Stellar atmospheres with opacity sampling methods
Frank Grupp
Current atmospheric models calculated at USM are obtained with opacity distribution functions (ODF) which include frozen-in metal abundance mixes and thus are not very good in representing real metal-poor stars. The new models instead refer to the opacity sampling (OS) method, which takes account of millions of absorption lines sampled at random frequencies. This method is substantially more expensive in terms of computing time but also produces a more reliable temperature stratification.
The new models lead to a temperature decrease of the solar stratification of ~50 K near continuum optical depths leading to a fit of the observed Balmer lines with a correspondingly smaller effective temperature. After compensation of that discrepancy using a parametrized self-broadening theory for the Balmer lines, the usual temperature of Teff = 5780 K is retained. Now metal-poor stars require an increase of their effective temperatures.
A set of spectroscopic "standard stars" will be observed and analyzed to establish a new Balmer line temperature scale for cool stars.
Frank Grupp
Melotte 111 and Pleiades belong to the few very young open clusters with a main sequence accessible to high-resolution spectroscopy from early A to late G spectral types. Their stars thus offer a unique approach to determine the chemical properties of clusters from both hot and cool stars. Assuming that all stars have the same He abundance, it will be possible to compare theoretical and observed main sequences and test the physics of stellar interiors.
Rare earths in metal-poor stars
Thomas Gehren (with Lyudmila Mashonkina, Moscow)
Synthesis of the rare earth elements is particularly interesting for extremely metal-poor subdwarfs and subgiants which are thought to be among the oldest objects of the Galaxy immediately following the mysterious Population III. It is therefore important to explore the possible sites of heavy element nucleosynthesis and the contributions of s- and r-process nucleosynthesis to the observed abundances of Ba and Eu. Comparison of the results for rare earths and magnesium seems to require the existence of two different sites of early Galactic massive star nucleosynthesis.
Ionization equilibria in turnoff stars
Thomas Gehren, Frank Grupp (with Lyudmila Mashonkina, Moscow; Shi Jianrong, Beijing; and Andreas J. Korn, Uppsala)
Decades of research in 1D atmospheric models have not lead to reliable ionization equilibria in cool stars around the turnoff and subgiants. In particular the calculation of statistical equilibria casts some doubt upon the current methods with which stellar parameters are determined. A small number of elements that allow the simultaneous spectroscopic analysis of two ionic stages (Mg, Si, Ca, and Fe) in extremely metal-poor stars is examined to determine necessary adjustments of free parameters in atomic models.
Maria Bergemann (now at MPA Garching), Thomas Gehren
The ionization equilibrium of iron and other iron-group elements in cool stellar atmospheres must not necessarily follow the Saha-Boltzmann equation. Observational evidence found in spectra of stars such as Procyon or metal-poor subdwarfs suggests that in fact NLTE plays a non-negligible role in the atmospheres of such stars. This project aims at a comprehensive and accurate study of the excitation and ionization of iron-group elements under the restrictions of different stellar parameters (temperature, gravity, and metal abundance). Preliminary results are discrepant in describing deviations from LTE. Most important will be the influence of hydrogen collisions.
Thomas Gehren (with Shi Jianrong, Zhao Gang, and Zhang Huawei, Beijing)
Synthesis of the elements beyond He is most interesting in the early universe that has left its imprints in the spectra of very metal-poor stars. These subdwarfs (in fact, they are often subgiants) show in their spectra significant enrichment of oxygen with respect to iron. A similar overabundance is found for Mg, while the neutron-rich elements Na and Al have often been claimed to be underabundant with respect to Fe. Using detailed kinetic (NLTE) analyses of these elements it became obvious that for Na, Mg, and Al the assumption of LTE leads to abundances that are wrong by 0.2 to 0.5 dex. While [Na/Mg] abundance ratios follow a typical "secondary element" behaviour with general metal abundance, [Al/Mg] reveals a step-like change between old metal-poor stars of halo and thick disk population. This offers a singular method to determine the population membership of single stars.
Frank Grupp
For years the FOCES échelle spectrograph offered high-resolution spectroscopy at the 2.2m telescope of the Calar Alto Observatory (see FOCES documentation). Due to the change of access rules at the DSAZ, this instrument now is available to German astronomers only during 20% of the observing nights, where it has to compete with other instruments at the 2.2m telesecope. Thus it was suggested by the PI to remove the spectrograph from the DSAZ and re-install it at the 2m telescope that is going to be built on the Wendelstein in the Bavarian Alps.
FOCES will be returned to the USM laboratory in spring 2008. It will be upgraded to prepare for a remote observing mode on the Wendelstein.
High-resolution spectrograph built for the LAMOST 4m survey telescope
Frank Grupp, together with Zhao G., Zhao Y. (Beijing) and Hu, Zhu (Nanjing)
This is a joint proposal of the Cool Star Group at the USM and the National Astronomical Observatories of the
Chinese Academy of Science. It will make possible high-resolution spectroscopy of stars up to 13th magnitude at
the new 4m Schmidt telescope built at Xinglong Observatory.
The spectrograph will be similar to the FOCES échelle spectrograph, however, using a special focal plane
interface to adapt to the telescope.
First light is assumed to be obtained late in 2008.
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