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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:2111.11552

We acquired H$\alpha$ spectroscopic observations from 2005 to 2019 showing Pleione has transitioned from a Be phase to a Be-shell phase during this period. Using the radiative transfer code \hdust\ we created a grid of $\sim100,000$ disk models for Pleione. We successfully reproduced the observed transition with a disk model that varies in inclination while maintaining an equatorial density of $\rho_0(r) = 3\times 10^{-11} (r/R_{eq})^{-2.7}~\rm{g~cm^{-3}}$, and an H$\alpha$ emitting region extending to $15~\rm{R_{eq}}$. We use a precessing disk model to extrapolate the changing disk inclination over $120$ years and follow the variability in archival observations. The best-fit disk model precesses over a line of sight inclination between $\sim25\rm{^{\circ}}$ and $\sim144\rm{^{\circ}}$ with a precessional period of $\sim80.5$ years. Our precessing models match some of the observed variability but fail to reproduce all of the historical data available. Therefore, we propose an ad-hoc model based on our precessing disk model inspired by recent SPH simulations of similar systems, where the disk tears due to the tidal influence of a companion star. In this model, a single disk is slowly tilted to an angle of $30^{\circ}$ from the stellar equator over $34$ years. Then, the disk is torn by the companion's tidal torque, with the outer region separating from the innermost disk. The small inner disk returns to the stellar equator as mass injection remains constant. The outer disk precesses for $\sim15$ years before gradually dissipating. The process repeats every $34$ years and reproduces all trends in Pleione's variability., Comment: 19 pages, 15 figures

One objective of the Polstar spectropolarimetry mission is to characterize the degree of nonconservative mass transfer that occurs at various stages of binary evolution, from the initial mass reversal to the late Algol phase. The proposed instrument combines spectroscopic and polarimetric capabilities, where the spectroscopy can resolve Doppler shifts in UV resonance lines with 10 km/s precision, and polarimetry can resolve linear polarization with 1e-3 precision or better. The spectroscopy will identify absorption by mass streams seen in projection against the stellar disk as a function of orbital phase, hot accretion spots, as well as scattering from extended splash structures, circumbinary disks, and other flows in and above/below the orbital plane (e.g. jets) that fail to be transferred conservatively. The polarimetry affects more the light coming from material not seen against the stellar disk, allowing the geometry of the scattering to be tracked, resolving ambiguities left by the spectroscopy and light-curve information. For example, nonconservative mass streams ejected in the polar direction will produce polarization of the opposite sign from conservative transfer accreting in the orbital plane. Also, time domain coverage over a range of phases of the binary orbit are well supported by the Polstar observing strategy. Combining these elements will significantly improve our understanding of the mass transfer process and the amount of mass that can escape from the system, an important channel for changing the final mass, and ultimate supernova, of the large number of massive stars found in binaries at close enough separation to undergo interaction., Comment: Refereed paper in Ap&SS 367:Topical Collection, 23 pages, 10 figures

As sources of chemical enrichment, ionizing radiation and energetic feedback, massive stars drive the ecology of their host galaxies despite their relative rarity, additionally to yielding compact remnants, which can generate gravitational waves. The evolution of massive stars is crucially informed by their detailed mass-loss history; however, wind structures on a variety of scales cause important uncertainties on their mass-loss rates. Binary systems can place further constraints on the mass-loss properties of massive stars, especially colliding-wind binaries. In this paper, we review how the proposed MIDEX-scale mission Polstar can critically constrain the physics of colliding winds (and hence radiatively-driven winds in general) with ultraviolet spectropolarimetric observations, providing an unprecedented improvement on the accuracy of the determination of both mass-loss rates and the velocity structure of the winds of massive stars. We propose a sample of 17 targets that will allow us to study a variety of wind-colliding systems spanning a large parameter space using the spatial information yielded by both spectroscopic and polarimetric data obtained with Polstar., Comment: 21 pages, 12 figures 1 table, Whitepaper

Trojan Technologies uses mercury-based ultraviolet (UV) lamps in reactors to purify water. When there are multiple closely spaced lamps in a reactor, significant UV emitted by the various lamps can reach, and 'interact' with neighbouring lamps. Although many of the optical phenomena occurring in a UV reactor are well understood and accounted for in current models, the fate of UV photons from one lamp reaching the plasma of a neighbouring lamp after travelling through the intervening air, quartz, and water has not been investigated in detail. These photons can be transmitted through the plasma, absorbed and re-emitted, or lost. The goal of this project was to develop a more accurate model, which accounts for these plasma interactions, to better predict the UV distribution throughout a reactor. We developed a Monte Carlo theoretical model of the photon-plasma interactions; predictions of this model agree well with work in the literature. We further validated our model by performing several optical experiments with UV lamps.

We use a smoothed particle hydrodynamics (SPH) code to examine the effects of a binary companion on a Be star disk for a range of disk viscosities and misalignment angles, i.e. the angle between the orbital plane and the primary's spin axis. The density structures in the disk due to the tidal interaction with the binary companion are investigated. Expanding on our previous work, the shape and density structure of density enhancements due to the binary companion are analyzed and the changes in observed interferometric features due to these orbiting enhancements are also predicted. We find that larger misalignment angles and viscosity values result in more tightly wound spiral arms with densities that fall-off more slowly with radial distance from the central star. We show that the orbital phase has very little effect on the structure of the spiral density enhancements. We demonstrate that these spiral features can be detected with an interferometer in H$\alpha$ and K-band emission. We also show that the spiral features affect the axis ratios determined by interferometry depending on the orientation of these features and the observer. For example, our simulations show that the axis ratios can vary by 20% for our co-planar binary disk system depending on the location of the disk density enhancements., Comment: 13 pages, 14 figures

We use a time-dependent hydrodynamic code and a non-LTE Monte Carlo code to model disk dissipation for the Be star 66 Ophiuchi. We compiled 63 years of observations from 1957 to 2020 to encompass the complete history of the growth and subsequent dissipation of the star's disk. Our models are constrained by new and archival photometry, spectroscopy and polarization observations, allowing us to model the disk dissipation event. Using Markov chain Monte Carlo methods, we find 66 Oph is consistent with standard B2Ve stellar properties. We computed a grid of 61568 Be star disk models to constrain the density profile of the disk before dissipation using observations of the H$\alpha$ line profile and SED. We find at the onset of dissipation the disk has a base density of $2.5\times10^{-11}\ \rm{g\ cm^{-3}}$ with a radial power-law index of $n=2.6$. Our models indicate that after 21 years of disk dissipation 66 Oph's outer disk remained present and bright in the radio. We find an isothermal disk with constant viscosity with an $\alpha = 0.4$ and an outer disk radius of $\sim$115 stellar radii best reproduces the rate of 66 Oph's disk dissipation. We determined the interstellar polarization in the direction of the star in the V-band is $p=0.63 \pm 0.02\%$ with a polarization position angle of $\theta_{IS}\approx85.7 \pm 0.7^\circ$. Using the Stokes QU diagram, we find the intrinsic polarization position angle of 66 Oph's disk is $\theta_{int}\approx98 \pm 3^\circ$., Comment: 20 pages, 16 figures

The observed emission lines of Be stars originate from a circumstellar Keplerian disk that are generally well explained by the Viscous Decretion Disk model. In an earlier work we performed the modeling of the full light curve of the bright Be star $\omega$ CMa (Ghoreyshi et al. 2018) with the 1-D time-dependent hydrodynamics code SINGLEBE and the Monte Carlo radiative-transfer code HDUST. We used the V -band light curve that probes the inner disk through four disk formation and dissipation cycles. This new study compares predictions of the same set of model parameters with time-resolved photometry from the near UV through the mid-infrared, comprehensive series of optical spectra, and optical broad-band polarimetry, that overall represent a larger volume of the disk. Qualitatively, the models reproduce the trends in the observed data due to the growth and decay of the disk. However, quantitative differences exist, e.g., an overprediction of the flux increasing with wavelength, too slow decreases in Balmer emission-line strength that are too slow during disk dissipation, and the discrepancy between the range of polarimetric data and the model. We find that a larger value of the viscosity parameter alone, or a truncated disk by a companion star, reduces these discrepancies by increasing the dissipation rate in the outer regions of the disk., Comment: 25 pages, 19 figures

We use a smoothed particle hydrodynamics (SPH) code to examine the effects of misaligned binary companions on Be star discs. We systematically vary the degree of misalignment between the disc and the binary orbit, as well as the disc viscosity and orbital period to study their effects on the density in the inner and outer parts of the disc. We find that varying the degree of misalignment, the viscosity, and the orbital period affects both the truncation radius and the density structure of the outer disc, while the inner disc remains mostly unaffected. We also investigate the tilting of the disc in the innermost part of the disc and find the tilt increases with radius until reaching a maximum around 5 stellar radii. The direction of the line of nodes, with respect to the equator of the central star, is found to be offset compared to the orbital line of nodes, and to vary periodically in time, with a period of half a orbital phase. We also compare the scale height of our discs with the analytical scale height of an isothermal disc, which increases with radius as $r^{1.5}$. We find that this formula reproduces the scale height well for both aligned and misaligned systems but underestimates the scale height in regions of the disc where density enhancements develop., Comment: 11 pages, 10 figures, to be published in MNRAS

Be stars are surrounded by outflowing circumstellar matter structured in the form of decretion discs. They are often members of binary systems, where it is expected that the decretion disc interacts both radiatively and gravitationally with the companion. In this work we study how various orbital (period, mass ratio and eccentricity) and disc (viscosity) parameters affect the disc structure in coplanar systems. We simulate such binaries with the use of a smoothed particle hydrodynamics code. The main effects of the secondary on the disc are its truncation and the accumulation of material inwards of truncation. We find two cases with respect to the effects of eccentricity: (i) In circular or nearly circular prograde orbits, the disc maintains a rotating, constant in shape, configuration, which is locked to the orbital phase. The disc is smaller in size, more elongated and more massive for low viscosity parameter, small orbital separation and/or high mass ratio. (ii) Highly eccentric orbits are more complex, with the disc structure and total mass strongly dependent on the orbital phase and the distance to the secondary. We also study the effects of binarity in the disc continuum emission. Since the infrared and radio SED are sensitive to the disc size and density slope, the truncation and matter accumulation result in considerable modifications in the emergent spectrum. We conclude that binarity can serve as an explanation for the variability exhibited in observations of Be stars, and that our model can be used to detect invisible companions., Comment: 15 pages, 30 figures

This work presents a novel method to estimate the effective opening angle of CBe star disks from projected axis ratio measurements, obtained by interferometry using Bayesian statistics. A Monte Carlo scheme was used to generate a large set of theoretical axis ratios from disk models using different distributions of disk densities and opening angles. These theoretical samples were then compared to observational samples, using a two-sample Kolmogorov-Smirnov test, to determine which theoretical distribution best reproduces the observations. The results suggest that the observed ratio distributions in the K-, H-, and N-band can best be explained by the presence of thin disks, with opening half-angles of the order of 0.15$^{\circ}$ to 4.0$^{\circ}$. Results for measurements over the H$\alpha$ line point toward slightly thicker disks, 3.7$^{\circ}$ to 14$^{\circ}$, which is consistent with a flaring disk predicted by the viscous disk model., Comment: Accepted for publication in ApJ, 10 pages, 13 figures

We report on the effects of two disk properties on the continuum linear polarization signature of classical Be stars. First, we investigate the effect of including metallicity in computing the thermal structure of the circumstellar gas on the resulting polarimetric Balmer jump. The Balmer jump is a distinguishing feature of the polarization signature in these objects and, as such, can be used as a tool for differentiating classical Be stars from similar H alpha-emitters identified through conventional photometric techniques. We find that although low-metallicity environments will have hotter disk temperatures on average, the temperature change alone cannot account for the discrepancy in the frequency of Balmer jumps between low-metallicity and solar-metallicity stellar populations. Second, we investigate the effect of including a global one-armed oscillation in the gas density distribution of the modeled disk. We find that a non-axisymmetric perturbation pattern yields discernible variations in the predicted polarization level. If these density oscillations are present in the inner region of classical Be star disks, the polarimetric variations should produce a periodic signature which can help characterize the dynamical nature of the gas near the star., Comment: Accepted to ApJS on July 22, 2013

We investigate the intrinsic continuum linear polarization from axisymmetric density distributions of gas surrounding classical Be stars during the formation and dissipation of their circumstellar disks. We implement a Monte Carlo calculation of the Stokes parameters with the use of the non-LTE radiative transfer code of Sigut & Jones (2007) to reproduce the continuous polarimetric spectra of classical Be stars. The scattering of light in the nonspherical circumstellar envelopes of classical Be stars produces a distinct polarization signature that can be used to study the physical nature of the scattering environment. In this paper, we highlight the utility of polarimetric measurements as important diagnostics in the modeling of these systems. We illustrate the effects of using self-consistent calculation of the thermal structure of the circumstellar gas on the characteristic wavelength-dependence of the polarization spectrum. In showing that the principal features of the polarization spectrum originate from different parts of the disk, we emphasize the capability of polarimetric observations to trace the evolution of the disk on critical scales. We produce models that approximate the disk formation and dissipation periods and illustrate how the polarimetric properties of these systems can have a pivotal role in determining the mechanism for mass decretion from the central star., Comment: Accepted to ApJ on January 15, 2013

We measured the mid-infrared extension of the gaseous disk surrounding seven Be stars in order to constrain the geometry of their circumstellar environments and to try to infer physical parameters characterizing these disks. We used the VLTI/MIDI instrument with baselines up to 130 m to obtain an angular resolution of about 15 mas in the N band and compared our results with previous K band measurements obtained with the VLTI/AMBER instrument and/or the CHARA interferometer. We obtained one calibrated visibility measurement for each of the four stars, p Car, zeta Tau, kappa CMa, and alpha Col, two for delta Cen and beta CMi, and three for alpha Ara. Almost all targets remain unresolved even with the largest VLTI baseline of 130m, evidence that their circumstellar disk extension is less than 10 mas. The only exception is alpha Ara, which is clearly resolved and well-fitted by an elliptical envelope with a major axis a=5.8+-0.8mas and an axis ratio a/b=2.4+-1 at 8 microns. This extension is similar to the size and flattening measured with the VLTI/AMBER instrument in the K band at 2 microns. The size of the circumstellar envelopes for these classical Be stars does not seem to vary strongly on the observed wavelength between 8 and 12microns. Moreover, the size and shape of Alpha Ara's disk is almost identical at 2, 8, and 12microns.

Within the last 10 years, long-baseline optical interferometry (LBOI) has benefited significantly from increased sensitivity, spatial resolution, and spectral resolution, e.g., measuring the diameters and asymmetries of single stars, imaging/fitting the orbits of multiple stars, modeling Be star disks, and modeling AGN nuclei. Similarly, polarimetry has also yielded excellent astrophysical results, e.g., characterizing the atmospheres and shells of red giants/supergiants, modeling the envelopes of AGB stars, studying the morphology of Be stars, and monitoring the short- and long- term behavior of AGNs. The next logical evolutionary step in instrumentation is to combine LBOI with polarimetry, which is called optical interferometric polarimetry (OIP). In other words, measurements of spatial coherence are performed simultaneously with measurements of coherence between orthogonal polarization states., Comment: This is a white paper