A new L5 brown dwarf member of the Hyades cluster with chromospheric activity

0.96 MB·

by  Antonio Pérez-Garrido

Astronomy&Astrophysicsmanuscriptno.paper_Antonio_Hyades_L5_v5 (cid:13)cESO2017
January13,2017
A new L5 brown dwarf member of the Hyades cluster with
chromospheric activity(cid:63)
A.Pérez-Garrido1,N.Lodieu2,3,andR.Rebolo2,3,4
1 Dpto.FísicaAplicada,UniversidadPolitécnicadeCartagena,E-30202Cartagena,Murcia,Spain
e-mail:[email protected]
2 InstitutodeAstrofísicadeCanarias(IAC),CalleVíaLácteas/n,E-38200LaLaguna,Tenerife,Spain
3 DepartamentodeAstrofísica,UniversidaddeLaLaguna(ULL),E-38206LaLaguna,Tenerife,Spain
4 ConsejoSuperiordeInvestigacionesCientíficas,CSIC,Spain
ReceivedJanuary13,2017;accepted(date)
7
1
0 ABSTRACT
2
Aims.OuraimistoidentifybrowndwarfmembersofthenearbyHyadesopenstarclustertodeterminethephotometricandspectro-
n
scopicpropertiesofbrowndwarfsatmoderatelyoldagesandextendtheknowledgeofthesubstellarmassfunctionofthecluster.
a
J Methods.Wecross-matchedthe2MASSandAllWISEpubliccataloguesandmeasuredpropermotionstoidentifylow-massstarsand
browndwarfcandidatesinanareaofradiuseightdegreesaroundthecentralregionoftheHyadescluster.Weidentifiedobjectswith
2 photometryandpropermotionsconsistentwithclustermembership.Forthefaintest(J=17.2mag)mostpromisingastrometricand
1 photometriclow-masscandidate2MASSJ04183483+2131275,withamembershipprobabilityof94.5%,weobtainedlow-resolution
(R=300–1000)andintermediate-resolution(R=2500)spectroscopywiththe10.4mGranTelescopioCanarias.
]
R Results.Fromthelow-resolutionspectrawedeterminedaL5.0±0.5spectraltype,consistentwiththeavailablephotometry.Inthe
intermediatedispersionspectrumwedetectedHαinemission(marginallyresolvedwithafullwidthhalfmaximumof∼2.8Å)and
S
determinedalog(L /L )=−6.0dex.FromHαweobtainedaradialvelocityof38.0±2.9km·s−1,whichcombinedwiththeproper
. Hα bol
h motionleadstospacevelocitieswhicharefullyconsistentwithmembershipintheHyadescluster.Wealsoreportadetectioninthe
p H2bandbytheUKIDSSGalacticPlaneSurvey.Usingevolutionarymodelswedeterminefromtheavailablephotometryoftheobject
- amassintherange0.039−0.055M(cid:12).Browndwarfswithmassesbelow0.055M(cid:12)shouldfullypreserveitsinitiallithiumcontent,and
o indeedthespectrumat6708Åmayshowafeatureconsistentwithlithiumpreservation;however,ahigherS/Nisneededtoconfirm
r
thispoint.
t
s Conclusions.Wehaveidentifiedanewhigh-probabilityL5browndwarfmemberoftheHyadescluster.Thisisthefirstrelativelyold
a L5browndwarfwithawell-determinedage(500−700Myr)andmeasuredchromosphericemission.
[

Keywords. Stars:low-mass—Galaxy:openclustersandassociation(Hyades)—techniques:photometric—techniques:spectro-
1
scopic—surveys
v
8
9
31. Introduction afewMyrstoafewhundredMyrs(seereviewsbyBastianetal.
3 2010;Luhman2012).
0Brown dwarfs are objects with masses below 0.075 M(cid:12) unable
Older star clusters (>500Myr) like the Hyades (Melotte
.to reach high enough temperatures in their cores to trigger hy-
1drogen fusion (Burrows & Liebert 1993; Chabrier & Baraffe 25,α2000=04h26m54s,δ2000=+15◦52(cid:48))offerauniqueopportu-
0 nity to find examples of relatively old brown dwarfs with well-
1997).Theabsenceofstablehydrogenburningcausesthephysi-
7 determinedageandmetallicity.TheHyadeshasameandistance
calpropertiesofbrowndwarfstochangedrasticallyasafunction
1 of46.3±0.3pcandistheclosestopenstarclustertotheSun(van
:of mass and age. Dynamical determination of masses has been
vpossible for a growing list of binary brown dwarfs (e.g. Zapa- Leeuwen 2009). The Hyades exhibit a significant mean proper
i motion:µ=∼74–140mas/yr,PA=90o-135o(Bryjaetal.1994),
XteroOsorioetal.2004;Dupuy&Liu2012),buttheageisalso
atidalradiusof∼10pc,andacoreradiusof2.5–3.0pc(Perry-
requiredtofullyconstrainevolutionarymodelsofsubstellarob-
arjects(Baraffeetal.1998;Siessetal.2000;Baraffeetal.2015; manetal.1998).Owingtoitsproximity,theclusterspansalarge
area over the sky (several hundred square degrees). The age of
Feidenetal.2015).Thecharacterizationofbrowndwarfmem-
theHyadesis625±50Myrbasedonthecomparisonoftheob-
bersofopenstarclustersofwell-knownageisofcapitalimpor-
servedclustersequencewithmodelisochrones,althoughawider
tance because we are provided with snapshots of the physical
agerangecannotbediscarded(Mermilliod1981;Eggen1998).
properties (colours, luminosities, spectral energy distributions)
The metallicity of high-mass members appears slightly super-
of theseobjects at givenages. Searches in star-formingregions
solar,around[Fe/H]∼0.13–0.14dex(Boesgaard&Friel1990)
andyoungclustershaverevealedbrowndwarfswithagesfrom
althoughamorerecentworkbyGebranetal.(2010)suggestsa
meansolarmetallicity.
(cid:63) BasedonobservationsmadewiththeGranTelescopiodeCanarias
(GTC)installedattheSpanishObservatoriodelRoquedelosMucha- Hogan et al. (2008) reported a number of L dwarf candi-
chos of the Instituto de Astrofísica de Canarias, on the island of La datesintheHyadesclusterbasedonphotometryandpropermo-
Palma. tions. They identified 12 objects with colours resembling those
Articlenumber,page1of9
A&Aproofs:manuscriptno.paper_Antonio_Hyades_L5_v5
of field L dwarfs. Recently, Casewell et al. (2014) and Lodieu 0.2
et al. (2014) presented spectroscopic follow-up confirming the Hyades members
0.15
cool nature of most of the candidates reported by Hogan et al. Object from Simbad 2 degrees
around 2M0418+21
(2008),althoughtheseauthorsdidnotunambiguouslydetermine 0.1 2M0418+21
whethertheseobjectsarebrowndwarfsorverylow-massstars.
Bouvieretal.(2008)discoveredthefirsttwoT-typedwarfsinthe 0.05
Hyades cluster based on low-resolution infrared spectra. These yr)
cthoeolmoabsjsecftusnacrteiovne,rByoliukveileyrbertoawl.n(d2w00a8rf)sa.rBgauseeddtohnatth∼e1s5hbarpoewonf (arcsec/µδ 0
dwarfscouldexistinthepresent-dayHyadesclusterandareyet -0.05
to be uncovered. In addition to 2MASSI023301.55+2470406
(Cruz et al. 2007), which was proposed as a L0 member of the -0.1
Hyades (Goldman et al. 2013), a few other known L/T dwarfs
-0.15
listed with spectra and proper motions in the compendium of
ultracool dwarfs1 could be associated with the Hyades moving
-0.2
group(Bannister&Jameson2007;Gagnéetal.2015)andthere- -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25
µ cos δ (arcsec/yr)
forecouldberelativelyold. α
Fig. 1. Proper motion diagram of the Hyades cluster. Green triangles
Inthispaper,wepresentthefindingofanewmid-Ldwarf,
2MASSJ04183483+2131275(hereafter2M0418+21)withpho- correspond to Hyades members (Goldman et al. 2013) and the solid
squarecorrespondsto2M0418+21.Blackdotscorrespondstoobjects
tometry, spectroscopy, proper motion, and radial velocity con-
in the Simbad database within a circular radius of 2 degrees around
sistent with membership in the Hyades cluster. Furthermore, it 2M0418+21.
presents signs of chromospheric activity with a clear detection
of Hα in emission on 19 December 2015 for a few hours. In
Sect.2wedescribeoursearchfornewverylow-massmembers WISE, and the Sloan Digital Sky Survey (SDSS; York et al.
oftheHyades.InSect.3wepresentspectroscopicobservations 2000) were indicative of a mid-L dwarf (Fig. 4; Table 1) that
of the most promising candidate found 2M0418+21 conducted iscoolerandsignificantlyfainterthanotherpreviouslyreported
withtheGranTelescopiodeCanarias(GTC).InSect.4weanal- HyadesLdwarfs.InSect.3wepresenttheopticalspectralclas-
ysetheopticalspectraandputourresultsincontextwithother sificationofthisobject.
low-massstarsandbrowndwarfs.
2.2. Propermotioncomputation
2. Newverylow-masspropermotionmemberofthe The proper motion of 2M0418+21 was obtained using the
Hyades 2MASS, AllWISE, and UKIDSS GCS catalogues and simply
calculatedasµ = ∆αcosδ/∆t andµ = ∆δ/∆t,where∆αand
α δ
2.1. Cataloguecross-match ∆δ are the differences between RA and Dec in any two cata-
logues. We note that we used the set of coordinates not cor-
We cross-matched the Two Micron All Sky Survey (2MASS;
rected for proper motion in the AllWISE catalogue. In order
Cutri et al. 2003; Skrutskie et al. 2006) point source catalogue
to assess the error in our measurement for each pair of cata-
and the AllWISE mid-infrared catalogue built upon the Wide-
logues we determined the proper motions of all objects placed
field Infrared Survey Explorer mission (WISE; Wright et al.
30 arcmin around 2M0418+21. We estimate the error, σ, from
2010) to identify new low-mass stars and substellar members
the full width half maximum of each bell-shaped proper mo-
in the Hyades cluster. Details of this cross-match will be pre-
tion histogram (2.3 times σ). We discarded the proper motion
sented in a future paper (Pérez-Garrido et al. 2016, in prep.).
measurements from the AllWISE vs UKIDSS GCS pairs ow-
Given the temporal baseline between the two surveys (∼11–14
ing to the short baseline between the two observations as it
yr)weconductedanastrometricsearchtoidentifyobjectswith
mean displacements larger than 100 mas/yr. In our search, we yielded large error bars (∼60 mas/yr). For the other two pairs
ofcataloguesweobtainederrorsof∼8mas/yrandtheinitialre-
looked for pairs of uncorrelated objects in 2MASS and WISE
in a Hyades region of radius 8 degrees centred at 04h26m54s, sulting2M0418+21propermotionswere(129,−53)mas/yrand
+15o52(cid:48). Uncorrelated objects are those which are present in (133,−44) mas/yr for 2MASS-AllWISE and 2MASS-UKIDSS
GCS, respectively. However, for the Hyades objects a correct
one catalogue with no counterpart in the other database within
estimation of the proper motion requires taking into account
a correlation radius of one arcsec. In a subsequent step, we se-
parallacticmotion.Weremovedthisparallacticeffectusingthe
lected objects with apparent proper motions measured between
both catalogues of 70 < µ cosδ < 130 mas/yr in right ascen- Earth’sbarycentrecoordinatesforeachcatalogueepoch,assum-
α
sion and −60 < µ < 0 mas/yr in declination. These pairs rep- ingadistanceof48.8pc.Wethenperformedalinearregression
δ
with the new coordinates from 2MASS, UKIDSS, and WISE
resent candidates with proper motions comparable to those of
AllSky(wedidnotuseAllWISEsinceitsastrometryisanaver-
Hyadesmembers(Fig.1).Thiscorrelationreturned130objects
agespanningseveralmonths),andtheresultingfityieldedafinal
with proper motions and near-infrared photometry compatible
propermotionof(124±7,-53±6)mas/yrforourobject.
withclustermembership(Fig.1;Fig.4).WerecoveredseveralL
dwarfcandidates(Hoganetal.2008)withspectroscopicfollow-
up(Casewelletal.2014;Lodieuetal.2014).Asubsetofthose 2.3. Hyadesmembership
130candidatesfoundinoursearchalsohaveopticalphotometry
intheSloanSkyDigitalSurvey(Yorketal.2000).Remarkably, All members of the Hyades cluster seem to move towards
weidentifiedanewobjectwhosecoloursmeasuredby2MASS, the cluster convergent point (CP) located at αCP =6h29.48m,
δ =6o53(cid:48).4 (Madsen et al. 2002). The angle θ from this ob-
CP
1 seehttp://spider.ipac.caltech.edu/staff/davy/ARCHIVE/index.shtml jecttotheconvergentpoint(measuredastheanglebetweenthe
Articlenumber,page2of9
PérezGarridoetal.:AnewL5memberoftheHyades
5
4 t = 600 Myr
t = 700 Myr
3 2M0418+12
K
J-2
1
0.8
W20.6
1-
W0.4
0.2
3.8
23.6
W
J-3.4
3.2
0.02 0.03 0.04 0.05 0.06 0.07 0.08
M/M
sun
Fig.3.J−K,W1−W2,andJ−W2colourasafunctionofmassfromthe
AMES-Dustyevolutionarymodel.Thedashedredlinewiththebottom
andtopredlinerepresenttheW1−W2(AllWISE)andJ−K(UKIDSS)
coloursof2M0418+21withtheirerrorbars.
Fig.2.Findingchartof2M0418+21inJbandfromUKIDSSGalactic
ClusterSurvey.
7
Hyades known L dwarfs
Hyades members
line pointing to the north and the line to the CP) can be calcu- Field L and T dwarfs
8 2M0418+21
latedasinHoganetal.(2008).Thisangleshouldbecompared
Dupuy & Liu 2012
tothepropermotionangle,θ .Valuesfortheseangleswithdif-
PM
ferencesbelow12o wouldindicatethatourobjectismovingto- 9
wardstheHyadesCP.Inourcasewegetθ=110oandθPM=113o. W2
Wealsocalculateapropermotiondistanceof41.7pcusingEq. M
10
(3) of Hogan et al. (2008). Both estimations are in good agree-
ment with what we expect for Hyades members (van Leeuwen
2007). 11
Furthermore, we use the maximum likelihood method de-
scribed in Sanders (1971) to assess the Hyades membership
12
probability of 2M0418+21. This method assumes two overlap-
pingnormalbivariatefrequencyfunctions,oneforfieldstarsand 0 0.25 0.5 0.75
one for the cluster. In our implementation we input the known W1-W2
proper motion of the Hyades cluster, µ = 115 and µ = −40 Fig.4.(W1−W2,M )colour-magnitudediagramdepictingknownL
α δ W2
(mas/yr),andletthealgorithmestimatethemeanpropermotion dwarfsintheHyades(redsquares;Hoganetal.2008;Casewelletal.
offieldobjects.Weconsideredallpointsourceswithinaradius 2014; Lodieu et al. 2014), Hyades high-mass and low-mass members
of 30 arcmin around 2M0418+21, finding a membership prob- (black dots; Goldman et al. 2013), the mean sequence of field L and
ability of 94.5% for 2M0418+21. Therefore, this object can be Tdwarfsshiftedto46.3pc(purpleline;Dupuy&Liu2012),andour
target2M0418+21(solidsquare;thiswork).
consideredfrombothphotometryandpropermotionasamem-
beroftheHyadescluster.
We checked the H (1–0)-band image obtained by the
2
UKIDSS Galactic Plane Survey on 29 November 2005. The
2.4. Additionalphotometry
DR10 catalogue gives a magnitude of 15.247±0.029 mag with
Inadditiontothe JHK near-infraredphotometryfrom2MASS anellipticitybelow0.06androundnessparametersindicatinga
s
andmid-infrareddatafromWISE,2M0418+21isalsocovered pointsource.However,2M0418+21standsoutinthe(J−H ,J)
2
by the Sloan Sky Digital Survey Data Release 9 (York et al. and (H − H ,H) colour-magnitude diagrams in comparison to
2
2000;Ahnetal.2012a)andtheUKIRTInfraredDeepSkySur- other point-like objects within five arcmin (Fig. 5). Its J − H
2
vey (UKIDSS; Lawrence et al. 2007) Galactic Clusters Survey andH−H coloursareredderthananyotherpointsourceinthe
2
andGalacticPlaneSurveys.2M0418+21isdetectedintheSloan field (Fig. 5). However, its location in the (K − H ,K) colour-
2
zbandonlyandinvariousfiltersfromUKIDSSwithtwodistinct magnitudediagramfitsthemainsequenceofpointsourceswell
epochs(1December2005forK1and29November2005forK). (right side plot in Fig. 5). The H (1–0) is centred at 2.12 mi-
2
Wesummarizethephotometryavailablefor2M0418+21inTa- crons with a 50% cut-off of 0.1 microns, and is usually used
ble 1 and display a J-band finding chart from UKIDSS in Fig. toprobethepresenceofmolecularemissionfromcircumstellar
2. disks.Wedonotseeanyobviouspresenceofajetinthe5arcmin
Articlenumber,page3of9
A&Aproofs:manuscriptno.paper_Antonio_Hyades_L5_v5
image.WehavecheckedtheHerschelandALMAarchives,but Table1.Compilationofthephotometryfor2M0418+21.
thesearchdidnotreturnanypublicimageatthetimeofwriting
so we cannot discuss further the presence or absence of a disk R.A.(2MASS) 04:18:34.83
around2M0418+21. dec(2MASS) +21:31:27.5
We added two samples of objects in Fig. 5 to place the H z(SDSSDR9) 20.338±0.116mag
2
photometry of 2M0418+21 in context. We cross-matched the J(2MASS) 17.152±0.239mag
fulllistofHyadesmembersfromGoldmanetal.(2013)withthe H(2MASS) 16.265±0.211mag
UKIDSSGPSDR10andfound79sourceswithH2 photometry Ks(2MASS) 15.381±0.198mag
(redopensquaresinFig.5).However,onlyahandfulhavemag- J(UKIDSSGPSDR10) 17.211±0.016mag
nitudesfainterthan12mag.Onlyonesourcehasan H magni- H(UKIDSSGPSDR10) 16.147±0.011mag
2
tudefainterthan13mag,safelybelowthesaturationlimitsofthe K (UKIDSSGPSDR10) 15.208±0.013mag
UKIDSSshallowsurveys(Lodieuetal.2007).Wealsoincluded H2(UKIDSSGPSDR10) 15.247±0.029mag
two L dwarfs covered by the GPS among all field L/T dwarfs K1(UKIDSSGCSDR10) 15.230±0.015mag
listed in the compendium of ultracool dwarfs2. One object is W1(AllWISE) 14.300±0.029mag
2MASSWJ0326137+295015 classified in the optical as a L3.5 W2(AllWISE) 13.894±0.045mag
dwarf(Kirkpatricketal.1999)withatrigonometricdistanceof µαcosδ 124±7mas/yr
32.2+1.49 pcandameanpropermotionof69.4±0.8mas/yrwith µδ −53±6mas/yr
−1.64
a position angle of 344.3±0.7 degrees (Dahn et al. 2002). The
Notes.Photometryintheoptical,near-infrared,andmid-infraredfrom
proper motion of this source is lower than the average motion
SDSSDR9(Yorketal.2000;Ahnetal.2012b),2MASSPointSource
ofHyadesmemberswithadiscrepantpositionangleanditlies
catalogue(Cutrietal.2003;Skrutskieetal.2006),UKIDSSGalactic
closer than the Hyades, arguing against its membership in the
PlaneSurveyandGalacticClustersSurvey(Lawrenceetal.2007;Lucas
Hyades.Theothersourceis2MASSIJ0409095+210439(L3.0;
et al. 2008), and AllWISE (Wright et al. 2010), as well as the mean
Kirkpatrick et al. 2000). This object lies close to the centre of propermotiondeterminedinthispaper.Thereisnoreliabledetection
theHyadesclusterandhasapropermotionof101±15,−148±12 intheSDSSifilterandtheWISEw3band.
mas/yr(Casewelletal.2008)andaspectrophotometricdistance
of 35.9–39.8 pc, which makes it a potential L dwarf member
We collected two new optical spectra of 2M0418+21 with
of the Hyades cluster. We observe a possible trend of redder
J − H and H − H colours with later spectral types in the L the R1000R grating on GTC/OSIRIS as part of filler program
2 2
GTC38–15A (PI Lodieu). We used the same configuration as
dwarfregime.
above, except for the grating which offers a resolution that is
higherbyapproximatelyafactorof3.Wesettheon-sourcein-
2.5. Massestimate tegration to 1800 sec, repeated twice with a shift of 10 arcsec
In Fig. 3 we compare the colours of 2M0418+21 with predic- (equivalent to 40 pixels) along the slit. The seeing was poor,
around1.8arcsec,buttheskywasclearandthemoonwaswithin
tionsforbrowndwarfsofvariousmassesattheageoftheHyades
7daysofnewmoon.
using the evolutionary models of the Lyon group (Allard et al.
2001;Chabrieretal.2000).Eachcolourgivesaslightlydifferent We reduced the optical spectra under the IRAF environ-
massestimation:0.044+0.005M ,0.046+0.010M ,and0.054+0.001 ment(Tody1986,1993)inastandardmanner.First,wemedian-
−0.005 (cid:12) −0.005 (cid:12) −0.001 combinedthebiasandflatfieldstakenduringtheafternoon.We
M for J −K, W1−W2, and J −W2, respectively. We infer a
(cid:12) subtractedthemeanbiasfromtherawspectrumofthetargetand
likely mass between 0.039–0.055 M with an average value of
0.048+0.007 M for2M0418+21.Tos(cid:12)ummarize,wehaveidenti- divided by the normalized flat field. We extracted the 1D spec-
−0.009 (cid:12) trumbyoptimallychoosingbackgroundlevel.Wecalibratedthe
fiedahighlyprobablebrowndwarfmemberoftheHyadesclus-
1D spectra with the response function derived from the spec-
ter.
trophotometricstandardstarRoss640(DZ5;Harrington&Dahn
1980;Monetetal.2003;Cutrietal.2003;Lépine&Shara2005;
3. Spectroscopicobservations Sion et al. 2009). The final spectra of 2M0418+21, normalized
at7500Å,aredisplayedinFigure6alongwithknownLdwarf
Wecollectedseveralopticalspectrawiththreedifferentgratings
spectra templates from the literature (Schmidt et al. 2010; Fan
installed on the OSIRIS (Optical System for Imaging and low-
etal.2000;Leggettetal.2002;Geballeetal.2002).
intermediate Resolution Integrated Spectroscopy; Cepa et al.
2000) spectrograph on the 10.4m GTC in La Palma to char-
acterize the target. OSIRIS is equipped with two 2048 × 4096 3.2. Medium-resolutionopticalspectroscopy
MarconiCCD42-82detectorsofferingafieldofviewofapprox-
imately7×7arcmin2withabinnedpixelscaleof0.25arcsec. Following the analysis of the low-resolution spectra, we re-
quested a dedicated program to take optical spectra with the
highestresolutiongratingavailable(R2500R)onGTC/OSIRIS,
3.1. Low-resolutionopticalspectroscopy offeringaresolutionof∼2400at6600Åwithaslitof1arcsec.
We obtained two low-resolution optical spectra of 2M0418+21 Wesecuredfivegroupsoftwospectraof30minon-sourceeach
withtheR300RgratingonGTC/OSIRISonthenightsof25and shiftedalongtheslitby10arcsec,yieldingatotalon-sourceex-
posure of 5h. The observations were taken on the night of 19
26January2015underfillerprogramGTC51–14B(PILodieu).
December2015underavariableseeingof1.3–1.5arcsec,spec-
We used an on-source integration of 1800 sec for both spectra
troscopic conditions, and the moon illuminated at 65% located
with the following configuration: slit of 1 arcsec and 2×2 bin-
at50degreesfromourtarget.
ning.Theobservationswereobtainedunderaseeingof1.3arc-
We reduced the R2500R spectra under the IRAF environ-
sec,withgreyskyandclearconditions.
ment (Tody 1986, 1993) taking advantage of the fact that all
2 seehttp://spider.ipac.caltech.edu/staff/davy/ARCHIVE/index.shtml spectrawereobtainedconsecutivelyonthesamenight.Wesub-
Articlenumber,page4of9
PérezGarridoetal.:AnewL5memberoftheHyades
13 13 13
14 14 14
15 15 15
J H K
16 16 16
17 17 17
18 18 18
19 19 19
0.5 1.0 1.5 2.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -0.4 -0.2 0.0 0.2 0.4
J − H H − H K − H
2 2 2
Fig.5.Left:(J−H ,J)colour-magnitudeforallpointsourceswithin5arcminof2M0418+21highlightedbyalargereddot.Middle:Samebut
2
for(H−H ,H).Right:Samebutfor(K−H ,K).HyadesmembersfromGoldmanetal.(2013)andfieldL/TdwarfswithH photometryfromthe
2 2 2
UKIDSSGPSareplottedasredandblueopensquares,respectively.
6
2M0418+21 (GTC/OSIRIS R300R)
5 2MASS J08251968+2115521 (L6)
x
u 4
Fl 2M0418+21 (GTC/OSIRIS R1000R)
d SDSS J053952-005902 (L5)
e
z
ali 3
m
or 2M0418+21 (GTC/OSIRIS R300R)
N
2 L5 (SDSS standard)
1
2M0418+21 (GTC/OSIRIS R300R)
2MASS J00361617+1821104 (L4)
0
6000 7000 8000 9000 10000
Wavelength (Angstroms)
Fig. 6. Low-resolution optical spectra of 2M0418 obtained with the
R300R (three spectra) and R1000R (third from bottom) gratings on Fig. 7. Bottom: Combined 1D optical spectrum of 2M0418+21 taken
GTC/OSIRIS (black lines). The continuum of the spectra are offset withtheR2500RgratingonGTC/OSIRISon19December2015.No
vertically by 1.5 for visualization purposes. From top to bottom: We smoothingwasapplied.Top:Finalcombined2DimageoftheR2500R
overplottedthespectraof2MASSJ00361617+1821104(L4;green),the spectrum of 2M0418+21. The white dot on the left-hand side corre-
Sloandwarftemplate(L5;red),thespectrumofSDSSJ053952−005902 spondstotheHαemission.Thefaintcontinuumisbarelyseen,butis
(L5; red), and 2MASSJ08251968+2115521 (L6; green). See text for present.
details.
the detector+grating derived from the spectrophotometric stan-
tractedeachpairofopticalspectratoremovethecontributionof dardstarFeige110(Høgetal.2000;vanLeeuwen2007;Drilling
thesky.Then,weaveragedthefivespectraatthenominalposi- etal.2013).Thefinal1Dspectrumof2M0418+21isdisplayed
tion(“firstframe”)andtheshiftedposition(“secondframe”)re- inFig.7.TheemissionlinecorrespondstoHαat6563Å.There
jectingthelowestandhighestvaluestocreateacombinedspec- is no evidence of Hα emission in the spectrum gathered with
trum at each position. Later, we combined the two frames by the R300R grism; however, data from the R1000R grism show
shiftingthesecondframetothefirstone.Thefinal2Dimageis signsofHαinemissionwithapseudo-equivalentwidth(pEW)
shownatthetopofFig.7,wheretheHαemissionlinestandsout between4–10Å,althoughitslowsignal-to-noiseratiodoesnot
asawhitedotwithasignal-to-noiseof10ontopofaveryfaint makeitareliabledetection.
continuum with a signal-to-noise of ∼2 per spectral resolution
elementof2.6Å.
Weoptimallyextractedthespectrumbyselectingthenomi- 4. Analysis
nalpositionfittingthebackgroundwithoutcosmicraysrejection.
4.1. Spectralclassificationandspectroscopicdistance
Wecalibratedourspectruminwavelengthbyfittinga1Dpoly-
nomialto20arclineswithadispersionof1.03Å/pixelandarms Theoverallappearanceofthe2M0418+21low-resolutionspec-
of 0.05 Å, corresponding to radial velocity precisions of ∼2.3 trum displayed in Fig. 6 suggests a mid-L dwarf. To de-
km/s.Wecalibratedthisspectrumwiththeresponsefunctionof rive a spectral type with better accuracy, we compared our
Articlenumber,page5of9
A&Aproofs:manuscriptno.paper_Antonio_Hyades_L5_v5
GTC/OSIRIS spectrum with the Sloan L dwarf template pub- age we can discard accretion as the origin of this emission and
licly available for each subclass (Schmidt et al. 2010, 2014). postulatechromosphericactivityasitscause.
We find the L5 SDSS template (red line) provides the best fit 2M0418+21representsanimportantadditiontothesample
to our spectrum (black line) up to 9000 Å (Fig. 6). We also of L5 dwarfs with detected Hα emission and log (L /L )
10 Hα bol
compared our optical spectrum to a set of spectral templates measurements because its age is well constrained to 625±50
listedonSandyLeggett’swebpage3anddiscoveredbytheSloan Myr. We collected a large number of L4.5–L5.5 with reported
team (Knapp et al. 2004; Golimowski et al. 2004; Chiu et al. Hα pEWs, yielding a wide range of values in log (L /L ).
2006),including2MASSJ00361617+1821104(L4; Reidetal. The highest values can reach up to −3.7 dex, while10theHfαainbtoelst
2000; Kirkpatrick et al. 2000; Knapp et al. 2004), LHS102B upperlimitliesaround−7.5dex.Reiners&Basri(2008)looked
(L4.5–L5(binary);EROSCollaborationetal.1999;Henryetal. at one L4.5 and three L5 and placed upper limits on the
2006; Kirkpatrick et al. 2001), SDSS05395199−0059020 (L5), log (L /L ). The sample of SDSS L5 dwarfs includes 10
10 Hα bol
SDSS13262982−0038315 (L5.5; Fan et al. 2000; Vrba et al. L4.5, 24 L5, and 2 L5.5 sources with seven detections of Hα
2004) and 2MASS08251968+2115521 (L6; Kirkpatrick et al. inemission,andthreeclassifiedasvariable(Schmidtetal.2007,
2000;Dahnetal.2002).WefindthebestmatchfortheL5dwarf 2015). In their sample, Pineda et al. (2016) included three L5
up to 10000 Å, as in the case of the SDSS templates. The L4 with Hα in emission in one of them. They inferred a fraction
andL6dwarfsarebluerandredderbeyond8800Å,respectively of emitters of 20% (15–35%; 68% confidence limit), consis-
(Fig.6).TheL6hasamuchstraighterredwardspectrumthanour tent with the interval from the SDSS sample (Schmidt et al.
target,markedbythepresenceoftheFeHbandsat∼8700Åand 2007,2015)althoughtodatetherearefewsources.Wealsonote
theH Obandat∼9300Å.Asaconsequence,wederiveanun- that a large number of L dwarfs show variability in their level
2
certaintyofhalfasubclassforouropticalspectralclassification of chromospheric activity (Liebert et al. 2003; Reiners & Basri
bycomparisonwiththemid-Ldwarfstandards. 2008;Schmidtetal.2015;Pinedaetal.2016),themoststriking
Assuming the spectral type vs absolute magnitude relation casebeingtheL5.5e+T7binary2MASSIJ1315309−264951AB
for L5 dwarfs from Dupuy & Liu (2012) in all five passbands (L5.5; Hall 2002a,b; Gizis 2002; Burgasser et al. 2011, 2013,
available for 2M0418+21, we derive spectrophotometric dis- 2015).
tances of 57.8, 53.1, 50.1, 42.3, and 40.6 pc in J, H, K, w1,
and w2, respectively, with typical errors in the 5–10 pc range.
4.3. Radialvelocityandspacemotion
These error bars take into account the uncertainty of 0.5 class
inthespectraltype.Weobserveatrendtowardscloserdistances
WemeasuredthepositionoftheHαlineinthe1Dspectrumwith
withredderwavelengths.Averagingallthesemeasurementswith
thesplottaskunderIRAF.Weidentifiedthelineat6564.11Å
equalweightsbecauseweassumethatthepolynomialfitsineach
withanerrorof0.04Å,whilethenominalairwavelengthfrom
filterrepresentindependentmeasurements,weinferameandis-
the NIST Atomic Spectra Database is 6562.819 Å. We should
tance of 48.8±4.0 pc. The error on the distance is rms on the
add in quadrature the error on our wavelength calibration (2.3
distancedividedbythesquarerootof5(thenumberofmeasure-
ments).Weassumethat2M0418+21issingle.Ourmeanvalue km/s, see Section 3.2) and uncertainty due to the timespan in
our observations (0.2 km/s). We inferred a radial velocity of
isclosetotheaccepteddistanceoftheHyadesclusterfromHip-
38.0±2.9 km/s (Table 2) after correcting for the rotation of the
parcos(46.3±0.3pc;vanLeeuwen2009).
Earth,themotionoftheEartharoundtheEarth-Moonbarycen-
tre,theorbitofthebarycentreabouttheSun,andmotionofthe
4.2. Hαemission Sunrelativetothespecifiedstandardofrest.Ourvalueisinclose
agreementwiththemeanvaluesderivedindependentlyfromthe
We detected a clear emission line present in each individual ground (39.1±0.2 km/s; Detweiler et al. 1984) and from space
spectrumatthenominalpositionofHα(6562.8Å)overthefull with Hipparcos (39.48±0.30 km/s; de Bruijne et al. 2001). We
time of observations, from UT=22h to UT=1h40 (Fig. 7). This inferaspacemotionof[U,V,W]=[−42.92,−23.96,−0.72]km/s
isthefirstdetectionofHαinemissioninamid-Ldwarfclassi- for 2M0418+21 (Table 2), in close agreement with the space
fiedasahigh-probabilityphotometric,astrometric,andspectro- motion of the cluster centre: [−41.1, −19.2, −1.4] km/s (van
scopiccandidatememberoftheHyadescluster.However,com- Leeuwen2009).
parable levels of activity have been reported in field L dwarfs AccordingtoVizier,2M0418+21shouldhaveaGaiaG-band
(Schmidtetal.2015;Pinedaetal.2016).
of19.096andshouldbeincludedintheInitialGaiaSourceList
We measured the ratio of Hα to bolometric luminosity
(Smart&Nicastro2014),implyingthatprecisecoordinatesand
for our target following the method outlined in Burgasser
propermotionswillbesoonavailablesincethesecondGaiadata
et al. (2011). We calibrated the flux of 2M0418+21 with
release is expected at the end of 2017. However, based on the
the spectrophotometric standard star Feige110 observed with equation G − J∼0.244×SpT−12.6332 kindly provided by R.
GTC/OSIRISonthesamenight.WeconsideredtheR-bandmag- Smart, 2M0418+21 might haveG∼20.4 mag, placing it at the
nitude of Feige110 (11.70 mag; Landolt & Uomoto 2007) be-
faint end of the Gaia catalogue. Therefore, it is unclear at this
cause it contains the Hα line. In this passband our target is
stagewhetheritsmembershiptotheHyadesclusterwillbeset-
∼30,000timesfainterthanFeige110.Weassumedabolometric
tledbyGaiainthenearfuture.
correctionintheKbandof3.3foraL5dwarf(Golimowskietal.
2004)andassumedasolarbolometricluminosityofM =4.74.
bol
We used the distance of 48.8 pc for 2M0418+21 (see previous 4.4. Lithiumabsorption
section).Weinferavalueoflog (L /L )=−6.0dex,consis-
10 Hα bol
tentwiththedropinactivityseenfromlate-Mtomid-Ldwarfs Thelithiumtestwasproposedintheearly1990stodistinguish
(see e.g. Figure 5 of Berger et al. 2010). Owing to the cluster between very low-mass stars and brown dwarfs (Rebolo et al.
1992; Magazzu et al. 1993). The technique relies on the detec-
3 http://staff.gemini.edu/∼sleggett/LTdata.html tionoflithiuminabsorptionat6707.8Åforlate-MandLdwarfs
Articlenumber,page6of9
PérezGarridoetal.:AnewL5memberoftheHyades
Table2.Resultsofourspectroscopicanalysis.
t = 400 Myr
SpT L5±0.5 1 t = 600 Myr
t = 800 Myr
Spectroscopicdistance 48.8±4.0pc
Radialvelocity 38.0±2.9km/s 0.8
LWUV 2.−−8−042023×...79912260k2kk9mmme/r//sgss/s Lithium fraction0.6
log (Lbol/L ) −6.0dex 0.4
10 Hα bol
Notes. We quote the spectral type, spectroscopic distance, radial ve- 0.2
locity, space motion, and ratio of the log (L /L ) luminosity for
10 Hα bol
2M0418+21.
0
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
M/M
sun
Fig.8.Fractionoflithiumasafunctionofmassforobjectsbelow0.08
straddlingthestellar-substellarboundary.Iflithiumisdetected,
M attheageoftheHyadescluster(625±50Myr)usingtheAMES-
theobjectissubstellar. (cid:12)
Dustymodel.Weaddedthesamecurvesfortwobracketingagesof400
Brown dwarf evolutionary models predict a lithium conser- Myrand800Myr.
vation of 95% for 0.050M brown dwarfs independent of age,
(cid:12)
whereas a 0.055 M should conserve 50% of its initial Li con-
(cid:12)
tent and a 0.060 M should have totally destroyed lithium at
(cid:12) 6
the age of the Hyades. In Fig. 8 we show the predicted lithium 2M0418+21 (GTC/OSIRIS R2500R) Binning 5 times
depletion/preservation as a function of mass at the age of the 5 Li Luhman16A (VLT/X-shooter)
Hyades. The observation of the lithium feature at 6707.8 Å in 4
msciosildtoe-unLrcsyd):wamnadorfdstehlcesopruehlpdyrsoipcdosuteconefttbihaoletlhyintpthereroivaotirmddeoesaspchcrierbirteiicdcapblryotepthsetertoileiftshci(oui.nme-. malized Flux 23
contentforthesamemass.ManynearbyoldfieldLdwarfsshow Nor
1
lithiuminabsorptionwithmorethan60%ofLdwarfswithspec-
traltypeslaterthanL5havingpseudo-equivalentwidthsgreater 0
than10–15Å,suggestingthatthegreatmajorityof>L5dwarfs -1
arebrowndwarfs. 6600 6800 Wave7l0e0n0gth (Angstroms)7200 7400 7600
Can lithium be detected in a mid-L dwarf at the age of the Fig. 9. GTC OSIRIS medium-resolution optical spectrum of
Hyades? There are several detections of the lithium resonance 2M0415+21 binned by a factor of five (black line) compared to the
doubletat6707.8ÅreportedintheliteratureforfieldLdwarfs smoothed(R∼730)VLTX-shooterspectrum(redline)ofLuhman16A.
Thepositionofthelithiumfeatureisindicatedbyabluedashedvertical
with pEWs ranging from 1 Å to 20 Å (Kirkpatrick et al. 2000,
line.
2008; Cruz et al. 2009; Zapatero Osorio et al. 2014). For in-
stance, Faherty et al. (2014) and Lodieu et al. (2015) have re-
ported the detection of lithium in the two components of the
nearestbinary(L7–T0)browndwarftotheSun,Luhman16AB
4.5. Alkalilines
(Luhman 2013). If preserved at the level predicted, there is a
high probability that lithium can be detected in Hyades mid-L
dwarfs. A determination of the lithium abundance in low-mass We measured the pEWs of two alkali lines present in the spec-
Hyadesmemberswouldprovideprecisedeterminationsoftheir traofmid-Ldwarfs,whichareknowntobesensitivetogravity.
massesandconfirmthemasgenuineHyadesbrowndwarfs. They are better detected than the NaI doublet at ∼8200 Å ow-
In Fig. 9 we compare our medium-resolution optical spec- ing to the low signal-to-noise of our spectra and the redness of
trum of 2M0418+21 smoothed by a factor of nine (black line) mid-Ldwarfs.Ontheonehand,wemeasuredconsistentpEWs
withtheunsmoothedVLTX-shooterspectrum(redline)ofLuh- of 5±2 Å for the RbI feature at 8946 Å in both low-resolution
man16A(L7;R∼11000;Lodieuetal.2015).Bydirectcompar- spectra.WecheckedthestrengthofthislineinseveralLdwarfs
ison with the clear lithium absorption present in the X-shooter withspectraltypesbetweenL3andL8downloadedfromSandy
spectrum of Luhman16A, we observe a wide feature around Leggett’shomepageandinLuhman16AB(Lodieuetal.2015),
6708Åthatmightbeconsistentinstrengthwiththefullpreser- yieldingpEWsinthe3–7Årange.Ontheotherhand,wemea-
vationoflithium.Wemeasuredanequivalentwidthintherange sured a pEW of 5.0±0.5 Å for the CsI line at 8521 Å, consis-
15–20Å.However,wecannotclaimthedetectionoflithiumdue tent with the 3–6 Å range for L4–L6 dwarfs (see Figure 17 of
tothelowsignal-to-noiseofthespectrum.Iftheabsorptionfea- Lodieuetal.2015),obtainedusingdatafromKirkpatricketal.
turewereconfirmedasbeingduetolithiumwithahigherqual- (2000)andBurgasseretal.(2003).ThefitsfromDupuy&Liu
ity spectrum., it would imply a mass below 0.050 M . If true, (2012)suggests M =13.40magforaL5dwarfwithanrmsof
(cid:12) J
2M0418+21wouldbecomethefirstrelativelyoldL-typebrown 0.39 mag. Our object would have M = 13.71±0.07 mag, con-
J
dwarf with well-determined mass and age. Its photometric and sistentwithfielddwarfwithintheuncertainties.Atthisspectral
spectroscopic properties would represent a benchmark to clas- type(mid-Ldwarfs),Figure25ofLiuetal.(2016)suggeststhat
sifyisolatedobjectsinthefieldforwhichneithermassnorage young objects have fainter absolute J-band magnitude, so our
canbereliablydetermined. targetismostconsistentwiththefieldpopulation.
Articlenumber,page7of9
A&Aproofs:manuscriptno.paper_Antonio_Hyades_L5_v5
5. Conclusions (MPAGarching),Max-Planck-InstitutfürExtraterrestrischePhysik(MPE),Na-
tionalAstronomicalObservatoryofChina,NewMexicoStateUniversity,New
Wecross-matchedthe2MASSandAllWISEdatabasesforiden- York University, University of Notre Dame, Observatário Nacional / MCTI,
tification of very low-mass stars and brown dwarfs with proper TheOhioStateUniversity,PennsylvaniaStateUniversity,ShanghaiAstronom-
motions consistent with membership in the Hyades cluster. We icalObservatory,UnitedKingdomParticipationGroup,UniversidadNacional
reportonthespectroscopicfollow-upwithGTC/OSIRISofone AutónomadeMéxico,UniversityofArizona,UniversityofColoradoBoulder,
UniversityofOxford,UniversityofPortsmouth,UniversityofUtah,University
faint proper motion candidate, 2M0418+21, discovered in this ofVirginia,UniversityofWashington,UniversityofWisconsin,VanderbiltUni-
search.Accordingtoitsphotometry,propermotion/radialveloc- versity,andYaleUniversity.Thispublicationmakesuseofdataproductsfrom
ity,andspectroscopicpropertiesweconcludethatthisobjectis the Wide-field Infrared Survey Explorer, which is a joint project of the Uni-
versityofCalifornia,LosAngeles,andtheJetPropulsionLaboratory/California
verylikelyabrowndwarfmemberoftheHyades.Themainre-
InstituteofTechnology,fundedbytheNationalAeronauticsandSpaceAdminis-
sults obtained from the astrometric, photometric, and spectro- tration.ThisresearchhasmadeuseoftheSimbadandVizierdatabases,operated
scopicanalysisofthe2M0418+21dataareasfollows: attheCentredeDonnéesAstronomiquesdeStrasbourg(CDS),andofNASA’s
AstrophysicsDataSystemBibliographicServices(ADS).
• apropermotionfullyconsistentwiththatoftheHyadesclus-
ter,wherethemaximumlikelihoodmethodsuggestsaprob-
abilitygreaterthan90%formembershipintheHyades;
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