Institut für Astronomie und Astrophysik Abteilung Astronomie Sand 1, D-72076 Tübingen, Germany

Stellar Atmospheres Group - Research Activities

Overview

Topics by keywords

• Hot stars, like PG1159 stars, white dwarfs, post-AGB stars, sdO stars, ...
• Stellar Atmospheres and radiation transfer
• Spectroscopy, from Infrared to X-Rays
• Computational Astrophysics
• Asteroseismology

Introduction

This is a very short description of our research interests and activities. For more details please see the numerous articles and reviews given in our publication lists (Klaus Werner [ADS author list], Stefan Dreizler, Institute preprints).

The general context of this work is the stellar evolution, especially the late stages of intermediate mass stars which end their life as white dwarfs. Several aspects of the evolution of the immediate progenitor stars of the white dwarfs as well as the transformation process into a white dwarf are far from understood. Our contribution to the solution of these problems is the determination of stellar parameters like the effective temperature, surface gravity, chemical composition, radius, mass, and mass loss rates by means of spectroscopic analyses. This requires high quality spectra from ground and space based observatories covering the stellar spectrum form X-rays to the infrared (see also Section Observation) as well as sophisticated computer simulations (see Section Theory) of the interaction between the radiation field with the hot plasma in the outer layers of the star.

We participate in several national and international cooperations.

A summary of the research activities of the institute is published at The Yearly Reports to the Astronomische Gesellschaft

Theory

Spectroscopic analyses require the comparison of stellar spectra with synthetic ones. Stellar spectra are obtained at ground or space based observatories described in the next section, the calculation of synthetic spectra are shortly described here.

Synthetic spectra are obtained from the solution of the radiative transport equations, describing the propagation of light through the outer layers of a star, the stellar atmosphere. On one hand, this solution necessitates the knowledge of the physical conditions like temperature, density and occupation of the atomic levels of the plasma in order to determine the opacity and emissivity. On the other hand the radiation field interacts with the stellar plasma and changes its physical conditions. While in stars similar to our sun, the occupation of the atomic energy levels can be determined with sufficient precision from the assumption of a local thermodynamic equilibrium (LTE) this is no longer the case for stars hotter than approximately 30000K effective temperature (see NLTE Spectral Analysis of Hot Stars). The level population in the non-LTE case is obtained by balancing all populating and de-populating processes from each atomic level assuming stationarity (statistical equilibrium). The complete problem is a non-linear system of integro-differential equations. Their solution requires a very efficient numerical iteration scheme which was developed within the last decade in our working group (Werner 1986 [ADS abstract: A&A 161, 177], Dreizler & Werner 1993 [ADS abstract: A&A 278, 199], Werner & Dreizler 1998). The latter reference also contains more elaborated introduction to the physical and numerical problems as well as a detailed description of our numerical implementation in a FORTRAN code. A WWW guide of our program package can be found at The Tübingen NLTE Model Atmosphere Package.

An independent stellar atmosphere code was developed by Ivan Hubeny (NASA/GSFC) and Thierry Lanz (Utrecht University). A description of this code can be found at the TLUSTY Home Page.

For interested readers we recommend the Lecture Notes in Stellar Atmospheres by Rob Rutten as a detailed introduction to this field.

Observation

All information about stars is obtained from the emitted light which can be detected as direct image or as spectrum.

We derive most of our results from stellar spectroscopy. The shape of the line profiles as well as the shape of the continuum allows the derivation of the physical condition in the outer layers of a star by a comparison with theoretical spectra. We obtain spectra at various ground and space based observatories which are listed in these links. The corresponding data reduction is partly done using our IDL software SPEX. Optical spectroscopy cover spectral lines of the most abundant elements, hydrogen and helium, and allows a classification of the star. However, hot stars emit most of their light in the far UV or X-ray range. A more precise temperature determination therefore requires observations in the UV up to the X-ray range which is only possible from space based observatories. In most cases, the important spectral lines of heavier elements like carbon, nitrogen, oxygen,... in hot stars reside in the UV which again requires space observations (e.g. HST).

Several aspects of the evolution of post-AGB stars and white dwarfs are hampered by the fact that not enough members of some interesting classes of stars are known. Sky surveys like the Hamburg Schmidt Survey, designed to be a Quasar survey, are also valuable sources of hot stars. In a collaborative project with the Dr. Remeis-Sternwarte, Bamberg, the Institute of Theoretical Physics and Astrophysics, Kiel and the Hamburg Observatory we explore the stellar component of Hamburg Schmidt and Hamburg ESO survey.

Direct imaging allows the search for Planetary Nebula around old post-AGB stars and white dwarfs using narrow band filters, the search for cool companions using infrared imaging, or the time dependent photometry of variable stars. The latter one can be used to derive stellar parameters of pulsating stars by means of Asteroseismology. An introduction to this field is given on the Whole Earth Telescope home page. We regularly participate in such WET runs, and independently perform a search for pulsating sdB stars, which again makes use of the stellar component of the Hamburg Schmidt and Hamburg ESO surveys. For the analysis of the resulting photometric data, we are developing the IDL software TRIPP. See also the nice visualization of stellar pulsations on the Delta Scuti Network page. Our participation on world wide observing campaigns of pulsating stars are shortly described in a press release of the University Tübingen as well as on the home page of the Whole Earth Telescope (Xcov17 and Xcov19).

The Tübingen Model Atmosphere Package (TMAP)

TMAP is the Tübingen NLTE-Model Atmosphere Package developed by Klaus Werner, Stefan Dreizler and Thomas Rauch during the last two decades.

The main body consists of a code which can compute non-LTE stellar model atmospheres in hydrostatic and radiative equilibrium, and in plane-parallel or spherically symmetric geometry. The code has been widely applied to hot compact stars (white dwarfs, subdwarfs, central stars on planetary nebulae). The basic advantage over earlier NLTE codes (e.g. by Auer and Mihalas) comes with the implementation of the Accelerated Lambda Iteration (ALI) method. This allows to include metal line blanketing effects, making NLTE models as sophisticated as LTE models concerning complexity of atomic data input to calculate opacities.

LINE1_PROF is a modified version of PRO2, which allows for the calculation of synthetic spectra using atomic data of refined spectral line broadening theories. LTE2 is a code which computes a LTE starting model for PRO2. Finally, Auxiliary contains some other small but useful routines. For a more detailed description of the Tübingen NLTE Model Atmosphere Package see the Users Guide.

IRONIC is a program which calculates atomic input data for the NLTE model code. It is described on the CSC page.

Acknowledgment

This research is partly granted by the Deutsche Forschungs Gemeinschaft and the Deutsches Zentrum für Luft- und Raumfahrt.

Further informations

For further informations, please contact Klaus Werner

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Thorsten Nagel | Impressum Last modified 14 Dec 2005