Joseph P. Cassinelli

Introduction to Stellar Winds

Preface 1. Historical overview 2. Observations of stellar winds 3. Basic concepts: isothermal winds 4. Basic concepts: non-isothermal winds 5. Coronal winds 6. Sound wave driven winds 7. Dust driven winds 8. Line driven winds 9. Magnetic rotator theory 10. Alfven wave driven winds 11. Outflowing disks from rotating stars 12. Winds colliding with the interstellar medium 13. The effects of mass loss on stellar evolution 14. Problems Appendices Bibliography Object index Index.

Conditions for the formation of massive stars

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. 63 references.

The temperature structure in accretion flows onto massive protostars

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. 56 references.

High-Resolution Chandra Spectroscopy of τ Scorpii: A Narrow-Line X-Ray Spectrum from a Hot Star

Long known to be an unusual early-type star by virtue of its hard and strong X-ray emission, τ Scorpii poses a severe challenge to the standard picture of O-star wind-shock X-ray emission. The Chandra HETGS spectrum now provides significant direct evidence that this B0.2 star does not fit this standard wind-shock framework. The many emission lines detected with the Chandra gratings are significantly narrower than what would be expected from a star with the known wind properties of τ Sco, although they are broader than the corresponding lines seen in late-type coronal sources. While line ratios are consistent with the hot plasma on this star being within a few stellar radii of the photosphere, from at least one He-like complex there is evidence that the X-ray-emitting plasma is located more than a stellar radius above the photosphere. The Chandra spectrum of τ Sco is harder and more variable than those of other hot stars, with the exception of the young magnetized O star θ1 Ori C. We discuss these new results in the context of wind, coronal, and hybrid wind-magnetic models of hot-star X-ray emission.

Early magnetic B-type stars: X-ray emission and wind properties

We present a comprehensive study of X-ray emission and wind properties of massive magnetic early B-type stars. Dedicated XMM-Newtonobservations were obtained for three early type B-type stars, ξ 1 CMa, V2052 Oph, and ζ Cas with recently discovered magnetic fields. We report the first detection of X-ray emission from V2 052 Oph and ζ Cas. The latter is one the softest X-ray sources among early type stars, while the former is one of the Xray faintest. The observations show that the X-ray spectra of our program stars are quite soft with the bulk of X-ray emitting material having a temperature of about 1 MK. We compile the complete sample of early B-type stars with detected magnetic fields to date and existing X-ray measurements, in order to study whether the X-ray emission can be used as a general proxy for stellar magnetism. We find that the X-ray properties of ea rly massive B-type magnetic stars are diverse, and that hard and strong X-ray emission does not necessarily correlate with the presence of a magnetic field corroborating similar conclusi ons reached earlier for the classical chemically peculiar magnetic Bp-Ap stars. We analyze the UV spectra of five non-supergiant B stars with m agnetic fields ( τ Sco, β Cep, ξ 1 CMa, V2052 Oph, and ζ Cas) by means of non-LTE iron-blanketed model atmospheres. The latter are calculated with the Potsdam Wolf-Rayet (PoWR) code, which treats the photosphere as well as the the wind, and also accounts for X-rays. With the exception of τ Sco, this is the first analysis of these stars by means of stell ar wind models. Our models accurately fit the stellar photospheric spectra in the optical and the UV. The parameters of X-ray emission, temperature and flux are included in the model in ac cordance with observations. We confirm the earlier findings that the filling factors of X-ray e mitting material are very high. Our analysis reveals that the magnetic early type B stars studied here have weak winds with velocities not significantly exceeding vesc. The mass-loss rates inferred from the analysis of UV lines are significantly lower than predicted by hydrody namically consistent models. We find that, although the X-rays strongly affect the ionisatio n structure of the wind, this effect is not sufficient in reducing the total radiative accelerati on. When the X-rays are accounted for at the intensity and temperatures observed, there is sti ll sufficient radiative acceleration to drive stronger mass-loss than we empirically infer from the UV spectral lines.

Stagnation and Infall of Dense Clumps in the Stellar Wind of τ Scorpii

Observations of the B0.2 V star q Scorpii have revealed unusual stellar wind characteristics: redshifted absorption in the far-ultraviolet O VI resonance doublet up to D)250 km s~1 and extremely hard X-ray emission implying gas at temperatures in excess of 107 K. We describe a phenomenological model to explain these properties. We assume the wind of q Sco consists of two components: ambient gas in which denser clumps are embedded. The clumps are optically thick in the UV resonance lines primarily responsible for accelerating the ambient wind. The reduced acceleration causes the clumps to slow and even infall, all the while being con—ned by the ram pressure of the out—owing ambient wind. We calcu- late detailed trajectories of the clumps in the ambient stellar wind, accounting for a line radiation driving force and the momentum deposited by the ambient wind in the form of drag. We show that these clumps will fall back toward the star with velocities of several hundred km s~1 for a broad range of initial conditions. The velocities of the clumps relative to the ambient stellar wind can approach 2000 km s~1, producing X-rayemitting plasmas with temperatures in excess of (1¨6) ) 107 K in bow shocks at their leading edge. The infalling material explains the peculiar redshifted absorption wings seen in the O VI doublet. Of order 103 clumps with individual masses g are needed to explain the m c D 1019¨1020 observed X-ray luminosity and also to explain the strength of the O VI absorption lines. These values correspond to a mass-loss rate in clumps of to 10~8 yr~1, a small fraction of the total M0 c D 10~9 M _ mass-loss rate yr~1). We discuss the position of q Sco in the H-R diagram, conclud- (M0 D 3 ) 10~8 M _ ing that q Sco is in a crucial position on the main sequence. Hotter stars near the spectral type of q Sco have too powerful winds for clumps to fall back to the stars, and cooler stars have too low mass-loss rates to produce observable eUects. The model developed here can be generally applied to line-driven out—ows with clumps or density irregularities. Subject headings: stars: early-typestars: individual (q Scorpii) ¨ stars: winds, out—ows ¨ ultraviolet: starsX-rays: stars

X-ray spectroscopy of the 2006 outburst of RS Ophiuchi

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 × 1036 ergs s−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 × 1037 ergs s−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 ~800,000 K, and a mass of at least 1.2 M☉. 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.

Rotating, magnetic, radiation-driven wind model for Wolf-Rayet stars

A radiation-driven wind model that incorporates the effects of rotation and an open magnetic field is applied to WR stars in order to address the wind momentum problem. The dependence of the mass-loss rate and terminal velocity on the rotation rate and surface magnetic field is studied for the flow in the equatorial region. The transition from a purely radiatively driven wind to a rotationally and magnetically driven wind is investigated for the case in which the stellar luminosity is consistent with Maeders mass-luminosity relation. An alternative picture for WR winds is developed in which there is a slower but denser equatorial flow and a fast radiation-driven wind at higher latitudes. Wind models for several stars are presented that are consistent with interior theory. If the stars have field of about 1500 G and rotate at rates greater than about 85 percent maximum, they can satisfy the radio and UV observations and explain the momentum problem, and also overcome the spindown problem. 52 references.

[ITAL]Chandra[/ITAL] Discovers a Very High Density X-Ray Plasma on the O Star ζ Orionis

We report on a Chandra line spectrum observation of the O supergiant ζ Orionis (O9.7 Ib). A 73.4 ks High-Energy Transmission Grating Spectrometer observation shows a wide range of ionization stages and line strengths over the wavelength range of 5-26 A. The observed emission lines indicate a range in temperature of 2-10 MK, which is consistent with earlier X-ray observations of ζ Ori. Many lines are spectrally resolved showing Doppler broadening of 900 ± 200 km s-1. The observed He-like ions (O VII, Ne IX, Mg XI, and Si XIII) provide information about the spatial distribution of the X-ray emission. Although the observations support a wind distribution of X-ray sources, we find three conflicting results. First, line diagnostics for Si XIII indicate that this line emission forms very close to the stellar surface, where the density is of order 1012 cm-3, but the velocity there is too small to produce the shock jump required for the observed ionization level. Second, the strong X-ray line profiles are symmetric and do not show any evidence of Doppler-blueshifted line centroids, which are expected to accompany an outwardly moving source in a high-density wind. Third, the observed velocity dispersions do not appear to correlate with the associated X-ray source radii velocities, contrary to expectations of wind-distributed source models. A composite source model involving wind shocks and some magnetic confinement of turbulent hot plasma in a highly nonsymmetric wind appears to be needed to explain the line diagnostic anomalies.

Wolf-Rayet stars as starting points or as endpoints of the evolution of massive stars

The paper investigates the evidence for the two interpretations of Wolf-Rayet stars suggested in the literature: (1) massive premain-sequence stars with disks and (2) massive stars which have lost most of their H-rich layers in a stellar wind is investigated. The abundance determinations which are done in two different ways and which lead to different conclusions are discussed. The composition is solar, which would suggest interpretation (1), or the CNO abundances are strongly anomalous, which would suggest interpretation (2). Results from evolutionary calculations, stellar statistics, the existence of Ofpe/WN9 transition stars and W-R stars with evolved companions show overwhelming evidence that W-R stars are not premain-sequence stars but that they are in a late stage of evolution. Moreover, the fact that W-R stars are usually in clear regions of space, whereas massive premain-sequence stars are embedded in ultracompact H II regions also shows that W-R stars are not young premain-sequence stars. 60 refs.