The sensitivity of H alpha profiles to rapid electron beam fluctuations

In order to understand the temporal relationship between H alpha and hard X-ray emission predicted by the nonthermal electron thick target model of impulsive-phase energy transport the time-dependent theoretical H alpha profiles were computed for the dynamic model atmospheres of Fisher, Canfield, and McClymont, which simulate the effects of an impulsively initiated power-law beam of electrons. On the basis of the physical analysis it was expected that a very rapid H alpha response to an instantaneous increase in the flux of a nonthermal deka-keV electron beam, as compared to the timescale associated with the propagation of these electrons over characteristic flare coronal loop spatial scales. It was concluded that observational efforts to test the thick target nonthermal electron model through detection of impulsive H alpha brightenings associated with impulsive hard X-ray or microwave bursts should initially focus attention on the H alpha line center. Additional simultaneous blue-wing measurements will have substantial diagnostic potential

A Porosity-Length Formalism for Photon-Tiring-Limited Mass Loss from Stars Above the Eddington Limit

We examine radiatively driven mass loss from stars near and above the Eddington limit (Ledd). We begin by reviewing the instabilities that are expected to form extensive structure near Ledd. We investigate how this "porosity" can reduce the effective coupling between the matter and radiation. Introducing a new "porosity-length'' formalism, we derive a simple scaling for the reduced effective opacity, and use this to derive an associated scaling for the porosity-moderated, continuum-driven mass loss rate from stars that formally exceed Ledd. For a simple super-Eddington model with a single porosity length that is assumed to be on the order of the gravitational scale height, the overall mass loss is similar to that derived in previous porosity work. This is much higher than is typical of line-driven winds, but is still only a few percent of the photon tiring limit--for which the luminosity becomes insufficient to carry the flow out of the gravitational potential. To obtain still stronger mass loss that approaches observationally inferred values near this limit, we introduce a power-law-porosity model in which the associated structure has a broad range of scales. We show that the mass loss rate can be enhanced by a factor that increases with the Eddington parameter Gamma, such that for moderately large Gamma (> 3-4), mass loss rates could approach the photon tiring limit. Together with the ability to drive quite fast outflow speeds, the derived mass loss could explain the large inferred mass loss and flow speeds of giant outbursts in eta Carinae and other LBV stars.Comment: 17 pages, 6 figures, to appear in Ap

Asymptotic Opening Angles for Colliding-Wind Bow Shocks: the Characteristic-Angle Approximation

By considering the advection and interaction of the vector momentum flux in highly supersonic spherically diverging winds, we derive a simple analytic description of the asymptotic opening angle of a wind-collision shock cone, in the approximation that the shocked gas is contained in a cone streaming out along a single characteristic opening angle. Both highly radiative and highly adiabatic limits are treated, and their comparison is the novel result. Analytic closed-form expressions are obtained for the inferred wind momentum ratios as a function of the observed shock opening angle, allowing the conspicuous shape of the asymptotic bow shock to be used as a preliminary constraint on more detailed modeling of the colliding winds. In the process, we explore from a general perspective the limitations in applying to the global shock geometry the so-called Dyson approximation, which asserts a local balance in the perpendicular ram pressure across the shock.Comment: 16 pages, 1 figur

Probing Wolf-Rayet Winds: Chandra/HETG X-Ray Spectra of WR 6

With a deep Chandra/HETGS exposure of WR 6, we have resolved emission lines whose profiles show that the X-rays originate from a uniformly expanding spherical wind of high X-ray-continuum optical depth. The presence of strong helium-like forbidden lines places the source of X-ray emission at tens to hundreds of stellar radii from the photosphere. Variability was present in X-rays and simultaneous optical photometry, but neither were correlated with the known period of the system or with each other. An enhanced abundance of sodium revealed nuclear processed material, a quantity related to the evolutionary state of the star. The characterization of the extent and nature of the hot plasma in WR 6 will help to pave the way to a more fundamental theoretical understanding of the winds and evolution of massive stars.Comment: Accepted by the Astrophysical Journa

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