GTC OSIRIS z-band imaging of Y dwarfs

0.31 MB·English

by  N. Lodieu

Astronomy&Astrophysicsmanuscriptno.20696 (cid:13)c ESO2013
January16,2013
LettertotheEditor
GTC OSIRIS z-band imaging of Y dwarfs
N.Lodieu1,2⋆,V.J.S.Be´jar1,2,andR.Rebolo1,2,3
1 InstitutodeAstrof´ısicadeCanarias(IAC),CalleV´ıaLa´cteas/n,E-38200LaLaguna,Tenerife,Spain
e-mail:nlodieu,vbejar,[email protected]
2 DepartamentodeAstrof´ısica,UniversidaddeLaLaguna(ULL),E-38205LaLaguna,Tenerife,Spain
3 ConsejoSuperiordeInvestigacionesCientficas,CSIC,Spain
3 January16,2013;January16,2013
1
0 ABSTRACT
2
Aims.Theaimoftheprojectistocontributetothecharacterisationofthespectralenergydistributionofthecoolestbrowndwarfs
n
discoveredtodate,theYdwarfs.
a
Methods.Weobtainedz-bandfar-redimagingforsixYdwarfsandaT9+Y0binarywiththeOSIRIS(OpticalSystemforImaging
J
andlowResolutionIntegratedSpectroscopy)instrumentonthe10.4-mGranTelescopiodeCanarias(GTC).
5 Results.WedetectfiveofthesevenknownYdwarfsinthez-band,infertheiroptical-to-infraredcolours,andmeasuretheirproper
1 motions.Wefindahigherdispersioninthez−J andz−H coloursofY0dwarfsthaninTdwarfs.Thisdispersionisfoundtobe
correlatedwith H−w2.Thehighdispersionintheoptical-to-infraredcoloursofYdwarfsandthepossibleturn-overtowardsbluer

] coloursmaybeaconsequenceofthepresenceofsulfidecloudswithdifferentthicknesses,thedepletionofalcalines,and/orgravity
R
effects.
S
Keywords.Stars:low-mass—Stars:browndwarfs—techniques:photometric
.
h
p
-
o 1. Introduction Ourgoalistocontributetothespectralenergycharacterisa-
r tionoftheY dwarfs,pushingthelimitsoftheGranTelescopio
st Theclassicalstellarsequence(Morganetal.1943)hasnowbeen de Canarias (GTC). This letter describes the optical imag-
a extended to include very cool objects that bridge the gap be- ing obtained for six Y dwarfs with OSIRIS (Optical System
[ tween brown dwarfs and planets, thanks to the advent of the
for Imaging and low Resolution Integrated Spectroscopy;
1 WideInfraredSurveyExplorer(WISE;Wrightetal.2010)satel- Cepaetal. 2000) on the 10.4-m GTC. We present the imaging
v lite. The “Y” class, originally proposed by Kirkpatricketal. campaignandtheassociateddatareductionandastrometry.We
0 (1999), now consists of 15 members (Cushingetal. 2011; exposeourresultsandplacethemintocontextwithrespecttothe
8 Liuetal.2011;Kirkpatricketal.2012;Tinneyetal.2012)with spectralenergydistributionsofTdwarfs(Leggettetal.2012b).
2 spectraltypesequalto,orlaterthan,Y0(tentativeclassification)
3 and effective temperaturesbelow 500K. Two additionalcandi-
. dates have been reported,but their faintness currentlyhampers 2. Far-redopticalimaging
1
0 spectraltyping(Liuetal.2012;Luhmanetal.2011).
2.1.Observations
3 The number of late-T dwarfs with spectral types later
1 than T6 has increased dramatically over the past few years The OSIRIS instrument is mounted on the 10.4-m GTC op-
: thankstolarge-scalesurveyssuchastheUKIRTInfraredDeep erating at the Observatory del Roque de Los Muchachos (La
v
i Sky Survey (UKIDSS; Lodieuetal. 2007; Pinfieldetal. 2008; Palma,CanaryIslands).Thedetectorconsistsoftwo2048×4096
X
Burninghametal. 2009, 2010a,b), the Canada-France-Hawaii MarconiCCD42-82separatedbyan8arcsecgapandoperatesat
r BrownDwarfSurvey(Delormeetal.2008a,b;Reyle´ etal.2010; opticalwavelengths,from365to1000nm.Theunvignettedin-
a
Albertetal.2011),andWISE(Kirkpatricketal.2012).TheT/Y strumentfield-of-viewisabout7by7arcminwithanunbinned
transitionischaracterisedbyarapidshiftofthepeakofthespec- pixelscaleof0.125arcsec.Weemployeda2×2binningbecause
tralenergydistributionfromthenear-infraredtothemid-infrared itiscurrentlythestandardmodeofobservations.
andbynarrowerpeaksinthe H band(Cushingetal.2011)with We imaged six of the seven Y dwarfs published by
the possible disappearanceof the potassiumabsorptionbeyond Cushingetal. (2011) and the T9+Y0 binary resolved by
0.7 microns (Leggettetal. 2012b). Because of to their nature Liuetal. (2012) using the Sloan z filter available on OSIRIS.
and low temperature, it is importantto characterise their spec- Table1providestheoriginalnameswith thefirstfourdigits of
tral energydistributionsfrom the optical to the mid-infraredto therightascensionanddeclinationasprovidedinCushingetal.
understandthechemistryatplayinthesecoolatmospheres. (2011)alongwiththeircurrentspectraltypes(Kirkpatricketal.
2012). This information is supplemented with the coordinates
⋆ Based on observations made with the Gran Telescopio Canarias measured on the GTC OSIRIS images (when the target is de-
(GTC), installed in the Spanish Observatorio del Roque de los tected), the z-bandmagnitudeswith theirassociated errorbars,
MuchachosoftheInstitutodeAstrof´ısicadeCanarias,intheislandof the dates of observations, and the numbers of individual im-
LaPalma. ages with their respective integration times in seconds. Figure
1
N.Lodieuetal.:GTCOSIRISz-bandimagingofYdwarfs
Table 1. GTC/OSIRIS astrometry, photometry (or 3σ lower limits), observing information, and photometry for six Y dwarfs
and a T9+Y0 binary. The near-infrared photometry is taken from Leggettetal. (2012a), except for the H-band magnitude of
WISEJ1828+2650(Kirkpatricketal.2012).
WISEJ... (SpT) R.A. dec z Date ExpT J H z−J µ cosδ µ
α δ
hh:mm:ss.sss ◦:’:′′ mag dd/mm/yy sec mag mag
0146+4234(Y0) 01:46:56.576 +42:34:09.80 24.10±0.13 03/09/12 50×50 19.40±0.25 18.71±0.24 5.39±0.27 −0.52±0.08 −0.11±0.08
0410+1502(Y0) 04:10:22.933 +15:02:42.91 22.66±0.09 15/09/12 40×50 19.44±0.03 20.02±0.05 3.22±0.09 +1.20±0.08 −2.17±0.08
1405+5534(Y0p) >23.85 06/08/12 45×50 21.06±0.06 21.41±0.08 >2.79 — —
1738+2732(Y0)a 17:38:35.585 +27:32:58.28 22.80±0.09 17/06/12 30×50 20.05±0.09 20.45±0.09 2.75±0.13 +0.32±0.10 −0.39±0.10
1828+2650(≥Y2) >24.46 09/09/12 40×50 23.48±0.23 22.85±0.24 >0.98 — —
2056+1459(Y0) 20:56:29.028 +14:59:54.64 23.09±0.08 15/06/12 40×50 19.94±0.04 19.96±0.04 3.66±0.09 +0.89±0.10 +0.61±0.10
1217+1626(T9+Y0)12:17:57.144 +16:26:35.99 21.60±0.03 15/12/12 15×60 17.83±0.02 18.18±0.05 3.77±0.04 +1.41±0.10 −1.96±0.10
aWealsotargetedthisobjectintheibandwithOSIRIS.Wemeasureda3σlowerlimitofi>25.3mag,translatingintoi−z>2.4magand
i−J>5.8mag.
1showstheGTCOSIRISfindingcharts(twoarcminaside)of tosomeofourtargets.We choseanapertureequalto ∼3times
all the observed objects (circled) with north up and east to the thefull-width-at-half-maximumandcheckedthatthetargetwas
left.Table1alsolistsrevisedJandHmagnitudes(Leggettetal. well-subtractedbyourPSFanalysiswithoutresiduals.Wetrans-
2012a)andtheassociatedz−J coloursplottedinFig.2. formed the instrumental magnitudes into apparent magnitudes
Theobservationswereconductedon15and17June,6and usingthezero-pointsderivedforthephotometricstandardfields
13 August, 3, 5, 9, and 16 September,and 15 December2012. observedonthesamenight.Thefinalcalibratedmagnitudesof
All observations were conducted under average seeing of 0.8– the fourY dwarfs detected on the OSIRIS images are given in
1.3 arcsec, photometric or clear conditions, grey time, and air- Table 1. Forthe remainingsources, we list the 3σlowerlimits
mass less than 1.5. Bias and skyflats were obtained during the (Table1)computedfromtheroot-mean-squareoftheskyatthe
afternoonormorningoftherespectivenights.ForeachYdwarf, nominalpositionofthe targetcomparedtothe peakof theflux
we obtained three to five series of ten frames with 50 sec on- ofthreenearby(non-saturatedandisolated)stars.Afteraverag-
source individual integrations, except for the coolest (WISE ingallimagesforWISE J1828+2650,we canseta lowerlimit
J1828+2650 tentatively classified as >Y2; Kirkpatricketal. ofz>24.46mag,translatingintoz−H>1.6mag(andz−J>0.9
2012) forwhich we repeated the series fourtimes to try to de- mag)forthis(tentative)Y2dwarf.
tecttheobjectinthez-bandandforWISEJ1217+1626AB,for
whichweobtainedthreeseriesoffiveframesof60sec.
2.3.Astrometry
Weastrometricallycalibratedthecombinedscienceframeswith
2.2.Datareductionandphotometry
IRAF(daofind,ccxymatch,andccmaproutines)usingbright
WereducedtheOSIRISSloanz-bandimagesinastandardman- pointsourcesfrom2MASS(Cutrietal.2003)orSDSSDR8in
nerwithIRAF1(Tody1993).First,wesubtractedthemeanbias thecaseofWISEJ1405p5534.Wetypicallyfound50–100stars
and divided by the normalised averaged master skyflat to each (dependingoncrowding)intheOSIRISfield-of-view,resulting
individualscienceframe.Then,wecombinedeachsetoftenim- in an astrometric calibration better than 0.1–0.15 arcsec. The
agestakenwithoutdithering,andfinallywecombinedthosesets, GTC OSIRIS coordinates (right ascension and declination) of
applyingtheoffsetstocreateamasterscienceframe.Inthecase the five objects detected in the z-band are listed in columns 2
of WISE J1217+1626AB, images were sky-subtracted before and3ofTable1.Wemeasuredpropermotions,consistentwith
thecombination. the values quoted by Marshetal. (2012) and Liuetal. (2012),
thankstothe2.0–2.5yearbaselinebetweentheGTCandWISE
Wephotometricallycalibratedthez-bandimageswithSloan
observations. The error bars of each component of the proper
standard fields (Smithetal. 2002) or with Sloan images of the
motiontakeintoaccounttheWISEastrometricaccuracy(0.15–
same field, using point sources with photometric errors below
0.17arcsec)andconservativeerrorsontheGTCcentroidof0.15
0.02–0.05mag.We measuredinstrumentalmagnitudesfor5 to
arcsec (Table 1). We find high motionsas expected forobjects
10 stars in those fields and derived zero points ranging from
27.976±0.037to28.091±0.049mag. lyingatdistancesofonlyafewparsecs.
WedetectedfourofthesixYdwarfsandtheT9+Y0binary
(unresolvedinourimages)withOSIRISinthez-band(Table1; 3. Discussionandconclusions
Fig.1).Wemeasuredthephotometryoftheclosestsourcetothe
nominalpositionoftheWISEsource(Table1),exceptforWISE We reported in this letter the first far-red optical detection of
J1410+1502andWISEJ1217+1626AB,whichhavemovedbe- fourY0dwarfsandtheunresolvedT9+Y0binaryfromLiuetal.
yond the five-arcsec circle drawn in Fig. 1 due to their high (2012), which are among the coolest brown dwarfs known to
propermotion(∼2.5arcsec/yr).Tomeasurethemagnitudes,we date.AllfourY0dwarfswithz-banddetectionhavemagnitudes
performedapertureandpoint-spreadfunction(PSF)photometry in the range 22.8–24.1mag, implying z− J and z−H colours
usingDAOPHOTinIRAFbecauseofthepresenceofobjectsclose spanning 2.7–5.2 mag and 2.3–5.4 mag, respectively (Fig. 2).
Wealsopresentedlowerlimitofz−Jof2.79magand0.98mag
1 IRAF is distributed by the National Optical Astronomy forWISEJ1405+5534(T0pec?)andWISEJ1828+2650(>T2),
Observatories, which are operated by the Association of Universities respectively.
forResearchinAstronomy,Inc.,undercooperativeagreementwiththe Figure2showsthez−Jandz−HcoloursoftheY0dwarfsas
NationalScienceFoundation afunctionofspectraltype(blackdotswitherrorbarsandarrows
2
N.Lodieuetal.:GTCOSIRISz-bandimagingofYdwarfs
Fig.1.FindingchartsforthesixYdwarfsandtheT9+Y0binaryimagedinSloanzwithGTC/OSIRIS.Northisupandeastisleft.
Imagesare1×1arcminTargetsareorderedbyincreasingrightascensionfromlefttorightstartingfromthetopleftcorner.Theopen
circleshavearadiusoffivearcsecandmarktheWISEposition oftheYdwarfs,showingthemotionoftheobjectwhendetected
(smallredcirclewithtwoarcsecradius).
6
0146
5
0146
5
UGPS0722
4
1217 2056
JMKO 0410 HMKO4
− B) 3 11743085 − B) UGPS0722
A A 1217
z( z(
2056
3
2
0410
1405
1738
1 1828 2
1828
T 2 T 4 T 6 T 8 Y 0 T 2 T 4 T 6 T 8 Y 0
Spectral Type Spectral Type
Fig.2. z − J (left) and z− H (right) colours of Y0 dwarfs (filled dots for detections and arrows for lower limits) as a function
of spectral type compared to the coloursof late-T dwarfs from UKIDSS (diamonds;Burninghametal. 2010b), Sloan (asterisks;
Golimowskietal.2004;Knappetal.2004;Chiuetal.2006),Canada-Francebrowndwarfsurvey(squares;Albertetal.2011),and
thesampleofLeggettetal.(2012a,triangles).
for lower limits) along with the colours of T dwarfs. Here we which looks peculiar in all diagrams. This trend seems to start
adoptedaspectraltypeofT9.5+/-0.5forthebinaryT9+Y0.The offaroundaspectraltypeof∼T8andagreeswiththebluerY−J
dispersionobservedforY0dwarfsisgreaterthanforTdwarfs. colours observed with cooler temperatures for the Y class and
WeaddedtothisplotUGPSJ072227.51−054031.2classifiedas theexcessoffluxinthe840–940nmregioninthespectrumof
T10 by Lucasetal. (2010) and Leggettetal. (2012b), but pro- WISE J2056+1459 compared to UGPS J072227.51−054031.2
posedastheT9standardbyCushingetal.(2011). (Leggettetal.2012a).TheT9+Y0binaryfromLiuetal.(2012)
showsanintermediatepositionbetweenlate-TandY0dwarfs.A
Figure 3 displays the (z − H,H − w2) colour-colour dia- possibleexplanationfortherelativechangesinthe z-bandmay
gramforthefourY0dwarfsandtheT9+Y0binary(filleddots) be associated to the disappearance of the strong atomic potas-
detected in z and for the two sources with upper limits (left- siumandsodiumbandsintomolecules(e.g.KCl orNaCl)and
2
pointing arrows). This plot takes into account the improved possiblyotheralkalisaswemovetowardscoolertemperatures.
near-infraredphotometryfromLeggettetal.(2012a),compared
to the original values in Table 2 of Kirkpatricketal. (2012). The spread in the z− J and z− H colours of Y0 dwarfs is
We note a trend towards bluercolours with increasing H −w2 ∼2.5-3.0mag,muchwiderthanthespreadobservedthroughout
fromlate-TdwarfstoY0dwarfs,exceptforWISEJ0146+4234, theTdwarfsequence(Fig.2)andgreaterthanthespreadinthe
3
N.Lodieuetal.:GTCOSIRISz-bandimagingofYdwarfs
tation efficiencies increases the spread in the J − H colours of
9
1828 brown dwarfs with temperatures below 600 K. These sulfide
cloudsmayberesponsibleforthespreadobservedintheoptical-
8
to-infraredcoloursofYdwarfs,suggestingthattheatmosphere
1405 of WISE J0146 may contain thicker clouds and higher gravity
7 thantheotherY0dwarfsinoursample.Table3ofLeggettetal.
(2012a)confirmsthatbrowndwarfswithatemperatureof400K
w2)MKO6 1738 0410 2056 ianfffleucetendcebdybtyhinthicclkoeurdsc.loAusdspowinotuelddobuetfaabinotveer,itnheYsathmaenethffoescet
− (H 1217 could occur in the z band. Moreover, we observe a correlation
5
between the z − H and Y − H colours of the three Y0 dwarfs
UGPS0722
(Fig. 4) commonto oursample andLeggettetal. (2012a). The
4 bluerobjectsinz−H turnouttobealsoratherblueinY−H.If
0146
thistrendholds,wewouldpredictY −H=0.96magforWISE
3 J0146+4234.
Finally, it is important to complete the sample of z-band
2 3 4 5 6 measurementsforYdwarfsinthesouthernhemisphereandob-
z(AB) − HMKO
tain far-red optical spectroscopy, which is only available for
Fig.3.(z−H,H−w2)colour-colourdiagramforourtargets(filled WISEJ2056+1459(Leggettetal.2012a).Thismayshowdras-
dotsandarrowsforlowerlimits),late-Tdwarfs(opentriangles; tic changes as indicated by the z-band photometry, which can
Leggettetal. 2012a),andUGPSJ0722227.51−054031.2(open probechangesintheatmosphericpropertiesresponsibleforthe
trianglewitherrorbars;Lucasetal.2010). far-red spectralenergydistribution of these browndwarfs. Our
photometry would also help the modellers to reproduce this
part of the spectral energy distribution, disentangling the ef-
1.5 fects of clouds,gravity, and pressure-broadenedpotassium and
sodium bands. Thirdly, the relatively blue optical-to-infrared
colours of Y0 dwarfs and the possible turnover towards even
1.0 Known T9
bluer colours with lower temperatures could have a signif-
icant impact on the strategies currently used to identify Y
UGPS0722 dwarfs in large-scale surveys, such as UKIDSS, CFBDS, Pan-
0.5
H)MKO 1405 S(LtaSrSrsT(SDceiaecnocneeCtoalll.a2b0o1ra1t)i,onanedtalal.rg2e00sy9n).opticsurveytelescope
− (Y 1217
0.0
Acknowledgements. NL was funded by the Ramo´n y Cajal fellowship num-
ber08-303-01-02.ThisresearchhasbeensupportedbytheSpanishMinistryof
2056 EconomicsandCompetitivenessundertheprojectsAYA2010-19136,AYA2010-
1828
-0.5 0410 21308-C3-02, AYA2010-21308-C03-03 and AYA2010-20535. We are very
grateful to the GTC operation team and to Antonio Cabrera-Lavers for his
support at all stages of this project. We thank Jose´ Alberto Rubin˜o, Ricardo
1738
Genova Santos, Rafael Barrena, and Angela Hempel forobtaining photomet-
-1.0 ric calibration at the Isaac Newton telescope, and Bartek Gauza, Mar´ıa Rosa
1 2 3 4 5
z(AB) − HMKO ZapateroOsorioandKarlaPen˜aRam´ırezfortheirhelpinobtainingsomeofthe
OSIRIS/GTCimages.
Fig.4.(z−H,Y−H)colour-colourdiagramforourtargets(filled ThisworkisbasedonobservationsmadewiththeGranTelescopioCanarias
dotsandarrowsforlowerlimits),UGPSJ0722227.51−054031.2 (GTC), operated on the island of La Palma in the Spanish Observatorio
del Roque de los Muchachos of the Instituto de Astrof´ısica de Canarias.
(open triangle with the error bars; Lucasetal. 2010), and
This research has been supported by the Spanish Ministry of Economy and
the known T9, CFBDSIR145829.0+101343.0 (open triangle; Competitiveness(MINECO)underthegrantAYA2010-19136.
Delormeetal.2010). ThisresearchhasmadeuseoftheSimbadandVizierdatabases,operated
attheCentredeDonne´esAstronomiquesdeStrasbourg(CDS),andofNASA’s
AstrophysicsDataSystemBibliographicServices(ADS).
J−KcoloursofLdwarfs(Hawleyetal.2002).Thisspreadcan
be qualitativelyexplainedusing recentmodelsby Morleyetal.
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