A search for H$\alpha$ absorption around KELT-3 b and GJ 436 b

TL;DR

This study searched for Hα absorption around hot exoplanets KELT-3 b and GJ 436 b, found no evidence, and provided upper limits, suggesting stellar activity influences detectability of such atmospheric features.

Contribution

First search for Hα absorption in KELT-3 b and GJ 436 b's atmospheres, establishing upper limits and comparing with Lyα observations to understand atmospheric conditions.

Findings

No Hα absorption detected during transit observations.

Upper limits set on hydrogen atmosphere density and extent.

Active star systems may be more conducive to Hα detection.

Abstract

Observations of extended atmospheres around hot planets have generated exciting results concerning the dynamics of escaping planetary material. The configuration of the escaping planetary gas can result in asymmetric transit features, producing both pre- and post-transit absorption in specific atomic transitions. Measuring the velocity and strength of the absorption can provide constraints on the mass loss mechanism and, potentially, clues to the interactions between the planet and the host star. Here we present a search for H$\alpha$ absorption in the circumplanetary environments of the hot planets KELT-3 b and GJ 436 b. We find no evidence for absorption around either planet at any point during the two separate transit epochs that each system was observed. We provide upper limits on the radial extent and density of the excited hydrogen atmospheres around both planets. The null detection for GJ 436 b contrasts with the strong Ly$\alpha$ absorption measured for the same system, suggesting that the large cloud of neutral hydrogen is almost entirely in the ground state. The only confirmed exoplanetary H$\alpha$ absorption to date has been made around the active star HD 189733 b. KELT-3 and GJ 436 are less active than HD 189733, hinting that exoplanet atmospheres exposed to EUV photons from active stars are better suited for H$\alpha$ absorption detection.