John C. Brown

Coulomb Energy Losses in the Solar Corona and the Proton Energy Budget in Flares

It has recently been proposed, on the basis of measurements of the flux in the 20Ne 1.634 MeV line, that the energy budget for nonthermal protons in solar flares may be significantly larger than previously assumed. The argument is founded on the fact that the 1.634 MeV feature has a (proton) excitation threshold energy significantly lower than that of the C and O lines in the 4-6 MeV range. Hence the observed enhanced level of emission in the 1.634 MeV line requires a higher flux of low-energy (~1 MeV) protons than would be obtained from a backward extrapolation of the ~10 MeV spectrum using canonical (i.e., modified Bessel function) spectral forms and so a greater overall energy content. In this paper we check the effects on this conclusion of two significant factors omitted from the previous analysis, which was based on a cold chromospheric target model. While such a model may be appropriate for protons of ~10 MeV energies, protons of ~1 MeV may undergo a significant part of their energy loss in the hot corona, which is ionized and also warm for beam protons of these energies. The ionization results in a Coulomb logarithm (and energy loss rate) almost 3 times higher than in the neutral chromosphere. On the other hand, the warm target effect results in energy losses a factor of 1-10 times lower than in a cold target. Thus, if beam protons underwent a substantial part of their energy loss in the corona (depending on the column density encountered), previous conclusions from the 20Ne line flux could be either enhanced or negated, depending on which effect dominates. We show that for likely flare coronal temperatures and column densities that the net consequences for the 20Ne flux are in fact small, unless the low-energy protons are preferentially trapped in an improbably hot dense magnetic island.

Stellar occultation of polarized light from circumstellar electrons. I: Flat envelopes viewed edge on

The depolarizing and occultation effects of a finite spherical light source on the polarization of light Thomson-scattered from a flat circumstellar envelope seen edge-on are analyzed. The analysis shows that neglect of the finite size of the light source leads to a gross overestimate of the polarization for a given disk geometry. By including occultation and depolarization, it is found that B-star envelopes are necessarily highly flattened disk-type structures. For a disk viewed edge-on, the effect of occultation reduces the polarization more than the inclusion of the depolarization factor alone. Analysis of a one-dimensional plume leads to a powerful technique that permits the electron density distribution to be explicitly obtained from the polarimetric data.

Stellar occultation of polarized light from circumstellar electrons. II, Flat envelopes viewed at arbitrary inclination

The treatment given in Brown J. C. and Fox G. K. (1989, Astrophys. J., 347) is extended for the linear polarization produced from a flat circumstellar envelope with a finite-size light source viewed edge-on to the case of arbitrary inclination. It is found that for an axisymmetric disk scattering region viewed at low inclination, stellar occultation enhances the net polarization, while for high inclination the polarization is reduced by occultation

Constraints on the physical properties of optical bullets in SS 433

The present study discusses possible mechanisms for continuously heating the H-alpha emitting bullets of SS 433 out to distances of 5 x 10 to the 14th cm and for turning off this emission at 10 to the 15th. Various observational contraints are used to establish bounds on permissible solutions in terms of the two key bullet parameters, mass, and angular radius seen from the central source. The analysis is carried out for the mathematically simplest case of uniform spherical bullets. For radiative heat of such bullets by starlight, solutions are found to exist only for the very massive bullets with about 0.03 radians, which are highly implausible on the grounds of the large implied mean kinetic luminosity of about 10 to the 41st ergs/s. It is concluded that collisional interaction is the most likely mechanism for heating the optical bullets of SS 433. The effects of these constraints being fragmented rather than uniform, and being elongated rather than spherical are discussed.

Using ISO to Probe the Acceleration of Wolf-Rayet Winds

Brown et al. (1997) and Ignace et al. (1997) have developed an inversion method to derive the wind velocity distribution from optically thin line profiles formed in spherically symmetric outflows. The method has been applied to three He ii lines (9-7 at 2.83µm, 7-6 at 3.09 µm, and 8-7 at 4.76 µm) observed with the ISO SWS for the stars WR 134 (WN6) and WR 136 (WN6). Of these, the 2.83 µm line probes deepest into the wind owing to the dominant free-free opacity. The winds of both WR 134 and WR 136 have attained only 75% of their respective terminal speeds at the radii where the 2.83 µm line is formed.

Polarization variability due to clumps in the winds of Wolf-Rayet stars

Wolf-Rayet (WR) stars are understood to have clumpy winds [1]. Robert et al. [2] found a statistical relation between the variations of the polarization and the scattering light intensity, R = σp/σphot ≈ 0.05. To explain this result, we propose a model in which clumps are ejected from the surface of WR stars uniformly in space with a Gaussian time interval distribution. According to the observed R along with the subpeaks on the emission lines of WR stars, we can obtain the parameters of the velocity law index β, and of the clump ejection rate in a flow time N. Also, the fraction η of the total mass loss rate contained in the clumps can be found from the observed polarization.