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We present a uniform forward-modeling analysis of 90 late-M and L dwarfs in nearby young (∼10–200 Myr) moving groups, the Pleiades, and the Hyades using low-resolution (R≈ 150) near-infrared (0.9–2.4μm) spectra and the BT-Settl model atmospheres. We derive the objects’ effective temperatures, surface gravities, radii, and masses by comparing our spectra to the models using a Bayesian framework with nested sampling and calculate the same parameters using evolutionary models. Assuming the evolutionary-based parameters are more robust, our spectroscopically inferred parameters from BT-Settl exhibit two types of systematic behavior for objects near the M-L spectral type boundary. Several objects are clustered aroundT_eff≈ 1800 K and$\log g\approx 5.5$dex, implying impossibly large masses (150–1400M_Jup), while others are clustered aroundT_eff≳ 3000 K and$\log g\lesssim 3.0$dex, implying unphysically low masses and unreasonably young ages. We find the fitted BT-Settl model spectra tend to overpredict the peakJ- andH-band flux for objects located near the M-L boundary, suggesting the dust content included in the model atmospheres is insufficient to match the observations. By adding an interstellar medium–like reddening law to the BT-Settl model spectra, we find the fits between models and observed spectra are greatly improved, with the largest reddening coefficients occurring at the M-L transition. This work delivers a systematic examination of the BT-Settl model atmospheres and constitutes the largest spectral analysis of benchmark late-M- and L-type brown dwarfs to date.

We present a L-band (2.98–3.96 $\mu$m) spectroscopic study of eight young L dwarfs with spectral types ranging from L2 to L7. Our spectra (λ/Δλ ≈ 250–600) were collected using the Gemini near-infrared spectrograph. We first examine the young L-band spectral sequence, most notably analysing the evolution of the Q-branch of methane absorption feature at 3.3 $\mu$m. We find the Q-branch feature first appears between L3 and L6, as previously seen in older field dwarfs. Secondly, we analyse how well various atmospheric models reproduce the Lband and published near-IR (0.7–2.5 $\mu$m) spectra of our objects by fitting five different grids of model spectra to the data. Best-fit parameters for the combined near-IR and L-band data are compared to best-fit parameters for just the near-IR data, isolating the impact that the addition of the L band has on the results. This addition notably causes a ∼100 K drop in the best-fit effective temperature. Also, when clouds and a vertical mixing rate (Kzz) are included in the models, thick clouds, and higher Kzz values are preferred. Five of our objects also have previously published effective temperatures and surface gravities derived using evolutionary models, age estimates, and bolometric luminosities. Comparing model spectra matching these parameters to our spectra, we find disequilibrium chemistry and clouds are needed to match these published effective temperatures. Three of these objects are members of AB Dor, allowing us to show the temperature dependence of the Q-branch of methane.

We present the characterization of the low-gravity M6 dwarf 2MASS J06195260-2903592, previously identified as an unusual field object based on its strong IR excess and variable near-IR spectrum. Multiple epochs of low-resolution (R≈ 150) near-IR spectra show large-amplitude (≈0.1–0.5 mag) continuum variations on timescales of days to 12 yr, unlike the small-amplitude variability typical for field ultracool dwarfs. The variations between epochs are well-modeled as changes in the relative extinction (ΔA_V≈ 2 mag). Similarly, Panoramic Survey Telescope and Rapid Response System 1 optical photometry varies on timescales as long as 11 yr (and possibly as short as an hour) and implies comparableA_Vchanges. Near Earth Object Wide-field Infrared Survey Explorer mid-IR light curves also suggest changes on 6 month timescales, with amplitudes consistent with the optical/near-IR extinction variations. However, near-IR spectra, near-IR photometry, and optical photometry obtained in the past year indicate that the source can also be stable on hourly and monthly timescales. From comparison to objects of similar spectral type, the total extinction of 2MASS J0619-2903 seems to beA_V≈ 4–6 mag, with perhaps epochs of lower extinction. Gaia Early Data Release 3 (EDR3) finds that 2MASS J0619-2903 has a wide-separation (1.′2 = 10,450 au) stellar companion, with an isochronal age of${31}_{-10}^{+22}$Myr and a mass of${0.30}_{-0.03}^{+0.04}$M_☉. Adopting this companion’s age and EDR3 distance (145.2 ± 0.6 pc), we estimate a mass of 0.11–0.17M_☉for 2MASS J0619-2903. Altogether, 2MASS J0619-2903 appears to possess an unusually long-lived primordial circumstellar disk, perhaps making it a more obscured analog to the “Peter Pan” disks found around a few M dwarfs in nearby young moving groups.