Kenneth G. Gayley

Inhibition of Wind-Compressed Disk Formation by Nonradial Line Forces in Rotating Hot-Star Winds

We investigate the effects of nonradial line forces on the formation of a wind-compressed disk (WCD) around a rapidly rotating B star. Such nonradial forces can arise both from asymmetries in the line resonances in the rotating wind and from rotational distortion of the stellar surface. They characteristically include a latitudinal force component directed away from the equator and an azimuthal force component acting against the sense of rotation. Here we present results from radiation-hydrodynamical simulations showing that these nonradial forces can lead to an effective suppression of the equatorward flow needed to form a WCD as well as a modest (~20%) spin-down of the wind rotation. Furthermore, contrary to previous expectations that the wind mass flux should be enhanced by the reduced effective gravity near the equator, we show here that gravity darkening effects can actually lead to a reduced mass loss, and thus lower density, in the wind from the equatorial region. Overall, the results here thus imply a flow configuration that is markedly different from that derived in previous models of winds from rotating early-type stars. In particular, a major conclusion is that equatorial wind compression effects should be effectively suppressed in any radiatively driven stellar wind for which, as in the usual CAK formalism, the driving includes a significant component from optically thick lines. This presents a serious challenge to the WCD paradigm as an explanation for disk formation around Be and other rapidly rotating hot stars thought to have CAK-type, line-driven winds.

Sudden Radiative Braking in Colliding Hot-Star Winds

Hot, massive stars have strong stellar winds, and in hot-star binaries these winds can undergo violent collision. Because such winds are thought to be radiatively driven, radiative forces may also play an important role in moderating the wind collision. However, previous studies have been limited to considering how such forces may inhibit the initial acceleration of the companion stellar wind. In this paper we analyze the role of an even stronger radiative braking effect, whereby the primary wind is rather suddenly decelerated by the radiative momentum flux it encounters as it approaches a bright companion. We further show that the braking location and velocity law along the line of centers between the stars can be approximated analytically using a simple one-dimensional analysis. The results of this analysis agree well with a detailed two-dimensional hydrodynamical simulation of the wind collision in the WR + O binary V444 Cygni and demonstrate that radiative braking can significantly alter the bow-shock geometry and reduce the strength of the wind collision. We then apply the derived analytic theory to a set of 14 hot-star binary systems, and conclude that radiative braking is likely to be of widespread importance for wind-wind collisions in WR + O binaries with close to medium separation, D 100 R?. It may also be important in other types of hot-star binaries that exhibit a large imbalance between the component wind strengths.

Impulsive H-alpha diagnostics of electron-beam-heated solar flare model chromospheres

Time-dependent H-alpha profiles were computed for the dynamic model atmospheres of Fisher, Canfield, and McClymont (1985) simulating the effects of an intense impulsively initiated power-law beam of electrons incident on the chromosphere. The temporal response of H-alpha arises from three separate physical mechanisms, whose relative importance varies over the line profile. The fastest variations (typically less than 0.1 s for the explosive heating discussed here) arise from energy imbalance; these are apparent on chromospheric heating and cooling time scales and have their greatest amplitude at line center. Slower variations arise from ionization imbalance and are most apparent in the blue wing. The slowest variations arise from hydrodynamic effects and are related to the formation of a chromospheric condensation; these are most apparent in the red wing. These results provide a basis for the design and analysis of observations of H-alpha, in coordination with hard X-rays or microwaves, to test mechanisms of energy transport in solar flares. 18 references.

The prototype colliding-wind pinwheel WR 104

Results from the most extensive study of the time-evolving dust structure around the prototype pinwheel nebula WR 104 are presented. Encompassing 11 epochs in three near-infrared filter bandpasses, a homogeneous imaging data set spanning more than 6 yr (or 10 orbits) is presented. Data were obtained from the highly successful Keck Aperture Masking Experiment, which can recover high-fidelity images at extremely high angular resolutions, revealing the geometry of the plume with unprecedented precision. Inferred properties for the (unresolved) underlying binary and wind system are orbital period -->241.5 ? 0.5 days and angular outflow velocity -->0.28 ? 0.02 mas day?1. An optically thin cavity of angular size -->13.3 ? 1.4 mas was found to lie between the central binary and the onset of the spiral dust plume. Rotational motion of the wind system induced by the binary orbit is found to have important ramifications: entanglement of the winds results in strong shock activity far downstream from the nose of the bow shock. The far greater fraction of the winds participating in the collision may play a key role in gas compression and the nucleation of dust at large radii from the central binary and shock stagnation point. Investigation of the effects of radiative braking points toward significant modifications of the simple hydrostatic colliding wind geometry, extending the relevance of this phenomenon to wider binary systems than previously considered. Limits placed on the maximum allowed orbital eccentricity of -->e 0.06 argue strongly for a prehistory of tidal circularization in this system. Finally, we discuss the implications of Earths polar ( -->i 16?) vantage point onto a system likely to host supernova explosions at future epochs.

Profile Shapes for Optically Thick X-Ray Emission Lines from Stellar Winds

We consider the consequences of appreciable line optical depth for the profile shape of X-ray emission lines formed in stellar winds. The hot gas is thought to arise in distributed wind shocks, and the line formation is predominantly via collisional excitation followed by radiative decay. Such lines are often modeled as optically thin, but the theory has difficulty matching resolved X-ray line profiles. We suggest that for strong lines of abundant metals, newly created photons may undergo resonance scattering, modifying the emergent profile. Using Sobolev theory in a spherically symmetric wind, we show that thick-line resonance scattering leads to emission profiles that still have blueshifted centroids, like the thin lines, but are considerably less asymmetric in appearance. We focus on winds in the constant-expansion domain and derive an analytic form for the profile shape in the limit of large line and photoabsorptive optical depths. In this limit the emission profile reduces to a universal shape and has a centroid shift of -0.24v∞, with a half-width at half-maximum (HWHM) of 0.63v∞. Using published data for Chandra observations of five emission lines from the O star ζ Pup, we find that the observed HWHMs are somewhat smaller than predicted by our theory; however, the centroid shifts of all five lines are consistent with our theoretical result. These optical depth effects can potentially explain the more nearly symmetric emission lines observed in ζ Ori, θ1 Ori C, and δ Ori by Chandra, although an alternative explanation is required to account for the unshifted peak line emission. We also consider enhanced reabsorption by continuous opacity as line photons multiply scatter within an optically thick line, and find, for lines with optical depths of a few, that such reabsorption can further reduce the line asymmetry. It also reduces the line equivalent width, but probably not enough to alleviate the problem of subsolar metallicities inferred from O star X-ray spectra by ASCA, unless the width of the resonance regions are superthermally enhanced.

Project RESUN, a radio EVLA search for UHE neutrinos

During the past decade there have been several attempts to detect cosmogenic ultra high energy (UHE) neutrinos by searching for radio Ĉerenkov bursts resulting from charged impact showers in terrestrial ice or the lunar regolith. So far these radio searches have yielded no detections, but the inferred flux upper limits have started to constrain physical models for UHE neutrino generation. For searches which use the Moon as a target, we summarize the physics of the interaction, properties of the resulting Ĉerenkov radio pulse, detection statistics, effective aperture scaling laws, and derivation of upper limits for isotropic and point source models. We report on initial results from the RESUN search, which uses the Expanded Very Large Array configured in multiple sub-arrays of four antennas at 1.45 GHz pointing along the lunar limb. We detected no pulses of lunar origin during 45 observing hours. This implies upper limits to the differential neutrino flux E2dN/dE < 0.003 EeV km−2 s−1 sr−1 and <0.0003 EeV km−2 s−1 at 90% confidence level for isotropic and sampled point sources respectively, in the neutrino energy range 1021.6 < E(eV ) < 1022.6. The isotropic flux limit is comparable to the lowest published upper limits for lunar searches. The full RESUN search, with an additional 200 hours observing time and an improved data acquisition scheme, will be be an order of magnitude more sensitive in the energy range 1021 < E(eV ) < 1022 than previous lunar-target searches, and will test Z burst models of neutrino generation. Subject headings: ultra-high energy neutrinos, cosmic rays, lunar interactions

The Importance of Radiative Braking for the Wind Interaction in the Close WR+O Binary V444 Cygni

We describe radiation-hydrodynamical simulations of the wind interaction in the close WR+O binary V444 Cygni, with special emphasis on the potential role of the O-star light in decelerating the approaching massive WR wind. We demonstrate that such radiative braking can significantly alter the strength and overall geometry of the wind interaction, leading, for example, to a substantially wider opening angle for the wind bow shock. It can also cause the X-ray production to fall far below previous theoretical estimates based on collision of the two winds at their terminal speeds. We further find that the importance of radiative braking in this system depends crucially on the effectiveness of the WR wind line opacity in reflecting O-star light. This suggests that observational estimates of quite gross system characteristics, like the bow-shock opening angle, can be used to infer the degree of radiative braking, and so provide a useful new contraint for line-driving models of WR winds.

Mass Loss from Rotating Hot-stars: Inhibition of Wind Compressed Disks by Nonradial Line-forces

We review the dynamics of radiatively driven mass loss from rapidly rotating hot-stars. We first summarize the angular momentum conservation process that leads to formation of a Wind Compressed Disk(WCD) when material from a rapidly rotating star is driven gradually outward in the radial direction. We next describe how stellar oblateness and asymmetries in the Sobolev line-resonance generally leads to nonradialcomponents of the driving force is a line-driven wind, including an azimuthal spin-down force acting against the sense of the wind rotation, and a latitudinal force away from the equator. We summarize results from radiation-hydrodynamical simulations showing that these nonradial forces can lead to an effective suppressionof the equatorward flow needed to form a WCD, as well as a modest (∼ 25%) spin-downof the wind rotation. Furthermore, contrary to previous expectations that the wind mass flux should be enhanced by the reduced effective gravity near the equator, we show here that gravity darkening effects can actually lead to a reducedmass loss, and thus lower density, in the wind from the equatorial region. Finally, we examine the equatorial bistability model, and show that a sufficiently strong jump in wind driving parameters can, in principle, overcome the effect of reduced radiative driving flux, thus still allowing moderate enhancements in density in an equatorial, bistability zone wind.

Line Forces in Keplerian Circumstellar Disks and Precession of Nearly Circular Orbits

We examine the effects of optically thick line forces on orbiting circumstellar disks, such as occur around Be stars. For radially streaming radiation (e.g., as from a point source), line forces are effective only if there is a strong radial velocity gradient, as occurs, for example, in a line-driven stellar wind. However, we emphasize here that, within an orbiting disk, the radial shear of the azimuthal velocity leads to strong line-of-sight velocity gradients along nonradial directions. As such, we show that, in the proximity of a stellar surface extending over a substantial cone angle, the nonradial components of stellar radiation can impart a significant line force to such a disk, even in the case of purely circular orbits with no radial velocity. Given the highly supersonic nature of orbital velocity variations, we use the Sobolev approximation for the line transfer, extending to the disk case the standard CAK formalism developed for line-driven winds. We delineate the parameter regimes for which radiative forces might alter disk properties; but even when radiative forces are small, we analytically quantify higher-order effects in the linear limit, including the precession of weakly elliptical orbits. We find that optically thick line forces, both radial and azimuthal, can have observable implications for the dynamics of disks around Be stars, including the generation of either prograde or retrograde precession in slightly eccentric orbits. However, our analysis here suggests a net retrograde effect, in apparent contradiction with observed long-term variations of violet/red line profile asymmetries from Be stars, which are generally thought to result from prograde propagation of a one-arm, disk-oscillation mode. We also conclude that radiative forces may alter the dynamical properties at the surface of the disk where disk winds originate, and in the outer regions far from the star, and may even make low-density disks vulnerable to being blown off completely.

Radiative Torque and Partial Spin-Down of Winds from Rotating Hot Stars

We examine the degree to which the azimuthal component of the line-driving force can remove angular momentum from the equatorial wind of a rapidly rotating hot star, using a straightforward extension of the standard CAK formalism. We illustrate how even in a wind that is azimuthally symmetric, such a net azimuthal line force results from the prograde/retrograde velocity gradient asymmetries that are inherent to a non-rigidly rotating outflow. In particular, we show that the sense of the associated line torque always acts against the rotation whenever (as is generally the case) the azimuthal velocity falls below the linear outward increase (v ~ r) associated with rigid rotation. Through a parameter study based on two-dimensional numerical hydrodynamical simulations, we find that the net loss of wind angular momentum is significant but generally quite moderate, about 30%-40%, for a wide range of conditions. We then present an extensive analytic analysis that further illuminates the physical nature of the wind spin-down and its robust net magnitude. This emphasizes the inherent dynamical feedback between line driving and flow acceleration, which allows the radiative force to effectively amplify the Coriolis force in the rotating frame, and so cause the rotation speed to decrease even more steeply with radius than required to conserve angular momentum. A general conclusion is that, while the moderate net spin-down of the wind is not likely to have a major impact on the overall wind dynamics, it should be observable from emission line diagnostics, and that doing so would provide an independent test of line-driven wind theory.