J. P. Cassinelli

ROSAT PSPC Observations of 27 Near-Main-Sequence B Stars

In this paper, we report on ROSAT Position Sensitive Proportional Counter (PSPC) observations of 27 near-main-sequence B stars made with unprecedented sensitivity. Contrary to the results of previous surveys, it is found that 75% of the sample stars are X-ray sources, albeit most at modest levels. The X-ray luminosities of the program stars range from 5.6 × 1027 up to 2.2 × 1032 ergs s-1. We find that LX/LBol decreases abruptly beyond about B0 and stabilizes at LX/LBol ≈ 10-8.5 by about B2, with seven nondetections at B2 and later. For the B0 and B1 stars, our modeling suggests that wind attenuation of the X-ray photons is significant, so that the emitted X-ray luminosity, corrected for this attenuation, actually exceeds 10-7LBol in some cases. Presumably, this situation is even more severe for O stars; thus, the well-known LX/LBol ≈ 10-7 law simply may be an artifact of the neglect of wind attenuation. The ROSAT PSPC observations of most of the B stars are very soft, with the notable exception of τ Sco (B0 V). The wind emission measure filling factors that we find for the very early B stars are rather large (roughly 0.1-1). This could be brought into line with theoretical calculations of the line-force instability, wind-shock mechanism if the mass-loss rates of these stars are a few times higher than theory currently predicts. However, the X-rays from stars later than B2 require filling factors greater than unity and thus cannot be produced by any radiation-driven wind-shock mechanism because there is simply not enough wind material to produce the observed X-rays. It is possible that mid- to late-B stars represent some kind of transition to, or hybrid of, wind and coronal X-ray mechanisms.

Magnetic Fields in Massive Stars. II. The Buoyant Rise of Magnetic Flux Tubes through the Radiative Interior

We present results from an investigation of the dynamical behavior of buoyant magnetic flux rings in the radiative interior of a uniformly rotating, early-type star. Our physical model describes a thin, axisymmetric, toroidal flux tube that is released from the outer boundary of the convective core and is acted on by buoyant, centrifugal, Coriolis, magnetic tension, and aerodynamic drag forces. We find that rings emitted in the equatorial plane can attain a stationary equilibrium state that is stable with respect to small displacements in radius, but is unstable when perturbed in the meridional direction. Rings emitted at other latitudes travel toward the surface along trajectories that largely parallel the rotation axis of the star. Over much of the ascent, the instantaneous rise speed is determined by the rate of heating by the absorption of radiation that diffuses into the tube from the external medium. Since the timescale for this heating varies like the square of the tube cross-sectional radius, for the same field strength, thin rings rise more rapidly than do thick rings. For a reasonable range of assumed ring sizes and field strengths, our results suggest that buoyancy is a viable mechanism for bringing magnetic flux from the core to the surface, being capable of accomplishing this transport in a time that is generally much less than the stellar main-sequence lifetime.

NEW CHALLENGES FOR WIND SHOCK MODELS: THE CHANDRA SPECTRUM OF THE HOT STARORIONIS

The Chandra spectrum ofOri A shows emission lines from hydrogen- and helium-like states of Si, Mg, Ne, and O, along with Nvii Lyand lines from ions in the range Fe xvii-Fexxi. In contrast to the broad lines seen inPup andOri (850 � 40 and 1000 � 240 km s � 1 half-width at half-maximum (HWHM), respec- tively), these lines are broadened to only 430 � 60 km s � 1 HWHM. This is much lower than the measured wind terminal velocity of 2000 km s � 1 . The forbidden, intercombination, and resonance ( fir) lines from He-like ions indicate that the majority of the X-ray line emission does not originate at the base of the wind, in agreement with the standard wind shock models for these objects. However, in that model the X-ray emission is distributed throughout an expanding, X-ray-absorbing wind, and it is therefore surprising that the emis- sion lines appear relatively narrow, unshifted, and symmetric. We compare the observed line profiles to recent detailed models for X-ray line profile generation in hot stars, but none of them offers a fully satisfac- tory explanation for the observed line profiles.

A Magnetically Torqued Disk Model for Be Stars

Despite extensive study, the mechanisms by which Be star disks acquire high densities and angular momentum while displaying variability on many timescales are still far from clear. In this paper, we discuss how magnetic torquing may help explain disk formation with the observed quasi-Keplerian ( as opposed to expanding) velocity structure and their variability. We focus on the effects of the rapid rotation of Be stars, considering the regime where centrifugal forces provide the dominant radial support of the disk material. Using a kinematic description of the angular velocity, v(phi)(r), in the disk and a parametric model of an aligned field with a strength B(r), we develop analytic expressions for the disk properties that allow us to estimate the stellar surface field strength necessary to create such a disk for a range of stars on the main sequence. The fields required to form a disk are compared with the bounds previously derived from photospheric limiting conditions. The model explains why disks are most common for main-sequence stars at about spectral class B2 V. The earlier type stars with very fast and high-density winds would require unacceptably strong surface fields ( gt 10(3) G) to form torqued disks, while the late B stars ( with their low mass-loss rates) tend to form disks that produce only small fluxes in the dominant Be diagnostics. For stars at B2 V the average surface field required is about 300 G. The predicted disks provide an intrinsic polarization and a flux at Halpha comparable to observations. The radial extent of our dense quasi-Keplerian disks is compatible with typical estimates. We also discuss whether the effect on field containment of the time-dependent accumulation of matter in the flux tubes/disk can help explain some of the observed variability of Be star disks.

EUVE spectroscopy of epsilon Canis Majoris (B2 II) from 70 to 730 A

We present spectra of the brightest stellar source of extreme ultraviolet (EUV) radiation longward of 400 A, the B2 II star, epsilon CMa. These data were taken with the three spectrometers aboard the NASA Extreme Ultraviolet Explorer satellite (EUVE) during the first cycle of pointed observations. We report on our initial studies of the continuum and line spectrum of the stellar photosphere in the 320 to 730 A region, and on the wind emission lines observed in the 170-375 A region. This is the first EUV spectrum of an early-type star, and thus makes epsilon CMa the most comprehensively observed B star from the X-ray to infrared regimes. The radiation in both the H Lyman continuum and He I continuum (shortward of 504 A) are found to be significantly greater than predicted by both Local Thermodynamic Equilibrium (LTE) and non-LTE model atmospheres. Since epsilon CMa also exhibits a mid-infrared excess, this points to the outer layers being warmer than the models indicate. The anomalously large Lyman continuum flux, combined with the very low column density measured in the direction toward this star implies that it is the dominant source of hydrogen ionization of the local interstellar medium in the immediate vicinity of the sun. All of the lines predicted to be strong from model atmospheres are present and several wind absorption features are also identified. We have detected emission lines from highly ionized iron that are consistent with the ROSAT Position Sensitive Proportional Counter (PSPC) observations if a multi-temperature emission model is used, and the assumption is made that there is significant absorption beyond that of the neutral phase of the ISM. The spectrum shows strong O III 374 A line emission produced by the Bowen flourescence mechanism, which has not previously been observed in the spectra of hot stars.

Predicted extreme-ultraviolet line emission for nearby main-sequence B stars

The source of the X-ray emission from O and B stars is currently the subject of debate. Shocks propagating through the winds of O and B stars have previously been proposed to explain their observed X-ray luminosities. Strong shocks with velocity jumps of several hundred km/s can heat portions of the wind to 10 6 -10 7 K, producing major sources of X-ray and extreme-ultraviolet (EUV) emission lines. Alternatively, a corona at the base of the wind might also be responsible for the X-ray emission. Here, the characteristics of EUV lines produced in high-temperature X-ray emitting regions of early B stars are investigated

The Hanle Effect as a Diagnostic of Magnetic Fields in Stellar Envelopes. I. Theoretical Results for Integrated Line Profiles

The Hanle effect concerns the modification of polarized resonance-line scattering by magnetic fields; thus, it can be used as a diagnostic of stellar magnetic fields. The Hanle effect has been used to determine the field strength and distribution of magnetic structures present in prominences of the Sun. To investigate its potential use in stellar astronomy, the simplified case of an optically thin axisymmetric ring illuminated by a stellar point source is considered. The results are then used to derive the polarization from polar plumes, equatorial disks, and spherical shells. The integrated line polarization is calculated for axisymmetric rings with a variety of magnetic field orientations, and in every case the polarization is proportional to sin2 i (where i is the viewing inclination), just as in the zero field case. It is also found that the Hanle effect can significantly alter the integrated line polarization. In some cases the position angle of the polarization in the line can be rotated by 90° relative to the zero field case. We consider the Hanle effect as a possible diagnostic of magnetic fields in stellar winds with prominent ultraviolet and visible resonance lines. For these lines the diagnostic has sensitivity in the range of 1-1000 G. The Zeeman effect is not normally applicable for diagnosing magnetic fields in stellar winds in the subkilogauss range; thus, the Hanle effect should provide an especially useful new method of determining magnetic fields in stars other than the Sun. Possibilities for measuring the fields in early-type stars using ultraviolet observations is discussed.

“WCFields”: A Magnetic Rotating Stellar Wind Model from Wind Compression Theory

A stellar wind model for a magnetic rotating star is presented. We use the semianalytic wind compression model that predicts the two-dimensional geometry of outNows from rotating stars and consider the addition of a magnetic Ðeld. In the limit of weak magnetic Ðelds, in such a way that the Ðelds are unimportant in accelerating the Now, the wind compression model can be used to predict the magnetic Ðeld distribution throughout the wind, which is shown to follow the mass Nux distribution. A compression of Ðeld lines near the equator results as the Now of material from higher latitudes brings magnetic Nux toward that region. As examples, wind compression models with magnetic Ðelds (““WCField II models) are computed for both a Wolf-Rayet star and a red supergiant star. In both cases an order of magnitude enhancement of the equatorial magnetic Ðeld can result within a few stellar radii for stellar rotation rates around 20% of critical. Such enhancements could have consequences for explaining (1) nonthermal emission observed from some Wolf-Rayet winds and (2) the ring structures observed in the ejecta of SN 1987a. Subject headings : stars : magnetic Ðelds E stars : mass loss E stars : rotation E stars : Wolf-Rayet E supernovae : individual (SN 1987A)

Effects of coronal and shock-produced X-rays on the ionization distribution in hot star winds

We investigate the effects of X-ray radiation on the ionization distribution in the wind of ζ Pup (O4 If). In particular, we have studied how the distribution of the superionization species O VI is affected by X-ray source characteristics. In our calculations, detailed statistical equilibrium and atomic physics models are used to ensure that critical processes such as photoionization out of excited states and Auger ionization are considered. For shock-produced X-ray sources, we show how the distribution of X-ray emission affects the O VI P Cygni profile and the resultant X-ray spectrum. For coronal sources, we examine the sensitivity of the X-ray spectrum and O VI profile to the mass-loss rate and attenuation by the overlying wind

Evidence for a disk in the wind of HD 93521: UV line profiles from an axisymmetric model

Recently it has been suggested (Massa 1992; Howarth & Reid 1993), from the C IV ultraviolet resonance line profile of HD 93521, that there is a high-speed component in the polar outflow from the star as well as a low-speed component in the equatorial regions. In this paper we present theoretical calculations of the line profiles that would be produced by such a model. We find from Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) observations of HD 93521 that the observed C IV profile can be produced if the star has an inclination angle very close to 90 deg and if the star is surrounded by a thin disk, whose half-width is approximately 3 deg in latitude. The geometry of this disk is similar to what one would expect from the wind-compressed disk model of Bjorkman & Cassinelli, so this star may provide an ideal observational test of that model. In addition to the C IV resonance line, we examine both the Si IV and N V resonance lines. The Si IV line exhibits low-velocity absorption that is similar to that seen in the C IV line, but the emission is much weaker. On the other hand, the N V line has weaker absorption and much stronger emission than the C IV line. N V is a higher ionization state than C IV, so it is likely that N V is one stage above the dominant state, N IV. Apart from fitting individual line profiles, we also examine the differences between the two Hubble Space Telescope observations of HD 93521. We find evidence for a pair of narrow absorption components, seen at low velocity, as well as evidence for a discrete emission feature in the blueshifted absorption cores of the lines. This blueshifted emission at low velocity can cause what instead appears to be an interstella absorption line. Without multiple observations that can reveal the temporary emission, one must be very careful when determining interstellar column densities to stars like HD 93521.