Structure and Evolution of Single Stars
Stars are the major source of information about the Universe. Understanding their properties and applying our knowledge is the aim of research about stellar structure and evolution at MPA. The theory of stellar structure and evolution has a long history at MPA, starting with the pioneering work by Kippenhahn, Meyer-Hofmeister and Weigert. Their stellar evolution code has continuously been updated, reshaped and extended until the present day, still being among the top codes available. It is now well known under the name of GARSTEC. It can be used to calculate the structure and evolution of single stars of all masses throughout most of their life, but is also accurate enough for the computation of solar models and for purposes of asteroseismology. Research in our group focuses on solar models, low and intermediate mass stars, asteroseismology, convection theory, and the application to stellar populations and questions related to cosmology and basic physics. While this code is a traditional hydrostatic 1-dimensional one, we are also investigating crucial aspects of stellar structure, which require a dynamical approach, with 2- and 3-dimensional hydro-codes. Such problems include core convection in massive and developed stars, envelope convection in the Sun and red giants, nuclear flashes, and rotation in stars. The aim is to extract fundamental properties of such effects and to include them in a realistic way in the 1-d code, which is still the only way to follow a star through its complete life.
GARSTEC - the Garching Stellar Evolution Code
Our stellar evolution code GARSTEC (Weiss & Schlattl 2008) with all necessary input data and analysis tools is available. However, to ensure proper scientific use, we insist on a training session (1-2 weeks) at MPA for those who intend to use it, and a sufficiently deep knowledge about stellar structure and evolution. In case of interest, contact Achim Weiss.
A. Weiss, M. Salaris, L. Cassara, L. Piovan, and C. Chiosi
A new set of AGB models, calculated by former PhD student A. Kitsikis, was used to model the properties of dusty envelopes around such stars, and to compute with a detailed radiation transfer code the emergent spectrum and stellar colours. AGB stars are very bright, and very cool. They are characterized by strong stellar winds which remove the largest part of the mass, leaving a proto White Dwarf behind. The mass lost contains lots of dust which enshroudes the central object in the optical and re-radiates the stellar luminosity in the infrared. For testing the physics of the stellar models one very often compares their colours with those of relatively young stellar clusters. We could explain the wide range of observed colours found for clusters of similar age in the Magellanic Clouds. It is a consequence of the colours of the individual stars, their strong variation during their short lifetime on the AGB, and the stochastic nature of their low numbers, even in massive clusters. The figure compares the contribution of infrared light to the total luminosity of ``superclusters'' (several clusters added) with the theoretical predictions for many Monte Carlo realization of AGB populations in clusters of the same mass (red: LMC, blue: SMC composition).
A. Weiss, L. Sbordone, M. Salaris, S. Cassisi
It is now well known that almost all galactic globular clusters host several, in most cases two, different stellar populations, characterized by different chemical compositions, most pronounced in the abundances of O, Na, Mg, Al, C, N, and to some degree He. The current understanding is that this is the result of internal chemical evolution with several phases of star formation, the second generation of stars inheriting the nuclear products of the first one. However, it is far from understood how this happened and which stars - low, intermediate or massive stars, single or binaries - provide the stellar matter for the second generation. To clarifiy this, element abundances are necessary, because different mass ranges produce different chemical mixtures. To guide the spectroscopic investigations it is very helpful to separate the different populations photometrically, as this allows to define fair samples of all sub-populations. In a seminal theoretical work we demonstrated that observations in blue photometric bands provide this separation of populations. To this end we computed both stellar models and stellar atmospheres of typical "first" and "second generation" composition, folded the predicted spectra with standard photometric filter functions (see figure for an example with Johnson-Cousins filters) and identified those filters where abundances variations show the largest colour variations (in the figure between the black and red theoretical spectrum for first and second generation mixture in U and B, the two left-most filters). Meanwhile, this method, based on our theoretical work, has become standard in separating the colour-magnitude-diagrams of globular clusters.
(L. Sbordone, M. Salaris, A. Weiss, S. Cassisi:, A&A 534, A9 (2011))