Fundamental Parameters of the Lowest Mass Stars to the Highest Mass Planets

The physical and atmospheric properties of ultracool dwarfs are deeply entangled due to the degenerate effects of mass, age, metallicity, clouds and dust, activity, rotation, and possibly formation mechanism on their observed properties. Accurate fundamental parameters for a wide range of substellar objects are crucial to testing stellar and planetary formation theories. To determine these quantities, we construct flux-calibrated spectral energy distributions (SEDs) for 234 M, L, T, and Y dwarfs and calculate bolometric luminosity (Lbol), effective temperature (Teff), mass, surface gravity, radius, spectral indexes, synthetic photometry, and bolometric corrections (BCs) for each object. We use these results to derive Lbol, Teff, and BC polynomial relations across the entire very-low-mass star/brown dwarf/planetary mass regime. We define a subsample of objects with age constraints based on nearby young moving group membership, companionship with a young star, or spectral signatures of low surface gravity. With this subsample, we derive new age-sensitive diagnostics and characterize the reddening of young substellar atmospheres as a redistribution of flux from the near-infrared into the mid-infrared. These results enable accurate, precise, and efficient characterization of very-low-mass objects with limited observational data

Signatures of Cloud, Temperature, and Gravity From Spectra of the Closest Brown Dwarfs

We present medium resolution optical and NIR spectral data for components of the newly discovered WISE J104915.57-531906.1AB (Luhman 16AB) brown dwarf binary. The optical spectra reveal strong 6708 A Li I absorption in both Luhman 16A (8.0+/-0.4 A) and Luhman 16B (3.8+/-0.4 A). Interestingly, this is the first detection of Li I absorption in a T dwarf. Combined with the lack of surface gravity features, the Li I detection constrains the system age to 0.1 - 3 Gyr. In the NIR data, we find strong KI absorption at 1.168, 1.177, 1.243, and 1.254 {\mu}m in both components. Compared to the strength of KI line absorption in equivalent spectral subtype brown dwarfs, Luhman 16A is weaker while Luhman 16B is stronger. Analyzing the spectral region around each doublet in distance scaled flux units and comparing the two sources, we confirm the J band flux reversal and find that Luhman 16B has a brighter continuum in the 1.17 {\mu}m and 1.25 {\mu}m regions than Luhman 16A. Converting flux units to a brightness temperature we interpret this to mean that the secondary is ~ 50 K warmer than the primary in regions dominated by condensate grain scattering. One plausible explanation for this difference is that Luhman 16B has thinner clouds or patchy holes in its atmosphere allowing us to see to deeper, hotter regions. We also detect comparably strong FeH in the 0.9896 {\mu}m Wing-Ford band for both components. Traditionally, a signpost of changing atmosphere conditions from late-type L to early T dwarfs, the persistence and similarity of FeH at 0.9896 {\mu}m in both Luhman 16A and Luhman 16B is an indication of homogenous atmosphere conditions. We calculate bolometric luminosities from observed data supplemented with best fit models for longer wavelengths and find the components are consistent within 1{\sigma} with resultant Teffs of 1310+/-30 K and 1280+/-75 K for Luhman 16AB respectively.Comment: 17 pages, 11 figures, 4 tables. Submitted to ApJ and revised after referee repor

The First Brown Dwarf Discovered by the Backyard Worlds: Planet 9 Citizen Science Project

The Wide-field Infrared Survey Explorer (WISE) is a powerful tool for finding nearby brown dwarfs and searching for new planets in the outer solar system, especially with the incorporation of NEOWISE and NEOWISE-Reactivation data. So far, searches for brown dwarfs in WISE data have yet to take advantage of the full depth of the WISE images. To efficiently search this unexplored space via visual inspection, we have launched a new citizen science project, called "Backyard Worlds: Planet 9," which asks volunteers to examine short animations composed of difference images constructed from time-resolved WISE coadds. We report the discovery of the first new substellar object found by this project, WISEA J110125.95+540052.8, a T5.5 brown dwarf located approximately 34 pc from the Sun with a total proper motion of $\sim$0.7 as yr$^{-1}$. WISEA J110125.95+540052.8 has a WISE $W2$ magnitude of $W2=15.37 \pm 0.09$, this discovery demonstrates the ability of citizen scientists to identify moving objects via visual inspection that are 0.9 magnitudes fainter than the $W2$ single-exposure sensitivity, a threshold that has limited prior motion-based brown dwarf searches with WISE.Comment: 9 pages, 4 figures, 1 table. Accepted for publication in the Astrophysical Journal Letter

Touchstone Stars: Highlights from the Cool Stars 18 Splinter Session

We present a summary of the splinter session on "touchstone stars" -- stars with directly measured parameters -- that was organized as part of the Cool Stars 18 conference. We discuss several methods to precisely determine cool star properties such as masses and radii from eclipsing binaries, and radii and effective temperatures from interferometry. We highlight recent results in identifying and measuring parameters for touchstone stars, and ongoing efforts to use touchstone stars to determine parameters for other stars. We conclude by comparing the results of touchstone stars with cool star models, noting some unusual patterns in the differences.Comment: Proceedings of the 18th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, Eds G. van Belle & H. Harri

Population Properties of Brown Dwarf Analogs to Exoplanets

We present a kinematic analysis of 152 low surface gravity M7-L8 dwarfs by adding 18 new parallaxes (including 10 for comparative field objects), 38 new radial velocities, and 19 new proper motions. We also add low- or moderate-resolution near-infrared spectra for 43 sources confirming their low surface gravity features. Among the full sample, we find 39 objects to be high-likelihood or new bona fide members of nearby moving groups, 92 objects to be ambiguous members and 21 objects that are non-members. Using this age-calibrated sample, we investigate trends in gravity classification, photometric color, absolute magnitude, color–magnitude, luminosity, and effective temperature. We find that gravity classification and photometric color clearly separate 5–130 Myr sources from \u3e3 Gyr field objects, but they do not correlate one to one with the narrower 5–130 Myr age range. Sources with the same spectral subtype in the same group have systematically redder colors, but they are distributed between 1 and 4σ from the field sequences and the most extreme outlier switches between intermediate- and low-gravity sources either confirmed in a group or not. The absolute magnitudes of low-gravity sources from the J band through W3 show a flux redistribution when compared to equivalently typed field brown dwarfs that is correlated with spectral subtype. Low-gravity, late-type L dwarfs are fainter at J than the field sequence but brighter by W3. Low-gravity M dwarfs are \u3e1 mag brighter than field dwarfs in all bands from J through W3. Clouds, which are a far more dominant opacity source for L dwarfs, are the likely cause. On color–magnitude diagrams, the latest-type, low-gravity L dwarfs drive the elbow of the L/T transition up to 1 mag redder and 1 mag fainter than field dwarfs at M J but are consistent with or brighter than the elbow at M W1 and M W2. We conclude that low-gravity dwarfs carry an extreme version of the cloud conditions of field objects to lower temperatures, which logically extends into the lowest-mass, directly imaged exoplanets. Furthermore, there is an indication on color-magnitude diagrams (CMDs; such as M J versus (J–W2)) of increasingly redder sequences separated by gravity classification, although it is not consistent across all CMD combinations. Examining bolometric luminosities for planets and low-gravity objects, we confirm that (in general) young M dwarfs are overluminous while young L dwarfs are normal compared to the field. Using model extracted radii, this translates into normal to slightly warmer M dwarf temperatures compared to the field sequence and lower temperatures for L dwarfs with no obvious correlation with the assigned moving group

Kepler Monitoring of an L Dwarf II. Clouds With Multiyear Lifetimes

We present Kepler, Spitzer Space Telescope, Gemini-North, MMT, and Kitt Peak observations of the L1 dwarf WISEP J190648.47+401106.8. We find that the Kepler optical light curve is consistent in phase and amplitude over the nearly two years of monitoring with a peak-to-peak amplitude of 1.4%. Spitzer Infrared Array Camera 3.6 µm observations are in phase with Kepler with similar light curve shape and peak-to-peak amplitude 1.1%, but at 4.5 µm, the variability has amplitude < 0.1%. Chromospheric Hα emission is variable but not synced with the stable Kepler light curve. A single dark spot can reproduce the light curve but is not a unique solution. An inhomogeneous cloud deck, specifically a region of thick cloud cover, can explain the multi-wavelength data of this ultracool dwarf and need not be coupled with the asynchronous magnetic emission variations. The long life of the cloud is in contrast with weather changes seen in cooler brown dwarfs on the timescale of hours and days.Astronom