Date

1-1-2018

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Physics

First Adviser

Pepper, Joshua

Abstract

The goal of this dissertation is to describe the behavior of the Be star population. To do this, we have studied large samples with high precision and long time baselines. The primary data type used is optical time-series photometry, but infrared and optical spectroscopy are also important. In the process of studying many hundreds of Be stars, we have characterized their diverse photometric variability, identified systems of particular interest, and established links between photometric and spectroscopic variations.Be stars and their disks have generally been characterized by the emission lines in their spectra, and especially the time variability of those spectroscopic features. They are known to also exhibit photometric variability at multiple timescales, but have not been broadly compared and analyzed by that behavior. We have taken advantage of the advent of wide-field, long-baseline, and high-cadence photometric surveys that search for transiting exoplanets to perform an analysis of brightness variations among a large number of known Be stars. The photometric data comes from the KELT transit survey, with a typical cadence of 30 minutes, baseline of up to ten years, photometric precision of about 1\%, and coverage of about 70\% of the sky. We analyze KELT light curves of 610 known Be stars in both the Northern and Southern hemispheres in an effort to study their variability in a comprehensive way. Consistent with other studies of Be star variability, we find most of the systems to be variable. We derive lower limits on the fraction of stars in our sample that exhibit features consistent with non-radial pulsation (25$\%$), disk-building events (`outbursts'; 36$\%$), and long-term trends in the circumstellar disk (37$\%$), and show how these are correlated with spectral sub-type. Other types of variability, such as those owing to binarity, are also explored. Simultaneous spectroscopy for some of these systems from the Be Star Spectra (BeSS) database allow us to better understand the physical causes for the observed variability, especially in cases of outbursts and changes in the disk. This sample is referred to as the ``BK sample'' (for BeSS-KELT).In order to study the growth and evolution of circumstellar disks around classical Be stars, we analyze optical time-series photometry from the KELT survey with simultaneous infrared and visible spectroscopy from the APOGEE survey and BeSS database for a sample of 160 Galactic classical Be stars. This sample is referred to as the ``AK sample'' (for APOGEE-KELT, since all systems have both APOGEE and KELT data). The systems studied here show variability including transitions from a disk-less to disk-possessing state (and vice versa), and persistent disks that vary in strength, being replenished at either regularly or irregularly occurring intervals. We detect disk-building events (outbursts) in the light curves of 28\% of this sample. Outbursts are more commonly observed in early- (57\%), compared to mid- (27\%) and late-type (8\%) systems. A given system may show anywhere between 0 -- 40 individual outbursts in its light curve, with amplitudes ranging up to $\sim$0.5 mag and timescales between $\sim$2 -- 1000 days. We study how both the photometry and spectroscopy change together during active episodes of disk growth or dissipation, revealing details about the evolution of the circumstellar environment. We demonstrate that photometric activity is linked to changes in the inner disk, and show that, at least in some cases, the disk growth process is asymmetrical. Observational evidence of Be star disks both growing and clearing from the inside out is presented. The duration of disk build-up and dissipation phases are measured for 70 outbursts, and we find that the average outburst takes about twice as long to dissipate as it does to build up in optical photometry. Our analysis hints that dissipation of the inner disk proceeds relatively slowly for late-type Be stars.Although both the BK and AK samples are comprised of Be stars, there are some minor differences. For each system in the AK sample, we have multiple high-resolution, infrared spectroscopic measurements, as well as optical light curves, and, in about a quarter of the sample, optical spectroscopy. Our analysis of the AK sample is mainly focused on studying disk creation, growth, and dissipation. Significant attention is given to systems with spectroscopic measurements that are near-contemporaneous with light curve variability. Each system in the BK sample has an optical light curve, but only about half of the sample has any spectroscopic data. Analysis of the BK sample emphasizes all types of photometric variability on all timescales, from hours to many years. The primary goal of our work with the BK sample is to generally classify photometric variability in Be stars as a population. Spectroscopic data is sometimes incorporated into the analysis of certain members of the BK sample, but is secondary to the photometric data. For these reasons, these two samples are kept separate in the text.

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