Showing papers on "Stellar-wind bubble published in 2000"

Winds from hot stars

Abstract: ▪ Abstract This review deals with the winds from “normal” hot stars such as O-stars, B- and A-supergiants, and Central Stars of Planetary Nebulae with O-type spectra. The advanced diagnostic methods of stellar winds, including an assessment of the accuracy of the determinations of global stellar wind parameters (terminal velocities, mass-loss rates, wind momenta, and energies), are introduced and scaling relations as a function of stellar parameters are provided. Observational results are interpreted in the framework of the stationary, one-dimensional (1-D) theory of line-driven winds. Systematic effects caused by nonhomogeneous structures, time dependence, and deviations from spherical symmetry are discussed. The review finishes with a brief description of the role of stellar winds as extragalactic distance indicators and as tracers of the chemical composition of galaxies at high redshift.

Solar and Stellar Magnetic Activity

Abstract: 1. Introduction: solar features and terminology 2. Stellar structure 3. Solar rotation and meridional flow 4. Solar magnetic structure 5. Solar magnetic configurations 6. Global properties of the solar magnetic field 7. The solar dynamo 8. The solar outer atmosphere 9. Stellar outer atmospheres 10. Mechanisms of outer-atmospheric heating 11. Activity and stellar properties 12. Stellar magnetic phenomena 13. Activity and rotation on evolutionary time scales 14. Activity in binary stars 15. Propositions on stellar dynamos Appendix I: unit conversions Index.

A numerical study of the evolution of the solar wind from Ulysses to Voyager 2

Abstract: Voyager 2 continues to explore the outer heliosphere as Ulysses studies the latitudinal dependence of the solar wind. During the year 1991 these spacecraft were within 2° latitude and their radial separation was larger than 30 AU. This alignment presents a good opportunity to investigate the evolution of the solar wind and, in particular, the effect of pickup ions, which are an important component of the solar wind in the outer heliosphere. A numerical simulation study is used to model the evolution of solar wind structures from the location of Ulysses (1-5 AU) to Voyager 2 (33-36 AU) in 1991. The Ulysses observations were used as input into the numerical models and the theoretical predictions were compared with the Voyager 2 observations. The model produces profiles and magnitudes of the plasma parameters (e.g., flow speed and density) which are in reasonable agreement with the observations. The inclusion of pickup ions slows the solar wind, reduces the amplitudes of the speed variations and density spikes, and changes the speed of shock propagation. However, within 35 AU, pickup ions do not change the solar wind structures dramatically. Our calculations favor a low interstellar neutral hydrogen density, i.e., n H ∞ = 0.05 cm -3 , for this time period (1991).

Nonstationary Phenomena in the Solar Wind and InterstellarMedium Interaction

Abstract: We discuss the results of numerical modeling of the solar wind with the inhomogeneous interstellar medium. The density of the plasma component in the interstellar cloud is supposed to be space periodic. The interaction pattern is shown to be highly unsteady with hydrodynamic instabilities developing on the side portion of the heliopause.

Radiatively Driven Stellar Winds from Hot Stars

Abstract: A stellar wind is the continuous, supersonic outflow of matter from a star. Among the most massive stars—which tend also to be the hottest and most luminous—the winds can be very strong, with important consequences both for the star’s own evolution as well as for the surrounding interstellar medium. Such hot-star winds are understood to be driven by the pressure of the star’s emitted radiation. Our Sun has a SOLAR WIND, but it is so tenuous and transparent that it would be difficult to detect directly from a more distant star. However, many stars have winds that are dense enough to be opaque at certain wavelengths of the star’s radiation, and this makes it possible to study them remotely through careful interpretation of the observed stellar spectra. The winds from massive, hot stars—with surface temperatures above about 10 000 K— form a well-defined class, distinct from the winds from cooler stars, and characterized by the central role of the star’s own radiation in driving the mass outflow. The high temperature of such stars means that they have a very high surface brightness. Because light carries momentum as well as energy, this high surface brightness imparts a force to the atoms that scatter the light. At the level of the star’s atmosphere where this force exceeds the inward force of the stellar gravity, material is accelerated upward and becomes the stellar wind. An important aspect of this radiative driving process is that it stems mostly from line scattering, wherein an electron is shuffled between two discrete, bound energy levels of an atom. In a static medium such scattering is confined to radiation with a photon energy near the energy difference between the levels, corresponding to a range of wavelengths near a distinct, line-center value. However, in an accelerating stellar wind flow, the Doppler effect shifts the resonance to increasingly longer wavelengths, allowing the line scattering to sweep gradually through a much broader portion of the stellar spectrum. This gives the dynamics of such winds an intricate feedback character, in which the radiative driving force that accelerates the stellar wind depends itself on that acceleration.