Physical properties and optical-infrared transmission spectrum of the giant planet XO-1b

Abstract

We present ten high-precision light curves of four transits in the XO-1 planetary system, obtained using $u$, $g$, $r$, redshifted H$\hat{I}\pm$, $I$ and $z$ filters. We use these to measure the physical properties, orbital ephemeris, and a transmission spectrum of the planet covering the full optical wavelength range. We augment this with published HST/WFC3 observations to construct a transmission spectrum of the planet covering 0.37 to 1.65 $\hat{I}¼$m. Our best-fitting model to this spectrum is for a H${}_2$/He-rich atmosphere containing water (3.05$\ddot{I}ƒ$ confidence), nitrogen-bearing molecules NH${}_3$ and HCN (1.5$\ddot{I}ƒ$) and patchy cloud (1.3$\ddot{I}ƒ$). We find that adding the optical to the near-infrared data does not lead to more precise constraints on the planetary atmosphere in this case. We conduct a detailed investigation into the effect of stellar limb darkening on our results, concluding that the choice of limb darkening law and coefficients is unimportant; such conclusions may not hold for other systems so should be reassessed for all high-quality datasets. The planet radius we measure in the $g$-band is anomalously low and should be investigated with future observations at a higher spectral resolution. From the measured times of transit we determine an improved orbital ephemeris, calculate a lower limit on the modified stellar tidal quality factor of $Q_{\star }^{\prime }>10^{5.6}$, and rule out a previously postulated sinusoidal variation in the transit times.

Accepted for publication in MNRAS. 16 pages, 11 figures, 7 tables