2MASS J035523.37+113343.7: A YOUNG, DUSTY, NEARBY, ISOLATED BROWN DWARF RESEMBLING A GIANT EXOPLANET

With masses intermediate between stars and planets (i.e., below the hydrogen burning and above the deuterium burning mass limit), brown dwarfs provide a natural link between stellar astrophysics and the planetary science of gas giants (Saumon et al. 1996; Chabrier & Baraffe 1997). Studies of the population have informed our understanding of low-mass star formation as well as the physical and chemical composition of low-temperature photospheres (e.g., Burrows et al. 2001, 1997; Chabrier et al. 2000). With an increasing number of brown dwarf discoveries, the diversity of the population in age, atmospheric properties, and chemical composition is becoming apparent.

Brown dwarfs are classified using red optical or near-infrared spectra and show characteristics which distinguish them as L (T_eff ∼ 2200–1300 K) or T/Y (T_eff < 1300) dwarfs (Kirkpatrick et al. 1999; Burgasser et al. 2002; Cushing et al. 2011; Tinney et al. 2012). The majority of spectrally classified field brown dwarfs within the literature are nearby isolated L dwarfs. Among the ∼1000 objects spanning this temperature regime, a significant portion exhibit near-infrared colors, spectral energy distributions (SEDs), and kinematics consistent with a field age population (e.g., Kirkpatrick et al. 2000; Knapp et al. 2004; Cruz et al. 2007; Chiu et al. 2006; Faherty et al. 2009; Schmidt et al. 2010). However, there are subsets exhibiting strong deviations in observational properties from the general population including low-metallicity subdwarfs, low surface gravity objects, and potentially cloudy/cloudless L dwarfs (Burgasser et al. 2003, 2007; Burgasser 2004; Looper et al. 2008; Cruz et al. 2009; Cushing et al. 2009; Kirkpatrick et al. 2010; Rice et al. 2010; Radigan et al. 2012; Gizis et al. 2012).

The most relevant sub-population to giant exoplanet studies is young (i.e., low surface gravity) isolated L dwarfs. The archetypal low surface gravity L dwarf, 2MASS J01415823−4633574 (2M0141), was discovered by Kirkpatrick et al. (2006). Its optical spectrum exhibits strong bands of VO but abnormally weak TiO, K, and Na absorption. In the near-infrared, its red J − K_s color (Two Micron All Sky Survey (2MASS) J − K_s = 1.73) and triangular H-band spectral morphology distinguish it from field L dwarfs (Kirkpatrick et al. 2010; Patience et al. 2012). It is suspected to be a member of the β Pictoris or Tucana–Horologium association, although the precise kinematics required to confirm association have not yet been determined (Kirkpatrick et al. 2010). After the discovery and characterization of 2M0141, additional isolated L dwarfs sharing similar photometric and spectral peculiarities attributed to a low surface gravity were reported (e.g., Reid et al. 2008; Cruz et al. 2009; Kirkpatrick et al. 2010). While the ages of these seemingly young L dwarfs remain largely unconstrained, there are kinematic and spatial indications that they represent the lowest mass members of nearby moving groups such as AB Doradus, β Pictoris, Tucana–Horologium (Cruz et al. 2009; Kirkpatrick et al. 2010).

Cruz et al. (2009) point out that the majority of objects defining the population of the lowest surface gravity L dwarfs show spectral deviations indicating that they are younger than the Pleiades. Therefore using an age range⁷ of <10–100 Myr and spectral classifications of early–mid type L dwarfs, these objects have masses close to—or in some cases below—the deuterium burning limit, making them exoplanet analogs. Since young brown dwarfs are nearby and isolated, they are ideal laboratories for detailed studies of cool, low-gravity, dusty atmospheres that are similar to directly imaged exoplanets.

In this paper we examine the kinematic, photometric, and spectral features of the low surface gravity L5γ dwarf 2MASS J035523.37+113343.7 (2M0355). In Section 2 we review published observations of 2M0355. In Section 3 we describe new near-infrared spectral and imaging data, and in Section 4 we evaluate indications of youth, including potential membership in nearby young moving groups. In Section 5 we discuss the SED for 2M0355 as well as the near-infrared color–magnitude diagram for the brown dwarf population, highlighting the location of 2M0355 compared to directly imaged exoplanets. Conclusions are presented in Section 6.

2M0355 was discovered by Reid et al. (2006) in a search of the 2MASS database for ultracool dwarfs, but its observational peculiarities were not discussed until Reid et al. (2008) and Cruz et al. (2009). 2M0355 is classified as an L5γ dwarf,⁸ demonstrating strong Li absorption (EW 7.0 Å) and other signatures of low surface gravity in the optical (Reid et al. 2008; Cruz et al. 2009). Notably this source is the reddest isolated L dwarf yet classified, with a 2MASS J − K_s color of 2.52 ± 0.03.

Reid et al. (2006) examined 2M0355 for a close companion with the Near-Infrared Camera and Multi-Object Spectrometer NIC1 on the Hubble Space Telescope and found it unresolved. Blake et al. (2007) examined this source for radial velocity (RV) variations but found no appreciable change over time and excluded the possibility of a companion with M sin i > 2.0 M_J at any separation. We note that Blake et al. (2007) assumed an L dwarf primary mass of 100 M_J, which is large for even a field aged object; therefore, given the RV constraints, the limit is likely below 2.0 M_Jup. Bernat et al. (2010) claimed the detection of a near-equal-mass companion at 82.5 mas using aperture masking interferometry; however, this result falls at the low end of their confidence limits (90%) and such a companion should have been detected by the Reid et al. (2006) imaging campaign (although Bernat et al. 2010 note this object may be at the limit of Reid et al. 2006 detections).

RVs of 12.24 ± 0.18 and 11.92 ± 0.22 km s⁻¹ were measured by Blake et al. (2007, 2010), respectively, using high-resolution K-band spectra from NIRSPEC on the Keck II telescope and forward modeling techniques for high precision. Proper motion measurements have been reported in Schmidt et al. (2007), Casewell et al. (2008), and Faherty et al. (2009). We present an updated proper motion as well as a parallax in Section 4.3.

We obtained near-infrared spectroscopy and imaging of 2M0355 and report new low- and medium-resolution spectroscopy of the source as well as a parallax and improved proper motion measurements.

3.1. Near-infrared Spectroscopy

3.2. Near-infrared Imaging

Youth indicators for isolated L dwarfs are not yet fully quantified or calibrated, but a number of distinguishing characteristics have been extrapolated from low-mass members (primarily late-type M dwarfs) of nearby young moving groups, open clusters, and star-forming regions or companions to young stars and confirmed by low-gravity atmosphere models (e.g., Lucas et al. 2001; Gorlova et al. 2003; Luhman et al. 2004; McGovern et al. 2004; Allers et al. 2007; Rice et al. 2010, 2011; Patience et al. 2012).

Among the strongest indicators is the shape of the near-infrared spectra of young brown dwarfs, which are subtly different than those of their field counterparts. Known brown dwarf members of the Chamaeleon II, Ophiuchus, Orion Nebula Cluster, TW Hydrae, and β Pictoris groups demonstrate various degrees of sharply peaked H-band spectra compared to field aged objects. The shape of the near-infrared continuum induced by steam absorption is sensitive to an objects surface gravity; therefore at younger ages, hence lower gravities, the H-band spectrum is peaked (Luhman et al. 2004 and Figure 6 from Rice et al. 2011).

An equally important indicator for brown dwarf members of young groups is a strong deviation in near-infrared color (significantly redder J − K_s) from the mean of a given spectral subtype. The clearest example is 2MASS J12073346−3932539 (2M1207b), a late-L dwarf member of the TW Hydrae association with J−K = 3.05, ∼0.5 mag redder than any other known L dwarf (Chauvin et al. 2004; Mohanty et al. 2007). Similar to the spectral deviations of young brown dwarfs, the photometric peculiarities can be explained as a consequence of lower surface gravity. At lower values—hence lower pressure at a given temperature in the photosphere—H₂ collision-induced absorption (CIA) is reduced leading to a reduction of the strongest absorption feature at 2.5 μm (less absorption at K band relative to J band) and a red J−K color (Kirkpatrick et al. 2006). An evolutionary model comparison of a large collection of low surface gravity or young companion brown dwarfs to tracks with differing cloud, metallicity, and gravity properties demonstrates that the change in near-IR color is attributed to changes in CIA H₂ affected by lower surface gravities (see Faherty et al. 2012 and references therein).

Additionally, the kinematics of young brown dwarfs as a population can be used as an indicator of youth as they are distinctly different from the kinematics of the field brown dwarf population. As discussed in Faherty et al. (2009, 2012) low surface gravity brown dwarfs have significantly smaller tangential velocities and dispersions than the overall brown dwarf population. The young age (likely <1 Gyr) of these sources means they have spent less time in the disk so they have had minimal interactions with nearby stars and giant molecular clouds that will eventually increase their overall velocity dispersion (e.g., Weinberg et al. 1987; Faherty et al. 2010; Dhital et al. 2010).

In the following subsections we compare the photometry, near-infrared spectral features, and kinematics of 2M0355 to known young brown dwarfs, directly imaged exoplanets, and the field population in order to evaluate signatures of youth for this unusual object.

4.1. Photometry

2M0355 is the reddest isolated L dwarf known. In Figure 1 we show the mean J − K_s color and standard deviation for L dwarfs (binned by 0.5 subtype) calculated from a compilation of field objects⁹ with photometric uncertainties <0.1, excluding known young objects and subdwarfs. For comparison, other confirmed low-gravity Lγ dwarfs are plotted as filled circles and 2M0355 as a filled five-point star. In Table 2 we list the average infrared photometric properties of field L dwarfs, and in Tables 3 and 4 we list the infrared photometry and colors of low-gravity L dwarfs, respectively.

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Table 2. Average Near-infrared and Mid-infrared Colors of L Dwarfs

SpT	NWISEa	N2MASSa	NLow-G	(J − Ks)avg	σ(J − Ks)	(W1 − W2)avg	σ(W1 − W2)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)
L0	143	102	11	1.30	0.15	0.27	0.06
L1	125	95	2	1.35	0.16	0.26	0.06
L2	58	60	3	1.48	0.17	0.28	0.07
L3	69	51	3	1.64	0.18	0.31	0.06
L4	37	33	5	1.69	0.24	0.34	0.08
L5	43	28	2	1.72	0.22	0.35	0.08
L6	25	13	0	1.84	0.25	0.42	0.11
L7	13	9	0	1.75	0.26	0.46	0.09
L8-9	19	10	0	1.85	0.17	0.54	0.08

Note. ^aOnly normal (non-low surface gravity, subdwarf, or young) L dwarfs with photometric uncertainty <0.1 were used in calculating the average.

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Table 3. Photometric Properties of Low Surface Gravity Lγ Dwarfs

Name	SpT (OpT)	Ja	Ha	Ksa	W1a	W2a	W3a	W4a	Ref.
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
2MASS J003255.84−440505.8	L0.0γ	14.78 ± 0.03	13.86 ± 0.03	13.27 ± 0.04	12.82 ± 0.03	12.49 ± 0.03	11.73 ± 0.19	9.29 ± null	1,2
2MASS J003743.06−584622.9	L0.0γ	15.37 ± 0.05	14.26 ± 0.05	13.59 ± 0.04	13.13 ± 0.03	12.74 ± 0.03	12.56 ± 0.38	9.32 ± null	1,2
2MASS J012445.99−574537.9	L0.0γ	16.31 ± 0.10	15.06 ± 0.09	14.32 ± 0.09	13.77 ± 0.03	13.34 ± 0.03	12.45 ± 0.31	8.91 ± null	1,2
2MASS J014158.23−463357.4	L0.0γ	14.83 ± 0.04	13.88 ± 0.02	13.10 ± 0.03	12.55 ± 0.02	12.17 ± 0.02	11.92 ± 0.21	9.24 ± null	3,2
2MASS J022354.64−581506.7	L0.0γ	15.07 ± 0.05	14.00 ± 0.04	13.42 ± 0.04	12.82 ± 0.02	12.43 ± 0.02	11.64 ± 0.15	9.47 ± null	1,2
2MASS J023400.93−644206.8	L0.0γ	15.33 ± 0.06	14.44 ± 0.06	13.85 ± 0.07	13.25 ± 0.03	12.91 ± 0.03	12.62 ± 0.28	9.49 ± null	4
2MASS J024111.51−032658.7	L0.0γ	15.80 ± 0.06	14.81 ± 0.05	14.04 ± 0.05	13.64 ± 0.03	13.26 ± 0.03	12.77 ± 0.42	9.00 ± null	2,5
2MASS J032310.02−463123.7	L0.0γ	15.39 ± 0.07	14.32 ± 0.06	13.70 ± 0.05	13.08 ± 0.02	12.67 ± 0.02	11.94 ± 0.16	9.18 ± null	1,2
2MASS J040626.77−381210.2	L0.0γ	16.77 ± 0.13	15.71 ± 0.10	15.11 ± 0.12	14.45 ± 0.03	14.10 ± 0.04	12.52 ± null	9.10 ± null	4
2MASS J195647.00−754227.0	L0.0γ	16.15 ± 0.10	15.04 ± 0.10	14.23 ± 0.07	13.69 ± 0.03	13.25 ± 0.03	12.68 ± null	9.17 ± null	1,2
2MASS J221344.91−213607.9	L0.0γ	15.38 ± 0.03	14.40 ± 0.06	13.76 ± 0.04	13.23 ± 0.03	12.83 ± 0.03	11.55 ± 0.20	9.07 ± null	2,5
2MASS J000402.88−641035.8	L1.0γ	15.79 ± 0.07	14.83 ± 0.07	14.01 ± 0.05	13.37 ± 0.03	12.94 ± 0.03	12.18 ± 0.24	9.16 ± null	4
2MASS J051846.16−275645.7	L1.0γ	15.26 ± 0.04	14.30 ± 0.05	13.62 ± 0.04	13.05 ± 0.02	12.66 ± 0.03	12.58 ± 0.35	9.22 ± null	5,6
2MASS J030320.42−731230.0	L2.0γ	16.14 ± 0.11	15.10 ± 0.09	14.32 ± 0.08	13.78 ± 0.03	13.35 ± 0.03	12.29 ± 0.17	9.34 ± 0.34	4
2MASS J053619.98−192039.6	L2.0γ	15.77 ± 0.07	14.69 ± 0.07	13.85 ± 0.06	13.26 ± 0.03	12.79 ± 0.03	12.55 ± 0.40	9.24 ± null	5,6
2MASS J232252.99−615127.5	L2.0γ	15.55 ± 0.06	14.54 ± 0.06	13.86 ± 0.04	13.24 ± 0.03	12.84 ± 0.03	12.68 ± 0.39	9.38 ± null	1,2
2MASS J172600.07+153819.0	L3.5γ	15.67 ± 0.06	14.47 ± 0.05	13.66 ± 0.05	13.07 ± 0.03	12.69 ± 0.03	11.56 ± 0.16	9.31 ± null	2,7
2MASS J212650.40−814029.3	L3.0γ	15.54 ± 0.06	14.41 ± 0.05	13.55 ± 0.04	12.91 ± 0.02	12.47 ± 0.02	11.89 ± 0.16	9.36 ± null	1,2
2MASS J220813.63+292121.5	L3.0γ	15.80 ± 0.08	14.79 ± 0.07	14.15 ± 0.07	13.35 ± 0.03	12.89 ± 0.03	12.58 ± 0.39	9.30 ± null	2,7
2MASS J012621.09+142805.7	L4.0γ	17.11 ± 0.21	16.17 ± 0.22	15.28 ± 0.15	14.24 ± 0.03	13.70 ± 0.04	12.38 ± null	9.13 ± null	6,8
2MASS J050124.06−001045.2	L4.0γ	14.98 ± 0.04	13.71 ± 0.03	12.96 ± 0.03	12.05 ± 0.02	11.52 ± 0.02	10.95 ± 0.11	9.17 ± null	1,2
2MASS J155152.37+094114.8	L4.0γ	16.32 ± 0.11	15.11 ± 0.07	14.31 ± 0.06	13.60 ± 0.03	13.12 ± 0.03	12.68 ± 0.48	9.16 ± null	1,6
2MASS J161542.55+495321.1	L4.0γ	16.79 ± 0.14	15.33 ± 0.10	14.31 ± 0.07	13.20 ± 0.02	12.62 ± 0.02	12.13 ± 0.13	9.31 ± null	5,6
2MASS J224953.45+004404.6	L4.0γ	16.59 ± 0.12	15.42 ± 0.11	14.36 ± 0.07	13.58 ± 0.03	13.14 ± 0.05	11.28 ± null	7.69 ± null	6,9,10,11
2MASS J035523.37+113343.7	L5.0γ	14.05 ± 0.02	12.53 ± 0.03	11.53 ± 0.02	10.53 ± 0.02	9.94 ± 0.02	9.29 ± 0.04	8.32 ± null	1,2
2MASS J042107.18−630602.2	L5.0γ	15.57 ± 0.05	14.28 ± 0.04	13.45 ± 0.04	12.56 ± 0.02	12.14 ± 0.02	11.60 ± 0.10	9.25 ± null	2,5

Note. ^aJHK_s photometry from the Two Micron All Sky Catalog (Skrutskie et al. 2006) and the W1, W2, W3, W4 from WISE (Wright et al. 2010). References. (1) Reid et al. 2008; (2) Cruz et al. 2009; (3) Kirkpatrick et al. 2006; (4) Kirkpatrick et al. 2010; (5) Cruz et al. 2007; (6) K. L. Cruz et al., in preparation; (7) Kirkpatrick et al. 2000; (8) Metchev et al. 2008; (9) Geballe et al. 2002; (10) Hawley et al. 2002; (11) Nakajima et al. 2004.

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Table 4. Colors of Low Surface Gravity Lγ Dwarfs

Name	SpT	(J − Ks)	(W1 − W2)	$\Delta _{(J-K_{s})}$a	Δ(W1 − W2)a
2MASS	OpT	2MASS	WISE
(1)	(2)	(3)	(4)	(5)	(6)
2MASS J0032−4405	L0.0γ	1.51 ± 0.05	0.33 ± 0.04	0.21	0.06
2MASS J0037−5846	L0.0γ	1.78 ± 0.06	0.39 ± 0.04	0.48	0.12
2MASS J0124−5745	L0.0γ	1.99 ± 0.13	0.43 ± 0.04	0.69	0.16
2MASS J0141−4633	L0.0γ	1.73 ± 0.05	0.38 ± 0.03	0.43	0.11
2MASS J0223−5815	L0.0γ	1.65 ± 0.06	0.39 ± 0.03	0.35	0.12
2MASS J0234−6442	L0.0γ	1.48 ± 0.09	0.34 ± 0.04	0.18	0.07
2MASS J0241−0326	L0.0γ	1.76 ± 0.08	0.38 ± 0.04	0.46	0.11
2MASS J0323−4631	L0.0γ	1.69 ± 0.09	0.41 ± 0.03	0.39	0.14
2MASS J0406−3812	L0.0γ	1.66 ± 0.18	0.35 ± 0.05	0.36	0.08
2MASS J1956−7542	L0.0γ	1.92 ± 0.12	0.44 ± 0.04	0.62	0.17
2MASS J2213−2136	L0.0γ	1.62 ± 0.05	0.40 ± 0.04	0.32	0.13
2MASS J0004−6410	L1.0γ	1.78 ± 0.09	0.43 ± 0.04	0.43	0.17
2MASS J0518−2756	L1.0γ	1.64 ± 0.06	0.39 ± 0.04	0.29	0.13
2MASS J0303−7312	L2.0γ	1.82 ± 0.14	0.43 ± 0.04	0.34	0.15
2MASS J0536−1920	L2.0γ	1.92 ± 0.09	0.47 ± 0.04	0.44	0.19
2MASS J2322−6151	L2.0γ	1.69 ± 0.07	0.40 ± 0.04	0.21	0.12
2MASS J1726+1538	L3.5γ	2.01 ± 0.08	0.38 ± 0.04	0.37	0.07
2MASS J2126−8140	L3.0γ	1.99 ± 0.07	0.44 ± 0.03	0.35	0.13
2MASS J2208+2921	L3.0γ	1.65 ± 0.11	0.47 ± 0.04	0.01	0.16
2MASS J0126+1428	L4.0γ	1.83 ± 0.26	0.54 ± 0.05	0.14	0.20
2MASS J0501−0010	L4.0γ	2.02 ± 0.05	0.53 ± 0.03	0.33	0.19
2MASS J1551+0941	L4.0γ	2.01 ± 0.12	0.48 ± 0.04	0.32	0.14
2MASS J1615+4953	L4.0γ	2.48 ± 0.16	0.58 ± 0.03	0.79	0.24
2MASS J2249+0044	L4.0γ	2.23 ± 0.14	0.43 ± 0.06	0.54	0.09
2MASS J0355+1133	L5.0γ	2.52 ± 0.03	0.59 ± 0.03	0.80	0.24
2MASS J0421−6306	L5.0γ	2.12 ± 0.06	0.42 ± 0.03	0.40	0.07

Note. ^aΔ values are calculated from the mean colors listed in Table 2.

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With a J − K_s color of 2.52 ± 0.03, 2M0355 is 0.8 mag redder than the average for L5 dwarfs, or nearly 4σ from the mean color. A similar deviation from the mean of the subtype is seen among other low surface gravity Lγ dwarfs listed in Tables 3 and 4, but 2M0355 is the most extreme example (although we note that the L4 dwarf 2MASS J1615+4953 shows very similar deviations in both its J − K_s and W1 − W2 colors). As discussed above, low surface gravity effects leading to a reduction in H₂ CIA are the likely cause for the extreme deviation. However we note that not all unusually red L dwarfs demonstrate low surface gravity spectral features; therefore this peculiarity alone is not conclusive about age (e.g., Looper et al. 2008).

In the same manner as Figure 1 we compile Wide-field Infrared Survey Explorer (WISE) photometry of known field L dwarfs with photometric uncertainties <0.1, excluding subdwarfs and confirmed young objects, to calculate the mean W1 − W2 color and corresponding standard deviation for spectral subtypes (again binned by 0.5 subtype) and highlight the photometry of 2M0355 (see also Table 2). As demonstrated in Figure 2, with a W1 − W2 color of 0.59, 2M0355 is 0.24 mag redder than the average of its spectral subtype or 3σ from the mean color. Comparing with the 25 similarly classified Lγ dwarfs, we find that 2M0355 is the reddest known isolated L dwarf in near- and mid-infrared colors.

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4.2. Spectral Features

2M0355 is classified as an L5γ in the optical by Cruz et al. (2009) based on its similarity to field L5's but with very weak FeH absorption and weak Na i and K i lines, which are typically interpreted as signatures of low surface gravity. In Figure 3 we show the SpeX prism spectrum for 2M0355 and compare it to the field L5 (presumed age >1 Gyr) near-infrared standard 2MASS J08350622+1953050 (2M0835) as well as the ∼10 Myr L dwarf 2M1207b (Chiu et al. 2006; Kirkpatrick et al. 2010; Patience et al. 2012). We normalize the spectra separately in each bandpass and smooth 2M1207b by a factor of three. The shape of 2M0355 in all three bands deviates significantly from the spectrum of the field standard. Compared to 2M1207b, the H and K bands are very similar, but the J band is intriguingly different. 2M0355 has a steeper slope from 1.1 to 1.25 μm and a wider peak at 1.30 μm that is more similar to the field object. In a forthcoming paper, we will present a detailed J-band spectral analysis of 2M0355 and other young brown dwarfs compared to their field counterparts.

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Several near-infrared spectral features are sensitive to surface gravity, including the H band where a sharp triangular peak is seen consistently for known young brown dwarfs at a range of ages (e.g., Lucas et al. 2001; Luhman et al. 2004; Allers et al. 2007; Rice et al. 2010, 2011). In Figure 4, we present higher-resolution (R ∼ 1200) H-band spectra of 2M0355 as well as the same comparative objects shown in Figure 3. There is an excellent match between the sharp peak of 2M1207b and 2M0355, distinct from the plateau at ∼1.55–1.70 μm of the field object. Combined with the photometric peculiarities, this is a strong indicator that 2M0355 is significantly younger than the field object (≪1 Gyr).

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4.3. Kinematics

Using multi-epoch ISPI data (see Figure 5), we report improved proper motion and parallax measurements for 2M0355. The proper motion was measured previously by Schmidt et al. (2007), Casewell et al. (2008), and Faherty et al. (2009). Our updated value is consistent with previous values but with 50%–60% smaller error bars. The new parallax measurement of π_abs = 122 ± 13 mas¹⁰ for 2M0355 places the L5γ dwarf at a distance of 8.2^+1.0_−0.8 pc. We list all astrometric and photometric properties in Table 5.

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Table 5. Properties of 2MASS J035523.37+113343.7

Parameter	Value	Reference
(1)	(2)	(3)
R.A., decl. (J2000)	03h55m2337 +11°33'437	1
Optical SpT	L5γ	2
J (2MASS)	14.05 ± 0.02	1
H (2MASS)	12.53 ± 0.03	1
Ks (2MASS)	11.53 ± 0.02	1
J (MKO)a	13.90 ± 0.03	4
H (MKO)a	12.60 ± 0.03	4
K (MKO)a	11.46 ± 0.02	4
MJ (MKO)	14.33 ± 0.24	4
MH (MKO)	13.03 ± 0.24	4
MK (MKO)	11.89 ± 0.23	4
W1	10.53 ± 0.02	3
W2	9.94 ± 0.02	3
W3	9.29 ± 0.04	3
W4	8.32 ± null	3
μα	218 ± 5 mas yr−1	4
μδ	−626 ± 5 mas yr−1	4
πabs	122 ± 13 mas	4
RV	11.92 ± 0.22 km s−1	5
Ub	−5.9 ± 1.5 km s−1	4
Vb	−23.6 ± 2.0 km s−1	4
Wb	−14.6 ± 1.3 km s−1	4
Xb	−7.0 ± 0.7 pc	4
Yb	0.2 ± 0.4 pc	4
Zb	−4.2 ± 0.4 pc	4
Agec	50–150 Myr	4
Massc	13–30 MJup	4

Notes. ^aMKO values calculated using the transformations in Stephens & Leggett (2004). ^bUVW and XYZ values are calculated in a left-handed coordinate system with U and X positive toward the Galactic center. ^cAge and mass are based on the fact that we estimate a 43% probability that 2M0355 is an AB Doradus member. References. (1) Cutri et al. 2003; (2) Cruz et al. 2009; (3) Wright et al. 2010; (4) This paper; (5) Blake et al. 2010.

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4.4. Moving Group Membership

At a distance of 8.2 pc and with spectral and photometric differences from the field population resembling those of the ∼10 Myr 2M1207b, we investigate whether 2M0355 could be kinematically associated with one of the nearby young moving groups. Using the proper motion and parallax measured in this work with the most recent RV from Blake et al. (2010), we calculate (U, V, W) = (−5.9 ± 1.5, −23.6 ± 2.0, −14.6 ± 1.3) km s⁻¹ for 2M0355.¹¹ These calculated space velocities are consistent with thin disk membership (age < ∼2–3 Gyr), and the tangential velocity of 21.5 ± 1.2 km s⁻¹ is consistent with the population of low gravity, kinematically young brown dwarfs (Faherty et al. 2009, 2012; Eggen 1989; Eggen & Iben 1989). In Figure 6 we show the UV velocities for a number of young stars or clusters within 200 pc of the Sun and find that 2M0355 is at the edge of a well-populated region of velocity space. Figure 7 shows Galactic space velocities compared to β Pictoris, and AB Doradus, the two closest moving groups to the Sun and the most likely groups of which 2M0355 might be a member. We find that 2M0355 overlaps within 1σ of the range in UVW values for probable members of AB Doradus.

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To examine the likelihood of 2M0355's membership in nearby moving groups, we determine the χ² probability for several known stellar groups within 75 pc. We include a field star model and nearby moving group parameters from Malo et al. (2012) and supplement with the parameters for the Ursa Major, Hyades, and Carina Near groups. For most groups, we adopt the centroid positions and dispersions calculated by Malo et al. (2012); however, we use velocity estimates either calculated by us or from the recent literature, where we split the uncertainties in the centroid velocities from their one-dimensional (1D) intrinsic velocity dispersions.¹²

We first determine a χ² probability that estimates the percentage of real members of a given group expected to have χ² values higher than that of 2M0355—allowing for 2M0355's observational errors and the estimated intrinsic velocity spread and spatial distribution of group members. Then we calculate a "final" probability, normalizing by the sum of the individual (marginal) star-group probabilities. At this time, equal weights are assigned to the field star and individual group models (further refinement would be beyond the focus of this study).

The initial χ² probability for six degrees of freedom is calculated as

$$\begin{equation} \chi ^{2} =A+B \end{equation} \tag{ 1 }$$

$$\begin{equation} A=\frac{(U_{o} - U_{g})^{2}}{\sigma ^{2}_{U}} + \frac{(V_{o} - V_{g})^{2}}{\sigma ^{2}_{V}} + \frac{(W_{o} - W_{g})^{2}}{\sigma ^{2}_{W}} \end{equation} \tag{ 2 }$$

$$\begin{equation} \sigma _{i}=\sqrt{\sigma _{i,o}^{2}+\sigma _{i,g}^{2}+\sigma _{i,d}^{2}}, \end{equation} \tag{ 3 }$$

where i is indexed as U, V, or W, o is the component for 2M0355, g is the component of the group, and d is the intrinsic 1D i-velocity dispersion of the group:

$$\begin{equation} B=\frac{(X_{o} - X_{g})^2}{\Delta _{X}^{2}} + \frac{(Y_{o} - Y_{g})^{2}}{\Delta _{Y}^{2}} + \frac{(Z_{o} - Z_{g})^2}{\Delta _{Z}^{2}} \end{equation} \tag{ 4 }$$

$$\begin{equation} \Delta _{j} = \sqrt{\Delta _{j,o}^{2} + \Delta _{j,g}^{2}}, \end{equation} \tag{ 5 }$$

where j is indexed as X, Y, or Z, Δ_j is defined as the 1σ dispersion in the Galactic Cartesian coordinates, o is the component for 2M0355, and g is the component for the group (we ignore the uncertainties in the group centroids which are negligible compared to the 1σ dispersions).

Using this method, we estimate that 73% of AB Doradus members would have velocities and positions more discrepant than that for 2M0355, while only 0.06% of β Pictoris members would have more discrepant values. Approximately 99.9% of field stars would have velocities and positions more discrepant than that of 2M0355, although this is likely skewed by the fact that the field star centroid (as well as our source) is so close to the Sun.

The χ² probabilities for the other groups investigated within 75 pc (Ursa Majoris, Carina Near, Tucana–Horologium, Hyades, Argus, TWA), all yielded probabilities of <10⁻¹⁷%. If one sums the individual marginal group and field star membership probabilities and assigns equal weights, then we estimate that 2M0355 has a 42% chance of being an AB Doradus member, a 58% chance of being a field star, and a <0.04% chance of being a β Pictoris group member. Further work calculating the relative densities of the young stellar groups could refine these probabilities, but at this point it appears most plausible that 2M0355 is either a member of the AB Doradus moving group or a field star. Given the photometric and spectroscopic evidence for youth shown herein combined with the low density of very young field stars, we believe that the kinematic evidence points toward 2M0355 being a likely member of the AB Doradus group.

Among the known population of low surface gravity L dwarfs, 2M0355+11 is the latest spectral type or one of the coolest isolated young brown dwarfs known. To extend the comparison of young brown dwarfs and planetary-mass objects, we investigate the colors and luminosities of 2M1207b and the directly imaged planets HR 8799bcd.

We calculated the absolute magnitude of 2M0355 from the new parallax as well as Mauna Kea Observatory (MKO; Tokunaga et al. 2002) apparent magnitudes converted from 2MASS photometry using the Stephens & Leggett (2004) relations. Comparing M_JHK for 2M0355 to the predicted values for an equivalent spectral type object based on the Faherty et al. (2012) polynomial, we find it to be [−0.9,−0.5,−0.1] mag underluminous at M_J, M_H, and M_K, respectively. As noted in Faherty et al. (2012), the population of low surface gravity L dwarfs is consistently red and underluminous—by up to 1.0 mag in M_JHK—compared to equivalent spectral type objects. As demonstrated in Figure 8, 2M0355 clearly follows this trend. As discussed in Faherty et al. (2012) evolutionary models trace low surface gravity objects at temperatures several hundred degrees lower than expected for equivalent spectral type objects on near-IR color–magnitude diagrams, providing a potential explanation for the deviation in absolute magnitudes of low-gravity L dwarfs. Extending this analysis to 2M0355 we conclude that one explanation for its peculiar near-IR absolute magnitudes is that this source is cooler than normal L5 field dwarfs.

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In Figure 9 we compare the full SED of 2M0355 to the field L5 dwarf 2MASS J1507476−162738 (2M1507; Reid et al. 2000; Dahn et al. 2002). Combining the optical spectra, MKO JHK, and WISE W1, W2, W3 absolute photometry for each we confirm that the SED for 2M0355 is underluminous compared to the field object through K band. However, redward of ∼2.5 μm, 2M0355 switches to being overluminous through at least 12 μm. Following the method described in Cushing et al. (2005), we combine the flux-calibrated optical and near-IR spectra as well as WISE photometry and calculate bolometric luminosities for both 2M0355 and 2M1507. We linearly interpolate between the centers of each WISE bandpass (W1: 3.4; W2: 4.6; W3: 11.6) and assume a Rayleigh–Jeans tail for λ >11.6 μm. We find that 2M0355 is slightly more luminous than 2M1507 by Δ log₁₀ (L_2M0355/L_2M1507) = 0.12 ± 0.1. The overall luminosity of our source is further evidence that it is young, and we surmise that enhanced photospheric dust which weakens molecular bands and shifts flux to longer wavelengths is the most likely cause of the red SED.

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In Figure 10 we show the near-infrared color–magnitude diagram for the field brown dwarf population (color-coded by spectral type), 2M1207b, the HR8799bcd planets, and 2M0355. The low luminosity and extremely red J−K color of 2M0355 place it at the red edge of the brown dwarf population, in a similar region as 2M1207b. Barman et al. (2011) find the positions of the giant exoplanets on this color–magnitude diagram—which are also redward and underluminous of the brown dwarf population—can be reproduced by allowing low T_eff models (typically assumed cloud-free) to have clouds extending across their photospheres (see also Bowler et al. 2010; Currie et al. 2011; Hinz et al. 2010; Marley et al. 2012; Madhusudhan et al. 2011; Skemer et al. 2012). 2M1207b and HR8799bcd are young (∼10 Myr and 30–160 Myr, respectively; Chauvin et al. 2004; Marois et al. 2008, 2010) so youth is thought to be correlated with enhanced photospheric dust among the low-luminosity, low-temperature brown dwarfs and giant exoplanets (see also Burgasser et al. 2010; Faherty et al. 2012).

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Consequently, the position of 2M0355 in Figure 10 leads us to conclude that in agreement with indications from the SED in Figure 9 this source is both young and dusty.

2M0355 is the reddest isolated L dwarfs yet characterized in the near- and mid-infrared. Cruz et al. (2009) classified 2M0355 as L5γ, indicating low surface gravity spectral signatures. The similarity of the near-infrared spectrum to that of the ∼10 Myr planetary-mass object 2M1207b supports the conclusion that the object is young. Furthermore, a comparison with the near- and mid-infrared colors of the known population of low surface gravity or Lγ dwarfs demonstrates that 2M0355 is the most extreme example of this class currently known.

Combining optical spectra and absolute near- to mid-IR photometry, we compared the full SED of 2M0355 with the field L5 dwarf 2M1507−16. We find that 2M0355 is underluminous in optical through the K band then switches to overluminous through at least 12 μm compared to 2M1507−16. Calculating the bolometric luminosity by integrating over the optical and near-IR spectra as well as WISE photometry shows that the overall luminosity of 2M0355 is overluminous compared to the field object. We conclude that enhanced photospheric dust, thought to be correlated with young, low-temperature, low-luminosity brown dwarfs, and giant exoplanets, shifts flux to longer wavelengths creating the red SED. The position of 2M0355 on the near-IR color–magnitude diagram supports this conclusion as it appears redward and underluminous of the known population in a similar region as 2M1207b and HR8799bcd.

Combining new proper motion and parallax measurements, we calculate UVW velocities to evaluate membership in nearby young moving groups. We find that the kinematics consistent with the young thin disk and the UV velocities for 2M0355 place it in a busy part of velocity space for young objects. A careful kinematic comparison with nearby young groups and the field population leads us to conclude that 2M0355 has a 42% chance of membership in AB Doradus. 2M0355 remains the brightest isolated low surface gravity L dwarf studied to date and will prove to be a useful comparative object in low-temperature atmosphere studies directly applicable to giant exoplanets.

Despite the spectral similarity to 2M1207b in H and K, 2M0355 is substantially different from the planetary-mass object in J band. This, combined with the older age estimate for 2M0355, causes the temperature and mass of 2M0355 to remain ambiguous. Nevertheless, we can use the object's absolute photometry and constrained age (assuming membership in AB Doradus) to estimate these key properties. Using the evolutionary tracks for young, low-mass objects of Baraffe et al. (2002), we estimate an effective temperature of ∼1500 K and a mass of ∼13 M_Jup for an age of 50 Myr (the lower limit for the age of AB Doradus). At the upper age limit for AB Doradus, ∼150 Myr, the mass of 2M0355 would be closer to ∼30 M_Jup. As a field object, the absolute magnitudes of 2M0355 correspond to an object of ∼70 M_Jup, slightly below hydrogen burning minimum mass.

We acknowledge receipt of observation time through NOAO and we thank 4.0 m telescope operators C. Aguilera, M. Gonzalez, and A. Alvarez. We also thank M. Cushing and the anonymous referee for useful comments regarding the manuscript. K.C. gratefully acknowledges support from the Research Initiative for Scientific Enhancement program at Hunter College funded by the National Institute of Health. This publication has made use of the data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has benefited from the M, L, and T dwarf compendium housed at DwarfArchives.org (http://spider.ipac.caltech.edu/staff/davy/ARCHIVE/index.shtml) and maintained by Chris Gelino, Davy Kirkpatrick, and Adam Burgasser. This research has made use of NASA's Astrophysics Data System. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.