Hot Star Wind Astrophysics

Overview: Winds from Hot Stars

Although few in number, hot massive stars are important constituents of the universe. Because of their extremely high luminosities (10,000 to a million times the sun's luminosity), they can be used as ``standard candles'' that allow us to determine distances to other galaxies. Hot stars also have prodigious supersonic winds (i.e., expanding outer envelopes) which inject large amounts of gas into the interstellar medium. Winds from O-type stars often have terminal outflow velocities of 1000 to 3000 km/sec (of the order of 1% of the speed of light) and mass loss rates of 10^(-8) to 10^(-5) solar masses per year. Because these stars only have main-sequence lifetimes of only several million years, they can lose a substantial fraction (typically about 50%) of their own mass over this time. This material contributes to the energy balance of the surrounding interstellar medium and can induce the formation of new stars, as well as have a strong impact on the star's own evolution.

The winds from hot stars are also important because they represent an ideal ``laboratory'' for the relatively unexplored field of radiation hydrodynamics. Often this term is used in a broad sense to refer to the common case where radiation plays an important role in the energy balance of a plasma; but here it applies in the stricter sense that the star's radiation imparts momentum (as well as energy) to the plasma, and so drives its supersonic outflow. In hot stars, both the continuum radiation and that due to spectral lines can transfer momentum to gas particles, via the absorption and scattering of photons. In fact, it is the opacity in the lines which dominates the momentum transfer, even though line transitions only occur in very narrow ranges of photon frequency. This efficiency comes from the presence of the rapidly accelerating wind, which Doppler shifts the line's opacity over a wider range of frequencies than it would have ``seen'' otherwise, thus providing a fresh supply of unattenuated flux from the star.

For further information about hot-star winds, see:

• Lamers, H., and Cassinelli, J. P. 1999, Introduction to Stellar Winds (Cambridge: Cambridge University Press).
• Kaper, L., and Fullerton, A. W. (eds.) 1998, Cyclical variability in stellar winds, proceedings of the ESO workshop held at Garching, Germany, 14-17 October 1997 (Berlin: Springer).
• Fullerton, A. W. 1997, ``Observations of Hot-Star Winds,'' in Stellar Atmospheres: Theory and Observations, (Proceedings of European Astronomical Doctoral Network Predoctoral School IX), ed. J.-P. DeGreve (Heidelberg: Springer).
• Moffat, A. F. J., et al., ed. 1994, ``Instability and Variability of Hot-Star Winds,'' Proceedings of an International Workshop held at Isle-aux-Coudres, Quebec, August 23-27, 1993, reprinted in Astrophys. Space Sci., vol. 221.
• Cassinelli, J. P. 1979, ``Stellar Winds,'' Ann. Rev. Astron. Astrophys., 17, 275-308.

Ongoing Research:

Over the past several years, I have been studying the interaction between rapid rotation and massive stellar winds in hot, early-type (O, B, Wolf-Rayet) stars. These stars are observed both to have strong radiatively driven stellar winds and to rotate rapidly enough for centrifugal and Coriolis forces to affect these winds.

Rapid rotation causes stars to become oblate, and this in turn causes the emitted radiation to be re-distributed over the distorted surface. This effect, known as gravity darkening, causes the polar regions to be brighter and the equatorial regions to be dimmer. The following image is of a B-type star rotating at 0, 300, 400, and 487 km/s, and the colormap is proportional to the surface flux, or effective temperature to the fourth power:

1. Centrifugal and Coriolis forces can deflect wind streamlines toward the equator, enhancing the density and possibly forming a shocked ``wind compressed disk'' (WCD) in the equatorial plane. There is a class of stars known as Be stars (see the excellent online Be star newsletter) which seem to exhibit this phenomenon, but there are still many unanswered questions. Some tentative recent results indicate that, in some situations, the gravity darkening may dominate the mass outflow to such an extent that the bright poles drive a denser wind than the dimmer equator, which is the exact opposite of the WCD picture! Research is ongoing....

2. The wind can enhance non-axisymmetric perturbations in the photosphere and create co-rotating structures in the circumstellar flow. Such ``co-rotating interaction regions'' (CIR's) are known to form an important component of the solar wind, and their existence has been proposed in hot stars to possibly explain the observed ``discrete absorption components'' in ultraviolet P Cygni line spectra. (Take a look at a 47K BW GIF of a theoretical model of a star emitting two spiral-shaped CIR's from its surface. Black is high density gas, white is low density gas.)

Much of this research is described in great detail in my Ph.D. Dissertation, which was completed in the summer of 1996 at the Bartol Research Institute, with the help of my advisor Stan Owocki.

Hot-Star Links:

• The Hot/Massive Star Newsletter
• The Be Star Newsletter
• UCL's Hot Home Page
• The home page for observations by the IUE MEGA Program
• The public-access version of Blondin's VH-1 Hydro Code
• An online database for atomic data from the Opacity Project at Strasbourg
• The GONG project's Adiabatic Stellar Pulsations Code
• The home page of the British CCP7 Project (Analysis of Astronomical Spectra), along with its American mirror at STScI
• Rob Rutten's Radiative Transfer Lecture Notes
• Jorgen Christensen-Dalsgaard's Lecture Notes on Stellar Oscillations

• Colleagues and collaborators have included researchers from: Universitäts Sternwarte München, University College London, the University of Wisconsin Astronomy Department, the Université de Montréal, and the Landessternwarte in Heidelberg.

• Additional hot-star colleagues with interesting and informative WWW pages: Jason Aufdenberg, Jacques Babel, Jon Bjorkman, Karen Bjorkman, David Cohen, Mike Corcoran, Achim Feldmeier, Alex Fullerton, Ken Gayley, Derck Massa, David McDavid, Frank Pijpers, Joachim Puls, Myron Smith, Richard Townsend, and many others (to be added soon...)

GO BACK to Steven Cranmer's Home Page, or to the Harvard-Smithsonian CfA Home Page.