Date of Award

Spring 1-1-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Astrophysical & Planetary Sciences

First Advisor

Kevin France

Second Advisor

Nils Halverson

Third Advisor

James Green

Fourth Advisor

David Brain

Fifth Advisor

Eric Wolf

Abstract

A low-mass star emits only 0.001-0.01% of its electromagnetic energy in the ultraviolet (100 – 1700 Å), yet the implications for planets are profound. Radiation at the short-wavelength end of this range, the extreme ultraviolet (100 – 912 Å), powers planetary atmospheric escape that can be observed through transit spectroscopy in the far ultraviolet (FUV; 912 – 1700 Å). In addition, FUV light that penetrates further into a planet’s atmosphere photolyzes molecules, driving nonthermal chemistry capable of producing O2 and O3 abiotically. I present results from extensive Hubble Space Telescope (HST) observations of the FUV emission from low-mass stars, focusing on its variability in time. Using all available archival data, I determined an astrophysical noise floor on detectable absorption in the C II, Si III, and Si IV FUV lines due to a transiting evaporating planet. I analyzed such a transit of the evaporating hot-Neptune GJ 436b, finding no detectable absorption in C II or Si III despite a large (>50%) transit in Lyα, but placing upper limits that agree with a photochemical-hydrodynamical model of the planetary outflow. These observations of GJ 436 were part of the MUSCLES Treasury Survey, a much larger dataset covering 7 M and 4 K dwarf stars. A time series analysis of all of these data constrained FUV flares on the surveyed M dwarfs, which I augmented with archival data on well-studied flare stars. The analysis confirmed that M dwarf flares are ubiquitous. In relative units, the FUV flares of the ``inactive’’ MUSCLES M dwarfs are equally as energetic and frequent as those of the M dwarf flare stars. Both flare ~3 orders of magnitude more frequently than the sun. Indeed, flares possibly (if not probably) dominate the energy budget of FUV emission from M dwarf stars. Highly energetic flares occurring roughly yearly could annihilate most of the ozone from an Earth-like atmosphere. However, the effect is short-lived, unless additional reactions not accounted for or particle events play a dominant role.

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