Stellar Binaries and Higher-Order Multiples In and Beyond the Milky Way

Author: ORCID icon
Lewis, Hannah, Astronomy - Graduate School of Arts and Sciences, University of Virginia
Majewski, Steven, AS-Astronomy, University of Virginia

Multiple star systems are a pervasive outcome of star formation. Within the Milky Way, it is well-established that multiplicity is a steep, increasing function with stellar mass; for solar-type stars, it is generally accepted that ~50% stars exist in binary or higher-order systems, with that fraction increasing to ~80% for stars with masses > 16 MSun. From simulations of star formation, it is also shown that star forming clouds with low metallicity have higher fragmentation rates than clouds with high metallicity, implying that stellar binaries should be more frequent in metal-poor environments. These findings are supported by large statistical studies of binary populations, which find that the close binary fraction is anticorrelated with metallicity.

Beyond the Milky Way, particularly in Local Group dwarf galaxies, the metallicity distribution function and the mass distribution of newly formed stars (i.e., the initial mass function) vary widely. The binary fraction in these dwarf galaxies should then not be identical to the fraction observed in the Milky Way. Prior studies of dwarf galaxy binary populations have been limited in their ability to constrain multiplicity by relatively small samples of stars with large radial velocity errors (> 1 km/s) and very few observations (at most three epochs) from which to assume the presence of a close companion.

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) provides high-resolution, multi-epoch spectra---from which precise radial velocities, stellar parameters, and elemental abundances can be derived---for hundreds of thousands of stars in the Milky Way, and thousands of stars in Local Group dwarf galaxies. With more than three observations per star (on average), spanning temporal baselines up to ten years, the APOGEE survey provides a promising opportunity for the study of binaries in Local Group dwarf galaxies.

In this dissertation, we exploit the APOGEE survey to find four substellar-mass companion candidates in close orbits (P < 100 days) about the components of wide stellar binaries (a > 200 AU) in the Solar Neighborhood (i.e., within 200 pc of the Sun). If confirmed, these candidate systems will represent an enhancement of an order of magnitude over the expected occurrence rate of stars having substellar companions > 2 MJup within ~100 day orbits. We also check for differences in the chemistries of wide binaries compared to those of the candidate higher-order systems, but are unable to discern any effect at the level of the APOGEE abundance uncertainties.

We report Keplerian orbital parameters for three interacting binary stars in several dwarf galaxies in the Local Group---the previously known symbiotic binaries, Draco C1, SMC N73, and LIN 358---derived from long-term radial velocity variations evident in the APOGEE data. The wide separations of the stars within these binary systems indicate that the mass transfer, from the giant star to the compact companion, is likely driven by the giant stellar wind. Strong variability in the strength and character (i.e., emission versus absorption) of the hydrogen lines in the APOGEE spectra lend evidence to the notion that LIN 358 is an eclipsing symbiotic binary, with an eccentric orbit. To date, very few eclipsing symbiotics are known, but such systems are required to understand the structure of the wind from the giant component of these wide binaries.

Utilizing the extensive survey of members of the Large and Small Magellanic Clouds provided by APOGEE, we present the first steps towards carrying out one of the first large-scale surveys of multiplicity in these nearby dwarf galaxies. We also present several systems of merit with BD-masses and masses that point to the presence of a faint, compact companion (i.e., black holes). Though this work represents only a pilot treatment of this large data set, we demonstrate the utility of the APOGEE survey for the study of binaries beyond the Milky Way.

PHD (Doctor of Philosophy)
astronomy, binary stars, binaries, radial velocity, spectroscopy, Milky Way, Local Group, dwarf galaxy, binary orbit, Keplerian orbit, symbiotic stars, survey
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