Structure and Evolution of Single Stars

A Hertzsprung-Russell-diagram with evolutionary tracks of low-mass stars. Indicated are the locations of these stars at two different ages. Connecting these points constitutes an isochrones, which is the theoretical counterpart of the colour-magnitude-diagram of a globular cluster.

A Hertzsprung-Russell-diagram with evolutionary tracks of low-mass stars. Indicated are the locations of these stars at two different ages. Connecting these points constitutes an isochrones, which is the theoretical counterpart of the colour-magnitude-diagram of a globular cluster.

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.

Recent projects

Updated models for Asymptotic Giant Branch stars and predictions for observed colours of star clusters

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).

(M. Salaris, A. Weiss, L.P. Cassara, L. Piovan, C. Chiosi: A&A 565, A9 (2014))

Improving the treatment of convective enveleopes in cool stars

Andreas C.S. Joergensen, Achim Weiss

Evolution of solar models with different treatments of the superadiabatic layers in the envelope. The blue line corresponds to the most advanced treatment. The kink in this track reflects the still insufficiently dense grid of underlying 3d-models.

Stellar evolution codes, such as GARSTEC, compute stellar structures under a set of simplifying assumptions. This enables the code to give a comprehensive picture of stars and their life cycles at a low computational expense. For instance, stellar evolution codes assume spherical symmetry and employ time-steps that are determined by changes in the chemical profile, which implies that they necessitate approximate treatments of phenomena that take place on shorter timescales. In other words, phenomena, such as turbulent convection, that are intrinsically three-dimensional (3D) and that lead to rapid fluctuations, are treated ad hoc. While the employed approximations are physically motivated and often work amazingly well, they lead to some discrepancies between model predictions and observations.

In order to overcome this shortcoming, we combine stellar evolution codes with 3D simulations of the outermost layers of stellar convective envelopes (Jørgensen et al. 2019). These simulations are a legacy of one of the group’s former PhD students, Zazralt Magic (Magic et al. 2013).

While 3Dsimulations yield a more realistic depiction of phenomena, such as turbulence, they come at a very high computational cost. However, due to the homology of the mean stratification of 3D simulations, we can easily recover the correct structure by interpolation. We hence interpolate in the existing 3D simulations and include these interpolated structures directly into GARSTEC at every single time-step throughout the evolution. By following this approach, weachieve stellar models that recover the physically more realistic description of convection from 3D simulations at the low computational cost of standard stellar models. Even better, we find that we overcome the tensionbetween seismic observations and model predictions. Finally, the evaluated stellar evolution tracks turn out to be sensitive to the improved depiction of convection (see figure). This means that our improvements of the standard procedure affects the predicted global stellar parameters (e.g. Age, Radius, effective temperature) and may play a role for any field, ranging from exoplanet research to galactic archaeology, that draws on parameter estimates from stellar models.

Picture: Evolution of solar models with different treatments of the superadiabatic layers in the envelope. The blue line corresponds to the most advanced treatment. The kink in this track reflects the still insufficiently dense grid of underlying 3d-models.