From Stars to Galaxies: Unveiling Their Properties, Connection, and Evolution With Next Generation Stellar Models

Abstract

Many areas of astrophysics spanning orders of magnitudes in physical scales—from exoplanets to high-redshift galaxies—are deeply rooted in our interpretation of light emitted by stars. Thus the importance of a well-tested and comprehensive set of stellar models extends well beyond the realm of stellar astrophysics. In this thesis, I use a new set of stellar evolution models to study the physical properties and the evolution of stars both as individual astrophysical objects and the constituents of stellar populations. In the first half of this thesis, I describe the construction of the new stellar evolution models and explore the implications of several model uncertainties. I begin by introducing the MESA Isochrones and Stellar Tracks (MIST) project, a single, self-consistent database of stellar evolutionary tracks and isochrones computed over a wide range of masses, ages, metallicities, and evolutionary states. The models are compared extensively with other models in the literature and a variety of observations. Next, I investigate one of the key model ingredients, the surface boundary condition, in the stellar evolution calculations. In particular, I critically evaluate its influence on the effective temperatures of red giant stars and assess the ramifications for inferring stellar ages from isochrones and placing constraints on the mixing length parameter, which describes convection in 1D stellar models. I find that even though the models I consider can reproduce the properties of the Sun, both the type of boundary condition and the location at which it is applied to the interior model yield ≈ 100 K, metallicity- and log g-dependent changes to the effective temperature distribution along the red giant branch. I close the first half by exploring the prospects for inferring the stellar ages of star clusters in the Gaia era and studying the effects of model uncertainties, for example the efficiency of mass loss, on the typical observations of star clusters, such the main sequence turn off colors. Case studies of three well-studied open clusters—NGC6819, M67, NGC6791—demonstrate that more precise data than are currently available are required for firmer constraints on these model parameters. With a combination of exquisite photometry, parallax distances, and cluster memberships from Gaia at the end of its mission, we expect to be able to differentiate between the subtle yet qualitatively distinct color-magnitude diagram morphologies induced by the model parameters, and to measure precise and accurate ages for these nearby open clusters. In the second half, I showcase two examples that highlight the star-galaxy connection. First, I infer the assembly histories of quiescent galaxies using their stellar populations as tracers. I present stellar ages and elemental abundances from modeling the optical spectra of a large sample of quiescent galaxies between 0.1 < z < 0.7. I find negligible evolution in the elemental abundances at fixed galaxy stellar mass over roughly 7 Gyr of cosmic time, and that the increase in stellar ages with time for massive galaxies is consistent with passive evolution since z = 0.7. Taken together, these results favor a scenario in which the inner regions (~0.3–3 Re) of massive quiescent galaxies have been passively evolving over the last half of cosmic time. Finally, I examine the role of stellar rotation on the ionizing photon production of young, massive stellar populations. I find that even in low-metallicity environments where rotation has a significant effect on the evolution of massive stars, stellar population models require substantial contribution from fast-rotating (initial speed > 40% of breakup speed) stars in order to sustain the production of ionizing photons beyond a few Myr following a starburst. These results have important implications for cosmic reionization by massive stars and the interpretation of nebular emission lines in high-redshift star-forming galaxies.