Winds of hot stars
Rolf Kudritzki and Joachim Puls (Institute for Astronomy, Hawaii and Universitätssternwarte München)
Quick outline for the impatient reader. Massive hot stars are extremely luminous and can be easily detected in distant galaxies. They have enormous stellar winds which provide the surrounding interstellar medium with mechanical energy and momentum and recycled nuclear burned material.
Blue supergiant stars in the spiral galaxy NGC 3621. The B,V,I images have been taken by RPK with the ESO Very Large Telescope and FORS1 (Focal Reducer Spectrograph). NGC 3621 is at a distance of about 6.7 Mpc. Recently we have taken spectra of the encircled targets again using FORS1 at the VLT to study chemical composition, stellar parameters and stellar wind properties of individual stars. We will determine the distance to this galaxy using the Wind Momentum - Luminosity Relationship.
The winds from hot stars are driven by radiation. The momentum of photospheric photons is absorbed by thousands of spectral lines and, thus, transfered to the atmospheric plasma. This physical process initiates and maintains stellar winds. The physical modelling of winds requires the tools of non-LTE radiative transfer combined with hydrodynamics. Below we show a a numerical simulation of the density distribution of a rapidly rotating wind (Petrenz and Puls, 2000).
As an example for the multitude of line transitions driving the wind, we show here the Energy level diagram of the most important levels of FeV. Solid, dashed and dotted line-style corresponds to strong, intermediate and low oscillator-strength of the indicated radiative transition. Note the large number of low-lying meta-stable levels in iron (-like) ions. A modern numerical stellar wind code includes the contributions of some 140 of such ions with all their spectral lines and all occupation numbers calculated in non-LTE.
The theory predicts a tight relationship between the momentum rate of the wind (multiplied with the square-root of the stellar radius) and the luminosity of the star, called the Wind momentum - Luminosity Relation. Observations and diagnostics of the winds from hot stars, performed in our own Milky way and Local Group galaxies (the Magellanic Clouds, M31, M33, NGC 6822) have confirmed this theoretical prediction. Since the winds are driven by radiative acceleration in metal lines, effective temperature and the stellar metal abundance affect the WLR. Again, the theory makes clear predictions concerning this dependence. We are presently carrying out a vigorous observing project to calibrate the WLR as function of spectral type and metallicity.
After the calibration phase will have been finished, the WLR can be used as a new and independent primary distance indicator, allowing for the measurement of extragalactic distances out to Virgo and Fornax, thus helping to further constrain the Hubble-Constant.
In the following, we will outline the basic theoretical and diagnostical tools accumulated over the last years and illuminate the importance of stellar winds and their application in the astrophysical context. To a major part, this introduction follows a recent review given by R.-R. Kudritzki and J. Puls in Annual Reviews of Astronomy and Astrophysics (2000).
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