Discovery of the benchmark metal poor T8 dwarf BD+01 2920B

0.91 MB·English

by  D. J. Pinfield

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed17January2012 (MNLATEXstylefilev2.2)
Discovery of the benchmark metal poor T8 dwarf BD+01
2920B
2 D. J. Pinfield1⋆, B. Burningham1, N. Lodieu2,3, S. K. Leggett4, C. G. Tinney5,
1
0 L. van Spaandonk1, F. Marocco1, R. Smart6, J. Gomes1, L. Smith1, P. W. Lucas1,
2
A. C. Day-Jones7, D. N. Murray1, A. C. Katsiyannis8, S. Catalan1, C. Cardoso6,1,9,
n
a J. R. A. Clarke1, S. Folkes10, M. C. Ga´lvez-Ortiz11, D. Homeier12, J. S. Jenkins7,
J
H. R. A. Jones1, Z. H. Zhang1
6
1 1Centre for Astrophysics Research, Science and Technology Research Institute,University of Hertfordshire, Hatfield AL10 9AB
2Instituto de Astrof´ısica de Canarias, V´ıaL´actea s/n, E-38205 La Laguna, Tenerife,Spain

] 3Departamento de Astrof´ısica, Universidad de La Laguna (ULL), E-38205 La Laguna, Tenerife,Spain
R 4Gemini Observatory,670 N. A’ohoku Place, Hilo, HI 96720, USA
S 5Department of Astrophysics, School of Physics, Universityof New South Wales, NSW 2052, Australia
. 6Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Torino, Strada Osservatrio 20, 10025 Pino Torinese, Italy
h 7Universidad de Chile, Santiago, Casilla 36-D, Chile
p 8Institute of Astronomy & Astrophysics, National Observatory of Athens, Penteli, GR-15236, Greece
- 9Astrophysics Group, College of Engineering, Mathematics, and Physical Sciences, University of Exeter,ExeterEX4 4QL, UK
o
10Departamento de F´ısica y Astronom´ıa, Universidad de Valpara´ıso, Av. Gran Bretan˜a 1111, Casilla 5030, Valpara´ıso, Chile
r
t 11Centro de Astrobiolog´ıa (CSIC-INTA), Ctra. Ajalvirkm 4, E-28850 Torrej´on de Ardoz, Madrid, Spain
s 12CRAL, UMR 5574, CNRS, Universit´e de Lyon, E´cole Normale Sup´erieure de Lyon, 46 All´ee d’Italie, 69364 LyonC´edex 07, France
a
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ABSTRACT
2
We have searchedthe WISE first data release for widely separated (610,000AU)late
3
. T dwarf companions to Hipparcos and Gliese stars.We have discovereda new binary
1
systemcontainingaK-bandsuppressedT8pdwarfWISEPJ1423+0116andthemildly
0 metal poor ([Fe/H]=−0.38±0.06)primary BD+01 2920(Hip 70319),a G1 dwarf at a
12 distanceof17.2pc.ThisnewbenchmarkhasTeff=680±55Kandamassof20−50MJup.
Its spectral properties are well modelled except for known discrepancies in the Y and
:
v K bands.Basedonthewelldeterminedmetallicityofitscompanion,thepropertiesof
Xi BD+012920BimplythatthecurrentlyknownT dwarfsaredominatedbyyounglow-
mass objects. We also present an accurate proper motion for the T8.5 dwarf WISEP
r
a J075003.84+272544.8.
Key words: surveys - stars: low-mass,brown dwarfs
1 INTRODUCTION andevolution(e.g.Bate et al.2002;Goodwin & Whitworth
2007; Stamatellos et al. 2007; Sumi et al. 2011).
Anaccurateunderstandingofthephysicsofultra-coolatmo-
Building on the samples of L (∼2300K–1500K) and
spheres(T <2300K)isamajorandongoingchallengefor
eff T dwarfs (∼1500K–500K) (Kirkpatrick 2005) identified in
theory(e.g.Allard et al.1997).Complexmolecularopacities
theTwo-Micron AllSkySurvey(Skrutskieet al. 2006),the
(e.g. Barber et al. 2006), condensateclouds and theirprop-
DENIS survey (Epchtein et al. 1997) and the Sloan Dig-
erties (e.g. Allard et al. 2001), and non-equilibrium chem-
ital Sky Survey (York et al. 2000), a new generation of
istry (i.e. vertical transport or mixing; Saumon et al. 2007)
infrared surveys is expanding our search-space into much
are significant sources of uncertainty in the models. How-
greater volumes. The UKIRT Infrared Deep Sky Survey
ever, it is crucial to improve ourunderstandingif we are to
(UKIDSS; Lawrence et al. 2007) is sensitive to mid L –
effectively measure the properties of substellar populations
mid T dwarfs out to ∼100pc over ∼15% of the sky. The
(browndwarfsandgiantplanets)andstudytheirformation
VISTA surveys (e.g. the Viking and VHS surveys) will ex-
pand this coverage to ∼50% of the sky in the next few
years.ForthelatestTdwarfs(T8-9;e.g.Warren et al.2007;
⋆ E-mail:D.J.Pinfi[email protected] Burningham et al. 2008; Delorme et al. 2008; Lucas et al.
2 D. J. Pinfield et al.
2010; Burningham et al. 2011c) the sensitivities of these of the WISE position) and those that are not. For 2MASS
surveys are matched by those of the Wide-field Infrared detected objects we required that either H −W2 >2.5 or
Survey Explorer (WISE; Wright et al. 2010), probing to J−W2>3.5soastoremoveLandearlyTdwarfs.Thefull
distances of ∼15–25pc. And for even lower temperatures complementofsearchesandthenumberofsourcesidentified
(T <500K) an increased mid-/near-infrared flux ratio in each is shown in Table 1.
eff
is allowing WISE to uncover the new Y dwarf class in ThesearchrequiringdetectioninonlytheW2-bandwill
the∼300–500K range(Cushing et al.2011).Togetherthese bethemostsensitivetofaintobjectswithredWISEcolours
surveys are characterising a rapidly growing population since the WISE sensitivity limits (all-sky 5-σ Vega limits
in the near- and mid-infrared (e.g. Lodieu et al. 2007; are W1=16.5, W2=15.5, W3=11.2, W4=7.9; Wright et al.
Pinfield et al. 2008; Burningham et al. 2010b; Reyl´e et al. 2010) mean that objects with W1−W2 >2 will generally
2010;Kirkpatrick et al. 2011). onlybedetectedinW2forW2=14.5-15.5(i.e.atleast∼75%
With sensitivity to a growing field L-, T- and Y- of the W2 survey volume). The other multi-band combina-
dwarfsearch-spaceitisbecomingfeasibletosearchformul- tionscoverthefullrangeofdetection/non-detectioncombi-
tiple systems (e.g. Burningham et al. 2009a; Zhang et al. nations that might be expected for T dwarfs.
2010; Burningham et al. 2010a; Day-Joneset al. 2011; For comparison, the recent large-scale WISE search
Murray et al.2011;Burningham et al.2011a;Leggett et al. made by Kirkpatrick et al. (2011) overlaps significantly
2010c)ormovinggroupassociations(e.g.Clarke et al.2010; with our search-space. However, they use a slightly bluer
G´alvez-Ortiz et al. 2010). The physical properties (mass, W1−W2 >1.5 selection, and where we require SNR>10
age and metallicity) of such objects can be constrained in the W2-band they require at least 8 separate detections
through association with more readily characterisable stel- (SNR>3)in theindividualW2 exposures.
larcompanionsormovinggroupmembers,establishingthem
as benchmark objects that can test the theory or more di-
rectly map physical properties onto spectral characteristics
3 IDENTIFYING CANDIDATE BINARY
(e.g.Pinfield et al.2006).Indeed,bysearchingforevenrarer
SYSTEMS
benchmarks with better physical constraints which span a
moreextremerangeofproperties,itwill bepossibletopro- To identify candidate binary systems we cross-matched our
vide the strongest tests for the model atmospheres, a goal candidate late-T sample with a list of potential primary
that absolutely requiressensitivity to large volumes. stars with measured parallax distances, and imposed sepa-
InthispaperwepresentasearchoftheWISEfirstdata rationconstraintsonthepotentialbinarypairingsaswellas
releaseforwidelyseparatedlateTdwarfcompanionstostars absolute magnitude constraints on the candidate T dwarfs
withknownparallaxes.Section2describesourWISEsample (where we assumed a common distance for components).
selection, and Section 3 the method used to identify candi- The list of potential primary stars was made by combin-
datebinaryassociations. Sections4and5presentourspec- ing together the latest Hipparcos catalogue (van Leeuwen
troscopicandadditionalphotometricdata,andinSection6 2007)withthemostrecentversionofthecatalogueofnearby
we derive candidate proper motions. Section 7 statistically stars(Gliese & Jahreiss1991).Hipparcosprovidesastromet-
assesses the expected level of false positives in our search, ric measurements (in position, parallax, and annual proper
and Sections 8, 9 and 10 discuss theproperties and charac- motion) with uncertainties in the range 0.7-0.9 milliarcsec
teristics of a newly discovered benchmark system. Conclu- (mas) for stars brighter than V=9. The catalogue is on the
sions and futurework are in Section 11. ICRS reference system and has proper motions consistent
with an inertial system at the level of ±0.25mas/yr. The
Third Catalogue of NearbyStars (CNS3) containsinforma-
tion on all known stars within 25 parsecs based on an ex-
2 WISE SAMPLE
tensiveliterature search duringalmost four decades.
Weidentifiedcandidatemid-Tandlatertypeobjectsinthe We required on-sky angular separation of candidate
WISEPreliminaryDataReleasesourcecatalogue,whichwe pairs to be 6300 arcseconds to reduce contamination from
accessed via the NASA/IPAC Infrared Science Archive’s random alignments, and physical separation of 610,000AU
catalogue query engine. We performed a series of all-sky (whereweusethedistanceoftheprimarytoconvertangular
searches using structured query language input to select separationintoaphysicalline-of-sightseparation),sincethe
sources with constraints on signal-to-noise and colour, and great majority of known wide ultracool stellar companions
with detections in various combinations of bands chosen to have separations below this limit (e.g. Zhang et al. 2010;
optimise sensitivity to late T dwarfs. We always required Faherty et al. 2010).Inaddition weusetheknowndistance
a detection in the W2-band with signal-to-noise (SNR) of of each candidate primary to estimate the MW2 that the
at least 10. If W1- and W2-band detections are available T dwarf candidate would have at this distance, and reject
then we require W1 − W2 >2.0 to select spectral types any associations where the T dwarf candidate would have
of >T5 (Kirkpatrick et al. 2011; Mainzer et al. 2011). If MW2 611.5(i.e.targetinglateTdwarfcompanions,e.g.Fig
W2- and W3-banddetections are available then we require 4 of Leggett et al. 2010a, where M is a good proxy for
[4.5]
W2− W3 62.5 in order to avoid dusty galaxies such as MW2). The candidate T dwarf components of the possible
ULIRGS,LINERSandobscuredAGN(Wright et al.2010). binary systems are distributed within our WISE source se-
As well as our WISE-band detection requirements we also lectionsassummarisedinTable1.TheseTdwarfcandidates
made use of the WISE catalogue cross-match with the werevisuallyinspected usingtheWISEimage serverat the
2MASS point source catalogue, to divide our searches into NASA/IPAC Infrared Science Archive, and candidates re-
objects that are detected in 2MASS (within 3 arcseconds jected if the source did not appear point-like in any of the
The benchmark metal poor T8 dwarf BD+01 2920B 3
WISE detection? Colours Selected Candidate Selected Candidate
W1 W2a W3 W4 sourcesd companionsd,e sourcesf companionsf
nb Yc n n - 2418 12(3) 289 0
Y Y n n W1−W2>2.0 5622 35(1) 1330 0
Y Y Y n W1−W2>2.0 1721 9(1) 283 0
W2−W362.5
Y Y Y Y W1−W2>2.0 1018 2 1642 0
W2−W362.5
n Y Y n W2−W362.5 174 0 54 0
n Y Y Y W2−W362.5 272 0 54 0
a W2signal-to-noisealways>10(w2snr>=10).
b Non-detections definedas(w⋆mproisnullorw⋆sigmproisnull).
c Detections definedas(w⋆mproisnotnullandw⋆sigmproisnotnull).
d 2MASSnondetections (tmass keyisnull).
e Thenumbersinbrackets areforcandidates thatpassedvisualinspection.
f 2MASSdetections withH−W2>2.5orJ−W2>3.5.
Table1.WISElateTcandidatesample.Twelveseparateselectionsweremade,sixrequiringnon-detectionin2MASSandsixrequiring
2MASSdetectionwithred2MASS-WISEcolour.ThenumberofcandidateTdwarfs,andthosethatbecamewidecompanioncandidates
(seeSection3)areindicatedforeachsearch,wherevariouscombinationsofdetectionandnon-detectionwereexploredinthefourWISE
bands.
bands,formedpartofablendedstructure,orwasclearlyan 4 SPECTROSCOPY
artefact (e.g. part of a diffraction spike).
Fivecandidatebinarysystemspassedvisualinspection.
Near-infrared spectroscopy of WISEP J1423+0116
One W2-only detected candidate remains an unconfirmed
(BD+012920B; see Section 7) was obtained using the
interesting candidate without any additional survey data
Gemini Near InfraRed Spectrograph (GNIRS; Elias et al.
(e.g.UKIDSS,VISTA)tofacilitatepropermotionmeasure-
2006) mounted on the Gemini-North telescope on the
ments. The other4 are listed below: night of 16th May 2011. The target was observed in
cross-dispersed mode capturing the full 0.8–2.5µm region
• WISEP J075003.84+272544.8 is a W2-only detected witha1.0′′ slit deliveringaresolvingpowerofR∼500.The
candidate 265 arcseconds from the star Hip 38228, a G5IV
data were reduced using GNIRS routines in the Gemini
star at 22pc. This candidate is a known T8.5 dwarf discov- IRAF package (Cooke & Rodgers 2005), using the nearby
ered (with WISE) by Kirkpatrick et al. (2011), though we
F5V star Hip 63976 for telluric correction. The telluric
subsequentlyshow(Section6)thatitisnotacompanionto
standard spectrum was divided by a black-body spectrum
Hip 38228.
of an appropriate T after removing hydrogen lines by
• WISEPJ142320.86+011638.1 (WISEPJ1423+0116) is interpolating the loceaffl continuum. The rectified standard
aW2-onlydetectedcandidate153arcseconds from thestar spectrum was then used to correct for telluric absorption
Hip 70319 (BD+012920), a G1V star at 17.2pc. Itwas not andtoproviderelativefluxcalibration. Theoverlapregions
identifiedbyKirkpatrick et al. (2011) because it isonly de- between the orders in the Y−,J− and H−bands agreed
tectedin7separateW2exposuresintheWISEPreliminary well suggesting that the relative flux of the orders is well
DataRelease.ThisTdwarfisthemainsubjectofthispaper. calibrated. The resulting YJHK spectra are shown in
• WISEP J145715.85-212207.6 is a W1+W2+W3 de- Figure 2.
tected candidate near the system Gl 570ABC (Hip 73182
and Hip 73184), a K4V+M1.5V+M3V triple system. This InFigure2wecompareourGNIRSspectrumofthenew
candidate is a known (discovered in 2MASS) T8 member T8withthatoftheT8spectraltemplate2MASSJ04151954-
of the multiple system (Burgasser et al. 2000). The WISE 0935066 from Burgasser et al. (2006b). The close similarity
cataloguedoesnotlistthesourceasa2MASSdetectionbe- of the spectra over most of the wavelength range argues
causeitshighpropermotionleadstotheWISEand2MASS strongly for T8 classification, which is reflected in the val-
positions being separated by more than 3 arcseconds. ues found for the spectral typing flux ratios (see Table 2).
• WISEP J150457.58+053800.1 is a W1+W2 detected AlthoughthenewT8closelytracestheT8templateoverthe
candidate 63 arcseconds from Hip 73786 (GJ 576), a K8V 0.9−1.9µmrange,itdisplaysaconsiderablymoredepressed
star at 18.6pc. This candidate is a known (discovered in K-band flux, which is interpreted as due to strong colli-
UKIDSS)T6pcompaniontothissomewhatmetalpoorstar sionally inducedabsorption byH (CIAH ; Saumon et al.
2 2
(Murray et al. 2011; Scholz 2010). 1994). Increased CIA H is typically attributed to higher
2
pressure atmospheres arising from lower-metallicity and/or
Figure 1 shows J- (VISTA) and W2-band images for high-gravity(e.g. Burgasser et al.2002;Knapp et al.2004;
WISEP J1423+0116, and indicates its separation from the Liu et al. 2007). For this reason we assign the type T8p to
nearby high propermotion star Hip 70319 (BD+01 2920). WISEPJ1423+0116, wherethe’p’suffixdenotesitispecu-
4 D. J. Pinfield et al.
Figure 1.J-bandandW2-bandimagesofWISEP J1423+0116.
Figure 2. GNIRS spectra for WISEP J1423+0116 (BD+012920B; see Section 7) compared to T8 spectral type template
2MASSJ04151954-0935066 takenfromBurgasseretal.(2006b).Theerrorspectrumisshownoffsetby−0.1.
liar,alludingtothepoormatchwiththetemplateintheK Near-infraredphotometrywasmeasuredusingtheNear
band. Infrared Camera Spectrometer (nics; Baffa et al. 2001) at
the 3.58-m optical/infrared tng located on La Palma, on
the night of the 7th of May 2011 for the H band and the
5 NEW PHOTOMETRY night of the 10th of June 2011 for the Y band. The data
were obtained in large field mode, with apixelscale of 0.25
Database photometry of WISEP J1423+0116 was obtained arcsec/pixels and a field of view of 4.2×4.2 arcmins. The
from the WISE Preliminary Data Release catalogue, the data were processed using the nics science pipeline snap
WFCAMScienceArchive(UKIDSSLargeAreaSurvey)and provided by the tng. H-band observations consisted of a
the VISTA Science Archive (VIKING proprietary data ac-
50 point jitter pattern with individual 10s exposures and 6
cess).Inaddition,observationsweretakenattheTelescopio
co-adds per jitter point, accumulating to a total exposure
Nazionale Galileo (tng) and with the Spitzer Space Tele-
timeof50minutes.IntheY-banda10pointjitterwasused
scope in its warm phase.
The benchmark metal poor T8 dwarf BD+01 2920B 5
2011). There are multiple measurements of Y-, J- and H-
band photometry, though no evidence of variability is seen
Index Ratio Value Type
(towithin theuncertainties) in thephotometric brightness.
1.165
R f(λ)dλ
H2O-J 11..12485 0.050±0.003 >T8
R f(λ)dλ
1.26
1.34
R f(λ)dλ 6 PROPER MOTIONS
CH4-J 11..321855 0.23±0.01 >T8
R f(λ)dλ
1.26 WemeasuredthepropermotionofWISEPJ1423+0116 us-
R1.23f(λ)dλ ingaVISTAVIKINGimageofApril2010andtwoUKIDSS
WJ 11.1.2885 0.32±0.01 T8 images from May 2008 (with lower signal-to-noise of ∼6).
2R f(λ)dλ
1.26 This avoids using the larger point-spread-functions inher-
R1.52f(λ)dλ ent in the WISE images (∼6.5 arcsecs in W2). The base-
H2O-H 11..4680 0.21±0.01 T7/8 line between the two near infrared epochs was 1.89 years.
R f(λ)dλ
1.56 We took the measured x,y coordinates from the standard
1.675
R f(λ)dλ CASUpipelinereductionsofallimagesandusing59objects
CH4-H R1.16.3650f(λ)dλ 0.15±0.01 T7/8 within4arcminutesofthetarget,transformedtheUKIDSS
1.56
framesontothestandardcoordinatesystemoftheVIKING
1.56
R f(λ)dλ frame using a simple linear model. The relative proper mo-
NH3-H R11..5630f(λ)dλ 0.68±0.01 ... tion for all objects were found from linear fits to the stan-
1.57
dard coordinates at the different epochs. A correction to
2.255
CH4-K R2.22.1152 f(λ)dλ 0.13±0.01 T6/7 an absolute system was estimated from the median differ-
R f(λ)dλ encebetweenmeasuredrelativepropermotionsand6SDSS
2.08
objects in the field with proper motions in the catalog of
Table 2. The spectral flux ratios for WISEP J1423+0116 Munn et al. (2004). The derived proper motion for WISEP
(BD+012920B; see Section 7). The locations of the numerators J1423+0116wascorrectedforanassumedparallaxof50mas
anddenominators areindicatedonFigure2. (see Section 7), and final uncertainties are based on the
formal uncertainties of the measured coordinates combined
withanadditionalallowanceforthecentroidingaccuracyin
forthesameexposuretimeandco-adds,resultinginatotal thelowsignal-to-noiseLASimage(∼±0.5pixelsestimated
exposuretimeof10minutes.Wecalibratedeachimageonto using Monte Carlo techniques) leading to a proper motion
themko system using ∼30 field stars in the frame. uncertainty of ±50 mas/yr. The proper motion of WISEP
Warm-Spitzer photometric data were obtained for J1423+0116 isµαcosδ =261±56masyr−1,µδ =−444±52
WISEP J1423+0116 on the 21st August 2011, via the Cy- masyr−1,whichiswithin0.7σ oftheHipparcospropermo-
cle 7 GO program 70058. Individual frame times were 30 s tionvectorofHip70319(BD+012920;µαcosδ =223.8±0.4
repeated six times, with a16-position spiral ditherpattern, mas yr−1, µδ =−477.4±0.4 mas yr−1). These objects are
foratotalintegrationtimeof48minineachofthe[3.6]and thusa common proper motion pair.
[4.5] bands. The post-basic-calibrated-data mosaics gener- We also measured the proper motion of WISEP
ated by version 18.18.0 of the Spitzer pipeline were used to J075003.84+272544.8 using two UKIDSS LAS J-band
obtain aperture photometry. The photometry was derived epochs with a baseline of 2 years. We applied a second or-
using a 7-arcsec aperture and the aperture correction was der polynomial transformation between the two epoch im-
taken from the IRAC handbook. The error is estimated by ages to correct for any non-uniformity in the focal plane.
the larger of either the variation with the sky aperture or Seventeen reference stars (J <18.1) were used, distributed
theerror implied bythe uncertaintyimages. aroundthetarget (with at least 3perquadrant)with sepa-
Tables 3 and 4 contain the available photometry and rationswithin2arcminutes.Acorrection wasapplied toan
colours,respectively,fortheTdwarf.WepresentWISEpho- absolute system using apparent proper motions of nearby
tometry where the signal-to-noise is positive and note that galaxies. The uncertainties were calculated using the stan-
theW1andW3magnitudesarebrightnessupperlimits.The dard deviations in theRA/Decresiduals of sources deemed
nearinfraredphotometryisontheMauna-KeaObservatory tohaveinsignificantmotion(<45milli-arcseconds)between
system (Leggett et al. 2006) except for the TNG Y-filter, epochs.ThepropermotionofWISEPJ075003.84+272544.8
whichisslightlydifferent(λc=1.02µm,FWHM=0.13µm)to is µαcosδ =−732±17mas yr−1, µδ =−194±17mas yr−1.
the MKO Y filter (λc=1.02µm, FWHM=0.10µm). In the By comparison, Kirkpatrick et al. (2011) used astrometric
absence of a measured K-band magnitude we have deter- fitstomultipleWISEobservationstoderiveapropermotion
mined a synthetic J −K colour using our GNIRS spectra (µ =−869±424masyr−1,µ =−1107±438masyr−1)
αcosδ δ
andaspectrumofVega(Bohlin & Gilliland2004)bothcon- withmuchlargeruncertainties.Theirvalueofµ iscon-
αcosδ
volved with the response functions for the pass-bands (e.g. sistent with our new measurement, however their value of
Hewett et al.2006).Thissyntheticcolour(seeTable4)com- µ istoolargeatthelevelof∼2σ.Althoughtherealsohap-
δ
binedwiththeJ-bandmagnitudeproducedourK bandes- penstobeanearbyHipparcosstar(Hip38228),ithasalow
timate.Forthemidinfraredphotometrywenotethatwhile proper motion (µ =−8mas yr−1, µ =−10mas yr−1)
αcosδ δ
similar to W1 and W2, the Spitzer [3.6] and [4.5] bands andtheTdwarfisnotacommonpropermotioncompanion
havesomesignificantdifferences(seeFig2ofMainzer et al. since its motion differs at a level >28σ.
6 D. J. Pinfield et al.
Source Ya J H K W1(snr)or[3.6] W2(snr)or[4.5] W3(snr) W4
WISE 17.75(1.5)b 14.76±0.09(11.8) 12.21(0.8)b -
UKIDSSLAS 19.51±0.14 18.76±0.12
VISTAVIKING 19.69±0.05 18.71±0.05
TNG 19.75±0.22 19.14±0.20
SyntheticEstimate (18.96±0.15) (19.89±0.33)
Spitzer 16.77±0.03 14.71±0.01
a PhotometryisontheMKOsystemexceptfortheTNGY filter(seetext).
b 95%confidence brightnessupperlimit.
Table 3.Photometricmagnitudes ofWISEP J1423+0116.
Y −J J−H H−K J−K W1−W2 W2−W3 J−W2 H−W2 [3.6]−[4.5] H−[4.5]
0.98±0.07 -0.38±0.23 -0.93±0.36a -1.27±0.34a >2.77b 62.55b 3.95±0.10c 4.38±0.22 2.06±0.03 4.43±0.20
a Synthetic photometry(seetext).
b 95%confidence limit.
c Usingthehighersignal-to-noiseVISTAJ-bandmagnitude.
Table 4.PhotometriccoloursofWISEP J1423+0116.
7 CONFIRMING BINARITY We must also assess these common proper motion sys-
temsforthechancethatcommonpropermotionsarealigned
To estimate the probability that WISEP J1423+0116 and
by random chance. Using the proper motion and direc-
BD+01 2920 may be a line-of-sight association as opposed
tion of WISEP J1423+0116 we estimated this probabil-
toagenuinebinary,wehaveperformedastatisticalanalysis
ity using a Hipparcos sample, downloading the proper mo-
toestimatetheexpectednumberofchancealignmentsinour
tions of Hipparcos stars within 45 degrees of the WISEP
search. We used the Burningham et al. (2010b) luminosity
J1423+0116/BD+01 2920 pair, and with distances from
function constraints toestimate thenumberof T6-9 dwarfs 10–40pc (the photometric distance range of a T8±1 dwarf
expected in the WISE first data release. In a sample with withJ ≃18.7allowing forpossible unresolvedbinarity).We
W2<15(akintoourW2signal-to-noiserequirement)weex-
counted the fraction of stars with proper motion within
pecttodetectT7±1andT9dwarfsouttodistances(Dmax) 55mas yr−1 (1σ) of the T dwarf motion, and thus esti-
of ∼25 and ∼15pc respectively, in the 57% sky coverage of
mate a chance of 1.3% that this high proper motion pair
the WISE first release. We adjusted the Burningham lumi-
could be common proper motion by random chance. We
nosity function to add back in the correction made for un-
therefore expect no more than 0.0015–0.014 false positive
resolvedbinarity(3-45%),sincethisremovedTdwarfsfrom
common proper motion systems in the search that we have
their magnitude limited samples, and estimate an expected
made, and conclude that all three of the common proper
28-251 T7±1 dwarfs and 26-111 T9dwarfs in theWISEse-
motionsystemsthatweidentifiedaregenuinebinaries.This
lectionusingthisluminosityfunction.Wethensummedthe
includesthetwopreviouslyreportedsystemsandtheassoci-
volumeinwhich line-of-sight associated starsmaybefound
ationbetweenWISEPJ1423+0116andBD+012920,which
using a set of cones (one per T dwarf) each with its apex
becomes the binary system BD+01 2920AB.
at theobserverand aTdwarf in thecentreofit’s base(us-
ingabaseradiusof10,000AUtomatchoursearchcriteria).
Since the number of T dwarfs is proportional to D3 and
the volume of a cone is proportional to D (where D is dis-
8 PROPERTIES OF BD+01 2920A
tance), the average cone volume will be 3 of the maximum
4
cone volume 31ADmax (where A is the base area of a cone Asearchoftheliteraturerevealsmultiplestudiesofthepri-
π×10,000AU2).ThetotalconevolumeforT6–9dwarfswas marystarBD+012920A.Itisanearbyhighpropermotion
thusestimated to be 2.0–14.6pc3. G1 dwarf (0.9M⊙) with thin disk kinematics. There is no
TheluminosityfunctionofReid et al.(2007)forthe8pc evidenceofanydebrisdiskorlow-masscompanions(includ-
and 20pc samples leads to a stellar density of 0.062–0.076 inggiant planets),and it has low activity.BD+012920A is
stars pc−3, and we thus expect 0.12–1.11 light-of-sight as- a mildly metal poor star, with a metallicity in the metal
sociations between stars and T dwarfs in our WISE selec- poor tail of the disk distribution rather than in the halo
tion. Amongst our fivecandidates we findthat one of them regime.Withoneexceptionpreviousestimatesof[Fe/H]are
(WISEPJ075003.84+272544.8)isinfactaline-of-sightasso- in the range -0.38±0.06 (only Lebreton et al. 1999, gives a
ciationwithalackofcommonpropermotion.Thisisconsis- slightly higher metallicity of -0.20 dex). The range of age
tentwithourestimatesabove.Anadditionalcandidatewas constraints covers 2.3–14.4 Gyr. This differs slightly from
identified without proper motion, though the above statis- the range quoted by Lawler et al. (2009) who give a lower
ticdoesnotprovideanyfurtherindicationsonthelikelihood limitof3.5Gyr.Thedifferenceisduetotheestimateof2.27
that this candidate may be genuine. Gyr from doNascimento et al. (2010), and we here adopt
The benchmark metal poor T8 dwarf BD+01 2920B 7
BD+012920A(Hip70319)
R.A.(J2000) 142315.285
Dec(J2000) +011429.65
PMαcosDec 223.8±0.4mas/yr
PMDec −477.4±0.4mas/yr
Spectraltype/class G1V
π 58.2±0.5mas(1)
Distance 17.2±0.2pc
m−M 1.18±0.03
Vr 19.6±0.3km/s(2−4)
Spacemotion UVW=22,15,39(5−8)
Population Thindisk(9,10)
Teff 5750±100K(3,7,8,11−21)
logg 4.45±0.05dex(3,7,12,16−18,20−24)
Mass 0.87±0.07M⊙ (3,13,23)
[Fe/H] -0.38±0.06dex(3,6−8,10,12,14−18,20−29)
Age 2.3–14.4Gyr(3,5,8,10,12,13,15,23,26,30−32)
vsini 1–2km/s(3,12,13)
Activity Lowactivitystar(33)
Debrisdisk None(15)
Closeincompanions No>70-75MJup at20-250AU(34)
Nogiantplanets
(>100m/s)at<5AU(24,35−39)
1 vanLeeuwen(2007),2 Gontcharov(2006),3 Valenti&Fischer(2005)
4 Latham etal.(2002),5 Holmbergetal.(2009),6 Ram´ırezetal.(2007)
7 Misheninaetal.(2004),8 Nordstr¨ometal.(2004),9 Borkova&Marsakov(2004)
10 Ibukiyama&Arimoto(2002),11 Casagrandeetal.(2010),12 Takedaetal.(2010)
13 doNascimentoetal.(2010),14 Holmbergetal.(2007),15 Lawleretal.(2009)
16 Luck&Heiter(2006),17 Shietal.(2004),18 Mashonkinaetal.(2007)
19 Kovtyukh etal.(2003),20 Giridhar&Goswami(2002),21 CayreldeStrobeletal.(2001)
22 Wuetal.(2011),23 Takedaetal.(2007b),24 Heiter&Luck(2003)
25 Mashonkina&Gehren(2001),26 Rocha-Pinto&Maciel(1998),27 Karata¸s etal.(2005)
28 Borkova&Marsakov(2005),29 Haywood(2001),30 Wrightetal.(2004)
31 Barry(1988),32 Takedaetal.(2007a),33 Halletal.(2007)34 Carsonetal.(2009)
35 Halbwachs etal.(2003),36 Halbwachsetal.(2000),37 Cummingetal.(1999)
38 Endletal.(2002),39 Nideveretal.(2002)
Table 5.PropertiesofBD+012920A (Hip70319).
aninclusiveagerange.ThepropertiesofBD+012920A are 4.50−5.5;T =500−900K).Wetakethescatterinthees-
eff
summarised in Table 5 and references therein. timatesresultingfromdifferentmodelchoicesasareflection
ofthesystematicuncertaintyintroducedbytheatmospheric
models. We have used a Monte Carlo method to determine
9 PROPERTIES OF BD+01 2920B the uncertainty in each estimate due to the noise in the
photometryusedforscalingthemodelsandthenoiseinour
9.1 Bolometric flux GNIRS spectrum. Our final estimate of F is the median
bol
ofourestimatesusingdifferentmodelextensions,whilstthe
Weestimatethebolometricflux(F )ofthenewT8pcom-
bol
uncertainty is the sum in quadrature of the systematic un-
panion BD+01 2920B following a similar method to that
certainty andthemean random uncertainty.This resultsin
outlined in Burningham et al. (2009a), by combining our
a determination of F =1.61±0.17×10−16 Wm−2.
YJHK spectrum(fluxcalibratedbyourfollow-upphotom- bol
etry) with model spectra (to allow us to estimate the flux
contributions from regions outside our near-infrared spec-
9.2 Luminosity, mass, radius, and effective
tral coverage). We have scaled the λ < 1.05µm region of
temperature
themodelstomatchthefluxlevelinourYJHK spectrum,
and we have used the Spitzer 3.6µm and 4.5µm photome- TheluminosityofBD+012920Bwasderivedfromthebolo-
try to scale the λ = 2.5−3.95µm and λ > 3.95µm regions metric flux and the distance. The on-sky separation of the
respectively (the transmission profiles of the Spitzer filters BD+012920ABcomponentsleadstoatangentialseparation
cross at 3.95 µm at a transmission level of ∼1%). To avoid of0.01 pc.Thisisnegligiblecompared withtheuncertainty
biasing our derived flux estimate with our choice of model, intheparallaxdistanceoftheprimary(±0.2pc)andwecan
we have produced estimates using Solar and [M/H]= −0.3 thereforeassumethattheTdwarfisatthesamedistanceas
metallicityBTSettlmodels(Allard et al.2011)thatbracket BD+01 2920A (17.2±0.2 pc). Taking the uncertainties as-
the likely range of gravities and T for our target (logg = sociated with thebolometric fluxanddistanceintoaccount
eff
8 D. J. Pinfield et al.
higher-metallicity(higher-cloud-thickness)atmospheresgiv-
BD+012920B(WISEP 1423+0116)
ing larger radii. However, a comparison between KOI-423b
R.A.(J2000) 142320.86 andCoRoT-3bsuggeststheconversetrend.KOI-423borbits
Dec(J2000) +011638.1 a metal poor star ([Fe/H]=−0.29±0.10) and is a relatively
PPMMαDceocsDec 2−6414±4±5652mmasaysry−r1−1 ClaorgReoT(1-3.22is+−00a..1120sRolJaurp)mbetraolwlincitydwsatrafr ((1[F8e/MHJ]=up-)0,.02w±h0er.0e6a)s
SSpepeactrraatliotnype 1T583parcsecs hosting a smaller (1.01±0.07RJup) brown dwarf (22MJup).
Giventhisuncertaintyintheradius-metallicitytrend,wedo
2630AUa
Fbol (1.61±0.17)×10−16 Wm−2 b not attempt to make a metallicity correction to our radius
m−M 1.18±0.02a estimate.WenotehoweverthatourCONDradiusconstraint
MY 18.51±0.04a already includesan uncertaintyat thelevelof 25%, compa-
MJ 17.53±0.05a rablewiththesizeofthetheoreticaltrendssuggestedbythe
MH 17.96±0.20a Burrows et al. (2011) models for a metallicity difference of
MK 18.71±0.33a,c 1.0 dex.
MM43..56 1135..5539±±00..0024aa natioAnsraelnieasdodnitaionnaaslsucamvpeatitownethnaottethtehoabtjoeuctriTsesffindgeltee,ramnid-
logL/L⊙ −5.83±0.05a notanunresolvedbinary.Unresolvedbinaritywouldleadto
[Fe/H] −0.38±0.06dexd
lower T for each unresolved component. If BD+01 2920B
Mass 0.019–0.047M⊙ e eff
20-50MJup e is an equal luminosity unresolved binary th1e Teff of each
Radius 0.080–0.099R⊙ e component would be a factor ∼0.8 less (14 with similar
2
0.80–0.99RJup e radii for the components). For unequal luminosity com-
logg 4.68–5.30dexe ponents the brighter component T would be closer to
eff
Teff 680±55Kf 680Kwiththefainterone<540K.Observationssuggest(e.g.
a Inferringadistanceof17.2±0.2pcfromBD+012920A. Burgasser et al. 2005) that the binary fraction of brown
b Integratingmeasuredfluxfrom1.0−2.4µmandaddinga dwarfs (resolved at ∼0.1 arcsec resolution) in widely sep-
theoretical correctionatother wavelengths (seetext). arated stellar – brown dwarf multiples is notably higher
c Synthetic photometryused(seetext). (45±15 per cent) than that of field brown dwarfs (18±7
d InferredfromBD+012920A. per cent), and unresolved binaries can also have separation
e Constraintsderivedfromstructuremodelsasafunctionof closer than 0.1 arcseconds (see Burningham et al. 2009a,
luminosityforages210Gyr.
andreferences therein).InFigure3weshowBD+012920B
f Derivedfromtheluminosityandradiusconstraints.
in absolute magnitude (MJ,H,K) spectral type diagrams,
along with the known population of late T and Y dwarfs
Table 6.PropertiesofBD+012920B (WISEP 1423+0116).
(seecaption).TheK-bandsuppressionisevidentintheMK
plot, though we also note that there is no clear indication
that the object is an unresolved binary (e.g. with compo-
leads to the determination of Lbol =5.69±0.60×10−20W, nents of near equal brightness) in the MJ and MH plots.
or logL/L⊙ = −5.83 ± 0.05. To determine the Teff of We cannot with high confidence however, rule out the pos-
BD+01 2920B we estimate itsradius using theCOND evo- sibility that BD+01 2920B is an unresolved binary.
lutionary models (Baraffe et al. 2003). These models repro-
duce the main trends of observed methane dwarfs in near-
infrared color-magnitude diagrams, though are only avail-
10 A METAL POOR BENCHMARK T8
able for solar metallicity.
DWARF
We used linear interpolation between the model
isochrones to estimate a range of possible mass, radii and Wenow assess some implications of thisbenchmarksystem
surface gravity for BD+01 2920B consistent with an age undertheassumptionthatitisasingleobject,andthrough
rangeof∼2–10Gyr.Accountingfortheuncertaintiesinthe comparison of its spectrum and colours to theoretical pre-
measured luminosity we obtained a mass range of 0.019– dictions and other ultra-cool objects. In Figure 4 we com-
0.047 M⊙ (20−50 MJup), a radius range 0.080−0.099 R⊙ pareour fluxcalibrated GNIRSspectrum of BD+01 2920B
(0.80–0.99 RJup), and a surface gravity range of logg = and warm-Spitzer photometry to mildly metal poor BT
4.68−5.30. Thecorresponding temperature(from luminos- Settl models (Allard et al. 2010) for our derived proper-
ityandradius)isT =680±55K.Asummaryoftheprop- ties,eachscaledtotheircorrespondingradiiandtheknown
eff
erties of BD+01 2920B is given in Table 6. distance to the primary star. The BT Settl atmospheric
Observations of transiting very low-mass stars and model grid spans the cool stellar to substellar temperature
brown dwarfs with mass>20 MJup (Pont et al. 2005b,a; regime using the BT2 water line-list (Barber et al. 2006)
Deleuil et al. 2008; Bouchy et al. 2011b; Anderson et al. andthereferenceSolarabundancesofAsplund et al.(2009).
2011;Johnson et al.2011)areallconsistentwiththeCOND In these models dust formation, cloud behaviour and verti-
mass-radius model data (see fig 10 of Bouchy et al. 2011a). cal mixing are parameterised with reference to the 2D ra-
Thesesystemsaresolar metallicity towithintheuncertain- diation hydrodynamic simulations of Freytag et al. (2010).
ties,though someoftheseuncertaintiesaresignificant.The The K−band suppression that is predicted by the models
effectsof metallicity on sub-stellar radiiare alittle unclear. for high-gravity and metal poorer brown dwarfs is seen in
Burrows et al. (2011) present evolutionary models with a our GNIRS spectrum of this benchmark T dwarf, although
spreadinradius(atagivenmassandage)of∼10–25%,with themodels predict this effect to be stronger than is seen in
The benchmark metal poor T8 dwarf BD+01 2920B 9
Figure 4. The YJHK spectrum of BD+01 2920B and mean
fluxes inferred from the Spitzer photometry compared to model
spectrathatstraddlethepropertiesestimatedinSection9.2.The
straightcolouredlinesindicatethemeanfluxes ofmodelspectra
intheIRACchannel 1and2photometricbandsandareplotted
toallowcomparisonwiththemeanfluxfromthetarget(straight
blacklines)throughthesamefilters.
H − K and H − [4.5] for the BT Settl models along
with colours of late T dwarfs from Leggett et al. (2010b)
with MKO and IRAC photometry, and BD+01 2920B.
It can be seen that BD+01 2920B lies in a similar re-
gion as other suspected mildly metal-poor T7.5/8 dwarfs
2MASS0939 and SDSS1416B, which have Teff 500-700K
(e.g.Burningham et al.2010a;Leggett et al.2010a).Theef-
fect of the poor match between the models and the data in
Figure 3. Absolute magnitude (MJ,H,K) spectral type plots the K−band is highlighted by the non-coincidence of the
showingBD+012920B(redsymbol)amongsttheexistingpopu- models and the observations in this colour space for the T
lationofmid-lateTdwarfsandYdwarfs.AlltheTdwarfswith dwarfswiththereddestH−[4.5]colours.However,themod-
spectral types 6T8.5 (Burninghametal. 2008) have distances
els correctly predict the colours for the young benchmark
fromparallax.TheT9.5andYdwarfsallhavespectroscopicdis-
Ross 458C. To provide an alternative comparison between
tances (except for WISEP J1541-2250 which has a parallax in
the models and the data we have shifted the model colours
Kirkpatricketal.2011), estimated via Teff and logg constraints
frommodelfitstonear-infraredspectroscopy. suchthattheymatchtheobservedcoloursforBD+012920B
for the parameters derived in Section 9. Figure 6 com-
pares these adjusted model colours to the same T dwarfs
this case. Similarly poor matches to the observed K−band shown in Figure 5. This plot broadly reproduces the result
spectroscopy have been seen in other benchmark systems of Leggett et al. (2010b), where it was noted that the ma-
(e.g.Burningham et al.2009b,2011b),soitisreasonableto jority of the coolest T dwarfs appear to have low-gravity
interpret this as a deficiency in the model atmospheres, al- and/orhigh-metallicity,suggestingthatthesampleisdomi-
thoughitsoriginisnotyetunderstood.Itisnoteworthythat natedbyyounglow-massbrowndwarfs(ages∼1Gyr).How-
the Y−band spectrum which has also been proposed as di- ever, we note that the sources with bluer H−[4.5] lie well
agnostic of metallicity variations (Burgasser et al.2006a) is below the adjusted model tracks in Figure 6, including the
alsopoorlymatchedbythemodels.Themodelatmospheres young benchmark Ross 458C (for which logg = 4.0−4.7;
provide good matches to the flux in theJ− and H−bands, Burningham et al. 2011a), which highlights that a simple
despiteknowndeficiencies in themethanelinelists in these offset correction to the models is not sufficient to allow
regions. The model predictions for the [3.6] and [4.5] fluxes the properties of the T dwarf population to be reliably as-
areconsistentwiththosethatwehaveobserved,anditinter- sessedthroughreferencetothesemodelcolours.Thesimilar
esting to note that in the [3.6] band, the 620K, logg =4.7, temperature of these two benchmarks (T ∼ 700K), but
eff
model fits the data best, whilst in the [4.5] band the two wideseparationinFigures5and6highlightstheimportant
warmer models providethe best match. roles that gravity and metallicity play in determining the
In Figure 5 we have plotted synthetic colours in H−K/H−[4.5] colours for cool T dwarfs.
10 D. J. Pinfield et al.
tiveoptics) observations ofBD+012920B will beimportant
to constrain multiplicity on a ∼0.1 arcsec (∼1.7AU) sepa-
ration scale. Aclose binary would beabletoprovidefuture
dynamical masses (since the orbital period would be just
a few years). Higher resolution spectroscopy would also be
able to assess multiplicity at closer separation, and confir-
mationofasingleobjectnaturewouldvalidatetheapproach
taken here to determination the physical properties of this
benchmark object. The existing constraints on the physical
properties of BD+012920B will be improved as we develop
a better understanding of how brown dwarf radii depend
oncomposition.Thiswillbeaidedbyanincreasingnumber
of transiting brown dwarfs from Kepler and other transit
surveys (e.g. Borucki et al. 2011; Pinfield et al. 2005), and
improved metallicity measurements for this sample.
Wecan expect additional late Tbenchmarksin thefu-
Figure 5. Near- to mid-infrared colours of cool T dwarfs com-
pared to those of the BT Settl model colours. BD+01 2920B is ture all-sky WISE data release, and a more encompassing
indicated with a cyan star symbol, whilst the young benchmark search of WISE, UKIDSS and VISTA (including at wider
Ross458C isindicatedinred. angular separations) should yield an expanded population
ofbenchmarksacrossthefullTdwarfT range.Asgreater
eff
survey volumes are searched for benchmark brown dwarfs
we can also expect to identify systems with more accu-
rately known ages. Evolved subgiants for example are less
numerousthattheirmainsequencecounterparts,butevolu-
tionary model comparisons can provide more accurate age
constraints (e.g. ±10%; Pinfield et al. 2006). And as the
range of well measured benchmarks expands into greater
parameter-space we will have the opportunity to compre-
hensively test the atmosphere models by directly mapping
thebenchmarkpopulation’sspectralvariations/trendsonto
a grid of tightly constrained physical properties.
ACKNOWLEDGMENTS
ThispublicationmakesuseofdataproductsfromtheWide-
Figure 6. Near- to mid-infrared colours of cool T dwarfs com- fieldInfraredSurveyExplorer,whichisajointprojectofthe
pared to those of the BT Settl model colours anchored to our University of California, Los Angeles, and the Jet Propul-
estimated properties for BD+01 2920B. BD+01 2920B is indi- sion Laboratory/California Institute of Technology, funded
catedwithacyanstarsymbol,whilsttheyoungbenchmarkRoss
by the National Aeronautics and Space Administration.
458Cisindicatedinred.
The UKIDSS project is defined in Lawrence et al. (2007).
UKIDSSusestheUKIRTWFCAM(Casali etal.2007) and
aphotometricsystemdescribedinHewettetal.(2006).The
11 CONCLUSIONS AND FUTURE WORK
pipeline processing and science archive are described in Ir-
Our cross-match between the WISE first data release and winet al.(2004) andHamblyetal. (2008). Based onobser-
theHipparcosandGliese catalogueshasresultedinthedis- vationsobtained at theGemini Observatory,which is oper-
covery of a new late T binary companion (BD+012920B) ated by the Association of Universities for Research in As-
andthere-discoveryoftwopreviouslyknownsystems.WISE tronomy,Inc.,underacooperativeagreementwiththeNSF
(in combination with UKIDSS and VISTA) is thus effec- on behalf of the Gemini partnership: the National Science
tivelyprobinganincreased volumeofverylowtemperature Foundation(UnitedStates),theScienceandTechnologyFa-
parameter-spaceforbenchmarkcompanions.Therearealso cilities Council (United Kingdom), the National Research
significant advantages that the primary star BD+012920A Council (Canada), CONICYT (Chile), the Australian Re-
is a nearby G dwarf rather than one of the more numerous search Council (Australia), Minist´erio da Ciˆencia, Tecnolo-
M dwarfs in the solar neighbourhood, and its lower metal- giaeInova¸c˜ao(Brazil)andMinisteriodeCiencia,Tecnolog´ıa
licity providesacrucial test fortheeffectsofreduced metal e Innovaci´on Productiva (Argentina). This work made use
contentonmodelsatmospheres.Themetallicitiesandabun- ofdataobtainedonGeminiprojectsGN-2011A-Q-73.Based
dances of bright Sun-like stars can be studied with much inparton observationsmadefor theVIKINGsurvey,using
moreconfidenceandinmuchgreaterdetailthanthoseofM VISTA at the ESO Paranal Observatory under programme
dwarfs,andlateTdwarfsinsuchsystemsoffertheopportu- ID179.A-2004.TheVISTADataFlowSystempipelinepro-
nity not only to test cool brown dwarf atmosphere physics, cessingandsciencearchivearedescribedinIrwinetal(2004)
but also to potentially studybrown dwarf abundances. and Hamblyet al (2008). Based on observations madewith
In the near future, high resolution imaging (e.g. adap- the Italian Telescopio Nazionale Galileo (A22TAC96) oper-