Schwerebeschleunigungen Kühler Sterne

A refined theory of line formation for neutral and singly-ionized iron in the atmospheres of cool stars in presented. It accounts for departures from local thermodynamic equilibrium –so-called non-LTE effects –an d serves the derivation of spectroscopic stellar gravities “g” via the ionization equilibrium constraint. The respective influence of various model assumptions (atomic data sets including photo-ionization cross-sections and transition probabilities; the efficiency of collisions with neutral particles and so forth) is investigated by means of extensive profile analyses of iron lines in the optical spectrum of the Sun. Due to the large scatter among laboratory transition probabilities the main free parameter of the model –th e strength of collisions with neutral hydrogen SH which govern excitation and ionization –can not be determined by inspecting the Sun alone. Depending on the choice of SH non-LTE effects in neutral iron range from 0.01 dex to 0.10 dex in the logarithmic abundance, while singly-ionized iron follows LTE. To calibrate SH a set of local standard stars is employed whose surface gravities can be inferred from astrometry (the Hipparcos satellite mission) on a given temperature scale. With the exception of the metal-rich standard star Procyon for which a mild temperature correction is needed to fulfil the ionization balance, excellent consistency is obtained for the metal-poor calibration stars in various evolutionary stages if the Balmer profile temperatures are combined with an SH of 3. Departures from LTE are moderate and amount to 0.1 dex in log g at most. The correct choice of the van-der-Waals damping constants is generally more important for these stars than non-LTE. The long-standing discrepancy with respect to an independent spectroscopic gravity scale discussed in the literature can be fully resolved. Recent modifications to the formation theory of Balmer lines are implemented, but do not yield consistent results: the temperature of the Sun is not recovered, the observed Balmer line profiles of metal-poor stars are not reproduced. The devised model is applied to two extreme objects, the most metal-poor star known (100 000 times less metals than the Sun) and a so-called r-process star, both recently identified in wide-angle sky survey. Here non-LTE effects are much more pronounced (0.3 dex in log g ). This result emphasizes the importance of considering departures from LTE for iron, especially when it comes to studying giant stars in the Galactic halo.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Ludwig-Maximilians-Universität

Year

2002

Author dc:creator

• Korn, Andreas

Repository record source_url

https://edoc.ub.uni-muenchen.de/352/

OAI identifier oai:identifier

oai:edoc.ub.uni-muenchen.de:352