We compile observations of early-type binaries identified via spectroscopy, eclipses, long-baseline interferometry, adaptive optics, common proper motion, etc. Each observational technique is sensitive to companions across a narrow parameter space of orbital periods P and mass ratios q. =. M-comp/M-1. After combining the samples from the various surveys and correcting for their respective selection effects, we find that the properties of companions to O-type and B-type main-sequence (MS) stars differ among three regimes. First, at short orbital periods P less than or similar to 20. days (separations a less than or similar to 0.4 au), the binaries have small eccentricities e... 0.4, favor modest mass ratios < q > less than or similar to 0.5, and exhibit a small excess of twins q. >. 0.95. Second, the companion frequency peaks at intermediate periods log P (days). approximate to. 3.5 (a approximate to 10 au), where the binaries have mass ratios weighted toward small values q. approximate to 0.2-0.3 and follow a Maxwellian " thermal" eccentricity distribution. Finally, companions with long orbital periods log P (days). approximate to 5.5-7.5 (a approximate to 200-5000 au) are outer tertiary components in hierarchical triples and have a mass ratio distribution across q. approximate to 0.1-1.0 that is nearly consistent with random pairings drawn from the initial mass function. We discuss these companion distributions and properties in the context of binary-star formation and evolution. We also reanalyze the binary statistics of solar-type MS primaries, taking into account that 30% +/-. 10% of single-lined spectroscopic binaries likely contain white dwarf companions instead of low-mass stellar secondaries. The mean frequency of stellar companions with q. >. 0.1 and log P (days). <. 8.0 per primary increases from 0.50. +/- 0.04 for solar-type MS primaries to 2.1. +/- 0.3 for O-type MS primaries. We fit joint probability density functions f (M-1, q, P, e) not equal f (M-1) f (q) f (P) f (e) to the corrected distributions, which can be incorporated into binary population synthesis studies.

We present a detailed search and analysis of chemically stratified hybrid (traces of helium and hydrogen) white dwarfs in the Sloan Digital Sky Survey (SDSS). Only one stratified white dwarf, PG 1305-017, was known prior to this analysis. The main objective is to confirm the existence of several new stratified objects. We first describe our new generation of stratified model atmospheres, where a thin hydrogen layer floats in diffusive equilibrium on top of a more massive helium layer. We then present the results of our search for hot (T-eff > 30,000 K) white dwarfs with a hybrid spectral type among the similar to 38,000 white dwarf spectra listed in the SDSS. A total of 51 spectra were retained in our final sample, which we analyze using spectroscopic fits to both chemically homogeneous and stratified model atmospheres. We identify 14 new stratified white dwarfs in the SDSS sample. From these results, we draw several conclusions regarding the physical processes that might explain the presence of helium in the atmospheres of all the stars in our sample.

Abstract not available

We use the multi-epoch radial velocities acquired by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey to perform a large-scale statistical study of stellar multiplicity for field stars in the Milky Way, spanning the evolutionary phases between the main sequence (MS) and the red clump. We show that the distribution of maximum radial velocity shifts (Delta RVmax) for APOGEE targets is a strong function of log g, with MS stars showing Delta RVmax as high as similar to 300 km s(-1), and steadily dropping down to similar to 30 km s(-1) for log g similar to 0, as stars climb up the red giant branch (RGB). Red clump stars show a distribution of Delta RVmax values comparable to that of stars at the tip of the RGB, implying they have similar multiplicity characteristics. The observed attrition of high Delta RVmax systems in the RGB is consistent with a lognormal period distribution in the MS and a multiplicity fraction of 0.35, which is truncated at an increasing period as stars become physically larger and undergo mass transfer after Roche Lobe overflow during H-shell burning. The Delta RVmax distributions also show that the multiplicity characteristics of field stars are metallicity-dependent, with metal-poor ([Fe/H] less than or similar to -0.5) stars having a multiplicity fraction a factor of 2-3 higher than metal-rich ([Fe/H] less than or similar to 0.0) stars. This has profound implications for the formation rates of interacting binaries observed by astronomical transient surveys and gravitational wave detectors, as well as the habitability of circumbinary planets.

We present new Chandra observations of the outer halo of the giant elliptical galaxy NGC 4472 (M49) in the Virgo Cluster. The data extend to 130 kpc (28'), and have a combined exposure time of 150 ks. After identifying optical counterparts using the Next Generation Virgo Cluster Survey to remove background active galactic nuclei and globular cluster (GC) sources, and correcting for completeness, we find that the number of field low-mass X-ray binaries (LMXBs) per unit stellar V-band light increases significantly with the galactocentric radius. Because the flux limit of the complete sample corresponds to the Eddington limit for neutron stars in NGC 4472, many of the similar to 90 field LMXBs in this sample could host black holes. The excess of field LMXBs at large galactocentric radii may be partially caused by natal kicks on black holes and neutron stars in binary systems in the inner part of the galaxy. Furthermore, because the metallicity in the halo of NGC 4472 strongly decreases toward larger galactocentric radii, the number of field LMXBs per unit stellar mass is anticorrelated with metallicity, opposite to what is observed in GCs. Another way to explain the spatial distribution of field LMXBs is therefore a reversed metallicity effect, although we have not identified a mechanism to explain this in terms of stellar and binary evolution.

Many young, massive stars are found in close binaries. Using population synthesis simulations. we predict the likelihood of a companion star being present when these massive stars end their lives as core-collapse supernovae (SNe). We focus on stripped-envelope SNe, whose progenitors have lost their outer hydrogen and possibly helium layers before explosion. We use these results to interpret new Hubble Space Telescope observations of the site of the broad-lined Type. Ic SN 2002ap, 14 years post-explosion. For a subsolar metallicity consistent with SN 2002ap, we expect a main-sequence (MS) companion present in about two thirds of all stripped-envelope SNe and a compact companion (likely a stripped helium star or a white dwarf/neutron star/black hole) in about 5% of cases. About a quarter of progenitors are single at explosion (originating from initially single stars, mergers, or disrupted systems). All of the latter scenarios require a massive progenitor, inconsistent with earlier studies of SN 2002ap. Our new, deeper upper limits exclude the presence of an MS companion star > 8-10 M., ruling out about 40% of all stripped-envelope SN channels. The most likely scenario for SN 2002ap includes nonconservative binary interaction of a primary star initially. 23 M.. Although unlikely (< 1% of the scenarios), we also discuss the possibility of an exotic reverse merger channel for broadlined Type. Ic events. Finally, we explore how our results depend on the metallicity and the model assumptions and discuss how additional searches for companions can constrain the physics that govern. the evolution of SN progenitors.

Model stellar atmospheres are fundamental tools for understanding stellar observations from interferometry, microlensing, eclipsing binaries and planetary transits. However, the calculations also include assumptions, such as the geometry of the model. We use intensity profiles computed for both plane-parallel and spherically symmetric model atmospheres to determine fitting coefficients in the BVRIHK, CoRot and Kepler wavebands for limb darkening using several different fitting laws, for gravity-darkening and for interferometric angular diameter corrections. Comparing predicted variables for each geometry, we find that the spherically symmetric model geometry leads to different predictions for surface gravities log g < 3. In particular, the most commonly used limb-darkening laws produce poor fits to the intensity profiles of spherically symmetric model atmospheres, which indicates the need for more sophisticated laws. Angular diameter corrections for spherically symmetric models range from 0.67 to 1, compared to the much smaller range from 0.95 to 1 for plane-parallel models.

Model stellar atmospheres are fundamental tools for understanding stellar observations from interferometry, microlensing, eclipsing binaries and planetary transits. However, the calculations also include assumptions, such as the geometry of the model. We use intensity profiles computed for both plane-parallel and spherically symmetric model atmospheres to determine fitting coefficients in the BVRIHK, CoRot and Kepler wavebands for limb darkening using several different fitting laws, for gravity-darkening and for interferometric angular diameter corrections. Comparing predicted variables for each geometry, we find that the spherically symmetric model geometry leads to different predictions for surface gravities log g < 3. In particular, the most commonly used limb-darkening laws produce poor fits to the intensity profiles of spherically symmetric model atmospheres, which indicates the need for more sophisticated laws. Angular diameter corrections for spherically symmetric models range from 0.67 to 1, compared to the much smaller range from 0.95 to 1 for plane-parallel models.

Accurate knowledge of stellar parameters such as mass, radius, effective temperature, and composition inform our understanding of stellar evolution and constrain theoretical models. Binaries and, in particular, eclipsing binaries make it possible to measure directly these parameters without reliance on models or scaling relations. In this dissertation we derive fundamental parameters of stars in close binary systems with and without (detected) tertiary companions to test and inform theories of stellar and binary evolution. A subsample of 41 detached and semi-detached short-period eclipsing binaries observed by NASA’s Kepler mission and analyzed for eclipse timing variations form the basis of our sample. Radial velocities and spectroscopic orbits for these systems are derived from moderate resolution optical spectra and used to determine individual masses for 34 double-lined spectroscopic binaries, five of which have detected tertiaries. The resulting mass ratio M2/M1 distribution is bimodal, dominated by binaries with like-mass pairs and semi-detached classical Algol systems that have undergone mass transfer. A more detailed analysis of KIC 5738698, a detached binary consisting of two F-type main sequence stars with an orbital period of 4.8 days, uses the derived radial velocities to reconstruct the primary and secondary component spectra via Doppler tomography and derive atmospheric parameters for both stars. These parameters are then combined with Kepler photometry to obtain accurate masses and radii through light curve and radial velocity fitting with the binary modeling software ELC. A similar analysis is performed for KOI-81, a rapidly-rotating B-type star orbited by a low-mass white dwarf, using UV spectroscopy to identify the hot companion and determine masses and temperatures of both components. Well defined stellar parameters for KOI-81 and the other close binary systems examined in this dissertation enable detailed analyses of the physical attributes of systems in different evolutionary stages, providing important constraints for the formation and evolution of close binary systems.

The β Cephei stars represent an important class of massive star pulsators that probe the evolution of B-type stars and the transition from main sequence to hydrogen-shell burning evolution. By understanding β Cep stars, we gain insights into the detailed physics of massive star evolution, including rotational mixing, convective core overshooting, magnetic fields, and stellar winds, all of which play important roles. Similarly, modeling their pulsation provides additional information into their interior structures. Furthermore, measurements of the rate of change of pulsation period offer a direct measure of β Cephei stellar evolution. In this work, we compute state-of-the-art stellar evolution models assuming different amounts of initial rotation and convective core overshoot and measure the theoretical rates of period change, that we compare to rates previously measured for a sample of β Cephei stars. The results of this comparison are mixed. For three stars, the rates are too low to infer any information from stellar evolution models, whereas for three other stars the rates are too high. We infer stellar parameters, such as mass and age, for two β Cephei stars: ξ1 CMa and δ Cet, which agree well with independent measurements. We explore ideas for why models may not predict the higher rates of period change. In particular, period drifts in β Cep stars can artificially lead to overestimated rates of secular period change.