212 research outputs found
Perfect Circles: A Study of the Scattering Regions of Wolf Rayet Binary Stars
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
A Spectropolarimetric Study of Southern WR + O Binaries
The classical Wolf-Rayet (WR) state is the evolved stage of a massive star, post main-sequence. They are characterized by their strong emission line spectra and stellar winds that are often more than 10 times denser than that of their progenitor O-type stars, which have mass loss rates of 10-6 MΘyr-1. The evolution of WR stars and their connection to specific types of supernovae (SNe) is an open question. Current theory suggests that rapidly rotating massive stars may be the progenitors of SNe that produce long-duration gamma-ray bursts. The interaction between WR stars and their companion in binary systems may provide sufficient angular momentum to create such progenitors.
Angular momentum (and therefore rotation) tends to create aspherical structures in astronomical objects (e.g. Be star disks, T Tauri jets caused by decretion and accretion respectively) that can be investigated using linear polarimetry, even for unresolved sources. I have investigated WR stars in detail to determine the geometric structure of their winds using spectropolarimetry. I began by using archival broadband polarimetric data to search for intrinsic polarization in a sample of more than 40 single and binary WR stars, finding that 12 of the stars exhibit intrinsic continuum polarization or line polarization effects that indicate aspherical or non-uniform winds.
In the later stages of the project, I used the Southern African Large Telescope (SALT) to obtain time-dependent spectropolarimetric observations of 10 of the stars in that sample, along with 8 additional targets. These targets are all WR + O binary systems, whose complex winds are best observed over time with spectropolarimetry to determine the geometry of the wind across different emission lines and the continuum. I investigated two stars in the sample, WR 42 and WR 79, and found that they exhibit classic continuum polarization signatures of binary orbits, as well as intriguing orbital line polarization effects. I compared the line polarization behaviour with the predictions of existing spectrally-derived models of the systems to obtain new information about the structure of the colliding wind regions.
Finally, I have modified an existing 3-D Monte Carlo radiative transfer code to include an additional source of photons that represents a companion star. This allows the code to treat the asymmetric structures seen in massive binary systems. I used this updated code to simulate the well-observed WR + O system V444 Cygni. I created a set of emission regions to simulate line emission from both the WR wind and wind-wind collision regions, finding that the wind-wind collision creates very strong polarimetric signals that appear similar to those in other systems in my SALT sample. The results shed new light on the relationships among WR + O binaries and yield clues to their subsequent evolution and potential roles as SN and GRB progenitors
Polarization simulations of stellar wind bow shock nebulae. II. The case of dust scattering
We study the polarization produced by scattering from dust in a bow
shock-shaped region of enhanced density surrounding a stellar source, using the
Monte Carlo radiative transfer code SLIP. Bow shocks are structures formed by
the interaction of the winds of fast-moving stars with the interstellar medium.
Our previous study focused on the polarization produced in these structures by
electron scattering; we showed that polarization is highly dependent on
inclination angle and that multiple scattering changes the shape and degree of
polarization. In contrast to electron scattering, dust scattering is
wavelength-dependent, which changes the polarization behaviour. Here we explore
different dust particle sizes and compositions and generate polarized spectral
energy distributions for each case. We find that the polarization SED behaviour
depends on the dust composition and grain size. Including dust emission leads
to polarization changes with temperature at higher optical depth in ways that
are sensitive to the orientation of the bow shock. In various scenarios and
under certain assumptions, our simulations can constrain the optical depth and
dust properties of resolved and unresolved bow shock-shaped scattering
regions.Constraints on optical depth can provide estimates of local ISM density
for observed bow shocks. We also study the impact of dust grains filling the
region between the star and bow shock. We see that as the density of dust
between the star and bow shock increases, the resulting polarization is
suppressed for all the optical depth regimes.Comment: 21 pages, accepted for publication in MNRA
Searching for a Hypervelocity White Dwarf Companion: A Proper Motion Survey of SN 1006
Type Ia Supernovae (SNe Ia) are securely understood to come from the
thermonuclear explosion of a white dwarf as a result of binary interaction, but
the nature of that binary interaction and the secondary object is uncertain.
Recently, a double white dwarf model known as the dynamically driven
double-degenerate double-detonation (D6) model has become a promising
explanation for these events. One realization of this scenario predicts that
the companion may survive the explosion and reside within the remnant as a fast
moving ( km s), overluminous ()
white dwarf. Recently, three objects which appear to have these unusual
properties have been discovered in the Gaia survey. We obtained photometric
observations of the SN Ia remnant SN 1006 with the Dark Energy Camera over four
years to attempt to discover a similar star. We present a deep, high precision
astrometric proper motion survey of the interior stellar population of the
remnant. We rule out the existence of a high proper motion object consistent
with our tested realization of the D6 scenario ( km
s with ). We conclude that such a star does not exist within the
remnant, or is hidden from detection by either strong localized dust or the
unlikely possibility of ejection from the binary system near parallel to the
line of sight.Comment: 15 pages, 10 figure
Modeling the Optical to Ultraviolet Polarimetric Variability from Thomson Scattering in Colliding-wind Binaries
peer reviewedAbstract
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
UV Spectropolarimetry with Polstar: Massive Star Binary Colliding Winds
The winds of massive stars are important for their direct impact on the
interstellar medium, and for their influence on the final state of a star prior
to it exploding as a supernova. However, the dynamics of these winds is
understood primarily via their illumination from a single central source. The
Doppler shift seen in resonance lines is a useful tool for inferring these
dynamics, but the mapping from that Doppler shift to the radial distance from
the source is ambiguous. Binary systems can reduce this ambiguity by providing
a second light source at a known radius in the wind, seen from orbitally
modulated directions. From the nature of the collision between the winds, a
massive companion also provides unique additional information about wind
momentum fluxes. Since massive stars are strong ultraviolet (UV) sources, and
UV resonance line opacity in the wind is strong, UV instruments with a high
resolution spectroscopic capability are essential for extracting this dynamical
information. Polarimetric capability also helps to further resolve ambiguities
in aspects of the wind geometry that are not axisymmetric about the line of
sight, because of its unique access to scattering direction information. We
review how the proposed MIDEX-scale mission Polstar can use UV
spectropolarimetric observations to critically constrain the physics of
colliding winds, and hence radiatively-driven winds in general. We propose a
sample of 20 binary targets, capitalizing on this unique combination of
illumination by companion starlight, and collision with a companion wind, to
probe wind attributes over a range in wind strengths. Of particular interest is
the hypothesis that the radial distribution of the wind acceleration is altered
significantly, when the radiative transfer within the winds becomes optically
thick to resonance scattering in multiple overlapping UV lines.Comment: 26 pages, 12 figures, Review in a topical collection series of
Astrophysics and Space Sciences on the proposed Polstar satellite. arXiv
admin note: substantial text overlap with arXiv:2111.1155
H-ATLAS/GAMA: quantifying the morphological evolution of the galaxy population using cosmic calorimetry
Using results from the Herschel Astrophysical Terrahertz Large-Area Survey (H-ATLAS) and the Galaxy and Mass Assembly (GAMA) project, we show that, for galaxy masses above ≃ 108 M⊙, 51 per cent of the stellar mass-density in the local Universe is in early-type galaxies (ETGs; Sérsic n > 2.5) while 89 per cent of the rate of production of stellar mass-density is occurring in late-type galaxies (LTGs; Sérsic n < 2.5). From this zero-redshift benchmark, we have used a calorimetric technique to quantify the importance of the morphological transformation of galaxies over the history of the Universe. The extragalactic background radiation contains all the energy generated by nuclear fusion in stars since the big bang. By resolving this background radiation into individual galaxies using the deepest far-infrared survey with the Herschel Space Observatory and a deep near-infrared/optical survey with the Hubble Space Telescope (HST), and using measurements of the Sérsic index of these galaxies derived from the HST images, we estimate that ≃83 per cent of the stellar mass-density formed over the history of the Universe occurred in LTGs. The difference between this value and the fraction of the stellar mass-density that is in LTGs today implies there must have been a major transformation of LTGs into ETGs after the formation of most of the stars
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