Departmental Doctoral Oral Examination (Thesis Title: “The Space Weather of Ultracool Dwarfs”)

Abstract:

Empirical trends in stellar X-ray and radio luminosities suggest that very low mass stars and brown dwarfs should not produce significant radio emission. Defying these expectations, strong non-thermal emission has been observed in a few UCDs in the 1-10 GHz range, often attributed to global aurorae. At higher radio frequencies, flux due to global aurorae becomes unphysical, and is instead attributed to gyrosynchrotron radiation. In my Ph.D. work I used observations in this frequency range (30-100 GHz) to infer the presence of gyrosynchrotron radiation from ultracool dwarfs in three stars, and to place upper limits on the radio flux on three other stars that were not detected. Prior to this work, only one ultracool dwarf had been detected at such high radio frequencies. My results suggest that gyrosynchrotron radiation from radio active ultracool dwarfs may be more common than previously assumed, indicating a threat to the atmospheric stability of surrounding planets. Another key component of my thesis has been an extensive radio study of the ultracool dwarf TRAPPIST-1. The TRAPPIST-1 system is notable for its system of seven terrestrial planets, at least three of which orbit within the habitable zone. I put upper limits on the quiescent radio emission from the star at 44 and 97.5 GHz using the VLA and ALMA, and monitored the long-term 3 GHz emission from the star over 50 hours to search for variability. I used these results to constrain the possible gyrosynchrotron radiation from the star, as well as the resulting space weather impacts on surrounding planets. My results from the TRAPPIST-1 studies suggest that while the TRAPPIST-1 planets are not regularly exposed to high populations of energetic particles due to stellar activity, supporting the case for planetary habitability.