Discovery of an L4 β Candidate Member of Argus in the Planetary Mass Regime: WISE J231921.92+764544.4
aa r X i v : . [ a s t r o - ph . S R ] D ec DISCOVERY OF AN L4 β CANDIDATE MEMBER OF ARGUSIN THE PLANETARY MASS REGIME: WISEJ231921.92+764544.4
Philip J. Castro and John E. Gizis
Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA;[email protected], [email protected]
Abstract
We present the discovery of a young L dwarf, WISE J231921.92+764544.4,identified by comparing the Wide-field Infrared Survey Explorer (WISE) All-SkyCatalog to the Two Micron All Sky Survey (2MASS). A medium-resolution op-tical spectrum provides a spectral type of L4 β , with a photometric distance esti-mate of . ± . pc. The red WISE W − W color provides additional evidenceof youth, while the 2MASS J − K s color does not. WISE J231921.92+764544.4is a candidate member of the young moving group Argus, with the space motionand position of WISE J231921.92+764544.4 giving a probability of 79% member-ship in Argus and a probability of 21% as a field object, based on BANYAN II.WISE J231921.92+764544.4 has a mass of 12.1 ± Jup based on membershipin Argus, within the planetary mass regime.
Subject headings: brown dwarfs - infrared: stars - proper motions - stars: dis-tances - stars: individual (WISE J231921.92+764544.4) - stars: late-type
1. INTRODUCTION
Brown dwarfs are objects that lie between the mass range of stars and planets (13 M
Jup . M .
75 M
Jup ), they have insufficient mass to sustain hydrogen fusion but enough mass toburn at least deuterium. Brown dwarfs are brightest when they are born and continously dimand cool thereafter (Basri 2000), and, as a consequence, evolve through the MLT spectralsequence. An early L dwarf could be an old very low mass star, a young brown dwarf, oran even younger planetary mass object (Cruz et al. 2009; Kirkpatrick 2005). Brown dwarfscontract as they age, with the radius of young brown dwarfs being as much as three timesgreater than their eventual equilibrium state (McGovern et al. 2004; Burrows et al. 2001). 2 –With g ∝ M/R , brown dwarfs evolve from low to high surface gravity as they age (McGovernet al. 2004; Kirkpatrick 2005).G 196-3B was discovered by Rebolo et al. (1998) as a companion to a young M2.5 dwarf(G 196-3A, ∼ Myr) by direct imaging. The low-resolution optical spectrum indicateda candidate L dwarf and showed weak Na I, Rb I, and Cs I, interpreted as an indicationof low surface gravity. Follow-up optical observations by Kirkpatrick et al. (2001) showedit to be an L2 dwarf. Observations of G 196-3B by McGovern et al. (2004) in the near-infrared ( J band) are supportive of low surface gravity, showing the presence of TiO andVO, weak K I lines, and weak FeH absorption. McGovern et al. (2004) showed that TiO,VO, K I, Na I, Cs I, Rb I, CaH, and FeH are gravity sensitive features in brown dwarfs bycomparing these features in late-type giants and in old field dwarfs using low-resolution J band and optical spectra. 2MASS J01415823-4633574 was discovered by Kirkpatrick et al.(2006), classified as L0 pec, and is another benchmark object that set the framework forunderstanding young L dwarfs in the field. Its optical spectrum showed very strong bands ofVO and weak absorptions by TiO, K I, and Na I, with low-gravity being an explanation forthese unique spectral signatures. The near-infrared spectrum showed a triangular-shaped H band, where this feature was reported in the spectra of young brown dwarf candidates in theOrion Nebula Cluster (Lucas et al. 2001), providing further evidence of youth. The spectrumwas also much redder than spectra of normal late-M/early-L dwarfs, its 2MASS J - K s colorbeing significantly redder than the average color for normal field L0 dwarfs. The spectrumshowed strong VO and weak Na I, K I, and FeH, analogous to the peculiarities seen in theoptical spectrum. Numerous other young L dwarfs have been identified in the field (Cruzet al. 2007; Reid et al. 2008; Kirkpatrick et al. 2008), culminating with a preliminary spectralsequence for low-gravity L dwarfs by Cruz et al. (2009). This sequence includes very low-gravity ( γ ) and intermediate-gravity ( β ) L dwarfs spanning L0 to L5, with the greek suffixappending the spectral type to indicate gravity type as suggested by Kirkpatrick (2005).The low-gravity ( γ ) are estimated to be closer to ≈ Myr, and the intermediate gravity( β ) are more likely ≈ Myr (Cruz et al. 2009). Young brown dwarfs play an importantrole in that they are analogs to giant exoplanets. Brown dwarfs and giant exoplanets shareoverlapping temperature regimes, condensate clouds in their atmospheres, and have similarphotometric and spectroscopic characteristics. Current technology only allows a handful ofplanetary systems to be directly studied. With young brown dwarfs being numerous, bright,and isolated in the field, they serve as excellent candidates for extensive studies that are notfeasible with exoplanets. Additionally, young brown dwarfs associated with young movinggroups provide a more precise age estimate than other young brown dwarfs in the field andwill therefore play a role in helping to constrain substellar evolutionary models (Fahertyet al. 2013a). 3 –The Wide-field Infrared Survey Explorer (WISE) all-sky data release (occurred on March14, 2012) covers the entire sky in four bands centered at wavelengths 3.4 µ m ( W ), 4.6 µ m( W ), 12 µ m ( W ), and 22 µ m ( W ), and achieves 5 σ detections for point sources (Wrightet al. 2010). The Two Micron All Sky Survey (2MASS) is a near-infrared survey performedfrom 1997 to 2001 covering virtually the entire sky at wavelengths 1.25 µ m ( J ), 1.65 µ m ( H ),and 2.16 µ m ( K s ), providing a 10 σ point-source detection level of better than 15.8, 15.1, and14.3 mag, respectively (Skrutskie et al. 2006). These two all-sky surveys, with a difference inepochs of ∼ yr, and photometry in the near and mid-infrared, provide an ideal setup tosearch for low-mass stars and brown dwarfs via their apparent motion. Multi-epoch searchesusing 2MASS and WISE have yielded numerous discoveries (e.g., Castro & Gizis 2012; Giziset al. 2012; Luhman et al. 2012; Luhman 2013; Castro et al. 2013).This manuscript presents the discovery of a young brown dwarf, with a mass estimatein the planetary mass regime based on membership in Argus. In Section 2 we discuss thediscovery and observations of WISE J231921.92+764544.4, Section 3 the analysis, whichincludes the optical spectrum, photometric colors, and the distance and physical properties,and lastly Section 4 with the conclusions and future work.
2. DISCOVERY AND OBSERVATIONS
We have performed a search for objects that have moderate apparent motion between2MASS and the WISE all-sky data release , complimentary to previous searches (Gizis et al.2011b,a; Castro & Gizis 2012; Gizis et al. 2012; Castro et al. 2013). Since WISE is alreadymatched to 2MASS within 3 ′′ , and we are searching for objects with moderate proper motion,we can use the 2MASS information within the WISE catalog in addition to the WISEparameters to help constrain the search. One of our searches required a detection in W , W , and W , an extended source flag of zero, ‘PH_QUAL’ of ‘AAA’ for W , W , and W , color constraints for WISE and 2MASS of . ≤ W - W ≤ . , . ≤ W - W ≤ . , W ≤ . , . ≤ J - K s ≤ . , and the distance between a WISE source and a 2MASSsource of ≥ . ′′ . WISE sources meeting our criteria were examined visually using 2MASS http://wise2.ipac.caltech.edu/docs/release/allsky/expsup/ The ‘PH_QUAL’ flag is a measure of the photometric quality in each band, with flags A,B, C, U, X, and Z. A to C represent detections with a decreasing signal to noise. Formore details refer to the Explanatory Supplement to the WISE All-Sky Data Release Products,http://wise2.ipac.caltech.edu/docs/release/allsky/expsup/sec2_2a.html in order to look for apparent motion of a source between thetwo surveys. This search criteria yielded six results, five were spurious, while one of themwas real, WISE J231921.91+764544.3. Although the original search that discovered WISEJ231921.91+764544.3 was done using the WISE all-sky data release, we present our resultsusing the most recent reprocessed AllWISE data release for WISE J231921.91+764544.3,using the AllWISE designation of WISE J231921.92+764544.4 (W2319+7645, hereafter).W2319+7645 is found a distance of 2.0 ′′ to the northeast of 2MASS source 2MASSJ23192137+7645437. The WISE source shows colors that are red, W − W . ± . ,consistent with that of a late L dwarf (Kirkpatrick et al. 2011); the 2MASS source hasred colors, J − K s = 1 . ± . , that are consistent with an L dwarf (Kirkpatrick et al.2000). Figure 1 shows the 2MASS K s and WISE W , W , W , and W images centered onW2319+7645, in all of the images the red circle shows the location of W2319+7645 basedon the WISE epoch and in the K s image the green circle shows the location of W2319+7645based on the 2MASS epoch.W2319+7645 was observed using the Gemini-North telescope. The Gemini-North ob-servations (Gemini program GN-2012B-Q-105) were on UT Date 11 September 2012 withthe GMOS spectrograph (Hook et al. 2004) using grating R831, and consisted of four 600second exposures. The wavelength coverage was 6340 to 8460Å with a resolution of ∼ Å.Conditions were non-photometric. All spectra were processed using standard IRAF tasks.
3. ANALYSIS3.1. Optical Spectrum
Figure 2 shows the Gemini GMOS-N spectrum of W2319+7645 (black) compared to theL0 through L5 optical standards (red). The standards are as follows: 2MASP J0345432+254023(L0; Kirkpatrick et al. 1999), 2MASSW J1439284+192915 (L1; Kirkpatrick et al. 1999), Kelu-1 (L2; Kirkpatrick et al. 1999), DENIS-P J1058.7-1548 (L3; Delfosse et al. 1997), 2MASSWJ1155009+230706 (L4; Kirkpatrick et al. 1999), and DENIS-P J1228.2-1547 (L5; Delfosseet al. 1997). The TiO head at 7053 Å has a maximum at about spectral type M8, anddisappears at about L2 (Kirkpatrick et al. 1999). The lack of TiO absorption at 7053 Å andred L dwarf J-K s colors indicates that W2319+7645 is an L dwarf later than L1. We per-formed a by-eye comparison of the overall continuum of W2319+7645 to the L0-L5 standards Finder charts at IRSA can be found at http://irsa.ipac.caltech.edu/ http://wise2.ipac.caltech.edu/docs/release/allwise/expsup/ ∼ L4 (Kirkpatricket al. 1999). The L2 standard shows VO present at 7334-7534 Å and 7851-7973 Å, however,W2319+7645 does not have any evidence of VO in these wavelength regions. The lack ofVO in W2319+7645 indicates it is an ∼ L4 or later, therefore, W2319+7645 is not consistentwith a spectral type of L2. With W2319+7645 not being consistent with any of the normalL dwarf standards we investigate the potential of it being a low-gravity L dwarf.As suggested by Cruz et al. (2009), we use a by-eye analysis to compare W2319+7645to L dwarf standards and low-gravity L dwarfs (from Cruz et al. (2009)), along with spectralindices, in order to investigate low-gravity and consequently determine spectral type. Wecompare the overall continuum in the relatively gravity-insensitive region (8000-8400 Å)(Cruz et al. 2009) of W2319+7645 to the L0-L5 standards. A comparison using the gravity-insensitive region will aid in constraining the low-gravity spectral subtype of W2319+7645.W2319+7645 best fits the the gravity-insensitive region of the L4 standard. W2319+7645 hastoo much flux for the K I doublet and the surrounding region compared to the L4 standard.We compare the overall continuum of W2319+7645 to the low-gravity L dwarf subtypes ofCruz et al. (2009); with the exception of the L3 β subtype whose spectrum was not availablefor comparison. Of the low-gravity L dwarfs compared to, W2319+7645 is a fit to the L4 β ,L4 γ , L5 β , and L5 γ . With a best fit to the gravity-insensitive region of the L0-L5 standardsbeing the L4 standard, and W2319+7645 being a fit to the low-gravity L4 dwarfs, we placethe focus on a spectral type of L4 β and L4 γ . Figure 3 shows the Gemini GMOS-N spectrumof W2319+7645 (black) compared to the L4 standard 2MASSW J1155009+230706 (red)(Kirkpatrick et al. 1999), the intermediate-gravity L4 β γ γ . However, theK I doublet of the L4 γ is much sharper than that of W2319+7645. W2319+7645 has aslight lack of flux in the region surrounding the K I doublet compared to the L4 β , in theregion most affected by gravity (7300-8000 Å) (Cruz et al. 2009), as well as for wavelengthsshortward of 7300 Å. The Na I doublet at 8183 and 8195 Å is either not present or very weakin low-gravity L dwarfs (Cruz et al. 2009). The Na I doublet of the L4 standard and theL4 β are similar, with W2319+7645 matching them quite well, while the L4 γ is very weakin comparison; see the zoomed in view of the Na I doublet to the right of each spectrum. 6 –The Na I doublet is a distinctive feature distinguishing the normal-gravity L4 standard andthe intermediate-gravity L4 β from the very low-gravity L4 γ . With the Na I doublet ofW2319+7645 being a poor match to the L4 γ , we conclude a by-eye spectral type of L4 β .Figure 4 shows the spectral indices for W2319+7645 (grey star), the L4 standard2MASSW J1155009+230706 (red circle), the L4 β γ β or L4 γ . Based on our by-eye analysis of the spectrum ofW2319+7645 and its spectral indices, we find a spectral type for W2319+7645 of L4 β .The unresolved absorption doublet at 6708 Å, Li I, is clearly present in the spectrum ofW2319+7645. W2319+7645 passes the lithium test (Rebolo et al. 1992). At about 1800 K(L4 spectral subtype), 50% lithium depletion occurs at about 1 Gyr and a mass of about 0.06M ⊙ (Kirkpatrick 2005). The lithium absorption of W2319+7645 indicates substellarity andyouth (< 1 Gyr). The lack of Li I absorption in the L4 β and the presence of Li I absorptionin the L4 γ cannot be used as an indicator of youth. Li I absorption is expected to be weakerfor lower gravity early to mid L dwarfs (Kirkpatrick et al. 2008), in contradiction to thesituation shown for the L4 β and L4 γ . Li I absorption is still not fully understood in youngbrown dwarfs (Kirkpatrick et al. 2008; Cruz et al. 2009). Low-gravity L dwarfs have been shown to have red near-infrared colors ( J - K s ), and thatthey tend to fall at the red end of the distribution within their spectral class (Cruz et al.2009). Faherty et al. (2013b) demonstrated that L γ dwarfs tend to lie at the red end ofthe distribution within their spectral class for near-infrared colors ( J - K s ) and analogouslyfor mid-infrared colors ( W - W ). Figure 5 shows 2MASS J - K s (top) and WISE W - W (bottom) colors as a function of spectral type for W2319+7645 (gray star), normal L dwarfs(red circle), L β dwarfs (blue squares), and L γ dwarfs (green diamonds) for spectral typesL0 to L5. The normal L dwarfs average values of J - K s and W - W , and their standarddeviations, are from Faherty et al. (2013b); we note that the W - W is All-sky WISE data.The L β and L γ dwarfs, and their J - K s colors are from Cruz et al. (2009), while the W - W colors were retrieved from AllWISE. Just as the L β dwarfs tend to be at the red end of thedistribution of their spectral types in J - K s , this appears to be the case as well for W - W β dwarfs, one L1 β and one L4 β , that lie at the averagecolor of the normal L dwarfs within their spectral class and are not red in W - W colors.The J - K s color of W2319+7645 is consistent with normal L4 dwarfs and does not provideadditional evidence of youth. This is similar to one of the L1 β dwarfs that has a J - K s colorconsistent with normal L1 dwarfs. The W - W color of W2319+7645 is at the red end ofthe distribution for normal L4 dwarfs; albeit with large error bars. While an L dwarf havingred colors alone does not imply youth, unusually thick clouds can also result in red colors(Cruz et al. 2009), the red W - W color of W2319+7645 combined with the optical spectraltype of L4 β provides additional evidence of youth. We provide a crude distance estimate by using the spectral-type-absolute-magnituderelationships of Looper et al. (2008) for 2MASS photometry and the spectral-type-absolute-magnitude relationships of Dupuy & Liu (2012) for 2MASS and WISE photometry. We find adistance of . ± . pc from 2MASS J photometry, . ± . pc from 2MASS H photometry,and . ± . pc from 2MASS K s photometry using the relations from Looper et al. (2008), . ± . pc from 2MASS J photometry, . ± . pc from 2MASS H photometry, . ± . pc from 2MASS K s photometry, . ± . pc from WISE W photometry, and . ± . pcfrom WISE W photometry using the relations from Dupuy & Liu (2012). The uncertaintyin the distance estimates comes from the uncertainty in the photometry and the RMS fromthe spectral-type-absolute-magnitude relationships. The mean of these estimates provides adistance of . ± . pc, assuming no binarity. Trigonometric parallax measurements areneeded for a reliable distance estimate.Based on a direct comparison of the 2MASS and WISE positions, W2319+7645 has aproper motion of µ α cos( δ ) = 0 . ± . ′′ yr − and µ δ = 0 . ± . ′′ yr − , with total apparentmotion . ± . ′′ yr − . Based on the estimated distance and apparent motion, W2319+7645has a tangential velocity of ± km s − . A radial velocity of ± km s − was measuredfor W2319+7645 using the narrow atomic lines in the Gemini spectrum, giving a total spacevelocity of ± km s − . Faherty et al. (2009) gives a median tangential velocity for L4dwarfs of 25 km s − with a dispersion of 20 km s − , while the median tangential velocityfor low surface gravity dwarfs is 18 km s − with a dispersion of 15 km s − . W2319+7645is consistent with both of these populations. Using the position, motion, and distance ofW2319+7645, from Gagné et al. (2014) we determine a heliocentric galactic position of (X, Gagné, Jonathan; Astrolib (2014): IDL routines to compute XYZ and UVW coordinates. figshare. ±
2, 22 ±
4, 7 ±
1) pc and a galactic motion of (U, V, W)=(-27 ±
5, -3 ±
5, 2 ± − , using a right-handed coordinate system with X and U positive toward the galacticcenter.In Figure 6, we compare the galactic position (XYZ) and galactic velocity (UVW) ofW2319+7645 (red star) with σ error bars to the mean galactic position and mean galacticvelocity of young moving groups with their σ dispersions. The σ (dashed line) and σ (dotted line) dispersion values are shown for Argus. The XYZ and UVW values for youngmoving groups are from Malo et al. (2013). W2319+7645 is consistent with the ∼ σ dis-persion values of the galactic position and galactic velocity of Argus if we allow σ errorbars for W2319+7645. We use the BANYAN II web tool (Malo et al. 2013; Gagné et al.2014) to determine the probability of W2319+7645 belonging to one of the young movinggroups. We used input parameters of right ascension, declination, proper motion in rightascension, proper motion in declination, radial velocity, distance, error in proper motion inright ascension, error in proper motion in declination, error in radial velocity, error in dis-tance, and include that W2319+7645 is younger than 1 Gyr. The probability of membershipfor W2319+7645 to the young moving groups shown in Figure 6 is 79.44% for membershipin Argus and 20.56% as a field star. W2319+7645 is a candidate for membership in Argus.The spectral-type-effective-temperature relationship (Looper et al. 2008) gives a T eff =1840 ± K, where the uncertainty in T eff is twice the RMS in the spectral-type-effective-temperature relation; we use double the RMS for the uncertainty in T eff because W2319+7645is not a normal L dwarf. We use an IDL routine (Gagné et al. 2014; Baraffe et al. 2003;Allard et al. 2013; Rajpurohit et al. 2013) to estimate the mass of W2319+7645. Thisroutine uses distance, an estimated age range, and 2MASS ( J , H , K s ) and WISE ( W , W )photometry to determine the most probable mass range in a likelihood analysis. Using ourestimated distance, an age range of 30 to 50 Myr (Gagné et al. 2014) based on membershipin Argus, and the 2MASS and WISE photometry, we find a mass of 12.1 ± Jup forW2319+7645, within the planetary mass regime. Using an age estimate of ∼ − Myr for an intermediate-gravity L dwarf in the field, we find a mass of 21.9 ± Jup forW2319+7645, this firmly places W2319+7645 in the substellar regime. Table 1 gives theproperties of W2319+7645. http://dx.doi.org/10.6084/m9.figshare.899753Retrieved 21:06, Aug 24, 2014 (GMT) Gagné, Jonathan; Lafrenière, David; Doyon, René; Malo, Lison (2014): An IDL routine to estimate themass of low-mass stars and brown dwarfs. figshare.http://dx.doi.org/10.6084/m9.figshare.899808Retrieved 02:10, Oct 23, 2014 (GMT)
Parameters WISE J231921.92+764544.4WISE R.A. (J2000) 23:19:21.93WISE Decl. (J2000) +76:45:44.5WISE Epoch 2010.412MASS R.A. (J2000) 23:19:21.372MASS Decl. (J2000) +76:45:43.82MASS Epoch 2000.462MASS J (mag) 15.35 ± H (mag) 14.39 ± K s (mag) 13.65 ± W (mag) 12.93 ± W (mag) 12.45 ± W (mag) 11.52 ± W (mag) > J − H (mag) 0.97 ± J − K s (mag) 1.71 ± H − K s (mag) 0.74 ± W − W (mag) 0.48 ± W − W (mag) 1.41 ± W − W (mag) 0.93 ± β Distance (pc) 26.1 ± µ α cos( δ ) (mas yr − ) 190 ± µ δ (mas yr − ) 70 ± ′′ yr − ) 0.20 ± v tan (km s − ) 25 ± v rad (km s − ) 10 ± v tot (km s − ) 27 ± U (km s − ) -27 ± V (km s − ) -3 ± W (km s − ) 2 ± X (pc) -12 ± Y (pc) 22 ± Z (pc) 7 ± T eff (K) 1800 ± − Jup ) 12.1 ±
10 –
4. CONCLUSIONS
W2319+7645 is a young L dwarf of spectral type L4 β , with a crude photometric distanceestimate of . ± . pc. An analysis of the 2MASS and WISE photometry of W2319+7645reveals that the red WISE W − W color provides additional evidence of youth while the2MASS J − K s color does not. W2319+7645 is a candidate member of the young movinggroup Argus. Based on BANYAN II, the space motion and position of W2319+7645 give aprobability of 79% membership in Argus and a probability of 21% as a field object. Basedon membership in Argus, W2319+7645 has a mass estimate of 12.1 ± Jup , within theplanetary mass regime.Future work should include a parallax measurement, a more accurate radial velocitymeasurement, and near-infrared spectroscopy. A parallax measurement will provide a reliabledistance estimate, since the photometric distance estimate is only a crude indicator foryoung L dwarfs. A more accurate radial velocity measurement combined with a parallaxmeasurement will help to provide a more accurate membership probability to the youngmoving group Argus. Near-infrared spectroscopy will help to further characterize the youthof this object, following the near-infrared classification and low-gravity scheme of Allers &Liu (2013).
5. ACKNOWLEDGMENTS
We thank the anonymous referee for a thorough report that significantly improved themanuscript. This publication makes use of data products from the Wide-field Infrared SurveyExplorer, which is a joint project of the University of California, Los Angeles, and the JetPropulsion Laboratory/California Institute of Technology, and NEOWISE, which is a projectof the Jet Propulsion Laboratory/California Institute of Technology. WISE and NEOWISEare funded by the National Aeronautics and Space Administration. This publication makesuse of data products from the Two Micron All Sky Survey, which is a joint project of theUniversity of Massachusetts and the Infrared Processing and Analysis Center/CaliforniaInstitute of Technology, funded by the National Aeronautics and Space Administration andthe National Science Foundation. This research has made use of the NASA/ IPAC InfraredScience Archive, which is operated by the Jet Propulsion Laboratory, California Institute ofTechnology, under contract with the National Aeronautics and Space Administration. Thisresearch has made use of the VizieR catalogue access tool, CDS, Strasbourg, France. 11 –
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This preprint was prepared with the AAS L A TEX macros v5.2.
14 –Fig. 1.— 2MASS K s and WISE W , W , W , and W images showing the location ofW2319+7645. The red circles show the location of W2319+7645 based on the WISE epochand the green circle shows the location of W2319+7645 based on the 2MASS epoch. Eachimage is 4 ′ x 4 ′ . North is up and east is to the left. Ks W1 W2 W3 W4
15 –Fig. 2.— Gemini GMOS-N spectrum of W2319+7645 (black) compared to the L0 throughL5 optical standards (red). The standards are as follows: 2MASP J0345432+254023 (L0;Kirkpatrick et al. 1999), 2MASSW J1439284+192915 (L1; Kirkpatrick et al. 1999), Kelu-1(L2; Kirkpatrick et al. 1999), DENIS-P J1058.7-1548 (L3; Delfosse et al. 1997), 2MASSWJ1155009+230706 (L4; Kirkpatrick et al. 1999), and DENIS-P J1228.2-1547 (L5; Delfosseet al. 1997). All spectra are normalized to the mean of the window 8240-8260 Å. The y-axis is logarithmic and the spectra are offset vertically by a multiplicative constant. Severalspectral features are labeled. The dashed lines delineate the relatively gravity-insensitiveregion (8000-8400 Å) (Cruz et al. 2009). N o r m a li z e d F λ x C o n s t a n t WISE J231921.92+764544.4 O ⊕ O ⊕VO VOTiO K I
16 –Fig. 3.— Gemini GMOS-N spectrum of W2319+7645 (black) compared to the L4 standard2MASSW J1155009+230706 (red) (Kirkpatrick et al. 1999), the intermediate-gravity L4 β γ N o r m a li z e d F λ x C o n s t a n t WISE J231921.92+764544.4
Li I Rb I Rb I Na IK IO ⊕ O ⊕
17 –Fig. 4.— Spectral indices of W2319+7645 (grey star), the L4 standard (red circle), the L4 β (blue square), and the L4 γ (green diamond), using the spectral indices defined by Kirkpatricket al. (1999); Cruz et al. (2009). For each index the data points are offset horizontally forclarity. The spectral indices overall show W2319+7645 to be low-gravity, and are mostconsistent with an L4 β or L4 γ . K-a K-bIndex3456789101112 I n d e x V a l u e Rb-a Rb-b Na-a Na-b VO-aIndex0.81.01.21.41.61.82.0 I n d e x V a l u e W2319+7645L4 StandardL4βL4γ
18 –Fig. 5.— 2MASS J - K s (top) and WISE W - W (bottom) colors as a function of spectraltype for L0 to L5 dwarfs. W2319+7645 (gray star) is compared to normal L dwarfs (redcircle) (Faherty et al. 2013b), L β dwarfs (blue squares), and L γ dwarfs (green diamonds)(Cruz et al. 2009). For each spectral type the data points (except for the normal L dwarfs)are offset horizontally for clarity. J - K s W2319+7645 Normal L dwarfs Lβ Lγ
L0 L1 L2 L3 L4 L5Spectral Type0.200.250.300.350.400.450.500.550.600.65 W - W
19 –Fig. 6.— The galactic position (XYZ) and galactic velocity (UVW) of W2319+7645 (redstar) compared to the mean galactic position and mean galactic velocity for young movinggroups from Malo et al. (2013). The σ error bars are shown for W2319+7645 and the σ dispersion values are shown for the young moving groups. The σ (dashed line) and σ (dotted line) dispersion values are shown for Argus. W2319+7645 is consistent with the ∼ σ dispersion values of the galactic position and galactic velocity of Argus if we allow σ error bars for W2319+7645. The probability of W2319+7645 being a member of Argus is79% based on BANYAN II. −30 −20 −10 0U (km s −1 )−30−25−20−15−10−50 V ( k m s − ) beta PictorisTucana−HorologiumAB DoradusColumbaCarinaTW HydraeArgus −30 −20 −10 0U (km s −1 )−15−10−505 W ( k m s − ) beta PictorisTucana−HorologiumAB DoradusColumbaCarinaTW HydraeArgus −30 −20 −10 0V (km s −1 )−15−10−505 W ( k m s − ) beta PictorisTucana−HorologiumAB DoradusColumbaCarinaTW HydraeArgus −40 −20 0 20 40X (pc)−80−60−40−20020 Y ( p c ) beta PictorisTucana−HorologiumAB DoradusColumba CarinaTW HydraeArgus −40 −20 0 20 40X (pc)−40−20020 Z ( p c ) beta PictorisTucana−HorologiumAB DoradusColumba CarinaTW HydraeArgus −80 −60 −40 −20 0 20Y (pc)−40−20020 Z ( p c ))