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.