Ulrike Heiter

Last update: 2014-11-26


Suggestions for undergraduate degree projects

Stellar temperatures from spectral line-depth ratios

The determination of effective temperature is a fundamental problem in stellar astrophysics. Different methods have been developed to use either photometric data or measurements of spectral lines as diagnostics for effective temperature. The ratios of spectral line-depths are particularly powerful indicators of the effective temperature. Some studies of this method can be found in the literature: Gray (1994) for dwarf stars, Gray and Brown (2001) for giant stars, and Kovtyukh and Gorlova (2000) for supergiants.

These should be extended and improved. The Nearby Stars Project has acquired high quality spectra of over 500 dwarf and giant stars in the solar vicinity. Spectral line-depth measurements over a large wavelength range are available for all these stars. Photometric and spectroscopic determinations of effective temperature for all stars have been published. For the project work, we suggest to find those pairs of spectral lines whose ratios are most sensitive to effective temperature. These may then be examined on sensitivity to other stellar parameters like gravity and metallicity. General relations for temperature with line-depth ratio may thus be derived.

Potential applications are the monitoring of stellar variability and precise determinations of relative temperatures of large stellar samples. By focussing on spectral lines with maximum sensitivity to temperature, we can save in observing and analysis time while retaining high precision.

Useful skills: Knowledge of stellar physics and programming experience.

Students working on this project: Louise Edstam (15 hp, VT 2013 period 4, Thesis)

Benchmark stars for abundance analysis - photometric calibrations of atmospheric parameters

The determination of stellar atmospheric parameters and abundances is often based on model spectra and other indirect methods. These parameter determination algorithms have to be calibrated against well-known stars. We have compiled a set of candidate benchmark stars, which are relatively bright stars for which direct atmospheric parameters and reliable metallicities are available. Direct effective temperatures and surface gravities are determined from measured stellar angular diameters and supplementary data, and metallicities from high-resolution spectroscopic studies.

For the project work, we suggest to search the literature and databases for the best available photometry of these stars. Calibrations such as those of Gonzalez Hernandez and Bonifacio (2009) and Worthey and Lee (2011) may then be applied to derive indirect atmospheric parameters. Finally, a comparison of the indirect and direct parameters will allow to evaluate the accuracy of the photometric calibrations.

Useful skills: Knowledge of stellar physics and programming experience.

Molecular data for stellar astrophysics

The calculation of synthetic stellar spectra requires a large amount of input data for atomic and molecular transitions. The VALD database, maintained in Uppsala, contains a convenient collection of atomic data. We are in the process of including data for the most important diatomic molecules visible in the spectra of cool stars. For various molecules, two or more different sources of line data are available.

For the project work, we suggest to choose a specific molecule (e.g. CH, CN, CO, C2, FeH, CrH, MgH) and search the literature for available data. The next step is a comparison of line parameters from different sources (e.g. wavelengths, line intensities), as shown in this example. In the case of a longer project (30 hp), the data may then be used to calculate synthetic spectra and compare these with observed spectra of cool stars. Finally, methods to select the best available data or to apply corrections to the data may be devised.

Useful skills: Knowledge of atomic and molecular physics and programming experience.

© 2014 Ulrike Heiter