The Hawaii Infrared Parallax Program. V. New T-Dwarf Members and Candidate Members of Nearby Young Moving Groups
Zhoujian Zhang, Michael C. Liu, William M. J. Best, Trent J. Dupuy, Robert J. Siverd
DD RAFT VERSION F EBRUARY
11, 2021Typeset using L A TEX twocolumn style in AASTeX61
The Hawaii Infrared Parallax Program.V. New T-Dwarf Members and Candidate Members of Nearby Young Moving Groups Z HOUJIAN Z HANG ( 张 周 健 ), M ICHAEL
C. L IU , W ILLIAM
M. J. B
EST , T RENT
J. D
UPUY ,
3, 4
AND R OBERT
J. S
IVERD Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822, USA The University of Texas at Austin, Department of Astronomy, 2515 Speedway, C1400, Austin, TX 78712, USA Gemini Observatory/NSF’s NOIRLab, 670 N. A‘ohoku Place, Hilo, HI, 96720, USA Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK (Received Nov 5, 2020; Revised Feb 4, 2021; Accepted Feb 5, 2021)
Submitted to ApJABSTRACTWe present a search for new planetary-mass members of nearby young moving groups (YMGs) using astrometry for 694T and Y dwarfs, including 447 objects with parallaxes, mostly produced by recent large parallax programs from UKIRT and
Spitzer . Using the BANYAN Σ and LACEwING algorithms, we identify 30 new candidate YMG members, with spectral typesof T0 − T9 and distances of 10 −
43 pc. Some candidates have unusually red colors and/or faint absolute magnitudes compared tofield dwarfs with similar spectral types, providing supporting evidence for their youth, including 4 early-T dwarfs. We establishone of these, the variable T1.5 dwarf 2MASS J21392676+0220226, as a new planetary-mass member (14 . + . − . M Jup ) of theCarina-Near group (200 ±
50 Myr) based on its full six-dimensional kinematics, including a new parallax measurement fromCFHT. The high-amplitude variability of this object is suggestive of a young age, given the coexistence of variability and youthseen in previously known YMG T dwarfs. Our four latest-type (T8 − T9) YMG candidates, WISE J031624 . + . . + .
2, WISEPC J225540 . − .
8, and WISE J233226 . − .
6, if confirmed, will be thefirst free-floating planets ( ≈ − Jup ) whose ages and luminosities are compatible with both hot-start and cold-start evolutionarymodels, and thus overlap the properties of the directly-imaged planet 51 Eri b. Several of our early/mid-T candidates have peculiarnear-infrared spectra, indicative of heterogenous photospheres or unresolved binarity. Radial velocity measurements needed forfinal membership assessment for most of our candidates await upcoming 20–30 meter class telescopes. In addition, we compileall 15 known T7 − Y1 benchmarks and derive a homogeneous set of their effective temperatures, surface gravities, radii, andmasses. INTRODUCTIONA plethora of planetary-mass objects have been discov-ered beyond our solar system in the past 25 years. Amongthese objects, gas-giant planets ( ≈ −
13 M
Jup ) that are ei-ther wide-separation companions to stars or brown dwarfs(e.g., Goldman et al. 2010; Naud et al. 2014; Miles-Páez et al.2017; Dupuy et al. 2018) or free-floating objects (e.g., Liuet al. 2013; Best et al. 2017; Schneider et al. 2017; Zhanget al. 2018) are a valuable subset for high-quality emissionspectroscopy, given the lack of the contaminating light fromhost stars. These objects thereby serve as excellent labora-tories to study self-luminous exoplanet atmospheres, as wellas exoplanet formation and evolution. As they are too lowin mass to fuse either hydrogen or deuterium in their cores,planetary-mass objects contract, cool, and fade after their initial formation (e.g., Burrows et al. 2001; Marley et al.2007). Consequently, searches for self-luminous giant plan-ets have focused on the nearest young ( ≈ −
200 Myr) mov-ing groups (YMGs) and stellar associations, where planetary-mass objects are bright enough to be directly detected. More-over, by virtue of their shared membership, these planetary-mass objects can adopt the age estimates inferred for the stel-lar members of the same groups, making them “age bench-marks” (e.g., Pinfield et al. 2006; Liu et al. 2007) for testingmodels of substellar evolution and ultracool atmospheres.Substantial progress has been made to identify new mem-bers of nearby YMGs and has spawned a variety of meth-ods for membership assessment (e.g., Zuckerman et al. 2004;Mamajek 2005; Torres et al. 2006; Shkolnik et al. 2012; Maloet al. 2013; Bowler et al. 2017, 2019; Riedel et al. 2017; a r X i v : . [ a s t r o - ph . E P ] F e b Gagné et al. 2018b; Crundall et al. 2019). These methods relyon the objects’ space motions to establish membership, alongwith spectrophotometric evidence to establish their youthful-ness. With precise proper motions and parallaxes,
Gaia
DR2(Gaia Collaboration et al. 2016, 2018) has enabled kine-matic studies of the solar neighborhood and greatly expandedthe stellar and substellar census of nearby associations (e.g.,Gagné & Faherty 2018). However, optical data from
Gaia arenot sensitive to planetary-mass objects, whose spectral en-ergy distributions peak at longer wavelengths. Deep opticaland near-infrared sky surveys are valuable resources to findfree-floating planets, including Pan-STARRS1 (PS1; Cham-bers et al. 2016), UKIDSS (Lawrence et al. 2007, 2012), andthe
WISE surveys (Wright et al. 2010; Cutri 2014; Eisen-hardt et al. 2020; Marocco et al. 2020). These catalogsprovide proper motions, but parallaxes are either lacking orlow-accuracy given the limited number of epochs and timebaseline, thus inhibiting the identification of new associationmembers.Given these challenges, the current planetary-mass censusof nearby associations is largely incomplete and has a sig-nificant deficit at T and Y spectral types. Mid- to late-Tdwarfs are among the most common field population in thesolar neighborhood (e.g., Kirkpatrick et al. 2012; Burning-ham et al. 2013; Marocco et al. 2015; Best et al. 2021), butwe still have limited census of such objects at young ages.Only a handful of T-dwarf (and no Y-dwarf) YMG membershave been found to date (Naud et al. 2014; Macintosh et al.2015; Gagné et al. 2015, 2017, 2018a), and a larger sam-ple of such objects is needed to investigate their atmospheres( T eff ≈ − IDENTIFICATION OF NEW YOUNG MOVINGGROUP MEMBERS2.1.
Data
We start our analysis using The UltracoolSheet (Best et al.2020b), a catalog of astrometry, photometry, spectroscopy,and multiplicity for over 3,000 ultracool dwarfs and im-aged exoplanets. Developed from compilations of ultracooldwarfs by Dupuy & Liu (2012), Dupuy & Kraus (2013),Liu et al. (2016), Best et al. (2018), and Best et al. (2021),The UltracoolSheet is complete for all spectroscopically con-firmed objects with (cid:62) L0 spectral types known prior to April15, 2015 and is further augmented by new ultracool dwarfsdiscovered by Best et al. (2015, 2017) and all imaged ex-oplanets discovered since then. Sky positions, proper mo-tions, parallaxes, and radial velocities of objects in The Ul-tracoolSheet are compiled from numerous catalogs, includ-ing
Gaia
DR2 (Gaia Collaboration et al. 2016, 2018), PS1(Chambers et al. 2016), UKIDSS (Lawrence et al. 2007,2012), AllWISE (Cutri 2014), the SIMBAD AstronomicalDatabase (Wenger et al. 2000), and recent large near-infraredparallax programs by Dahn et al. (2017), Smart et al. (2018),Kirkpatrick et al. (2019), and Best et al. (2020a).For one particular T dwarf, 2MASS J21392676 + + J band, we achieved signal-to-noise ratios (S/Ns) of 90 −
160 on the target in individ-ual frames, obtaining an average of 18 frames per epoch,from which we measured the ( x , y ) positions of it and 126reference stars. Using our custom pipeline (Dupuy & Liu2012; Dupuy et al. 2015), we reduced these individual mea-surements into high-precision multi-epoch relative astrome-try, with the absolute calibration provided by 91 low-proper-motion 2MASS stars (Cutri et al. 2003). We derived the rela-tive parallax and proper motion for 2MASS J2139+0220 us-ing our standard MCMC approach and then, in order to beconsistent with our many previously published CFHT par-allaxes, converted to an absolute reference frame using theBesançon galaxy model to simulate the distances of the ref-erence stars (Robin et al. 2003). Our thirteen epochs of as-trometry spanning 4.94 years yield an absolute parallax of96 . ± . ( . ± . , . ± . ) mas yr − . We found a very reasonable reduced χ = .
04 for our best-fit solution with 21 degrees of freedom (Fig-ure 1).Multiple astrometric and kinematic measurements for thesame objects in The UltracoolSheet are unified following theapproach described below (see Best et al. 2020b for more de-tails). For objects that are companions in binary systems, weassume the companion has the same sky position, proper mo- http://bit.ly/UltracoolSheet − O − C ( m a s ) rms = 3.4 mas − − ∆ α c o s δ ( m a s ) − − ∆ δ ( m a s ) CFHT (J band)
Figure 1.
Relative astrometry of 2MASS J2139 + ∆ δ residuals to more clearly show error bars. Our best-fit solution has areduced χ = .
04 with 21 degrees of freedom. tion, and parallax as its host stars if (1) this binary system hasan angular separation of (cid:54) (cid:48)(cid:48) , or (2) the companion has nodirect astrometry from the existing catalogs. We also adoptthe host stars’ radial velocities if these values are lacking orhave lower precision for the companions.We computed J2000 sky positions of all ultracool dwarfsusing the following preferences (from highest to lowest): Gaia
DR2, PS1, UKIDSS, AllWISE, and SIMBAD. Thecoordinates of PS1 and UKIDSS are given at the observed epoch, so we computed J2000 coordinates using the reportedepochs and proper motions. Such calculation is also per-formed by SIMBAD for
Gaia
DR2.The final adopted proper motions and parallaxes of ultra-cool dwarfs are taken from
Gaia
DR2 if available and oth-erwise from the most precise measurements among PS1 andthe literature. The objects’ PS1 parallaxes are required tohave the S/N of at least 5. We allow the adopted proper mo-tions and parallaxes of a given object to come from differentreferences. The radial velocities are mostly obtained fromSIMBAD, available for nearly 1,000 objects.We identify new members and candidate members ofnearby YMGs by using the resulting compilation of astrom-etry and radial velocities of 694 T and Y dwarfs in TheUltracoolSheet (447 objects with parallaxes), including bothsingle objects and components of resolved binary/multiplesystems. For objects without trigonometric parallaxes, weuse the photometric distances available in The Ultracool-Sheet, with final values calculated from W K , or J band .Photometric distances are computed using the Dupuy & Liu(2012) relation between absolute magnitudes and spectraltypes established for field-age, high-gravity objects. Thisrelation differs for young, low-gravity objects, especially atthe L/T transition (e.g., Liu et al. 2013, 2016; Faherty et al.2016; Zhang et al. 2020a), so we treat with caution candidateYMG members identified using photometric distances.2.2. Membership Assessment
We use both BANYAN Σ (version 1.2; Gagné et al. 2018b)and LACEwING (Riedel et al. 2017) to evaluate whethera given object in The UltracoolSheet is a YMG member.BANYAN Σ is a Bayesian inference framework that com-pares an object’s sky position and proper motion (as well asits parallax and radial velocity when available) to those ofbona fide members of 29 young moving groups and associa-tions ( ≈ −
800 Myr) within 150 pc and field stars simulatedby the Besançon Galactic model (Robin et al. 2003), andthen computes a membership probability based on the ob-ject’s Galactic coordinates and space velocity (
XY ZUVW ).A threshold value for the computed Bayesian probabilitiesis needed to assess the objects’ membership and the robust-ness of such a threshold can be tested against known YMGmembers and synthetic field stars, with the results describedby the confusion matrix and derived quantities, including thetrue-positive rate (i.e., the fraction of known members re-covered) and false-positive rate (i.e., the fraction of contami-nating field stars that are incorrectly classified as members).In principle, different probability thresholds are needed fordifferent associations in order to achieve the same recov-ery/contamination rate, given that the YMGs have a varietyof sizes, distances, and membership completeness. To re-duce such association dependence for the threshold, Gagnéet al. (2018b) customized their Bayesian priors and designedBANYAN Σ to produce similar recovery rates for all 29 We adopt the objects’ W WISE photometry ex-ists and is not contaminated by nearby sources (i.e., “nb == 1”). Otherwise,we adopt photometric distances computed from K (more preferred) or K MKO band for < T4.5 dwarfs, and from J (more preferred) or J MKO band for later-type objects. With the 90% probability threshold, BANYAN Σ can recover 50%(proper motion only), 68% (proper motion and radial velocity), 82% (proper YMGs at a 90% threshold value. Therefore, the 90% prob-ability reported by BANYAN Σ is not a metric for the truemembership, but rather a value chosen to allow the classifi-cation performance among different YMGs to converge (seeSection 7 of Gagné et al. 2018b).LACEwING is a frequentist inference framework thatcompares an object’s kinematics with those of nearby YMGsusing the observed quantities (sky position, proper motion,parallax, radial velocity) rather than XY ZUVW as done byBANYAN Σ . LACEwING incorporates 16 young movinggroups and associations ( ≈ −
800 Myr) within 100 pc,which are all included in BANYAN Σ but with slightly dif-ferent lists of bona fide members. The resulting LACEwINGprobability directly describes the likelihood that a given ob-ject is a kinematic member in each YMG, and the probabil-ities among all YMGs do not necessarily add up to 100%by design. Riedel et al. (2017) suggested probability thresh-olds of 66%, 40%, and 20% to select high-, moderate-, andlow-probability candidate members, respectively. UnlikeBANYAN Σ , a given probability threshold does not promisethe similar recovery rate or contamination rate among dif-ferent YMGs. Also, within the same YMG, any (positive)probability threshold can lead to a very wide range of recov-ery rates (spanning 0% − Σ and LACEwING, and then select T and Y dwarfs with (cid:62)
80% BANYAN Σ or (cid:62)
66% LACEwING membershipprobabilities. For BANYAN Σ , our chosen threshold willlead to a higher recovery rate of known YMG members thanusing 90%, with the specific enhancement depending on as-sociations (see Figure 12 of Gagné et al. 2018b), but false-positive rates will increase as well. Using the BANYAN Σ and LACEwING results, we recover all the 5 T-dwarf YMGmembers known to date and find 30 new T-type candidatemembers (Table 1). In the following section, we study the motion and parallax), and 90% (proper motion, radial velocity, and parallax)of bona fide members of all associations (Gagné et al. 2018b). The false-positive rates for different associations depend on their angular sizes andcharacteristic kinematics but are usually (cid:54) − . One of our candidate members, CFHT-Hy-20 (T2.5), was previouslysuggested as a Hyades member by Bouvier et al. (2008) based on the pho-tometry and proper motion. The more precise proper motion, as well as thenew parallax, measured by Liu et al. (2016) supported the object’s Hyadesmembership. Here we also identify this object as a candidate, but do notconsider it to be confirmed, given that a radial velocity measurement is lack-ing. In addition, while this paper was under review, new astrometry of 5YMG candidates and 1 previously known YMG member became availablefrom Kirkpatrick et al. (2020), which does not alter these objects’ candi-dacy but does lead to slightly different membership probabilities (see foot-note b of Table 1). Also, two other objects (WISE J033651.90 + − ULAS J0047+15462MASS J1324+6358ULAS J1316+031251 Eri bWISE J2255–3118ULAS J1302+130851 Eri b2MASS J1324+6358WISE J2255–3118ULAS J1302+1308 51 Eri b2MASS J1324+6358PSO J049+262MASS J2139+02202MASS J2139+0220 2MASS J2139+0220PSO J069+04PSO J049+26 WISE J0316+4307WISE J2332–4325
Figure 2.
Near-infrared photometry of our newly confirmed Carina-Near member 2MASS J2139 + > photometric, spectroscopic, and physical properties of ourcandidates and discuss their membership. PROPERTIES OF CANDIDATE MEMBERS are now excluded from our analysis, since their YMG membership proba-bilities do not pass our criteria by using these objects’ new and more preciseastrometry from Kirkpatrick et al. (2020).
Photometric Properties
Figure 2 and Table 2 present near-infrared photometryof our YMG candidates. Several early-T dwarfs exhibit ≈ . J − K colors than field dwarfs with simi-lar spectral types, including four of our candidate members,2MASS J2139 + . + . + . + . +
04; T2), and ULAS J131610 . + . + + + + + ≈ . J -band abso-lute magnitudes than the field sequence. The anomalous pho-tometry of these objects provides evidence for their youth,given that the L/T transition of ultracool dwarfs is surface-gravity dependent, with young, lower-gravity objects havingfainter, redder near-infrared photometry than their older,higher-gravity counterparts at the same spectral type (e.g.,Metchev & Hillenbrand 2006; Barman et al. 2011; Fahertyet al. 2016; Liu et al. 2016).The gravity dependence of the L/T transition likely di-minishes from early to later T types (Zhang et al. 2020a).Therefore the membership of our remaining candidates,whose photometry follows the field sequence, is still plau-sible. Despite this, two of our late-T candidate members,WISEPC J225540 . − . − . + . + β Pictoris mem-ber, 51 Eri b (T6.5; Macintosh et al. 2015), have redder J − K colors than field dwarfs by 0 . − . . + . + J -band absolute magnitudes thanthe field sequence by 1 . − . Spectroscopic Properties
Among our 30 candidate YMG members, 20 objects havelow-resolution ( R ∼ . − . µ m) spec-tra observed by the NASA Infrared Telescope Facility (IRTF)with the facility spectrograph SpeX (Rayner et al. 2003)in prism mode. Here we investigate if any candidates ex-hibit spectral peculiarity, which is indicative of atmosphericvariability or unresolved binarity, with the former related tothe rotation of ultracool dwarfs with inhomogeneous photo-spheric condensate clouds (e.g., Radigan et al. 2014) and/ortemperature fluctuations (e.g., Tremblin et al. 2020). Inboth the variability and binary scenarios, a peculiar spectrummight be described by the composite of two (parts of) pho-tospheres with different effective temperatures. Therefore,empirical spectral indices designed to identify unresolved bi-naries (e.g., Burgasser et al. 2010a; Bardalez Gagliuffi et al.2014) can also find objects with high-amplitude photometricvariability (e.g., Radigan et al. 2012; Khandrika et al. 2013;Heinze et al. 2015; Yang et al. 2016; Manjavacas et al. 2019).We have visually compared the IRTF/SpeX spectra of our20 candidates to spectral standards from Burgasser et al.(2006) and Cushing et al. (2011) to identify any spec-tral peculiarity. We have also computed the Burgasseret al. (2010a) quantitative spectral indices to identify ob- jects with spectra indicative of composite photospheres orunresolved binarity. As a result, we find 4 “strong” com-posite candidates, meeting at least 3 out of 6 Burgasseret al. (2010a) criteria (with updates by Bardalez Gagliuffiet al. 2015): PSO J069 . + . + . + . +
26; T2.5),PSO J168 . − . −
27; T2.5), andULAS J1316 + − + . + . + − − . − . − . − . − (cid:62)
30 perpixel in J band, as well as K MKO magnitudes and parallaxes.We also exclude subdwarfs, resolved binaries, likely unre-solved binaries identified in literature (using the same quan-titative spectral indices as in this work), and all our iden-tified YMG candidate members, leading to a total of 193SpeX templates. We flux-calibrate each template using its K MKO -band absolute magnitude, with the WFCAM K -bandfilter and the corresponding zero-point flux from Hewett et al.(2006) and Lawrence et al. (2007), respectively. This step isdifferent from Burgasser et al. (2010a), who estimated ab-solute magnitudes from spectral types using empirical rela-tions. We then combine all possible pairs of flux-calibratedtemplates, resulting in 18,528 composite spectral templates.For the secondary component of each composite system, wefurther allow its absolute K MKO magnitude to vary by 7 stepsof 0 mag, ± . ± . ± . K MKO -bandphotometric scatter of ultracool dwarfs at a given spectraltypes (e.g., Figure 25 of Dupuy & Liu 2012). We thereforeobtain 129,696 composite spectral templates, expanded fromthe original set by a factor of 7. For a given binary can-didate, we compare its spectrum with each composite tem-plate over wavelengths of 0 . − . µ m, 1 . − . µ m,and 2 . − . µ m, and then compute the flux scale factor Figure 3.
Near-infrared spectra of our 4 strong composite candidates, compared with spectral standards (left), the best-fit single-object spectraltemplate (middle), and the best-fit composite spectral template (right), with χ values labeled. In the left panel for each object, we show onlyone spectral standard if the object has an integer spectral type and two standards for objects with half types. In the right panel, we use purpleand green for the primary and the secondary components, respectively, with the composite spectra shown in blue. The value in the brackets(green) indicates the magnitude offset we have added to the absolute K MKO of the secondary when flux-calibrating its spectrum and generatingthe composite spectral template. that minimizes the χ (Equation 1 of Burgasser et al. 2010a).In addition to this synthetic composite fitting, we also fitthe single-object spectral templates to our candidates’ spectrawith the same method. We have in total 246 single templatesfor such analysis as we do not require them to have paral-laxes or K MKO magnitudes. The best-fit single and compositetemplates for our 4 strong and 6 weak composite candidatesare shown in Figures 3 and 4, respectively. We do not quantitatively assess whether composite tem-plates provide better spectral matches than the single-objectones for our objects. Instead, we visually examine thebest-fit single and composite templates to study the spec-tral peculiarity of each candidate. We find the best-fitsingle-object templates of 6 candidates do not match theirobserved spectra, 2MASS J0013 − + +
04, ULAS J1316 + − Figure 4.
Near-infrared spectra of our 6 weak composite candidates with the same format as Figure 3. and 2MASS J2139 + absorption at 1 . µ m relative to 2 . µ m (PSO J049 + + J -band peakSand/or deeper H O and CH absorption around 1 . µ m(2MASS J0013 − +
04, ULAS J1316 + − + Y band (2MASS J0013 − + JH -band only) and spectroscopic follow-up is needed to obtainspectra with wider, contiguous wavelength coverage for de-tailed atmospheric analysis. Among these 10 objects, onlyULAS J0047 + + T7 composite.Among our 30 candidate YMG members, photometricvariability has been reported for two objects, 2MASS J0013 − + − . ± .
2% in J band (Eriks-son et al. 2019a). 2MASS J2139 + ±
1% in J band (Radigan et al. 2012) and11 −
12% in
Spitzer /IRAC [3.6] and [4.5] bands (Yang et al.2016; Vos et al. 2017). Therefore, the unusual spectral fea-tures of these two objects are likely related to their vari-
Figure 4.
Continued ability. Variability monitoring for all our remaining com-posite candidates would be helpful to further investigatethese objects’ spectral peculiarity. To summarize, we haveidentified a total of 7 T1.5–T4.5 candidates with peculiarspectra indicative of either atmospheric variability or un-resolved binarity: 2MASS J0013 − + +
04, ULAS J1316 + − + + Physical Properties
We derive physical properties of our 30 candidates by as-suming they are all YMG members. We first estimate theobjects’ bolometric luminosities from their broadband pho-tometry. There are 19 objects with T0 − T7 spectral types andparallaxes, and 12 of them have K , which we convertinto L bol using the Filippazzo et al. (2015) bolometric correc-tion for young ultracool dwarfs based on the objects’ spectraltypes. The other 7 ( = −
12) objects do not have K data so we first convert their K MKO (6 objects) or J MKO (1 ob-ject) photometry into 2MASS photometry using their spec-tra and then apply the corresponding Filippazzo et al. (2015)bolometric correction for young objects. We also have 4 T8 − T9 candidates with parallaxes, and these objects’ spec-tral types exceed the applicable range (M7 − T7) of the Fil-ippazzo et al. (2015) bolometric corrections. Therefore, weuse the super-magnitude method of Dupuy & Kraus (2013)as updated by W. Best et al. (in preparation). Briefly, thismethod computes bolometric luminosities for a set of abso-lute magnitudes composed of (1) J MKO , H MKO , Spitzer /IRAC[3 . Spitzer /IRAC [4 .
5] bands, (2) J MKO , H MKO , W W − L bol by integrating its observed1 . − . µ m SpeX spectrum and fitted model spectra toshorter and longer wavelengths spanning 0 . − µ m. Theresulting spectroscopic L bol = − . ± .
03 dex is consistentwith our super-magnitude L bol = − . ± .
06 dex adoptedin this work. Given that the Filippazzo et al. (2015) bolomet-ric corrections were also based on integrating spectral energydistributions of objects, there does not seem to be signifi-0
Jup
Jup
Jup
Jup
10 M
Jup
20 M
Jup
40 M
Jup
70 M
Jup
Jup
Jup
Jup
Jup
L6—Y1 BenchmarksKnown YMG T DwarfsCandidate YMG T Dwarfs2MASS 2139+0220
Figure 5.
Derived bolometric luminosities and ages of our identified candidates (light red), by assuming they are all YMG members. We usethe red star to mark our newly confirmed Carina-Near member 2MASS J2139 + − Y1 benchmarks (blue open circles) compiled by Zhang et al. (2020a) and this work (Tables 4 and 5), the hot-startSaumon & Marley (2008) hybrid evolutionary models (dashed grey lines), and the cold-start Fortney et al. (2008) evolutionary models (blacksolid lines). We find WISE J0316 + − + − L bol than the other benchmark ultracool dwarfs with similar ages. cant systematics in L bol between our subsets of T0 − T7 andT8 − T9 candidates. For the remaining 7 ( = − −
4) ob-jects without trigonometric parallaxes, we use their photo-metric distances and convert their K MKO -band absolute mag-nitudes into K band using their spectra or polynomi-als provided by Filippazzo et al. (2015). Then, we applythe Filippazzo et al. (2015) bolometric correction for youngultracool dwarfs to compute these objects’ bolometric lumi-nosities. We have propagated all uncertainties in magnitudes, parallaxes, spectral types, and empirical relations into our re-sulting L bol values in a Monte Carlo fashion.We adopt YMG ages of 149 + − Myr for AB Doradus(Bell et al. 2015), 40 −
50 Myr for Argus (Zuckerman 2019),24 ± β Pictoris (Bell et al. 2015), 200 ±
50 Myrfor Carina-Near (Zuckerman et al. 2006), 750 ±
100 Myr forHyades (Brandt & Huang 2015) , and 414 ±
23 Myr for the The Hyades open cluster mentioned here is the core of, and thereby dis-tinct from, the Hyades supercluster (a.k.a. Hyades stream or Hyades moving L bol and ages, we then interpolate the hot-start Saumon& Marley (2008) hybrid evolutionary models and computetheir effective temperatures ( T eff ), surface gravities (log g ),radii ( R ), and masses ( M ) in a Monte Carlo fashion. We as-sume the objects’ bolometric luminosities follow a normaldistribution and assume their ages follow a uniform distribu-tion (for Argus members) or a Gaussian distribution (for allother YMGs) constrained to 0 −
10 Gyr. The derived physi-cal properties of our 30 candidates are listed in Table 3 andshown in Figure 5. In total, 22 objects have planetary masses(2 −
13 M
Jup ) if their memberships are confirmed.We compare bolometric luminosities and ages of our can-didates with all previously known L6 − Y1 ultracool dwarfswith independently determined ages or masses (75 totalbenchmarks), including YMG members, wide-orbit com-panions to stars or white dwarfs, and ultracool binary sys-tems with measured dynamical masses. We obtain proper-ties of L6 − T6 benchmarks (60 objects) from Zhang et al.(2020a), and we compile a catalog for T7 − Y1 benchmarks(15 objects) in this work (Tables 4 and 5). We obtain L bol of 11 T7 − Y1 benchmarks from the literature, computedby integrating the objects’ spectral energy distributions.Bolometric luminosities of the remaining 4 objects are ei-ther lacking (WISEU J005559 . + .
0, Wolf 1130C,WD 0806 − W . + .
9; see Wrightet al. 2013), and therefore we (re-)compute their L bol usingthe aforementioned super-magnitude method.Among these 15 T7 − Y1 benchmarks, dynamical masseshave been measured for Gl 229B (Brandt et al. 2020) andGl 758B (Bowler et al. 2018; Brandt et al. 2019). Follow-ing the rejection sampling analysis in Dupuy & Liu (2017),Brandt et al. (2020) combined the dynamical mass and L bol ofGl 229B to derive its age, T eff , log g , and R using the Saumon& Marley (2008) hybrid evolutionary models. In this work,we conduct the same analysis for Gl 758B by using the morerecent dynamical mass measured by Brandt et al. (2019;also see Bowler et al. 2018; Calissendorff & Janson 2018).For the remaining 13 benchmarks with no independently in-ferred masses, we obtain their ages from their host stars asdetermined in the literature. We then derive these objects’ T eff , log g , R , and M by using their L bol , ages, and the inter-polated Saumon & Marley (2008) hybrid evolutionary mod- group) proposed by Olin Eggen (e.g., Eggen 1958). It was initially hypoth-esized that members of the Hyades supercluster are coeval, but in fact thissupercluster is composed of both young and field-age stars with a range ofelemental abundances (e.g., Chereul & Grenon 2001; Famaey et al. 2005,2007; Bovy & Hogg 2010; de Silva et al. 2011). Throughout this work, wefollow Gagné et al. (2018b) and use the Hyades designation to refer to theyoung open cluster (750 ±
100 Myr; Brandt & Huang 2015), with memberscompiled by Perryman et al. (1998). els as done for our YMG candidates. For the two bench-marks older than ∼
10 Gyr, WISEU J005559 . + . ± >
10 Gyr;Mace et al. 2018), we derive their physical properties at anage of 10 Gyr.As shown in Figure 5, we find our 4 latest-type can-didates, WISE J0316 + − + . − . − Individual Notable Objects + : A Newly Confirmed Member of theCarina-Near Moving Group + . ±
50 Myr) based on BANYAN Σ . Its member-ship probability is only 1% based on LACEwING, whichis known to produce a very low recovery rate for Carina-Near (see Section 7.15 of Riedel et al. 2017). This object’s XY ZUVW position lines up well with those of previouslyknown Carina-Near members (Figure 6), with
UVW spacemotion very close to a member of the Carina-Near stream(GJ 907.1; K8) as defined by Zuckerman et al. (2006).Previous work has studied near-infrared spectra of 2MASS J2139 + g = . g value where the models are defined. Apai et al.(2013) compared this object’s HST /WFC3 G141 grism spec-tra (1 . − . µ m, R ∼ g of 4 . . g = . + . − . dex based on evolutionary models and theassumption of its YMG membership (Section 3.3). Morerecent work by Vos et al. (2017) inferred a much higher log g of 5 . ± .
02 dex based on the Keck/NIRSPEC spectra(2 . − . µ m, R ∼ , g might be lessreliable given the narrow wavelength coverage and the lackof gravity-sensitive lines in the data.2 Figure 6.
Galactic
XY ZUVW coordinates of our newly confirmed member 2MASS J2139 + Σ multivariate Gaussian model to generate1 σ/ σ/ σ extent of this group’s XY ZUVW properties (black contours), with these contours encompassing 39 .
3% (1 σ ), 86 .
5% (2 σ ), and98 .
9% (3 σ ) of the cumulative volume for each bivariate Gaussian distribution. For our candidate members with no radial velocity, we use lightred circles to mark their optimal UVW space motions as Carina-Near members as computed by BANYAN Σ . + g ) is in accord with a tentative correlation between lowsurface gravity and high-amplitude variability of mid-L andL/T transition dwarfs (e.g., Metchev et al. 2015; Biller et al.2015; Lew et al. 2016; Vos et al. 2018; Schneider et al. 2018;Bowler et al. 2020; Zhou et al. 2020). Photometric vari-ability has been also detected in previously known T-dwarfYMG members. SIMP J013656.5+093347.3 (SIMP J0136 + ≈ J band (Artigau et al. 2009; Radigan et al.2014; Vos et al. 2017). GU Psc b (Naud et al. 2014) and2MASS J1324 + ≈
4% in J band (for GU Psc b detected by Naud et al. 2017a) or ≈
3% in the mid-infrared (for 2MASS J1324 + + + + + . ± .
03 pc] and GJ 358[9 . ± .
004 pc]).3.4.2.
Young Late-T Candidates: WISE J + ,ULAS J + , WISE J − , andWISE J − As seen in Figure 5, our 4 latest-type candidates havemuch fainter L bol than other ultracool benchmarks with simi-3lar ages. WISE J0316 + + ±
50 Myr) with BANYAN Σ probabilities of95 .
4% and 97 . + J and H bands (Mace et al. 2013a) which show no anomalieswhen compared to NIRSPEC spectra (McLean et al. 2003)of the T8 spectral standard, 2MASSI J0415195 − − + ∼ J - and H -band abso-lute magnitudes than typical field T8 dwarfs. ULAS J1302 + J − K and H − K colors than otherT8 − T9 field dwarfs (Figure 2), and its Subaru/IRCS spec-trum has slightly enhanced fluxes near the Y -band peak ascompared to 2MASS J0415 − − β Pictoris moving group (24 ± . Σ probability and a 32% LACEwING probabil-ity. Similar to ULAS J1302 + − J − K and H − K colors (Figure 2),with a slightly enhanced Y -band peak flux compared to theT8 spectral standard 2MASS J0415 − − g = . + . − . dex of WISE J2255 − g = . + . − . dex in this workusing the evolutionary models with assumed YMG member-ship (Section 3.3 and Table 3).WISE J2332 − + − Myr) with aBANYAN Σ probability of 98 . J -band Keck/NIRSPEC spectra (Tinney et al. 2018) consistentwith the T9 spectral standard UGPS J072227 . − . − J -band absolute magnitude and redder J − H color than typical field T9 dwarfs (Figure 2).The anomalous spectrophotometric appearance of these 4objects is very similar to the β Pictoris moving group exo-planet 51 Eri b (T6.5; Macintosh et al. 2015), which also hasunusually faint absolute magnitudes and red near-infraredcolors. Moreover, all these objects have distinctly faint L bol as compared to other ultracool benchmarks with similar ages(Figure 5). Radial velocity follow-up is needed to assess the YMG membership of these 4 candidates, and if theiryoung ages are confirmed, they will become the latest-typekinematic members of any young moving groups or associa-tions. Most notably, they will be the first free-floating plan-ets, whose physical properties are compatible with formationby both hot-start (with high initial entropy and no subsequentaccretion; e.g., Burrows et al. 1997; Chabrier 2001; Baraffeet al. 2003; Saumon & Marley 2008) and cold-start (with lowinitial entropy and core accretion; e.g., Marley et al. 2007;Fortney et al. 2008) conditions. These late-type YMG mem-bers may therefore shed insight on the formation pathwaysof directly-imaged and free-floating planets. SUMMARYWe have identified new and candidate T-dwarf membersof nearby young moving using astrometry for 694 T and Ydwarfs, including 447 objects with parallaxes, mostly pro-duced by recent large near-infrared astrometric programs byKirkpatrick et al. (2019) and Best et al. (2020a). Using theBANYAN Σ and LACEwING algorithms, we have recoveredall 5 previously known T-dwarf YMG members and identi-fied 30 new candidate members.We find 4 early-T (including 2MASS J2139 + . − . J − K colors and/or 0 . − . J -band absolute mag-nitudes than field dwarfs with similar spectral types. Suchanomalous photometry is in accord with previously knownYMG T dwarfs (e.g., 2MASS J1324 + −
13 M
Jup ,firmly in the planetary-mass regime. We establish the high-amplitude variable T1.5 dwarf 2MASS J2139 + . + . − . M Jup ) of the Carina-Near (200 ±
50 Myr) moving group, making it the sec-ond T dwarf and the third closest member of this group.2MASS J2139 + − T9. If confirmed, these objects will be the first free-floating planets whose ages and luminosities are compati-ble with both hot-start and cold-start evolutionary models,and thereby overlap the planetary-mass companion 51 Eri b.The low surface gravity of these objects are also supportedby their anomalous spectrophotometry and our recent atmo-spheric modeling for one of them (WISE J2255 − − Y1 benchmarks and derived a homoge-neous set of their effective temperatures, surface gravities,radii, and masses.Radial velocity measurements are needed to assess themembership of our YMG candidates except for 2MASS J2139 + −
10 meter-class telescopes (e.g., Gemini/GNIRS and Keck/NIRSPEC),but the majority of our candidates are too faint ( J (cid:38) . K (cid:38)
15 mag) and await 20 − Facilities:
IRTF (SpeX), CFHT (WIRCam)
Software:
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Membership Probability ReferencesObject SpT R.A. a Dec. a µ α cos δ µ δ Parallax b RV c BANYAN Σ LACEwING SpT Coord. PM Parallax RV Membership(hh:mm:ss.ss) (dd:mm:ss.ss) (mas yr − ) (mas yr − ) (mas) (km s − )AB Doradus • New Candidate Members (with trigonometric parallax)WISE J163645.56 − − − . ± . − . ± . . ± . − . ± .
4] 81 .
6% 7 .
0% 21 31 36 41 ··· − − − . ± . − . ± . . ± . + . ± .
0] 99 .
0% 30 .
0% 13 31 40 40 ··· − d T9 23:32:26.54 − . ± . − . ± . . ± . + . ± .
5] 98 .
9% 8 .
0% 15 22 40 40 ··· • New Candidate Members (with photometric parallax)ULAS J081918.58+210310.4 T6 08:19:18.62 21:03:12.60 − . ± . − . ± . . ± .
2] [ + . ± .
8] 86 .
3% 12 .
0% 18 31 18 42 ··· • Recovered Previously-Known Members2MASS J13243553+6358281 d T2.5 13:24:35.50 63:58:27.84 − . ± . − . ± . . ± . − . ± . .
8% 64 .
0% 10 31 36 38 38 38GU Psc b T3.5 01:12:35.04 17:03:55.44 96 . ± . − . ± .
11 21 . ± . − . ± . .
1% 72 .
0% 24 37 29,37 29,37 23 24SDSSp J111010.01+011613.1 T5.5 11:10:10.01 01:16:12.72 − . ± . − . ± . . ± . + . ± . .
3% 46 .
0% 4 31 14 14 26 26Argus • New Candidate Members (with trigonometric parallax)SDSS J152103.24+013142.7 T3 15:21:03.24 01:31:42.60 − . ± . . ± . . ± . − . ± .
3] 82 .
3% 0 .
0% 28 31 41 41 ··· − − . ± . − . ± . . ± . + . ± .
3] 96 .
8% 0 .
0% 33 31 41 41 ··· . ± . − . ± . . ± . + . ± .
3] 95 .
6% 0 .
0% 4 31 11 11 ··· − d T7 02:41:24.74 − . ± . . ± . . ± . + . ± .
5] 87 .
7% 5 .
0% 39 22 40 40 ··· • New Candidate Members (with photometric parallax)ULAS J004757.41+154641.4 T2 00:47:57.43 15:46:41.16 147 . ± . − . ± . . ± .
9] [ + . ± .
1] 80 .
1% 0 .
0% 19 22 16 42 ··· − − − . ± . . ± . . ± .
8] [ + . ± .
7] 83 .
4% 0 .
0% 25 22 25 42 ··· − . ± . . ± . . ± .
7] [ − . ± .
8] 95 .
3% 0 .
0% 8 22 18 42 ··· − . ± . . ± . . ± .
5] [ + . ± .
5] 90 .
6% 0 .
0% 9 22 18 42 ··· − . ± . − . ± . . ± .
6] [ + . ± .
4] 92 .
7% 1 .
0% 18 22 18 42 ··· β Pictoris • New Candidate Members (with trigonometric parallax)WISEPA J081958.05 − − − . ± . − . ± . . ± . + . ± .
0] 83 .
9% 1 .
0% 13 31 41 41 ··· − − . ± . − . ± . . ± . − . ± .
9] 89 .
1% 1 .
0% 12 31 41 41 ··· − d T8 22:55:40.75 − . ± . − . ± . . ± . + . ± .
9] 98 .
7% 31 .
0% 13 22 40 40 ··· • Recovered Previously-Known Members51 Eri b T6.5 04:37:36.14 − . ± . − . ± . . ± . + . ± . .
9% 50 .
0% 34 29,37 29,37 29,37 6 1,27Carina-Near • Newly Confirmed Member2MASS J21392676+0220226
T1.5 21:39:27.10 02:20:24.00 489 . ± . . ± . . ± . − . ± . .
9% 1 .
0% 4 22 43 43 35 43
Table 1 continued Table 1 (continued)
Membership Probability ReferencesObject SpT R.A. a Dec. a µ α cos δ µ δ Parallax b RV c BANYAN Σ LACEwING SpT Coord. PM Parallax RV Membership(hh:mm:ss.ss) (dd:mm:ss.ss) (mas yr − ) (mas yr − ) (mas) (km s − ) • New Candidate Members (with trigonometric parallax)ULAS J131610.13+031205.5 T3 13:16:10.22 03:12:05.76 − . ± . − . ± . . ± . − . ± .
8] 91 .
7% 0 .
0% 28 31 41 41 ··· . ± . . ± . . ± . − . ± .
0] 90 .
8% 0 .
0% 41 31 41 41 ··· d T5 22:36:16.80 51:05:48.12 708 . ± . . ± . . ± . − . ± .
0] 98 .
0% 0 .
0% 17 31 41 41 ··· − . ± . − . ± . . ± . − . ± .
1] 99 .
0% 0 .
0% 4 31 2 2,40 ··· − . ± . . ± . . ± . − . ± .
0] 89 .
6% 0 .
0% 4 31 14 14 ··· d T8 03:16:24.41 43:07:08.76 372 . ± . − . ± . . ± . + . ± .
0] 95 .
4% 0 .
0% 21 22 40 40 ··· − . ± . . ± . . ± . − . ± .
8] 97 .
7% 0 .
0% 9 22 20 20 ··· • Recovered Previously-Known MembersSIMP J013656.5+093347.3 T2.5 01:36:56.57 09:33:47.16 1239 . ± . − . ± . . ± . + . ± . .
6% 0 .
0% 3 29,37 29,37 29,37 32 32Hyades • New Candidate Members (with trigonometric parallax)PSO J069.7303+04.3834 T2 04:38:55.18 04:23:00.24 132 . ± . . ± . . ± . + . ± .
8] 84 .
7% 78 .
0% 41 31 41 41 ··· . ± . − . ± . . ± . + . ± .
8] 80 .
3% 45 .
0% 25 31 41 41 ··· . ± . − . ± . . ± . + . ± .
8] 92 .
8% 63 .
0% 41 31 41 41 ··· • New Candidate Members (with photometric parallax)WISEPA J030724.57+290447.6 T6.5 03:07:24.60 29:04:47.64 100 . ± . − . ± . . ± .
7] [ + . ± .
2] 39 .
3% 82 . e
13 22 13 42 ··· • Recovered Previously-Known Candidate MembersCFHT − Hy −
20 T2.5 04:30:38.71 13:09:56.88 142 . ± . − . ± . . ± . + . ± .
0] 98 .
7% 99 .
0% 30 31 30 30 ··· • New Candidate Members (with trigonometric parallax)SDSS J125011.65+392553.9 T4 12:50:11.71 39:25:55.55 − . ± . − . ± . . ± . − . ± .
2] 0 .
0% 78 .
0% 5 31 41 41 ··· a Coordinates are provided at epoch J2000 with equinox J2000. b Parallaxes inside brackets are derived from photometric distances. c Radial velocities inside brackets are optimal values with the assumed YMG membership as inferred by BANYAN Σ (all candidates with membership probabilities > . + . . + . d These six objects (5 YMG candidates and 1 previously known YMG member) also have parallaxes and proper motions from Kirkpatrick et al. (2020), which became availablewhile our paper was under review. The new astrometry does not alter the candidacy of these objects, but does slightly change their BANYAN Σ membership probabilities to 99 . + .
5% (Argus) for WISE J024124.73 − .
4% (Carina-Near) for WISE J031624.35 + .
5% (Carina-Near)for WISE J223617.59 + .
7% (AB Doradus) for WISE J233226.49 − .
1% ( β Pictoris) for WISEPC J225540.74 − − + β Pictoris)for WISEPC J225540.74 − e Based on LACEwING, this object is also likely a member of the AB Doradus moving group with a probability of 40%.
References —(1) Zuckerman et al. (2001), (2) Tinney et al. (2003), (3) Artigau et al. (2006), (4) Burgasser et al. (2006), (5) Chiu et al. (2006), (6) Kharchenko et al. (2007), (7) Bouvieret al. (2008), (8) Pinfield et al. (2008), (9) Burningham et al. (2010a), (10) Kirkpatrick et al. (2010), (11) Marocco et al. (2010), (12) Albert et al. (2011), (13) Kirkpatrick et al.(2011), (14) Dupuy & Liu (2012), (15) Kirkpatrick et al. (2012), (16) Lawrence et al. (2012), (17) Best et al. (2013), (18) Burningham et al. (2013), (19) Day-Jones et al. (2013), (20)Manjavacas et al. (2013), (21) Mace et al. (2013a), (22) Cutri (2014), (23) Malo et al. (2014), (24) Naud et al. (2014), (25) Best et al. (2015), (26) Gagné et al. (2015), (27) Macintoshet al. (2015), (28) Marocco et al. (2015), (29) Gaia Collaboration et al. (2016), (30) Liu et al. (2016), (31) Magnier et al. (2016), (32) Gagné et al. (2017), (33) Kellogg et al. (2017),(34) Rajan et al. (2017), (35) Vos et al. (2017), (36) Best et al. (2018), (37) Gaia Collaboration et al. (2018), (38) Gagné et al. (2018a), (39) Tinney et al. (2018), (40) Kirkpatrick et al.(2019), (41) Best et al. (2020a), (42) Best et al. (2020b), (43) This Work Table 2 . Photometry of T-Dwarf YMG Members and Candidates
Near-Infrared MKO Photometry AllWISE Photometry
Spitzer /IRAC PhotometryObject SpT Y MKO J MKO H MKO K MKO
References W W • New Candidate Members (with trigonometric parallax)WISE J163645.56 − . ± .
05 16 . ± .
02 16 . ± .
05 16 . ± .
05 25,26 15 . ± .
06 14 . ± .
06 16 – – ···
WISEPA J062720.07 − . ± .
07 15 . ± .
05 15 . ± .
18 15 . ± .
18 26 14 . ± .
03 13 . ± .
03 16 14 . ± .
02 13 . ± .
02 9WISE J233226.49 − . ± .
10 19 . ± .
18 – 10,18 17 . ± .
24 14 . ± .
07 16 17 . ± .
06 15 . ± .
02 24 • New Candidate Members (with photometric parallax)ULAS J081918.58+210310.4 T6 18 . ± .
03 16 . ± .
011 17 . ± .
04 17 . ± .
06 11 16 . ± .
11 15 . ± .
09 16 – – ···•
Recovered Previously-Known Members2MASS J13243553+6358281 T2.5 16 . ± .
08 15 . ± .
07 14 . ± .
06 14 . ± .
06 26 13 . ± .
02 12 . ± .
02 16 12 . ± .
03 12 . ± .
03 5GU Psc b T3.5 19 . ± .
05 18 . ± .
03 17 . ± .
03 17 . ± .
03 17 17 . ± .
33 15 . ± .
22 17 – – ···
SDSSp J111010.01+011613.1 T5.5 17 . ± .
012 16 . ± .
008 16 . ± .
02 16 . ± .
03 11 15 . ± .
04 13 . ± .
04 16 – – ···
Argus • New Candidate Members (with trigonometric parallax)SDSS J152103.24+013142.7 T3 17 . ± .
02 16 . ± .
010 15 . ± .
009 15 . ± .
015 11 14 . ± .
03 13 . ± .
04 16 – – ··· − . ± .
02 15 . ± .
22 15 . ± .
22 25,26 15 . ± .
05 14 . ± .
05 16 – – ···
SDSS J020742.48+000056.2 T4.5 18 . ± .
03 16 . ± .
013 16 . ± .
04 16 . ± .
05 11 16 . ± .
06 15 . ± .
07 16 – – ···
WISE J024124.73 − . ± .
04 17 . ± .
07 – 23,24 16 . ± .
08 14 . ± .
04 16 15 . ± .
03 14 . ± .
02 24 • New Candidate Members (with photometric parallax)ULAS J004757.41+154641.4 T2 19 . ± .
07 17 . ± .
05 17 . ± .
05 16 . ± .
04 11 15 . ± .
04 14 . ± .
07 16 – – ···
PSO J168.1800 − . ± .
04 17 . ± .
03 16 . ± .
03 16 . ± .
06 15,26 15 . ± .
05 14 . ± .
07 16 – – ···
ULAS J154701.84+005320.3 T5.5 19 . ± .
06 18 . ± .
03 18 . ± .
07 18 . ± .
10 6 16 . ± .
10 15 . ± .
15 16 – – ···
ULAS J120744.65+133902.7 T6 19 . ± .
05 18 . ± .
05 18 . ± .
05 18 . ± .
05 7 17 . ± .
25 15 . ± .
14 16 – – ···
ULAS J075829.83+222526.7 T6.5 18 . ± .
04 17 . ± .
02 17 . ± .
02 17 . ± .
12 13 16 . ± .
09 15 . ± .
08 16 – – ··· β Pictoris • New Candidate Members (with trigonometric parallax)WISEPA J081958.05 − . ± .
05 14 . ± .
02 14 . ± .
05 14 . ± .
05 25,26 14 . ± .
03 13 . ± .
03 16 13 . ± .
02 13 . ± .
02 9CFBDS J232304.41 − . ± .
02 17 . ± .
03 17 . ± .
04 17 . ± .
03 8 16 . ± .
09 15 . ± .
09 16 – – ···
WISEPC J225540.74 − . ± .
02 17 . ± .
011 17 . ± .
03 17 . ± .
05 20,26 16 . ± .
08 14 . ± .
05 16 15 . ± .
03 14 . ± .
02 9 • Recovered Previously-Known Members51 Eri b T6.5 – 19 . ± .
40 18 . ± .
21 18 . ± .
19 22 – – ··· – – ···
Carina-Near • Newly Confirmed Member2MASS J21392676+0220226
T1.5 16 . ± .
07 15 . ± .
05 14 . ± .
05 13 . ± .
05 26 12 . ± .
02 12 . ± .
02 16 – – ···
Table 2 continued Table 2 (continued)
Near-Infrared MKO Photometry AllWISE Photometry
Spitzer /IRAC PhotometryObject SpT Y MKO J MKO H MKO K MKO
References W W • New Candidate Members (with trigonometric parallax)ULAS J131610.13+031205.5 T3 18 . ± .
03 16 . ± .
02 16 . ± .
02 15 . ± .
02 11 14 . ± .
03 13 . ± .
04 16 – – ···
PSO J004.6359+56.8370 T4.5 – 16 . ± .
02 16 . ± .
02 16 . ± .
02 25 – – ··· – – ···
WISE J223617.59+510551.9 T5 15 . ± .
014 14 . ± .
011 14 . ± .
02 14 . ± .
05 12,26 13 . ± .
03 12 . ± .
03 16 – – ···
SDSSp J162414.37+002915.6 T6 16 . ± .
05 15 . ± .
05 15 . ± .
05 15 . ± .
05 1 15 . ± .
04 13 . ± .
03 16 14 . ± .
02 13 . ± .
02 242MASSI J1553022+153236 T7 16 . ± .
06 15 . ± .
03 15 . ± .
03 15 . ± .
03 2 15 . ± .
04 13 . ± .
03 16 14 . ± .
02 13 . ± .
02 24WISE J031624.35+430709.1 T8 – 19 . ± .
04 19 . ± .
09 – 14 17 . ± .
22 14 . ± .
05 16 16 . ± .
04 14 . ± .
02 14ULAS J130217.21+130851.2 T8.5 19 . ± .
03 18 . ± .
04 18 . ± .
06 18 . ± .
03 7 17 . ± .
23 14 . ± .
07 16 16 . ± .
010 14 . ± .
02 24 • Recovered Previously-Known MembersSIMP J013656.5+093347.3 T2.5 14 . ± .
003 13 . ± .
002 12 . ± .
002 12 . ± .
002 3,11 11 . ± .
02 10 . ± .
02 16 – – ···
Hyades • New Candidate Members (with trigonometric parallax)PSO J069.7303+04.3834 T2 – 16 . ± .
02 15 . ± .
02 15 . ± .
02 25 14 . ± .
03 13 . ± .
03 16 – – ···
PSO J049.1159+26.8409 T2.5 17 . ± .
05 16 . ± .
02 15 . ± .
02 15 . ± .
05 19,26 14 . ± .
04 13 . ± .
04 16 – – ···
PSO J052.2746+13.3754 T3.5 17 . ± .
05 16 . ± .
02 15 . ± .
05 15 . ± .
05 25,26 15 . ± .
04 14 . ± .
05 16 – – ···•
New Candidate Members (with photometric parallax)WISEPA J030724.57+290447.6 T6.5 18 . ± .
06 17 . ± .
03 17 . ± .
14 18 . ± .
12 3,11,26 17 . ± .
14 15 . ± .
08 16 16 . ± .
04 14 . ± .
02 9 • Recovered Previously-Known Candidate MembersCFHT − Hy −
20 T2.5 18 . ± .
05 17 . ± .
05 16 . ± .
05 16 . ± .
05 4,21 15 . ± .
05 14 . ± .
08 16 – – ···
Ursa Major • New Candidate Members (with trigonometric parallax)SDSS J125011.65+392553.9 T4 – 16 . ± .
02 16 . ± .
25 16 . ± .
25 25,26 15 . ± .
05 14 . ± .
05 16 – – ···
References —(1) Strauss et al. (1999), (2) Knapp et al. (2004), (3) Lawrence et al. (2007), (4) Bouvier et al. (2008), (5) Metchev et al. (2008), (6) Pinfield et al. (2008), (7) Burninghamet al. (2010a), (8) Albert et al. (2011), (9) Kirkpatrick et al. (2011), (10) Kirkpatrick et al. (2012), (11) Lawrence et al. (2012), (12) Best et al. (2013), (13) Burningham et al. (2013),(14) Mace et al. (2013a), (15) McMahon et al. (2013), (16) Cutri (2014), (17) Naud et al. (2014), (18) Tinney et al. (2014), (19) Best et al. (2015), (20) Edge et al. (2016), (21) Liuet al. (2016), (22) Rajan et al. (2017), (23) Tinney et al. (2018), (24) Kirkpatrick et al. (2019), (25) Best et al. (2020a), (26) Best et al. (2021) Table 3 . Properties of T-Dwarf YMG Members and Candidates
Physical Properties b PeculiarityObject SpT log ( L bol / L (cid:12) ) a,b T eff log g R M Photometry c Spectroscopy d Variability e (dex) (K) (dex) (R Jup ) (M
Jup )AB Doradus • New Candidate Members (with trigonometric parallax)WISE J163645.56 − − . ± .
144 1090 + − . + . − . . + . − . . + . − . N WCC g WISEPA J062720.07 − − . ± .
103 910 + − . + . − . . + . − . . + . − . N NWISE J233226.49 − − . ± .
040 454 + − . + . − . . + . − . . + . − . N ? • New Candidate Members (with photometric parallax)ULAS J081918.58+210310.4 T6 − . ± .
158 926 + − . + . − . . + . − . . + . − . N ? • Recovered Previously-Known Members2MASS J13243553+6358281 T2.5 − . ± .
100 1093 + − . + . − . . + . − . . + . − . Red WCC f ± . − . ± .
100 1002 + − . + . − . . + . − . . + . − . N WCC f ± − . ± .
020 948 + − . + . − . . + . − . . + . − . N N < . • New Candidate Members (with trigonometric parallax)SDSS J152103.24+013142.7 T3 − . ± .
149 1083 + − . + . − . . + . − . . + . − . N WCC2MASS J00132229 − − . ± .
121 1039 + − . + . − . . + . − . . + . − . N WCC f,g . ± . − . ± .
134 1024 + − . + . − . . + . − . . + . − . N NWISE J024124.73 − − . ± .
075 754 + − . + . − . . + . − . . + . − . N N • New Candidate Members (with photometric parallax)ULAS J004757.41+154641.4 T2 − . ± .
161 1057 + − . + . − . . + . − . . + . − . Red SCC g PSO J168.1800 − − . ± .
108 1032 + − . + . − . . + . − . . + . − . N SCC f ULAS J154701.84+005320.3 T5.5 − . ± .
170 831 + − . + . − . . + . − . . + . − . N ?ULAS J120744.65+133902.7 T6 − . ± .
159 736 + − . + . − . . + . − . . + . − . N ?ULAS J075829.83+222526.7 T6.5 − . ± .
163 727 + − . + . − . . + . − . . + . − . N ? β Pictoris • New Candidate Members (with trigonometric parallax)WISEPA J081958.05 − − . ± .
078 1004 + − . + . − . . + . − . . + . − . N WCCCFBDS J232304.41 − − . ± .
147 894 + − . + . − . . + . − . . + . − . N ?WISEPC J225540.74 − − . ± .
057 577 + − . + . − . . + . − . . + . − . Red N • Recovered Previously-Known Members51 Eri b T6.5 − . ± .
150 588 + − . + . − . . + . − . . + . − . Red+Faint NCarina-Near • Newly Confirmed Member2MASS J21392676+0220226
T1.5 − . ± .
056 1111 + − . + . − . . + . − . . + . − . Red+Faint WCC f,g ± Table 3 continued Table 3 (continued)
Physical Properties b PeculiarityObject SpT log ( L bol / L (cid:12) ) a,b T eff log g R M Photometry c Spectroscopy d Variability e (dex) (K) (dex) (R Jup ) (M
Jup ) • New Candidate Members (with trigonometric parallax)ULAS J131610.13+031205.5 T3 − . ± .
131 1284 + − . + . − . . + . − . . + . − . Red SCC g PSO J004.6359+56.8370 T4.5 − . ± .
093 960 + − . + . − . . + . − . . + . − . N NWISE J223617.59+510551.9 T5 − . ± .
069 950 + − . + . − . . + . − . . + . − . N NSDSSp J162414.37+002915.6 T6 − . ± .
062 820 + − . + . − . . + . − . . + . − . N N2MASSI J1553022+153236 T7 − . ± .
093 927 + − . + . − . . + . − . . + . − . N NWISE J031624.35+430709.1 T8 − . ± .
038 473 + − . + . − . . + . − . . + . − . Faint ?ULAS J130217.21+130851.2 T8.5 − . ± .
077 514 + − . + . − . . + . − . . + . − . Red ? • Recovered Previously-Known MembersSIMP J013656.5+093347.3 T2.5 − . ± .
005 1126 + − . + . − . . + . − . . + . − . N WCC ≈ • New Candidate Members (with trigonometric parallax)PSO J069.7303+04.3834 T2 − . ± .
160 1445 + − . + . − . . + . − . . + . − . Red SCC g PSO J049.1159+26.8409 T2.5 − . ± .
096 1331 + − . + . − . . + . − . . + . − . Bright SCC f,g
PSO J052.2746+13.3754 T3.5 − . ± .
136 1154 + − . + . − . . + . − . . + . − . N N • New Candidate Members (with photometric parallax)WISEPA J030724.57+290447.6 T6.5 − . ± .
123 755 + − . + . − . . + . − . . + . − . N ? • Recovered Previously-Known Candidate MembersCFHT − Hy −
20 T2.5 − . ± .
071 1220 + − . + . − . . + . − . . + . − . N WCCUrsa Major • New Candidate Members (with trigonometric parallax)SDSS J125011.65+392553.9 T4 − . ± .
128 1023 + − . + . − . . + . − . . + . − . N ? a Bolometric luminosities for our candidate members with photometric parallaxes should be used with caution. b Bolometric luminosities and physical properties of previously-known YMG members and candidate members are from Table 4 of Zhang et al. (2020a). c Objects with “Red” have much redder J − K colors than field dwarfs with similar spectral types, and those with “Faint” or “Bright” have much fainter or brighter J -band absolutemagnitudes than the field sequence (see Figure 2). d Objects with “SCC” and “WCC” are strong and weak composite candidates, respectively, based on the Burgasser et al. (2010a) and Bardalez Gagliuffi et al. (2015) criteria (seeSection 3.2). We use “?” for objects whose spectra have only partial wavelength coverage in the near-infrared and/or have not been vetted for spectral peculiarity by previous work.We use “N” for objects with normal spectra. e We provide peak-to-peak amplitudes in J band for variable ultracool dwarfs 2MASS J00132229 − + . + . Spitzer /IRAC [4.5] band forSDSSp J111010 . + . + f Potential binarity of our 4 YMG candidates and 2 previously-known YMG members have been previously noted by using the same quantitative spectral indices as in this work:2MASS J21392676 + + . + . . − . − g These 7 objects have peculiar spectra indicative of either atmospheric variability or unresolved binarity (Section 3.2). Table 4 . T7 − Y1 Benchmarks: Spectral Type, Photometry, Parallax, and Age
MKO Photometry ReferencesObject SpT Y MKO J MKO H MKO K MKO
Parallax Age Primary SpT Separation Discovery SpT Phot. π Age(mag) (mag) (mag) (mag) (mas) (Gyr) ( (cid:48)(cid:48) )Gl 229B T7 pec 15 . ± .
10 14 . ± .
05 14 . ± .
05 14 . ± .
05 173 . ± .
05 8 . + . − . M1V 7 . . ± .
06 16 . ± .
03 16 . ± .
04 16 . ± .
05 89 . ± .
06 4 . − . . . ± .
03 17 . ± .
02 17 . ± .
03 18 . ± .
09 107 . ± .
30 0 . −
10 sd L7 9 . . ± .
10 14 . ± .
05 15 . ± .
05 15 . ± .
05 170 . ± .
09 1 . − . . . ± .
03 18 . ± .
03 18 . ± .
03 18 . ± .
07 50 . ± . > . . . ± .
03 16 . ± .
01 17 . ± .
04 16 . ± .
06 86 . ± .
15 0 . − . . ◦ . ± .
14 18 . ± .
03 18 . ± .
07 19 . ± .
33 58 . ± .
50 2 . − . . . ± .
05 – – 43 . ± .
07 10 ± . − . ± .
13 18 . ± .
10 – – 41 . ± .
28 0 . −
10 L1 14 . . ± .
09 19 . ± .
08 – 60 . ± . >
10 sdM1 + WD 188 . . ± .
12 17 . ± .
05 18 . ± .
06 18 . ± .
15 114 . ± . > a . . ± .
03 18 . ± .
02 18 . ± .
03 18 . ± .
05 80 . ± .
11 3 . − . . − T8 – 18 . ± .
20 19 . ± .
20 – 64 . ± .
02 8 . ± . . . ± .
10 19 . ± .
10 19 . ± .
11 57 . ± .
25 0 . − . . − . ± .
10 25 . ± .
14 – 51 . ± .
02 2 ± . . a The two primary stars of WISE J111838 . + . References —(1) Nakajima et al. (1995), (2) Burgasser et al. (2000), (3) Leggett et al. (2002), (4) Burgasser et al. (2006), (5) Mugrauer et al. (2006), (6) Luhman et al. (2007), (7) vanLeeuwen (2007), (8) Burningham et al. (2009), (9) Thalmann et al. (2009), (10) Burgasser et al. (2010b), (11) Burningham et al. (2010b), (12) Burgasser et al. (2010c), (13) Goldmanet al. (2010), (14) Leggett et al. (2010), (15) Scholz (2010), (16) Cushing et al. (2011), (17) Janson et al. (2011), (18) Luhman et al. (2011), (19) Lawrence et al. (2012), (20) Luhmanet al. (2012), (21) Pinfield et al. (2012), (22) Burningham et al. (2013), (23) Janson et al. (2013), (24) Kuzuhara et al. (2013), (25) Mace et al. (2013b), (26) Mace et al. (2013a), (27)Wright et al. (2013), (28) Filippazzo et al. (2015), (29) Leggett et al. (2015), (30) Gaia Collaboration et al. (2016), (31) Skemer et al. (2016), (32) Leggett et al. (2017), (33) Nilssonet al. (2017), (34) Gaia Collaboration et al. (2018), (35) Mace et al. (2018), (36) Kirkpatrick et al. (2019), (37) Brandt et al. (2020), (38) Faherty et al. (2020), (39) Meisner et al.(2020), (40) Zhang et al. (2020b), (41) Best et al. (2021), (42) This Work Table 5 . T7 − Y1 Benchmarks: Bolometric Luminosity, Effective Temperature, Surface Gravity, Radius, and Mass
Physical Properties b ReferencesObject SpT log ( L bol / L (cid:12) ) T eff log g R M L bol Phys.(dex) (K) (dex) ( R Jup ) ( M Jup )Gl 229B T7 pec − . ± .
007 1011 + − . + . − . . + . − . . + . − . − . ± .
03 802 + − . + . − . . + . − . . + . − .
10 11ULAS J141623.94+134836.3 sd T7.5 − . ± .
01 691 + − . + . − . . + . − . . + . − . − . ± .
03 786 + − . + . − . . + . − . . + . − .
10 11ULAS J095047.28+011734.3 T8 − . ± .
07 774 + − . + . − . . + . − . . + . − . − . + . − . + − . + . − . . + . − . . + . − .
10 11BD +01 ◦ − . ± .
05 677 + − . + . − . . + . − . . + . − . b T8 − . ± .
008 753 5 .
317 0 . . − − . ± .
02 758 + − . + . − . . + . − . . + . − . b sd T8 − . ± .
007 647 5 .
219 0 . . − . ± .
008 584 + − . + . − . . + . − . . + . − .
11 11Wolf 940B T8.5 − . ± .
04 574 + − . + . − . . + . − . . + . − . − T8 − . ± .
03 594 + − . + . − . . + . − . . + . − . − . ± .
03 540 + − . + . − . . + . − . . + . − . − − . ± .
017 328 + − . + . − . . + . − . . + . − .