HORuS transmission spectroscopy and revised planetary parameters of KELT-7b

TL;DR

This study uses high-resolution spectroscopy to refine the parameters and orbital alignment of KELT-7b, investigates stellar activity's impact on atmospheric measurements, and emphasizes the importance of stellar monitoring.

Contribution

It provides updated stellar and planetary parameters, measures the system's 3D obliquity, and assesses stellar activity effects on transmission spectroscopy of KELT-7b.

Findings

KELT-7b's orbit is well-aligned with its star's rotation.

Upper limits on atmospheric features are established due to stellar variability.

Stellar activity significantly impacts high-resolution planetary atmosphere observations.

Abstract

We report on the high-resolution spectroscopic observations of two planetary transits of the hot Jupiter KELT-7b ($M_{\rm p}$ $=$ 1.28 $\pm$ 0.17 M$_{\rm Jup}$, $T_{\rm eq}$ $=$ 2028 K) observed with the High Optical Resolution Spectrograph (HORuS) mounted on the 10.4-m Gran Telescopio Canarias (GTC). A new set of stellar parameters are obtained for the rapidly rotating parent star from the analysis of the spectra. Using the newly derived stellar mass and radius, and the planetary transit data of the Transiting Exoplanet Survey Satellite (TESS) together with the HORuS velocities and the photometric and spectroscopic data available in the literature, we update and improve the ephemeris of KELT-7b. Our results indicate that KELT-7 has an angle $\lambda$ = $-$10.55 $\pm$ 0.27 deg between the sky projections of the star's spin axis and the planet's orbital axis. By combining this angle and our newly derived stellar rotation period of 1.38 $\pm$ 0.05 d, we obtained a 3D obliquity $\psi$ = 12.4 $\pm$ 11.7 deg (or 167.6 deg), thus reinforcing that KELT-7 is a well-aligned planetary system. We search for the presence of H$\alpha$, Li I, Na I, Mg I, and Ca II features in the transmission spectrum of KELT-7b but we are only able to determine upper limits of 0.08-1.4 % on their presence after accounting for the contribution of the stellar variability to the extracted planetary spectrum. We also discuss the impact of stellar variability in the planetary data. Our results reinforce the importance of monitoring the parent star when performing high-resolution transmission spectroscopy of the planetary atmosphere in the presence of stellar activity.