Time-resolved Ultraviolet Spectroscopy of the M-dwarf GJ 876 Exoplanetary System

TL;DR

This study provides detailed ultraviolet spectra of the M-dwarf GJ 876, revealing its strong UV radiation, flare activity, and implications for exoplanet habitability modeling.

Contribution

It offers the first high-resolution UV spectral data of GJ 876, including LyA reconstruction and flare analysis, improving atmospheric models of planets around low-mass stars.

Findings

LyA flux is about 70% of total UV flux, 2500 times solar value.

Detected strong fluorescent H2 emission at >2000 K.

Observed a significant UV flare with flux ratios >= 10.

Abstract

Extrasolar planets orbiting M-stars may represent our best chance to discover habitable worlds in the coming decade. The ultraviolet spectrum incident upon both Earth-like and Jovian planets is critically important for proper modeling of their atmospheric heating and chemistry. In order to provide more realistic inputs for atmospheric models of planets orbiting low-mass stars, we present new near- and far-ultraviolet (NUV and FUV) spectroscopy of the M-dwarf exoplanet host GJ 876 (M4V). Using the COS and STIS spectrographs aboard the Hubble Space Telescope, we have measured the 1150-3140A spectrum of GJ 876. We have reconstructed the stellar HI LyA emission line profile, and find that the integrated LyA flux is roughly equal to the rest of the integrated flux (1150-1210A + 1220-3140A) in the entire ultraviolet bandpass (F(LyA)/F(FUV+NUV) ~0.7). This ratio is ~ 2500x greater than the solar value. We describe the ultraviolet line spectrum and report surprisingly strong fluorescent emission from hot H2 (T(H2) > 2000 K). We show the light-curve of a chromospheric + transition region flare observed in several far-UV emission lines, with flare/quiescent flux ratios >= 10. The strong FUV radiation field of an M-star (and specifically LyA) is important for determining the abundance of O2 -- and the formation of biomarkers -- in the lower atmospheres of Earth-like planets in the habitable zones of low-mass stars.