Because M dwarfs make up 75% of the stars in our galaxy and 50% of them host small planets in the habitable zone (HZ), most of the detectable HZ planets must orbit M dwarfs. The star's UV radiation can destroy or alter the planet's atmosphere, which is a necessary condition for detectable surface life. Assessing the lifetime exposure of a planet to stellar UV radiation is critical to our understanding of both the evolution of life and our ability to identify it. We propose to observe a statistical sample of early M stars with well-determined ages to map the evolution of the full UV spectral range. We will use the diagnostic near-UV and far-UV emission lines accessible only with HST's COS to guide new upper-atmosphere models and produce the full stellar spectrum for a wide range of ages. These empirically-guided models will predict the unobservable extreme-UV fluxes, which most strongly affect the heating and erosion of planetary atmospheres. Our carefully-constructed sample consists of cluster or young moving group (YMG) members at ages during the greatest UV evolution. We will target 12 highly-active Tuc-Hor YMG members (40 Myr), 10 intermediately-active Hyades members (650 Myr), and 10 weakly-active old M dwarfs (~Gyr). This program will provide the stellar and planetary communities with a comprehensive study of the UV history of M stars, a realistic full-wavelength grid of model spectra, and tell us which planets in the canonical HZ are most likely to be habitable.
- Pub Date:
- June 2016