HST/STIS Lyman-alpha observations of the quiet M dwarf GJ436: Predictions for the exospheric transit signature of the hot neptune GJ436b

TL;DR

This study presents the first high-quality Lyman-alpha emission spectrum of a quiet M dwarf star GJ436, predicts the exospheric transit signature of its hot Neptune planet GJ436b, and establishes a correlation between X-ray and Lyman-alpha emissions for M dwarfs.

Contribution

It provides the first Lyman-alpha observations of a quiet M dwarf hosting an exoplanet and predicts the exospheric transit signature of GJ436b, enhancing understanding of planetary evaporation signatures.

Findings

Detected bright Lyman-alpha emission from GJ436, a quiet M dwarf.

Estimated an ~11% Lyman-alpha absorption signature of GJ436b's exosphere.

Derived a correlation between X-ray and Lyman-alpha emissions for M dwarfs.

Abstract

Lyman-alpha (Lya) emission of neutral hydrogen (1215.67 Angstr\"om) is the main contributor to the ultraviolet flux of low-mass stars such as M dwarfs. It is also the main light source used in studies of the evaporating upper atmospheres of transiting extrasolar planets with ultraviolet transmission spectroscopy. However, there are very few observations of the Lya emissions of quiet M dwarfs, and none exist for those hosting exoplanets. Here, we present Lya observations of the hot-neptune host star GJ436 with the Hubble Space Telescope Imaging Spectrograph (HST/STIS). We detect bright emission in the first resolved and high quality spectrum of a quiet M dwarf at Lya. Using an energy diagram for exoplanets and an N-body particle simulation, this detection enables the possible exospheric signature of the hot neptune to be estimated as a ~11% absorption in the Lya stellar emission, for a typical mass-loss rate of 10^10 g/s. The atmosphere of the planet GJ436b is found to be stable to evaporation, and should be readily observable with HST. We also derive a correlation between X-ray and Lya emissions for M dwarfs. This correlation will be useful for predicting the evaporation signatures of planets transiting other quiet M dwarfs.