Kavli Affiliate: Bruce Macintosh
| First 5 Authors: Kielan K. W. Hoch, Quinn M. Konopacky, Travis S. Barman, Christopher A. Theissen, Laci Brock
| Summary:
We present moderate-resolution ($Rsim4000$) $K$ band spectra of the
planetary-mass companion VHS 1256 b. The data were taken with the OSIRIS
integral field spectrograph at the W.M. Keck Observatory. The spectra reveal
resolved molecular lines from H$_{2}$O and CO. The spectra are compared to
custom $PHOENIX$ atmosphere model grids appropriate for young, substellar
objects. We fit the data using a Markov Chain Monte Carlo forward modeling
method. Using a combination of our moderate-resolution spectrum and
low-resolution, broadband data from the literature, we derive an effective
temperature of 1240 K, with a range of 1200–1300 K, a surface gravity of
$log{g}=$ 3.25, with a range of 3.25–3.75 and a cloud parameter of $log
P_{cloud}=$ 6, with a range of 6.0–6.6. These values are consistent with
previous studies, regardless of the new, larger system distance from GAIA EDR3
(22.2$^{+1.1}_{-1.2}$ pc). We derive a C/O ratio of 0.590$_{-0.354}^{+0.280}$
for VHS 1256b. Both our OSIRIS data and spectra from the literature are best
modeled when using a larger 3 $mu$m grain size for the clouds than used for
hotter objects, consistent with other sources in the L/T transition region. VHS
1256 b offers an opportunity to look for systematics in the modeling process
that may lead to the incorrect derivation of properties like C/O ratio in the
high contrast regime.
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