Uniform Forward-Modeling Analysis of Ultracool Dwarfs. I. Methodology and Benchmarking. (arXiv:2011.12294v2 [astro-ph.SR] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Zhang_Z/0/1/0/all/0/1">Zhoujian Zhang</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Liu_M/0/1/0/all/0/1">Michael C. Liu</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Marley_M/0/1/0/all/0/1">Mark S. Marley</a> (2 and 3), <a href="http://arxiv.org/find/astro-ph/1/au:+Line_M/0/1/0/all/0/1">Michael R. Line</a> (4), <a href="http://arxiv.org/find/astro-ph/1/au:+Best_W/0/1/0/all/0/1">William M. J. Best</a> (5) ((1) Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI, USA, (2) NASA Ames Research Center, Moffett Field, CA, USA, (3) The University of Arizona, Tuscon AZ, USA, (4) School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA, (5) Department of Astronomy, University of Texas at Austin, Austin, Texas, USA)
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.
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.
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