Search for: All records

Abstract Massive-star binaries are critical laboratories for measuring masses and stellar wind mass-loss rates. A major challenge is inferring viewing inclination and extracting information about the colliding-wind interaction (CWI) region. Polarimetric variability from electron scattering in the highly ionized winds provides important diagnostic information about system geometry. We combine for the first time the well-known generalized treatment of Brown et al. for variable polarization from binaries with the semianalytic solution for the geometry and surface density CWI shock interface between the winds based on Cantó et al. Our calculations include some simplifications in the form of inverse-square law wind densities and the assumption of axisymmetry, but in so doing they arrive at several robust conclusions. One is that when the winds are nearly equal (e.g., O+O binaries) the polarization has a relatively mild decline with binary separation. Another is that despite Thomson scattering being a gray opacity, the continuum polarization can show chromatic effects at ultraviolet wavelengths but will be mostly constant at longer wavelengths. Finally, when one wind dominates the other, as, for example, in WR+OB binaries, the polarization is expected to be larger at wavelengths where the OB component is more luminous and generally smaller at wavelengths where the WR component is more luminous. This behavior arises because, from the perspective of the WR star, the distortion of the scattering envelope from spherical is a minor perturbation situated far from the WR star. By contrast, the polarization contribution from the OB star is dominated by the geometry of the CWI shock. 

Anthropogenic disturbances associated with urban ecosystems can create favorable conditions for populations of some invasive plant species. Light pollution is one of these disturbances, but how it affects the growth and establishment of invasive plant populations is unknown. Cheatgrass (Bromus tectorum) is a problematic invasive species where it has displaced native grassland communities in the United States, but to our knowledge, there have been no studies of the ecological factors that affect cheatgrass presence in urban ecosystems. We conducted field surveys in urban alleys in Denver, Colorado, to compare the presence of cheatgrass at sites with and without artificial light at night (hereafter artificial light) from streetlights. These streetlights are mounted on utility poles, which cause ground disturbance when installed in alleys; we were able to test the independent effect of poles on cheatgrass establishment because not all poles have streetlights on them. We found that cheatgrass was positively associated with the presence of streetlights and to a lesser extent poles. In addition to cheatgrass, we also found that other plants were positively associated with the presence of both poles and streetlights. Our results suggest that artificial light may benefit the occurrence of cheatgrass and other plant species in urban settings. While invasive populations of cheatgrass in wild habitats attract the most attention from managers, we suggest more consideration for this grass in urban environments where its growth and establishment benefit from anthropogenic changes. 

Massive Wolf-Rayet (WR) stars in binary systems may produce supernovae capable of emitting long duration gamma ray bursts. Characterizing the structure of the colliding winds in these systems may help constrain the mass loss and transfer properties and help predict their future evolution. I will present new spectropolarimetric data for the possible WR+O binary system WR 71, collected using RSS at the Southern African Large Telescope. WR 71 is a WN6 whose binary status is unknown, but it displays similar spectropolarimetric variations to the known WR+O binary system V444 Cygni. I investigate the orbital and rotational velocity of WR 71's winds by analyzing its polarized emission line profiles as a function of phase, the first analysis of its kind. I compare the line polarization behavior with predictive models of both colliding wind binaries and single stars with co-rotating interaction regions. Describing the wind structure of WR 71 will help determine the rate of mass loss from the system, an important indicator for LGRB progenitors, and shed light on its binary status. 

Massive Wolf-Rayet (WR) stars in binary systems may produce supernovae capable of emitting long-duration gamma-ray bursts (LGRB). The canonical WR+O eclipsing binary is V444 Cygni, which is a WN5+O system that has X-ray emitting colliding winds and a well-constrained geometry. I will present new time-dependent spectropolarimetric data, collected using RSS at the Southern African Large Telescope, from several southern WN+O binary systems that may be analogs to V444 Cygni. By analyzing their polarimetric variations with respect to V444 Cygni, I investigate their wind geometries and assess the similarities among the WN subclass. Characterizing the mass loss and transfer structures within these systems will help to constrain the future evolution of these WN stars and their roles as LGRB prognitors 

Although we have been able to develop an understanding of many aspects of stellar evolution and formation, a few key gaps remain. One is the fate of massive binary star systems composed of Wolf-Rayet (WR) and O-type stars. In these WR + O binaries, the stellar winds surrounding these stars collide, creating a complex interaction region in which light from the stars scatters and becomes polarized. To study these scattering regions, I employ a technique that allows me to map the polarization of the light emitted from these stars and track its variation over the binary orbit. I found that although we have some models for this behavior, they do not fully reproduce the observed data, suggesting these systems are more complex that previously known. The unexplained behaviors give clues to the complexity of these systems and shows how these models can be improved upon in the future. Understanding the structure and evolution of this scattering region could be the key to understanding the lives and eventual deaths of these stars. 

https://www.bhsu.edu/Research/CUWiP#Home-1947 

https://nexsci.caltech.edu/workshop/2019/