SDSS J141624.08+134826.7: A Nearby Blue L Dwarf From the Sloan Digital Sky Survey

We present the discovery of a bright (J = 13.1 mag) nearby L6 dwarf found in a search for L-type ultracool subdwarfs in the Sloan Digital Sky Survey (SDSS) Data Release 7. SDSS J141624.08+134826.7 exhibits blue near-infrared colors compared to other optically-typed L6 objects, but its optical and near-infrared spectra do not show metal-poor features characteristic of known L-type ultracool subdwarfs. Instead, SDSS J141624.08+134826.7 is probably a nearby example of the class of L dwarfs with low condensate opacities which exhibit unusually blue near-infrared colors for a given spectral type. Its deep 1.4 and 1.9 um H2O absorption bands, weak 2.3 um CO feature, strong 0.99 um FeH band, and shallow optical TiO and CaH bands resemble the spectra of other blue L dwarfs which are believed to have unusually thin or large-grained cloud structure. The luminosity of SDSS J141624.08+134826.7 implies that it is either a high-mass brown dwarf or a low mass star, depending on its age, and its UVW space motion suggests a thin-disk membership. With a spectrophotometric distance of 8.4 +/- 1.9 pc, SDSS J141624.08+134826.7 is one of the nearest L dwarfs to the Sun and is therefore an excellent target for high resolution imaging, spectroscopic, and astrometric follow-up observations.Comment: Accepted by ApJ; 9 pages, 3 figure

The Early Evolution of Stars and Exoplanet Systems: Exploring and Exploiting Nearby, Young Stars

Our knowledge of the population of young (age ≾750 Myr) stars that lie within ~120 pc of the Sun is rapidly accelerating. The vast majority of these nearby, young stars can be placed in kinematically coherent groups (nearby, young moving groups; NYMGs). NYMGs and their member stars afford unmatched opportunities to explore a wide variety of aspects of the early evolution of stars and exoplanet systems, including stellar initial mass functions and age determination methods; the magnetic activities and high-energy radiation environments of young, late-type stars; the dynamics of young binary and hierarchical multiple systems; the late evolutionary stages of circumstellar disks; and, especially, direct-imaging discovery and characterization of massive young exoplanets. In this Astro2020 Science White Paper, we describe how our understanding of these and many other aspects of the early lives of stars and planetary systems is ripe for progress over the next decade via the identification and study of NYMG members with present and next-generation facilities and instruments

Studying the Physical Diversity of Late-M Dwarfs with Dynamical Masses

We present a systematic study of the physical properties of late-M dwarfs based on high-quality dynamical mass measurements and near-infrared (NIR) spectra. We use astrometry from Keck NGS and LGS AO imaging to determine orbits for late-M binaries. We find that LP 349-25 (M7.5+M8) is a pair of young brown dwarfs (Mtot = 0.120 Msun) for which Lyon and Tucson evolutionary models jointly predict an age of 140+/-30 Myr. This is consistent with the age of the Pleiades, but at least LP 349-25A defies the empirical Pleiades lithium depletion boundary, implying that the system is in fact older and that evolutionary models underpredict the component luminosities. We find that LHS 1901AB (M6.5+M6.5) is a pair of very low-mass stars (Mtot = 0.194 Msun) with model-derived ages consistent with limits from its lack of activity (> 6 Gyr). Our improved orbit for Gl 569Bab (M8.5+M9) results in a higher mass for this binary (Mtot = 0.140 Msun) compared to previous work (0.125 Msun). We use these masses along with our published results for 2MASS J2206-2047AB (M8+M8) to test four sets of ultracool model atmospheres currently in use. Fitting these models to our NIR integrated-light spectra provides temperature estimates warmer by ~250 K than those derived independently from Dusty evolutionary models given the measured masses and luminosities. We propose that model atmospheres are more likely to be the source of this discrepancy, as it would be difficult to explain a uniform temperature offset over such a wide range of masses, ages, and activity levels in the context of evolutionary models. Our results contrast those of Konopacky et al. as we find an opposite and smaller mass discrepancy from what they report when we adopt their model-testing approach since our Teff estimates from fitting spectra are ~650 K higher than from their fitting of broadband photometry alone.Comment: 53 pages, 12 figures, accepted to Ap

HIP 38939B: A New Benchmark T Dwarf in the Galactic Plane Discovered with Pan-STARRS1

We report the discovery of a wide brown dwarf companion to the mildly metal-poor ([Fe/H]=-0.24), low galactic latitude (b = 1.88 deg) K4V star HIP 38939. The companion was discovered by its common proper motion with the primary and its red optical (Pan-STARRS1) and blue infrared (2MASS) colors. It has a projected separation of 1630 AU and a near-infrared spectral type of T4.5. As such it is one of only three known companions to a main sequence star which have early/mid-T spectral types (the others being HN Peg B and eps Indi B). Using chromospheric activity we estimate an age for the primary of 900{+1900,-600} Myr. This value is also in agreement with the age derived from the star's weak ROSAT detection. Comparison with evolutionary models for this age range indicates that HIP 38939B falls in the mass range 38+/-20 Mjup with an effective temperature range of 1090+/-60 K. Fitting our spectrum with atmospheric models gives a best fitting temperature of 1100 K. We include our object in an analysis of the population of benchmark T dwarfs and find that while older atmospheric models appeared to over-predict the temperature of the coolest objects compared to evolutionary models, more recent atmospheric models provide better agreement.Comment: ApJ, in press. Tiny changes incorporated into final version: added analysis of likelihood of companionship, clarified the fitting proceedure, and updated the benchmark analysis to highlight when the quoted evolutionary models use the atmospheric model they are being compared to as a boundary conditio

Surveying Nearby Brown Dwarfs with HGCA: Direct Imaging Discovery of a Faint, High-Mass Brown Dwarf Orbiting HD 176535 A

Brown dwarfs with well-measured masses, ages and luminosities provide direct benchmark tests of substellar formation and evolutionary models. We report the first results from a direct imaging survey aiming to find and characterize substellar companions to nearby accelerating stars with the assistance of the Hipparcos-Gaia Catalog of Accelerations (HGCA). In this paper, we present a joint high-contrast imaging and astrometric discovery of a substellar companion to HD 176535 A, a K3.5V main-sequence star aged approximately $3.59_{-1.15}^{+0.87}$ Gyrs at a distance of $36.99 \pm 0.03$ pc. In advance of our high-contrast imaging observations, we combined precision HARPS RVs and HGCA astrometry to predict the potential companion's location and mass. We thereafter acquired two nights of KeckAO/NIRC2 direct imaging observations in the $L'$ band, which revealed a companion with a contrast of $\Delta L'_p = 9.20\pm0.06$ mag at a projected separation of $\approx$0.\!\!''35 ($\approx$13 AU) from the host star. We revise our orbital fit by incorporating our dual-epoch relative astrometry using the open-source MCMC orbit fitting code $\tt orvara$. HD 176535 B is a new benchmark dwarf useful for constraining the evolutionary and atmospheric models of high-mass brown dwarfs. We found a luminosity of $\rm log(L_{bol}/L_{\odot}) = -5.26\pm0.06$ and a model-dependent effective temperature of $980 \pm 35$ K for HD 176535 B. Our dynamical mass suggests that some substellar evolutionary models may be underestimating luminosity for high-mass T dwarfs. Given its angular separation and luminosity, HD 176535 B would make a promising candidate for Aperture Masking Interferometry with JWST and GRAVITY/KPIC, and further spectroscopic characterization with instruments like the CHARIS/SCExAO/Subaru integral field spectrograph

Dynamical Mass of the M8+M8 Binary 2MASS J22062280-2047058AB

We present Keck laser guide star adaptive optics imaging of the M8+M8 binary 2MASS J2206-2047AB. Together with archival HST, Gemini-North, and VLT data, our observations span 8.3 years of the binary's 35 year orbital period, and we determine a total dynamical mass of 0.15 (+0.05,-0.03) Msun, with the uncertainty dominated by the parallax error. Using the measured total mass and individual luminosities, the Tucson and Lyon evolutionary models both give an age for the system of 0.4 (+9.6, -0.2) Gyr, which is consistent with its thin disk space motion derived from the Besancon Galactic structure model. Our mass measurement combined with the Tucson (Lyon) evolutionary models also yields precise effective temperatures, giving 2660+-100 K and 2640+-100K (2550+-100 K and 2530+-100 K) for components A and B, respectively. These temperatures are in good agreement with estimates for other M8 dwarfs (from the infrared flux method and the M8 mass benchmark LHS 2397aA), but atmospheric model fitting of the NIR spectrum gives hotter temperatures of 2800+-100 K for both components. This modest discrepancy can be explained by systematic errors in the atmospheric models or by a slight underestimate of the distance (and thus mass and age) of the system. We also find the observed near-infrared colors and magnitudes do not agree with those predicted by the Lyon Dusty models, given the known mass.Comment: ApJ, in pres

The Early Evolution of Stars and Exoplanet Systems: Exploring and Exploiting Nearby, Young Stars

Our knowledge of the population of young (age ≾750 Myr) stars that lie within ~120 pc of the Sun is rapidly accelerating. The vast majority of these nearby, young stars can be placed in kinematically coherent groups (nearby, young moving groups; NYMGs). NYMGs and their member stars afford unmatched opportunities to explore a wide variety of aspects of the early evolution of stars and exoplanet systems, including stellar initial mass functions and age determination methods; the magnetic activities and high-energy radiation environments of young, late-type stars; the dynamics of young binary and hierarchical multiple systems; the late evolutionary stages of circumstellar disks; and, especially, direct-imaging discovery and characterization of massive young exoplanets. In this Astro2020 Science White Paper, we describe how our understanding of these and many other aspects of the early lives of stars and planetary systems is ripe for progress over the next decade via the identification and study of NYMG members with present and next-generation facilities and instruments

Astrometric accelerations as dynamical beacons : discovery and characterization of HIP 21152 B, the First T-dwarf companion in the Hyades * * Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

Benchmark brown dwarf companions with well-determined ages and model-independent masses are powerful tools to test substellar evolutionary models and probe the formation of giant planets and brown dwarfs. Here, we report the independent discovery of HIP 21152 B, the first imaged brown dwarf companion in the Hyades, and conduct a comprehensive orbital and atmospheric characterization of the system. HIP 21152 was targeted in an ongoing high-contrast imaging campaign of stars exhibiting proper-motion changes between Hipparcos and Gaia, and was also recently identified by Bonavita et al. (2022) and Kuzuhara et al. (2022). Our Keck/NIRC2 and SCExAO/CHARIS imaging of HIP 21152 revealed a comoving companion at a separation of 0.″37 (16 au). We perform a joint orbit fit of all available relative astrometry and radial velocities together with the Hipparcos-Gaia proper motions, yielding a dynamical mass of 24−4+6MJup , which is 1–2σ lower than evolutionary model predictions. Hybrid grids that include the evolution of cloud properties best reproduce the dynamical mass. We also identify a comoving wide-separation (1837″ or 7.9 × 104 au) early-L dwarf with an inferred mass near the hydrogen-burning limit. Finally, we analyze the spectra and photometry of HIP 21152 B using the Saumon & Marley (2008) atmospheric models and a suite of retrievals. The best-fit grid-based models have f sed = 2, indicating the presence of clouds, T eff = 1400 K, and logg=4.5dex . These results are consistent with the object’s spectral type of T0 ± 1. As the first benchmark brown dwarf companion in the Hyades, HIP 21152 B joins the small but growing number of substellar companions with well-determined ages and dynamical masses