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Be stars are rapidly rotating B stars whose spectra contain Balmer emission lines originating from circumstellar decretion disks. The decretion disks also produce in- frared (IR) excesses in their spectral energy distributions, and the slope of the excess reveals information about the density of the disk. The decretion disks serve as medi- ums through which mass and angular momentum are transported away from the star. Be stars rotate at or near the critical rotation rate, and the angular momentum lost through the disks prevents them from breaking up. Determinations of the amount mass and angular momentum lost through the decretion disks are model dependent. The best accepted model describing the decretion disk is the Viscous Decretion Disk (VDD) model. However, there is a discrepancy between the current measurements of mass-loss rates and those predicted by stellar evolutionary models. The projects in this thesis were chosen to better constrain the models in an effort to reconcile this discrepancy.First, it has been suggested that Be stars are spun up through previous mass- transfer from a binary companion. Also, binary companions are observed to be common. However, the VDD model assumes an isolated Be star. In this thesis, I investigate the density structure of the disks in the presence of binary companions. The Be disk is truncated by the companion, and while most of the disk material accretes onto the binary companion, we find that a portion flows past the binary in what is likely a wind-like outflow.Second, the radiation from the central Be star can launch a line-driven ablation wind that flows radially outward along the surface of the disk. This disk wind is an additional source of mass and angular momentum loss from the central star. I investigate how the disk wind would be observed in the ultraviolet (UV) wind line profiles. I find that the disk wind can be seen as a low velocity narrow absorption component in the UV wind line when the observing geometry is within a few degrees of edge-on.

Variable asymmetric double-peaked hydrogen emission line profiles in the optical in classical Be stars have been interpreted as evidence of one-armed density waves (or other density structures) within the circumstellar disks. Contemporaneous optical and IR spectroscopy can provide a means of exploring the density structure of any such density variations as a function of radius, since models have shown in the infrared lines serve as a probe of the inner disk, while the hydrogen alpha line probes a much larger region. While variability has been detected in these stars in the optical spectral region over both short (days to weeks) and longer (months) timescales, the variability of infrared line profiles has not been similarly investigated. Presented are contemporaneous Ritter Observatory and IRTF SpeX (0.8-5.4 microns) spectroscopy of 28 classical Be stars observed between September 2005 and January 2011. The data illustrate a range of line profiles common in Be stars and show significant variability in both the optical and the near-infrared. IR hydrogen line profile variability is found to be similar both in character and variety to that observed in the optical. Several targets show asymmetric line profiles with phase differences between the optical and the IR hydrogen lines at least once. For at least 4 targets, this asymmetry suggests a phase delay between the periodic variation in the \ha\ and the IR hydrogen lines. Three additional targets suggest the same type of phase delay exists between the hydrogen alpha and iron lines. For other targets no evidence of a phase delay is detected, indicating that a variety of disk morphologies may be present. A handful of targets are observed in the process of disk formation or loss.

While rapid rotation is likely the dominant mechanism which influences the development of classical Be circumstellar disks, recent observational and theoretical work suggest that evolutionary age and/or metallicity may also influence the onset of the Be phenomenon. We use a simple 2-color diagram photometric technique to identify the candidate Be population in 16 Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic clusters having a wide range of ages and metallicities. We detect an enhancement in the fractional early-type candidate Be star population relative to the fractional later-type candidate population in clusters whose early-type stars are near the end of their main sequence lifetimes, suggesting the Be phenomenon is enhanced with evolutionary age. Furthermore, in contrast to the suggestion of Fabregat & Torrejon (2000) that the Be phenomenon should begin at least 10 Myr after the zero-age-main-sequence, we detect a substantial number of candidate Be stars in clusters as young as 5 Myr. Follow-up photo-polarimetric observations of these young candidates reveal many are true classical Be stars, indicating that a significant number of zero-age-main-sequence stars must be rotating close to their critical breakup velocities. The improved statistics offered by our study also reveal clear evidence of an enhancement of the Be phenomenon in low metallicity environments. It is commonly assumed in the literature that all B-type objects detected as excess H alpha emitters via 2-color diagrams are ''Be stars''. We explore the nature of many of these candidate Be stars with additional photo-polarimetric observations, and find that ~25% of these objects exhibit properties which aren't consistent with those expected from classical Be stars. We also find that the prevalence of polarization Balmer jumps in Be stars located in low metallicity environments is lower than that typically observed for Galactic Be stars. One interpretation of this result is that disk systems in low metallicity environments have fundamentally different properties, i.e. smaller disks and/or lower disk temperatures, than their Galactic counterparts. We also detect evidence of cluster-wide alignment of Be circumstellar disks in 2 LMC clusters.

In all areas of astronomy a multi-wavelength approach is crucial, the work presented here demonstrates this. The thesis is divided into two parts. Part I is a study of the physics of accretion from a young hot Be star, onto a neutron star. The general properties of high mass X-ray binaries are reviewed and two systems are investigated. A detailed pulse phase-resolved analysis of the X-ray spectrum observed by EXOSAT during the 1983-84 outburst from V0332+ 53 is given. We suggest that the low energy absorption is predominantly interstellar in origin. If our line of sight is not close to the neutron star magnetic axis then we would not expect to see any dependency on pulse phase of either the cutoff energy or the e-folding energy, as observed. The pulse phase dependency of the spectral index and normalisation may be due to emission from two hotspots. This could explain the change in the profile shape from a single to a double pulse. The discovery of occasional excess power at low frequencies is probably caused by inhomogeneities in the accretion stream during the onset of an X-ray flare. The correlation between the infrared excess, Hα emission and X-ray outbursts over the past decade is also investigated. We suggest the circumstellar disk is at a low inclination angle and that the larger X-ray outbursts in 1989 was the result of enhanced mass loss from the circumstellar disk. Part II presents the results from an investigation designed to investigate the nature of a sample of highly variable radio sources (HVRSs) positioned in the galactic plane.