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

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 on board the Hubble Space Telescope, we have measured the 1150-3140 Å spectrum of GJ 876. We have reconstructed the stellar H I Lyα emission line profile, and find that the integrated Lyα flux is roughly equal to the rest of the integrated flux (1150-1210 Å + 1220-3140 Å) in the entire ultraviolet bandpass (F(Lyα)/F(FUV+NUV) ≈ 0.7). This ratio is ~2500× 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 Lyα) 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.