Models of stellar atmospheres
Model atmospheres of cool stars
Models of stellar atmospheres are necessary for studies of many of the properties of stars. Prominently the composition of all the chemical elements is encoded in their spectra. The light emanating from the stellar surface is created by the interaction of light with the atoms and molecules in the stellar surface layers. This light originates in the heat that continuously escapes from the nuclear reactions in the stellar centre.
A one-dimensional (1D) model atmosphere is actually a large table which describes the temperatures, pressures and many other properties of the gas as they vary with depth below the stellar surface. Uppsala astronomers maintain and develop a large data base of 1D model atmospheres called MARCS which is freely available on the www. The models are used for the interpretation of spectra of cool stars. They are also used as boundary conditions for models of stellar interiors.
Advanced 1D model atmospheres of early-type stars
One-dimensional theoretical codes are commonly employed to model atmospheres of early-type stars, assuming solar or scaled-solar chemical composition. Such tools are generally not applicable to B, A, and F stars with anomalous surface abundances. In these stars line opacity differs considerably from normal stars and also varies substantially from one star to another, depending on individual chemical composition and occasional presence of strong magnetic fields. The properties of stellar atmospheres may also vary significantly across the stellar surface due to horizontal chemical and magnetic inhomogeneities.
We use an advanced line-by-line opacity sampling 1-D stellar model atmosphere code LLMODELS which allows to treat the effects of non-solar abundances, modification of the line opacity due to Zeeman splitting, magneto-hydrostatic equilibrium, and an inhomogeneous vertical and horizontal distribution of chemical elements. Inclusion of these effects enables a considerably more accurate modelling of stellar spectra than possible with standard solar or scaled-solar composition models.
Our theoretical work with the LLMODELS code is closely coupled with the observational spectroscopic studies of early-type stars. We systematically apply advanced model atmospheres to derive fundamental parameters, chemical composition, and to study a non-uniform vertical and horizontal distribution of chemical elements on the surfaces of peculiar B-F stars.
The “weather” in the atmosphere of a cool star like the Sun is governed by overturning convective flows, pulsating waves, rotating tornados, and small-scale turbulent motions. The movie above is produced from a 3D computer simulation of these processes produced by Uppsala astronomers with the CO5BOLD code. It shows the light intensity of a tiny patch of the solar surface: bright granules bring hot material from the stellar interior up to the surface, where it cools down and sinks back in the dark intergranular lanes. Just like stationary 1D model atmospheres, dynamical 3D models are used to analyse observed stellar spectra, allowing in addition to study the effects of temperature fluctuations and motions. From these comparisons of observed and synthetic spectra, not only the properties of the stellar atmospheres themselves can be deduced, but also those of the interior and the outer envelope.
Uppsala astronomers study the atmospheres of stars similar to the Sun as well as atmospheres of stars with more extreme properties: Stars much cooler than the Sun, as well as brown dwarfs and planets form clouds composed of minerals in their atmospheres that are transported horizontally, just like water-droplet clouds on Earth. During its future evolution, the Sun will grow enormously and turn into a cool red giant star that pulsates and has huge convective cells at the surface (see the movie above, based on another computer model). It develops a strong wind and looses a good fraction of its mass. Much later, it will shrink again and turn into a white dwarf – a very dense, bright Earth-size object. Its large gravity will suck all heavy elements into its interior, with an atmosphere made of pure hydrogen floating on top.