On the Evolutionary Pathways of Stars and Extrasolar Planets

Citation

David, Trevor Justin (2018) On the Evolutionary Pathways of Stars and Extrasolar Planets. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9J67F42. https://resolver.caltech.edu/CaltechTHESIS:09132017-153137420

Abstract

In this thesis, I present several studies aimed at determining the evolutionary states of stars and the planets that orbit them. Multiple approaches are undertaken to determine the physical parameters of stars over a wide range of masses, and in the process I evaluate current theoretical models which are commonly used to indirectly determine the properties of stars and planets.

Chapter II concerns the ages of nearby stars more massive than the Sun. These stars, because they are bright and young (less than 1 Gyr) on the average, constitute attractive targets for surveys aiming to directly capture light from planets in wide orbits. The precise masses of directly imaged companions are important for constraining star and planet formation theories, but rely critically on the host star ages. I show that sky-projected rotational velocity is a vital parameter in age-determination for intermediate-mass stars. Rapid rotation induces large pole-to-equator gradients in the photospheric temperature and surface gravity, such that a star seen pole-on appears hotter and higher gravity (and thus younger) than a star with identical properties seen edge-on. I use intermediate-band photometry centered on the hydrogen Balmer series and projected rotational velocities to determine atmospheric parameters and ages for approximately 3500 nearby stars with masses in the range of 1-10 solar masses. I validate the method using four open clusters, in the process finding ages for α Persei and the Hyades that are younger at ~70 Myr and older at ~830 Myr, respectively, than canonical values.

In Chapters III through V, I present orbital solutions and fundamental parameters for eclipsing binaries (EBs), newly discovered from the K2 mission, in the Pleiades open cluster (125 Myr) and the Upper Scorpius OB association (5-10 Myr). EBs, particularly those in coeval stellar populations, are valuable benchmarks for evaluating evolutionary models. Such benchmarks are particularly rare at low masses and young ages, and my work has increased the sample by 80% for stars less massive than the Sun and younger than 150 Myr. By jointly fitting eclipse observations and radial velocity measurements, one can directly determine the masses and radii of stars in an EB with percent-level precision. I use newly determined masses and radii to demonstrate a systematic temperature offset of approximately 200 K between empirical relations and model predictions. This result is in agreement with literature on the temperature suppression observed in low-mass stars, believed to be related to magnetic activity and inhibited convection. I show that stellar ages determined from the mass-radius diagram appear systematically older than those determined for the same stars in a Hertzsprung-Russell (H-R) diagram. A precise distance determination for the Pleiades EB HCG 76 is in agreement with the literature consensus and formally excludes the now discredited trigonometric measurement from Hipparcos. The orbital periods, eccentricities, and stellar spin periods determined from K2 photometry are compared with theoretical expectations from tidal dissipation. In Chapter VI, I present preliminary results on more recently discovered EBs in Upper Scorpius, and updated interpretations of previously published systems. I use the combined data to pave the way for an empirical pre-main-sequence mass-radius relation over a broad range of masses and determine an age of Upper Scorpius which is intermediate to the canonical age (3-5 Myr, as derived from low-mass pre-main-sequence stars) and a more recent estimate (9-13 Myr, as derived from intermediate-mass stars) from H-R diagram analyses.

In Chapter VII, I present observations of the low-mass pre-main-sequence star RIK-210. This star shows a variable eclipse-like signature, which persisted through 78 days of K2 photometry but was apparently absent in archival photometry. Follow-up observations demonstrate that RIK-210 is a single star with no massive companion orbiting at the period of the eclipse-like signature. The flux diminutions are in phase with the stellar rotation, behavior seen in some young stars that are periodically obscured by an accretion disk, but RIK-210 lacks such a protoplanetary disk. I consider various explanations for the observations and favor a model in which charged dust is trapped in a rigidly-rotating magnetosphere. The source of such dust could be from one or more close-in planets or residual planet-forming material drifting in towards the star.

In Chapter VIII I present the discovery and characterization of K2-33 b, a Neptune-sized planet closely orbiting a low-mass star in Upper Scorpius and one of the youngest exoplanets currently known. K2-33 b provides evidence that planets with substantial gaseous envelopes can be found close to their stars shortly after dispersal of the primordial protoplanetary disk. Given the planet's age (5-10 Myr), in situ formation or migration through the protoplanetary disk are the only plausible formation scenarios. K2-33 b is unusually large in size when compared to planets with similar orbital periods around low-mass field stars. I interpret this as tentative evidence that the planet is still contracting, experiencing photoevaporative atmospheric mass-loss, or both. In Chapter IX I present preliminary results from a study aimed at determining the prevalence of close-in planets at the epoch of primordial disk dispersal. Using K2 photometry for hundreds of pre-main-sequence stars in Upper Scorpius I search for transiting planets, assess survey completeness, and determine the occurrence rates or upper limits to such rates for large planets in close orbits around low-mass stars. With the singular detection of K2-33 b, I determine a rate of close-in Neptune- to Jupiter-sized planets higher than that for low-mass field stars but in closer agreement with the rate for sub-Neptune planets. Given the extreme youth of K2-33 b, I tentatively interpret these results as an indication that the planet is a progenitor of the abundant class of close-in sub-Neptunes.

Considered collectively, my results highlight the importance of benchmark systems. Nearly all branches of astrophysics are reliant in some regard on stellar evolutionary models, but the magnitude and sense of any particular systematic offset contained in these models is often poorly understood. Eclipsing binaries and planets located in stellar clusters provide firm anchors to theoretical models aiming to predict the evolution of such objects. The need for benchmarks and model calibration is particularly acute at young ages, when stars and planets are rapidly evolving and where theoretical evolutionary models are highly uncertain. While statistical studies of large populations can yield far-reaching results that move fields forward, it is the detailed characterization of individual systems that often reveal salient clues about the inaccuracies of assumptions underlying larger scale studies. Benchmark systems, whether they are clusters, eclipsing binaries, or well-characterized planets, offer the best path forward in refining our understanding of the evolution of stars and planets.

Thesis Files