Moderate-Resolution $K$-Band Spectroscopy of Substellar Companion $\kappa$ Andromedae b

Abstract

We present moderate-resolution ($R\sim 4000$) $K$ band spectra of the "super-Jupiter," $\kappa$ Andromedae b. The data were taken with the OSIRIS integral field spectrograph at Keck Observatory. The spectra reveal resolved molecular lines from H${}_2$O and CO. The spectra are compared to a custom $PHOENIX$ atmosphere model grid appropriate for young planetary-mass 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 $T_{\mathrm{eff}}$ = 1950 - 2150 K, a surface gravity of $\log g=3.5-4.5$, and a metallicity of [M/H] = $-0.2-0.0$. These values are consistent with previous estimates from atmospheric modeling and the currently favored young age of the system ($<$50 Myr). We derive a C/O ratio of 0.70${}_{-0.24}^{+0.09}$ for the source, broadly consistent with the solar C/O ratio. This, coupled with the slightly subsolar metallicity, implies a composition consistent with that of the host star, and is suggestive of formation by a rapid process. The subsolar metallicity of $\kappa$ Andromedae b is also consistent with predictions of formation via gravitational instability. Further constraints on formation of the companion will require measurement of the C/O ratio of $\kappa$ Andromedae A. We also measure the radial velocity of $\kappa$ Andromedae b for the first time, with a value of $-1.4\pm 0.9\mathrm{km}{\mathrm{s}}^{-1}$ relative to the host star. We find that the derived radial velocity is consistent with the estimated high eccentricity of $\kappa$ Andromedae b.

23 pages, 18 figures. Accepted for publication in AJ