A cool supergiant star
In 2004 Dr Kjell Eriksson, Department of Astronomyand Space Physics, Uppsala University, received a STINT Institutional Grant for cooperation with Dr Alexander Brown, Center for Astrophysics and Space Astronomy, University of Colorado at Boulder in USA. The project received SEK 600 000 for the first two years, the cooperation continues and it intends to develop the experience of young researchers in astronomy.
On clear winter nights the constellation Orion is prominent in the sky. As always, the great hunter is chasing various animals along with his dogs. Above his conspicuous belt, at the left shoulder, is the red and bright star Betelgeuse.
It is what astronomers call a cool supergiant star - a massive star (more than ten times as massive as our Sun) which has already spent most of its allotted lifetime. As the name suggests, supergiants are very large - if our Sun were replaced by Betelgeuse the Earth would be inside the star. The outer parts of the star - the atmosphere - are very tenuous, more like a laboratory vacuum than the atmosphere of the Earth. The light we see coming from Betelgeuse is coming from its atmosphere and to learn about the conditions there we must interpret the detailed information in the light. We usually do this by building models in our computers and then produce synthetic spectra which are compared with observed spectra. For a very successful model there is of course a good match between them, and we can claim that our model shares many properties with the real star. However, there are many complications on the route to the perfect model.
In Uppsala we have been constructing models for stellar atmosphere since the 1970s starting with normal solar-type stars and then proceeding towards more difficult models like the cool supergiants where formation of molecules and even dust grains plays an important role. Also, inhomogeneities are more easily formed in a low gravity environment which creates the need for 3-dimensional hydrodynamical models. Thanks to the dramatic increase in computer speed and memory storage it is nowadays possible to construct 3D radiation-hydrodynamical models with a reasonable degree of realistic physics.
In Boulder, Colorado, there is another group of astronomers interested in stellar physics. They are one of the leading groups in exploring the outer atmospheres of stars, regions where the conditions are in many ways controlled by the stellar magnetic fields and/or the flows of matter away from the star, the stellar winds. The Boulder group has a solid observational basis using several astronomical satellites as well as ground-based observatories, they have e.g. observed Betelgeuse in "all" wavelength regions from X-rays to radio.
Thanks to the STINT institutional grant it was possible to strengthen the contacts we have had since the 1980s between the Uppsala and Boulder groups. It has allowed Prof. Jeff Linsky, Drs. Alex Brown and Graham Harper to visit us several times. It also enabled us, Prof Nik Piskunov, Drs. Kjell Eriksson, Bernd Freytag, Nils Ryde and graduate student Michelle Mizuno-Wiedner, to go to Boulder in October 2005 for a focussed attack on the problem of how to interpret the observations of Betelgeuse. They show variations of the details in the spectra over several years which now can be reproduced almost astonishingly well. This is done in several steps, starting with a "star-in-a-box" radiation-hydrodynamical simulation of the whole star for many years (stellar time, requiring many months of computer time). Then the transport of light through this model is computed taking into account all spectral lines and the different temperatures, densities and movements at every point in the model, another time-consuming and cumbersome task.
The main results obtained so far is summarized in the figure.
The left panel shows the spectroscopic observations of Betelgeuse over a period of 3 years. The top shows the average spectrum while the time series below represent differences between individual spectra and the average. Only a small wavelength region is shown here. The middle panel shows similar plots obtained by computing 3D hydrodynamical models of
Betelgeuse and carrying out accurate calculations of the synthetic spectra. Snapshots of the 3D calculations reveal the peculiar appearance of Betelgeuse and shows how much the surface of this supergiant star changes in time (the images in the right panel are separated by a few months). For this reason we do not expect an accurate reproduction of the spectra but study if the variation pattern in the simulations matches the observations.
More details on the radiative transfer through the models can be found at http://www.astro.uu.se/~piskunov/RESEARCH/RT_3D/
Department of Astronomy and Space Physics
Senast uppdaterad: 07-06-18 10:55