We present X-ray grating spectra of the recurrent nova RS Ophiuchi during its 2006 outburst, obtained with XMM-Newton and Chandra. For the first month after optical maximum, the X-ray spectrum was ...hard and dominated by emission lines of H-like and He-like ions. The X-ray luminosity was 2.4 x 10 super(30) ergs s super(-1) in the 0.33-10 keV range. The spectra indicate a collisionally dominated plasma with a broad range of temperatures and an energy-dependent velocity structure. During an observation obtained in week 4, a soft X-ray flare occurred in which a new system of soft, higher velocity emission lines appeared in the spectrum. Then, during weeks 6-10, the supersoft continuum of the hot white dwarf atmosphere was the dominant emission component. The X-ray luminosity reached at least 9 x 10 super(37) ergs s super(-1) in the 0.2-1 keV range, while the intrinsic nebular absorption decreased by a factor of 5 since the first observation. Preliminary model fitting indicates a white dwarf temperature of similar to 800,000 K, and a mass of at least 1.2 M unk. Therefore, RS Oph may be an important Type Ia supernova progenitor. We show that the data are consistent with mass loss ending before day 54 of the outburst, and nuclear burning ending around day 69. A rapid decay in X-ray luminosity followed after week 10. The X-ray luminosity 5, 7, and 8 months after optical maximum dropped by more than 2 orders of magnitude. The spectra do not appear to be consistent with emission from an accretion disk.
The Hanle effect has been proposed as a new diagnostic of circumstellar magnetic fields for early-type stars, for which it is sensitive to field strengths in the 1-300 G range. In this paper we ...compute the polarized P Cygni line profiles that result from the Hanle effect. For modeling the polarization, we employ a variant of the "last scattering approximation." For cases in which the Sobolev optical depths are greater than unity, the emergent line intensity is assumed to be unpolarized, while for smaller optical depths, the Stokes source functions for the Hanle effect with optically thin line scattering are used. For a typical P Cygni line, the polarized emission forms in the outer wind, because the Sobolev optical depth is large at the inner wind. For low surface field strengths, weak P Cygni lines are needed to measure the circumstellar field. For high values of the surface fields, both the Zeeman and Hanle diagnostics can be used, with the Zeeman effect probing the photospheric magnetic fields and the Hanle effect measuring the magnetic field in the wind flow. Polarized line profiles are calculated for a self-consistent structure of the flow and the magnetic geometry based on the WCFields model, which is applicable to slowly rotating stellar winds with magnetic fields drawn out by the gas flow. For surface fields of a few hundred gauss, we find that the Hanle effect can produce line polarizations in the range of a few tenths of a percent up to about 2%.
Conditions for the formation of massive stars Wolfire, Mark G.; Cassinelli, Joseph P.
Astrophysical journal/The Astrophysical journal,
08/1987, Letnik:
319, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Upper limits on the masses of stars that can form are reexamined and models for the inflow of matter through cocoons around stars of 60, 100, and 200 solar masses are calculated. Consideration is ...given to the general conditions that must hold at the inner and outer boundaries of a protostellar cocoon; limits on the dust-to-gas ratios and mass inflow rates that will permit inflow onto very massive stars are determined. It is found that inflow can occur if intermediate-sized grains (0.05-0.25 micron) are missing from the initial gas/dust mixture. The existence of massive stars in certain locations in galaxies indicates that preconditioning of the interstellar medium by shocks or turbulence is necessary for massive star formation.
" Neutron stars are the most compact astronomical objects in the universe which are accessible by direct observation. Studying neutron stars means studying physics in regimes unattainable in any ...terrestrial laboratory. Understanding their observed complex phenomena requires a wide range of scientific disciplines, including the nuclear and condensed matter physics of very dense matter in neutron star interiors, plasma physics and quantum electrodynamics of magnetospheres, and the relativistic magneto-hydrodynamics of electron-positron pulsar winds interacting with some ambient medium. Not to mention the test bed neutron stars provide for general relativity theories, and their importance as potential sources of gravitational waves. It is this variety of disciplines which, among others, makes neutron star research so fascinating, not only for those who have been working in the field for many years but also for students and young scientists. The aim of this book is to serve as a reference work which not only reviews the progress made since the early days of pulsar astronomy, but especially focuses on questions such as: ""What have we learned about the subject and how did we learn it?"", ""What are the most important open questions in this area?"" and ""What new tools, telescopes, observations, and calculations are needed to answer these questions?"".All authors who have contributed to this book have devoted a significant part of their scientific careers to exploring the nature of neutron stars and understanding pulsars. Everyone has paid special attention to writing educational comprehensive review articles with the needs of beginners, students and young scientists as potential readers in mind. This book will be a valuable source of information for these groups."
Radiation transfer problems involved in the infall of dust and gas during star formation are studied. Dust properties are discussed, and modifications of spherical radiative transfer equations are ...presented that permit forward scattering by dust to be treated for the small size of the star relative to the inner radius of the shell. A procedure for deriving the stellar radiation field incident on the inner edge of the shell is developed. The temperature correction procedure of Cassinelli and Hartmann (1975) for extended stellar atmospheres is modified so that the multitemperature nature of the grains in the cloud may be derived. Temperature distributions for three schematic models in which the density is prespecified are discussed. Radiative acceleration of grains is addressed, showing that the proper mean opacity differs by a large factor from the Rosseland mean opacity that is commonly used. Emergent fluxes for the models are given.
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