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Register a new userIndividual Dynamical Masses of Ultracool Dwarfs
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Trent J. Dupuy
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We present the full results of our decade-long astrometric monitoring programs targeting 31 ultracool binaries with component spectral types M7-T5. Joint analysis of resolved imaging from Keck Observatory and Hubble Space Telescope and unresolved astrometry from CFHT/WIRCam yields parallactic distances for all systems, robust orbit determinations for 23 systems, and photocenter orbits for 19 systems. As a result, we measure 38 precise individual masses spanning 30-115 $M_{rm Jup}$. We determine a model-independent substellar boundary that is $approx$70 $M_{rm Jup}$ in mass ($approx$L4 in spectral type), and we validate Baraffe et al. (2015) evolutionary model predictions for the lithium-depletion boundary (60 $M_{rm Jup}$ at field ages). Assuming each binary is coeval, we test models of the substellar mass-luminosity relation and find that in the L/T transition, only the Saumon & Marley (2008) hybrid models accounting for cloud clearing match our data. We derive a precise, mass-calibrated spectral type-effective temperature relation covering 1100-2800 K. Our masses enable a novel direct determination of the age distribution of field brown dwarfs spanning L4-T5 and 30-70 $M_{rm Jup}$. We determine a median age of 1.3 Gyr, and our population synthesis modeling indicates our sample is consistent with a constant star formation history modulated by dynamical heating in the Galactic disk. We discover two triple-brown-dwarf systems, the first with directly measured masses and eccentricities. We examine the eccentricity distribution, carefully considering biases and completeness, and find that low-eccentricity orbits are significantly more common among ultracool binaries than solar-type binaries, possibly indicating the early influence of long-lived dissipative gas disks. Overall, this work represents a major advance in the empirical view of very low-mass stars and brown dwarfs.
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Brown dwarf spectra are rich in information revealing of the chemical and physical processes operating in their atmospheres. We apply a recently developed atmospheric retrieval tool to an ensemble of late T-dwarf (600-800K) near infrared spectra. With these spectra we are able to place direct constraints the molecular abundances of H$_2$O, CH$_4$, CO, CO$_2$, NH$_3$, H$_2$S, and Na+K, gravity, thermal structure (and effective temperature), photometric radius, and cloud optical depths. We find that ammonia, water, methane, and the alkali metals are present and well constrained in all 11 objects. From the abundance constraints we find no significant trend in the water, methane, or ammonia abundances with temperature, but find a very strong ($>$25$sigma$) increasing trend in the alkali metal abundances with effective temperature, indicative of alkali rainout. We also find little evidence for optically thick clouds. With the methane and water abundances, we derive the intrinsic atmospheric metallicity and carbon-to-oxygen ratios. We find in our sample, that metallicities are typically sub solar and carbon-to-oxygen ratios are somewhat super solar, different than expectations from the local stellar population. We also find that the retrieved vertical thermal profiles are consistent with radiative equilibrium over the photospheric regions. Finally, we find that our retrieved effective temperatures are lower than previous inferences for some objects and that our radii are larger than expectations from evolutionary models, possibly indicative of un-resolved binaries. This investigation and methodology represents a paradigm in linking spectra to the determination of the fundamental chemical and physical processes governing cool brown dwarf atmospheres.
We present a large forward-modeling analysis for 55 late-T (T7-T9) dwarfs, using low-resolution ($Rapprox150$) near-infrared spectra and cloudless Sonora-Bobcat model atmospheres. We derive the objects effective temperatures, surface gravities, metallicities, radii, masses, and luminosities using our newly developed Bayesian framework, and use the resulting population properties to test the model atmospheres. We find (1) our objects fitted metallicities are 0.3-0.4 dex lower than those of nearby stars; (2) their ages derived from spectroscopic parameters are implausibly young; (3) their fitted temperatures show a similar spread as empirical temperature scales at a given spectral type but are $sim100$ K hotter for $geqslant$T8 dwarfs; and (4) their spectroscopically inferred masses are unphysically small. These results suggest the Sonora-Bobcat assumptions of cloudless and chemical-equilibrium atmospheres do not adequately reproduce late-T dwarf spectra. We also find a gravity- and a metallicity-dependence of temperatures. Combining the resulting parameter posteriors of our sample, we quantify the degeneracy between surface gravity and metallicity such that an increase in $Z$ combined with a $3.4times$ increase in $log{g}$ results in a spectrum that has similar fitted parameters. We note the systematic difference between our 1.0-2.5 $mu$m spectra and the Sonora-Bobcat models is $approx$2-4% of the objects peak $J$-band fluxes, implying modeling systematics will exceed measurement uncertainties when analyzing data with $J$-band S/N $gtrsim50$. Using our large sample, we examine the fitting residuals as a function of wavelength and atmospheric properties to discern how to improve the models. Our work constitutes the largest analysis of brown dwarf spectra using multi-metallicity models and the most systematic examination of ultracool model atmospheres to date.
In this paper, we describe how to use the Maximum Reduced Proper Motion method (Phan-Bao et al. 2003) to detect 57 nearby L and late-M dwarfs (d_phot <= 30 pc): 36 of them are newly discovered. Spectroscopic observations of 43 of the 57 ultracool dwarfs were previously reported in Martin et al. (2010). These ultracool dwarfs were identified by color criteria in ~5,000 square degrees of the DENIS database and then further selected by the method for spectroscopic follow-up to determine their spectral types and spectroscopic distances. We also report here our newly measured proper motions of these ultracool dwarfs from multi-epoch images found in public archives (ALADIN, DSS, 2MASS, DENIS), with at least three distinct epochs and time baselines of 2 to 46 years.
We present a forward-modeling framework using the Bayesian inference tool Starfish and cloudless Sonora-Bobcat model atmospheres to analyze low-resolution ($Rapprox80-250$) near-infrared ($1.0-2.5$ $mu$m) spectra of T dwarfs. Our approach infers effective temperatures, surface gravities, metallicities, radii, and masses, and by accounting for uncertainties from model interpolation and correlated residuals due to instrumental effects and modeling systematics, produces more realistic parameter posteriors than traditional ($chi^2$-based) spectral-fitting analyses. We validate our framework by fitting the model atmospheres themselves and finding negligible offsets between derived and input parameters. We apply our methodology to three well-known benchmark late-T dwarfs, HD 3651B, GJ 570D, and Ross 458C, using both solar and non-solar metallicity atmospheric models. We also derive these benchmarks physical properties using their bolometric luminosities, their primary stars ages and metallicities, and Sonora-Bobcat evolutionary models. Assuming the evolutionary-based parameters are more robust, we find our atmospheric-based, forward-modeling analysis produces two outcomes. For HD 3615B and GJ 570D, spectral fits provide accurate $T_{rm eff}$ and $R$ but underestimated $log{g}$ (by $approx1.2$ dex) and $Z$ (by $approx0.35$ dex), likely due to the systematics from modeling the potassium line profiles. For Ross 458C, spectral fits provide accurate $log{g}$ and $Z$ but overestimated $T_{rm eff}$ (by $approx120$ K) and underestimated $R$ (by $approx1.6times$), likely because our model atmospheres lack clouds, reduced vertical temperature gradients, or disequilibrium processes. Finally, the spectroscopically inferred masses of these benchmarks are all considerably underestimated.
High resolution spectroscopy of the lowest-mass stars and brown dwarfs reveals their origins, multiplicity, compositions and physical properties, with implications for the star formation and chemical evolution history of the Milky Way. We motivate the need for high-resolution, infrared spectroscopic surveys to reach these faint sources.
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