobjectswiththehigh- fromMay 2006,althoughthe differenceseems too large to
tan
est v only marginally lowers the age of the sample. On the beexplainedbytheorbitalmotiononly.
tot
otherhand,theremovalofsixobjectswiththehighesttangential – 2MASS1441-09:Bouyetal. (2003) foundthis objectto be
velocity(v >70kms−1)lowersthemeanageofthesampleto a close binary (ρ ∼ 0.4′′) composed of two L1 dwarfs.
tan
3.0Gyr,ascomputedfromσ .Thiseffectmaybeexpectedbut Bouyetal. (2008) present follow-up astrometric observa-
tot
aremovalofthoseobjectscanonlybejustifiedifthedistribution tionsandestimate the orbitalperiodto be 120-230yr. This
of tangentialvelocitiesidentifies them as clear outliers. This is period is too long and the brightness difference in the two
not evident for our sample. We note that the error in the spec- components too small to bias our radial velocity measure-
trophotometricdistances of some targets mightbe much larger ment in any significant way. Seifahrtetal. (2005) demon-
than the ±1pc given in Fahertyetal. (2009), which could con- strate that this L dwarf binary forms a common proper
tribute to a few moderate v outliers. In any case, it remains motion pair with the M4.5V star G124-62. The radial ve-
tan
truethateven43objectsmustbeconsideredtoosmallasample locity of G124-62 reported by the same authors is v =
rad
when robustness to kinematic outliers is important, especially −29.3 kms−1, which is in close agreement with our radial
when the sample is not volume-limited and prone to selection velocity for the L dwarf binary.Seifahrtetal. (2005) argue
effects. thatthe system is a probablememberof the Hyades super-
In summary,an age of ∼3 Gyr forour sample seems likely cluster,whichweconfirmwithimprovedpropermotionfor
given the non-Gaussian distribution of the U and V velocities thesystem.
andtheagedeterminedfromσ alone.Furthermore,thatthere- – 2MASS1854+84:This objecthas no publishedproper mo-
W
movalofthemostsignificantoutliersinthe distributionoftan- tion and distance. Hence, we have to exclude it from our
gentialvelocitiesreducestheageofthesampletoasimilarvalue analysisofspacemotions,reducingtheoriginalsamplesize
mayindicatethatahandfuloffastmovingoutliersarecontained to 43 objects. We obtain a radial velocity of v = −3.5±
rad
inthesample.Theseoutliersmayhavebeenacceleratedduring 1.0kms−1forthistarget.
a very early phase of their evolution, perhapsby ejection from – 2MASS2200-30:Burgasser&McElwain(2006)resolvethis
multiplesystems. target into a close binary (ρ ∼ 1′′) with spectral types
of M9V+L1V and estimate the orbital period to be 750–
1000yrs.Hence,ourradialvelocitymeasurementshouldbe
3.5.Notesonindividualobjects
unaffectedbythebinarity,giventhelongperiodandsimilar
– 2MASS 0036+18 and 2MASS 0825+21 are both listed in brightnessofbothcomponents.
Bannister&Jameson (2007) as belonging to the Hyades
MovingGroupbasedonthemovingclustermethod.Weare
4. Summary
unable to confirm this finding. Although both objects be-
long to the young disk population, their space motions are We have measured the radial velocities of 44 L0–L8 dwarfs
inconsistent with the space vector of the Hyades in V and in the solar neighbourhood from high resolution optical
W (2MASS0036+18)andU andW (2MASS0825+21),re- VLT/UVES and Keck/HIRES spectra. We combined these
spectively. measurements with distance and proper motion data to com-
– 2MASS 0255-47:We reporta radialvelocityfor thisobject pute space motions for 43 of our targets. For one target
ofv = 25±4.1kms−1, whichisinconsistentwith v = (2M1854+84),no propermotion and distance was available in
rad rad
17.5±2.8kms−1 reportedinZapateroOsorioetal.(2007), theliteratureandwehadtoexcludethetargetfromthesample.
and v = 13.0± 3.0 kms−1 in Basrietal. (2000). Given We formally derived an age of 5.1 Gyr from the |W|-weighted
rad
the late spectral type, ranging from L6 to L8, and the high totalvelocitydispersionofthesampleandtheage-velocityrela-
rotationalvelocityofthesystem(vsini ≈ 67kms−1,RB08) tion of Wielen (1977). However,the space motioncomponents
the uncertaintiesin all the reportedradial velocity values – UandVofoursamplearenotnormallydistributed,compromis-
includingours–mightbeunderestimated.Nevertheless,we ingtheagecomputationfromσ .Wefoundthatthedistribution
tot
noteaslighttrendwithtimeintheradialvelocitythatshould inW is,however,quiteconsistentwithaGaussianshape.From
beverifiedwithadditionalobservations. W alone,wecomputeanageof∼3Gyr.
– 2MASS1045-01and2MASS1047-18:Jamesonetal.(2008) We note thatthe formallyderivedage of 5.1 Gyr fromσ
tot
listbothobjectsasprobableHyadesmembers,basedonnew for the whole sample is in disagreement with the mean age of
propermotionmeasurements.Wecanconfirmtheirmember- the lowest mass stars in the solar neighbourhood (∼ 3 Gyr,
shipbasedonthefullspacemotionsofbothobjects. Reidetal.2002;Reiners&Basri2009)andtheexpectationthat
– 2MASS1305-25:Thisobject,alsoknownasKelu1,isatight early-tomid-Ldwarfsshouldbeyoungerthanthelowestmass
binary(ρ∼ 0.3′′,Liu&Leggett2005)consistingofaL1.5- stars of spectral type M7.5–M9. We speculate that either the
L3andaL3-L4.5component.Itsorbitishighlyeccentricand initial velocity dispersion of brown dwarfs is notably different
the orbitalperiodis estimatedto be ∼ 38yrs(Stumpfetal. fromthatofsolar-typeandlower-massstars,orthatourparticu-
2008). Using the orbital elements of Stumpfetal. (2008), larsampleisbiasedtowardsolderand/orkinematicallypeculiar
we estimate that our radial velocity measurementmight be objects.Thelatterseemsthemostlikelyexplanationsinceafew
affected by the orbital motion of the binary at a level of a high-velocityobjectscouldbiasourstatistics.Suchabiaswould
few kms−1 depending on the fractional light contribution be enhancedby the |W|-weightingof the dispersionswhencal-
from each component in our optical spectra. Because of culatingσ andthusthemeanageofoursample.
tot
6 A.Seifahrtetal.:Spacemotionsofbrowndwarfs
Fig.3.Toprow:HistogramsofthespacemotioncomponentsinU,V,andW (fromlefttoright).Bottomrow:Probabilityplotsfor
inU,V,andW (fromlefttoright).Dataareshownwiththeiruncertaintiesaserrorbars.Thedashedlinemarksthe1.5σcutofffor
thefitshownasasolidredline.TheslopeofthelinearfitistheσoftheclosestmatchingGaussiandistribution.
The most likely age of nearby early- to mid-L type dwarfs Blake,C.H.,Charbonneau,D.,White,R.J.,Marley,M.S.,&Saumon,D.2007,
is of the order of 3Gyr, and we have found no evidence of an ApJ,666,1198
agelowerthanthis.However,astatisticallyclearlydefined,and Bouy,H.,Brandner,W.,Mart´ın,E.L.,Delfosse,X.,Allard,F.,&Basri,G.2003,
AJ,126,1526
perhaps larger sample of L and T dwarfs with accurate radial
Bouy,H.,etal.2008,A&A,481,757
velocities would be necessary to robustly determine the mean Burgasser,A.J.2004,ApJS,155,191
ageofbrowndwarfsfromakinematicanalysis. Burgasser,A.J.2009,IAUSymposium,258,317
Burgasser,A.J.,&McElwain,M.W.2006,AJ,131,1007
Casewell,S.L.,Jameson,R.F.,&Burleigh,M.R.2008,MNRAS,390,1517
Acknowledgements. This work is based on observations obtained from the Chabrier,G.,&Baraffe,I.1997,A&A,327,1039
EuropeanSouthernObservatory,PIDs077.C-0449and078.C-0025,andonob- Chen,B.,Asiain,R.,Figueras,F.,&Torra,J.1997,A&A,318,29
servationsobtainedfromtheW.M.KeckObservatory,whichisoperatedasasci- Chereul,E.,Cre´ze´,M.,&Bienayme´,O.1999,A&AS,135,5
entificpartnershipamongtheCaliforniaInstituteofTechnology,theUniversity Costa,E.,Me´ndez,R.A.,Jao,W.-C.,Henry,T.J.,Subasavage,J.P.,&Ianna,
of California and the National Aeronautics and Space Administration. Based P.A.2006,AJ,132,1234
onobservations madewiththeEuropeanSouthernObservatorytelescopes ob- Cruz,K.L.,Reid,I.N.,Liebert,J.,Kirkpatrick,J.D.,&Lowrance,P.J.2003,
tainedfromtheESO/ST-ECFScienceArchiveFacility.Thisresearchhasbene- AJ,126,2421
fittedfromtheM,L,andTdwarfcompendiumhousedatDwarfArchives.organd Cruz,K.L.,Kirkpatrick,J.D.,&Burgasser,A.J.2009,AJ,137,3345
maintainedbyChrisGelino,DavyKirkpatrick,andAdamBurgasser.Wethank Faherty, J. K.,Burgasser, A. J.,Cruz, K. L.,Shara, M. M.,Walter, F. M., &
the referee, Kelle Cruz, forher constructive report. AS and ARacknowledge Gelino,C.R.2009,AJ,137,1
financial support from the Deutsche Forschungsgemeinschaft under DFG RE Fuchs, B., Dettbarn, C., Jahreiß, H., & Wielen, R. 2001, Dynamics of Star
1664/4-1.ASfurtheracknowledges financialsupportfromNSFgrantAST07- ClustersandtheMilkyWay,228,235
08074.GBthankstheNSFforgrantsupportthroughAST06-06748. Henry,T.J.,Jao,W.-C.,Subasavage,J.P.,Beaulieu,T.D.,Ianna,P.A.,Costa,
E.,&Me´ndez,R.A.2006,AJ,132,2360
Jameson, R. F., Casewell, S. L., Bannister, N. P., Lodieu, N., Keresztes, K.,
Dobbie,P.D.,&Hodgkin,S.T.2008,MNRAS,384,1399
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Table2.Radialvelocitiesandspacemotionsofourtargets.Dataondistanceandpropermotionsareobtainedfromtheliterature. 8
Object SpT Distance µ cosδ µ RV U V W Ref. Comments
α δ
(pc) (mas/yr) (mas/yr) (kms−1) (kms−1) (kms−1) (kms−1)
2MASSJ03140344+1603056 L0.0 14.0±1.0 -241±18 -76±19 -8.0±1.1 16.4±1.4 5.4±1.4 -7.0±1.5 1 UMamovinggroup
2MASSJ12212770+0257198 L0.0 19.0±1.0 -115±30 -18±27 -9.0±1.4 -9.2±2.8 -2.6±2.5 -10.0±1.7 1 Youngdisk
2MASSJ17312974+2721233 L0.0 12.0±1.0 -82±15 -240±17 -30.5±1.1 -5.8±1.4 -30.0±1.3 -14.3±1.0 1 ABDormovinggroup
2MASSJ22000201-3038327 L0.0 35.0±2.0 210±48 -64±21 -25.3±1.0 -39.7±6.7 -19.7±3.8 -1.6±5.2 1 Hyadesmovinggroup
2MASSJ07464256+2000321 L0.5 12.2±0.04 -374± 1 -57± 0 53.0±1.1 -55.3±1.0 -15.1±0.4 -1.0±0.4 1
2MASSJ14122449+1633115 L0.5 25.0±2.0 29±16 -80±30 5.0±1.1 10.0±2.8 -4.8±3.1 1.7±1.5 1 Youngdisk
2MASSJ14413716-0945590 L0.5 27.5±2.7 -198± 3 -16± 4 -28.3±1.1 -34.7±1.8 -12.5±2.0 -10.4±1.3 1 Hyadesmovinggroup
2MASSJ23515044-2537367 L0.5 18.0±2.0 387±21 163± 9 -3.0±1.1 -35.7±4.4 -2.1±1.1 -4.1±1.4 1 Youngdisk
2MASSJ02355993-2331205 L1.0 21.3±0.5 84± 1 13± 0 15.3±1.1 -11.7±0.4 -7.7±0.3 -10.5±1.1 2 Youngdisk
2MASSJ06023045+3910592 L1.0 10.6±0.3 146±10 -501±10 7.6±1.1 -11.4±1.1 -24.4±0.9 -4.4±0.6 1 Pleiadesmovinggroup
2MASSJ10224821+5825453 L1.0 20.0±1.0 -360±99 -780±90 19.7±1.1 -40.0±8.1 -69.7±9.3 23.7±6.2 1
2MASSJ10452400-0149576 L1.0 17.0±1.0 -495±18 12±12 7.0±1.1 -35.7±2.5 -14.2±1.3 -12.9±1.6 1 Hyadesmovinggroup
2MASSJ10484281+0111580 L1.0 15.0±1.0 -442±13 -209±12 24.0±1.1 -24.9±1.6 -34.0±1.7 -2.2±1.8 1
2MASSJ13004255+1912354 L1.0 14.0±1.0 -820±18 -1244±19 -17.8±1.0 -1.6±1.3 -96.8±7.4 -27.0±1.3 1 A
.
2MASSJ13595510-4034582 L1.0 21.0±1.0 38±11 -485±15 49.8±1.0 39.3±1.1 -50.5±1.5 -27.0±2.6 1 Se
2MASSJ14392836+1929149 L1.0 14.4±0.1 -1229± 1 407± 2 -27.2±1.1 -82.6±0.7 -39.2±0.3 14.1±1.1 1 ifa
h
2MASSJ15551573-0956055 L1.0 13.0±1.0 950±15 -767±15 14.5±1.1 52.2±3.4 2.3±1.0 -56.1±5.2 1 r
t
2MASSJ11455714+2317297 L1.5 44.0±3.0 155±16 -56± 6 3.3±1.0 32.9±3.9 2.4±1.9 10.3±1.4 1 et
2MASSJ13340623+1940351 L1.5 46.0±3.0 -58±12 98±16 2.0±4.1 -22.0±3.5 9.3±3.4 6.9±4.2 1 al.:
2MASSJ16452211-1319516 L1.5 12.0±1.0 -364±18 -804±16 26.4±1.0 32.6±1.2 -46.4±4.3 -0.3±1.3 1 S
p
2MASSJ18071593+5015316 L1.5 14.0±1.0 35±19 -126±14 -2.0±3.2 7.6±1.3 -1.7±2.9 -4.3±1.9 1 Youngdisk a
c
2MASSJ20575409-0252302 L1.5 16.0±1.0 10±20 -80±20 -25.0±3.2 -12.9±2.3 -20.3±2.4 9.0±2.1 1 Youngdisk e
m
2MASSJ08283419-1309198 L2.0 11.6±1.4 -592± 6 14± 7 26.2±1.0 -33.1±2.5 -16.6±1.1 -19.4±3.3 3 Youngdisk o
2MASSJ09211410-2104446 L2.0 12.0±1.0 244±16 -908±17 80.0±1.1 18.1±3.8 -94.4±2.3 5.1±2.2 1 tio
n
2MASSJ11553952-3727350 L2.0 13.0±1.0 50±12 -767±15 45.0±1.1 34.4±1.8 -50.6±1.5 -22.9±3.4 1 s
o
2MASSJ13054019-2541059 L2.0 18.7±0.7 -285± 1 11± 1 5.0±2.2 -18.7±1.4 -17.0±1.6 5.2±1.4 1 Youngdisk f
b
2MASSJ05233822-1403022 L2.5 13.0±1.0 90±17 166±17 11.3±1.0 -15.7±1.2 -2.6±1.1 3.4±1.3 1 Youngdisk ro
2MASSJ10292165+1626526 L2.5 23.0±2.0 359±15 -348±17 -28.2±1.4 58.5±4.5 -14.3±2.9 -11.8±1.9 1 w
n
2MASSJ10473109-1815574 L2.5 22.0±2.0 -347±17 54±14 6.0±1.1 -34.0±3.6 -11.7±1.5 -9.3±1.9 1 Hyadesmovinggroup d
w
2MASSJ09130320+1841501 L3.0 46.0±4.0 32±16 -187±17 -1.0±2.2 17.1±3.5 -37.1±5.0 -6.4±3.2 1 a
r
2MASSJ12035812+0015500 L3.0 19.0±2.0 -1209±18 -261±15 -1.5±2.2 -83.7±9.2 -66.3±7.5 -31.9±3.9 1 fs
2MASSJ15065441+1321060 L3.0 14.0±1.0 -1087±13 14±11 -0.9±1.2 -41.4±3.2 -47.4±3.6 35.3±2.9 1
2MASSJ16154416+3559005 L3.0 24.0±2.0 -17±12 -512±15 -21.0±1.2 42.1±4.6 -42.5±3.0 -16.1±1.3 1
2MASSJ21041491-1037369 L3.0 17.0±2.0 614±16 -281±15 -20.5±2.2 -37.9±3.5 -28.9±2.7 -33.4±5.7 1
2MASSJ00361617+1821104 L3.5 8.8±0.1 899± 1 120± 2 21.0±1.8 -41.0±0.7 -3.2±1.2 -13.3±1.3 1 Youngdisk
2MASSJ07003664+3157266 L3.5 12.2±0.3 130± 1 -546± 1 -43.5±2.2 43.0±2.2 -28.6±0.8 -16.8±0.6 1
2MASSJ17054834-0516462 L4.0 11.0±1.0 129±14 -103±15 12.2±1.1 14.2±1.1 2.8±0.8 -3.7±1.1 1 UMamovinggroup
2MASSJ22244381-0158521 L4.5 11.4±0.1 457± 2 -871± 1 -39.0±1.1 -10.4±0.4 -64.9±0.8 -5.0±0.9 1
2MASSJ00043484-4044058 L5.0 13.0±0.7 643± 2 -1494± 1 30.0±1.4 11.4±0.5 -102.9±5.4 -15.3±1.6 4
2MASSJ08251968+2115521 L5.0 10.7±0.1 -510± 2 -288± 2 20.0±3.2 -27.5±2.6 -16.9±1.1 -15.5±1.6 1 Youngdisk
2MASSJ08354256-0819237 L5.0 7.0±1.0 -530±30 300±30 26.5±2.2 -31.2±1.6 -11.8±1.9 -0.5±1.2 1 Hyadesmovinggroup
2MASSJ15074769-1627386 L5.0 7.3±0.03 -161± 2 -888± 0 -40.4±1.8 -26.4±1.5 -16.7±0.4 -40.5±1.1 1
2MASSJ02550357-4700509 L8.0 5.0±0.1 999± 2 -565± 3 25.0±4.1 -5.4±0.4 -35.7±2.2 -7.3±3.6 5
References.– Distancesandpropermotion:(1)Fahertyetal.(2009,andreferencestherein),(2)HIPPARCOScatalogue
(Perrymanetal.1997),(3)Lodieuetal.(2005),(4)Henryetal.(2006),(5)Costaetal.(2006)