UV and X-Ray Emission from Exoplanet Host Stars: Results of the MUSCLES HST Survey and Photochemistry in Exoplanet Atmospheres
Jeff Linsky, Colorado University, Boulder
Exoplanets evolve in the radiation environment created by their host stars. Of particular interest is the UV and X-ray emission produced in the chromospheres and coronae of cooler stars such as M dwarfs. I will summarize the results of our HST MUSCLES Treasury Survey, consisting of complete UV spectra of 7 M-type dwarf stars and 4 K-type dwarf stars, together with coordinated X-ray and optical observations of these stars. The spectral energy distributions of the M stars are very different from G-type stars like the Sun with much stronger far-UV, extreme-UV, and X-ray emission compared to the optical emission. The far-UV emission, in particular the very bright Lyman-alpha line, photodissociates H20, CO2, and CH4 in the outer atmospheres of exoplanets, and the extreme-UV and X-ray emission is the energy source for exoplanet mass loss. I will describe how we correct the observed Lyman-alpha emission from interstellar absorption and present our new model of the chromosphere and corona of the M1.5 V host star GJ 832.
Boundary Conditions for Abiogenesis: Constraints on the UV Environment Relevant to Prebiotic Chemistry on Young Planets
Sukrit Ranjan, Harvard
Recent laboratory studies (e.g., Ritson et al 2012, Patel et al 2015) suggest UV light may have played a critical role in the synthesis of molecules relevant to abiogenesis (prebiotic chemistry), such as RNA. UV light interacts with prebiotic chemistry in ways that are sensitive to wavelength and fluence level (e.g., Ranjan & Sasselov 2016a). Here, we use radiative transfer models to constrain the UV environment on Earth in the era of abiogenesis (~3.9 Ga). We exhaustively explore the available surface fluence as a function of surface conditions and atmospheric composition, exploring all plausible areas of parameter space. We convolve the resultant spectra against action spectra for prebiotically relevant processes to estimate the effect of these variables on prebiotic chemistry. Some workers have also suggested the possibility of abiogenesis on Mars and subsequent panspermic transfer to Earth (e.g, Benner+2015). Therefore, we also explore plausible UV spectral fluences on Mars at 3.9 Ga to evaluate its hospitability for prebiotic chemistry (Ranjan, Wordsworth & Sasselov 2016). Lastly, we calculate the UV fluences on exoplanets orbiting M-dwarfs, and evaluate the implications for the hospitability of these worlds for abiogenesis events.