Spots and Flares: Stellar Activity in the Time Domain Era

Abstract

Thesis (Ph.D.)--University of Washington, 2015

Time domain photometric surveys for large numbers of stars have ushered in a new era of statistical studies of astrophysics. This new parameter space allows us to observe how stars behave and change on a human timescale, and facilitates ensemble studies to understand how stars change over cosmic timescales. With current and planned time domain stellar surveys, we will be able to put the Sun in a Galactic context, and discover how typical or unique our parent star truly is. The goal of this thesis is to develop techniques for detecting and analyzing the most prominent forms of magnetic activity from low-mass stars in modern time domain surveys: starspots and flares. Magnetic field strength is a fundamental property that decays over a star's life. As a result, flux modulations from both flares and starspots become smaller amplitude and more infrequent in light curves. Methods for detecting these forms of magnetic activity will be extensible to future time domain surveys, and helpful in characterizing the properties of stars as they age. Flares can be detected in sparsely sampled wide field surveys by searching for bright single-point outliers in light curves. Using both red optical and near infrared data from ground-based surveys over many years, I have constrained the rate of flares in multiple wavelengths for an ensemble of M dwarfs. Studying flares in these existing ground-based datasets will enable predictions for future survey yields. Space-based photometry enables continuous and precise monitoring of stars for many years, which is crucial for obtaining a complete census of flares from a single star. Using 11 months of 1-minute photometry for the M dwarf GJ 1243, I have amassed over 6100 flare events, the largest sample of white light flares for any low-mass star. I have also created the first high fidelity empirical white light flare template, which shows three distinct phases in typical flare light curves. With this template, I demonstrate that complex multi-peaked flares can be decomposed into their constituent flare events. This is the first modern study of the detailed white light morphology of stellar flares. Space-based survey data is also ideal for studying starspots, whose photometric modulation amplitude is typically much smaller than for flares. Using 4 years of 30 minute photometry for GJ 1243, I have traced the sizes and longitudes for multiple large starspots. A primary starspot is found that is stable in position and size over the 4 years of data, as well as secondary starspot features that decay on 100 to 500 day timescales and evolve in longitude. The secular longitude evolution of the secondary starspots indicates a very low rate of differential rotation on this rapidly rotating low-mass star. The presence of a transiting exoplanet can provide a great deal of information about the sizes and locations of starspots on the host star. When the planet crosses in front of a starspot, a small deviation in the predicted transit light curve is observed. By tracing these transit anomalies, I have detected more than 100 distinct starspots in 4 years of data on the young G2 dwarf, Kepler 17. These starspots are up to an order of magnitude larger than those on the Sun, and this star shows an almost four times larger amplitude of differential rotation than on the Sun.