Revealing star and planet formation with stellar multiplicity
Studies of star and planet formation work to understand the processes that produced the Solar System and the many other systems now known to host exoplanets. Understanding star and planet formation requires measurement of accurate stellar properties at all evolutionary stages of stellar and planetary systems. These stellar properties include age, mass, effective temperature (T [subscript eff]), stellar radius, and stellar multiplicity. Binary stars and higher-order multiples comprise about half of the population of main-sequence solar-type stars, and stellar multiplicity impacts the observed properties of stars across their lifetimes. Because exoplanet and stellar demographics are typically inferred from stellar properties, incorrect stellar characterization because of binaries feeds into biases and errors in stellar populations and exoplanet demographics. In this dissertation, I explored the impact of binary stars in the two scientific contexts of young stellar associations and binary stars that host exoplanets. In my studies of young stellar associations, I developed a simulation suite to perform synthetic spectroscopic surveys. I implemented mass-dependent binary properties to explore the origins of apparent mass-dependent age gradients previously observed in star-forming regions. My subsequent work added starspots to the simulation. I found that although binary stars can explain mass-dependent age gradients, starspots become the dominant contributor to the gradient in populations with Gaia distances. I also explored the nature of the relationship between accretion and circumstellar disks in young stars and found that the inner disks of binaries and single stars are probably similar, and that the inner rim of the dust disk is related to the accretion rate as a result of mass transfer through the disk. These studies demonstrated the importance of considering binary stars when attempting to measure ages or understand star formation histories in young stellar associations. In my studies of main sequence binary star exoplanet hosts, I developed an algorithm to accurately characterize the individual components of binary stars that are unresolved in most observations. As an initial step, I tested this code with an archival sample of M stars. Then, I performed a spectroscopic survey of binary stars from the Kepler sample using the Hobby-Eberly Telescope, and carried out two targeted studies of subsamples from the survey. The first study explored binary stars that supposedly host rocky Earth-analog planets and found that most of them are actually gaseous planets, which has implications for exoplanet demographics and attempts to measure the frequency of Earth analogs. The second study explored the radius distribution of small exoplanets and found that the gap in the radius distribution separating rocky and gaseous exoplanets in single systems was not present in binary stars. This result suggested that the location of the gap may be binary-separation-dependent and therefore “blurred out” by a range of stellar separations in the sample. This series of papers has demonstrated the power of using binary stars that host planets as a laboratory for controlled experiments in planet formation and evolution, because the binary properties leave a record of the planet-forming environment. The work presented in this dissertation has shown the ability of binary stars to influence observations of young stars and exoplanet hosts, and has demonstrated the potential of binary stars to provide a direct link between formation environment and exoplanet properties for the first time.