Winds from Hot Stars

Hot Star Winds

Find lots of papers in English and German at Universitäts-Sternwarte München.

The wind on Earth blows largely parallel to the surface. Caused by differences in atmospheric pressure and deflected by the Coriolis effect. wp The massive stellar winds from hot stars in contrary largely blow perpendicular to the surface of the star. It may be deflected, modulated through magnetic fields. The rotating star gives some horizontal momentum. But anyhow we see radial velocities up to 3500 km/s. (Unsöld, Baschek, 1988, p.201)

There are at least two questions:

  1. How do we know there is a strong radial wind on an object hundreds of light years away?
  2. What is the force driving those winds to that exceptional speeds?
  3. What is it good for?yt

What is it good for?

It is good for enriching the interstellar medium with heavier elements without having a super nova, a nova, a planetary nebula. Rocky planets need this. For more see Joachim Puls, USM, AstroLab A, p.8.

How do we know?

We cannot stick out our hand at our spaceship to feel the wind, but we see the signature in the spectrum at several wave lengths. This signature is an emission line with a blueward absorption trough. It was first found at novae and in thespectrum of the star P Cygni aavso. Now it is called P Cygni profile wherever it is found. Here we see a spectrum of the star P Cygni with Hα and HeI6678.

P Cygni at H-alpha
A spectrum from Sept. 2000. Made with my early fiber coupled spectrograph. Resolution is only 2000.

J. K. E. Halm then in Edinburgh succests in 1904 that an expanding atmosphere causes this spectral feature. See Halm 1906. The PDF file is behind a pay wall and I don't want to pay for file carrying a more than one hundred years old message. So I will not verify if this is true. We wait thirty years. C. S. Beals describes the status of the observations an the physical analysis of the P Cygni spectra. He wrote:

The absorption line has its origin in the gases moving outward between the star and the observer, while the emission line represents the integrated radiation from atoms not directly in the line of vision between the observer and the stellar surface. The star will, accordingly, be surrounded by a nebulous envelope of outmoving gases …(Beals1934)
Voila! Here it is!

Another twenty years later J.A. Rottenberg figures out a theoretical P cygni profile under three assumptions:

  1. An expanding atmosphere.
  2. A scattering mechanism. That is scattering photons in arbitrary directions without altering the wave length of the photons in the system of the scattering atom.
  3. Recombination of electrons and ions. Creating different wave length while the captured electron falls down some energy levels in the capturing atom.
Due to the fast outward moving atoms in the extending atmosphere the observer far away from the system realizes some Doppler shifts and sees rather broad spectral features. (Rottenberg1952).

Now, putting it all together into a coherend scene, with the observer at the bottom of the page. We have a star (black), a radial extending atmosphere partly moving towards us (blue) and away form us (red). The star occults the fastest receding atoms in this wind (grey) so we miss the most red-shifted lines in our spectrum. The lines of the fastest approaching atoms (spotted) add some absorption features to the photospheric spectrum blue-shifted to various speeds. The material alongside the star only emits light and cannot absorb light from the star at least from our point of view. It therefore appeares in emission with Doppler shift depending on the projected radial velocitys e.g v+ and v-. (Böhm-Vitense, 1989, p.219f.)

Formation of a P Cygni profile
The observer at the bottom of the page sees lots of Doppler shifted waves.

We measure the speed of expanding shell of our star α Cam with a line of the threefold ionised carbon atom CIV in an UV spectrum from the archive of the IUE satellite. Why UV? These lines are resonance lines that appear also in thin gas. The hydrogen lines are from recombination, a collisional process in relatively dense gas in the vicinity of the stellar surface (Unsöld, Baschek, 1988, p.201). See here the spectrum out of the data base of the International Ultraviolet Explorer

UV spectrum around 1550
											Angstroms of alpha Cam
Obs Date = 03/02/91 Exposure Time = 109.643
This P Cygni profile of two CIV lines has a prominent absorption trough at 1540 Ångstroms and a not so distict emission at 1550 Ångstroms. The fastest approaching material is found at the left steep edge of the large trough. The rightmost of the two CIV ions marks hopefully the wave length of the ion at rest in the stellar atmosphere. Measuring some pixel distances we get as a Doppler shift between this peak and the left edge of the large trough a speed of 2200 km/s. Puh. How can this ions become so really fast?

Line-driven stellar winds

See again Joachim Puls, USM, AstroLab A, p.11.

The ions in the wind material are accelerated due to absorbing photons from the immense bright photosphere of the hot star. These photons carry not only a specific amount of energy \(E_{phot}=\frac{hc}{\lambda}\) dependent on its wave length \(\lambda\) but also a momentum \(p_{phot} = \frac{h}{\lambda}\) that is transferred to the ion. A ion that absorbs a photon from the photosphere gains a momentum outward and accelerates away from the star. Soon the excited ion emits a photon in an arbitrary direction but this emission of a photon and its momentum cancels out. The net effect on a large number of ions is an acceleration outward.

This effect to transfer the momentum of photons to ions is very effective on metals, i.e. ions heavier than helium. These metals have lots of absorption lines to absorb the photons. The metals take the ions of hydrogen and helium with them.

Last modified: 2022 Jul 20