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:

* 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:

von Zeipel

Rotation can affect the outflow of mass in many ways. Two basic pieces of the puzzle are outlined below:

  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.

NEW WORK since the late 1990s includes a new observational determination of the true rotation rates of classical Be stars, which takes gravity darkening into account (Cranmer 2005) and a theoretical exploration of how stellar pulsations might be able to feed angular momentum into the upper atmospheres of Be stars and produce Keplerian "decretion disks" (Cranmer 2009).

* Hot-Star Links:

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