Volume Limited Study of Hot Subdwarfs

• Applicant: Professor Dr. Martin Matthias Roth
• Subject Area: Astrophysics and Astronomy
• Term: Funded in 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 564320067

Hot subdwarfs (sdO/B's) are evolved, core-helium burning stars that have lost their Hydrogen envelope while ascending the first red giant branch. The details of this mass loss and the precise origin of sdO/B stars are poorly understood but rely on scenarios involving binary evolution with stellar or substellar companions. Hot subdwarf stars are ideal objects to study binary interactions as observational evidence suggest that they can only be formed in multiple systems. All previous studies attempting to disclose reliable population and binary properties are limited by strong selection biases caused by the initial target selection. In contrast to the thin disc population, very small close binary fractions have been found in the halo and in old globular clusters. It has been predicted that in old populations the majority of the sdO/Bs are formed by mergers. This motivates us to extend our unique, unbiased and volume-complete sample, which is dominated by thin disc stars down to 500 pc, to the thick disc and the halo. This ambition is observationally feasible within a single semester at Calar Alto. Such a sample would not only buffer our number statistics, but also would provide the next generation of binary population synthesis studies key population property prescriptions. The goal is to obtain spectra for classification and to conduct quantitative analyses for all candidates within our cone sample above 50 degrees that have not been covered by other spectroscopic surveys such as SDSS and LAMOST. 137 out of 672 target candidates require a spectrum (20%), which will fully classify this sample. A complete characterization of this unbiased sample will give insights on the properties of hot subdwarf stars in environments of lower metallicies compared to our thin-disk 500 pc sample.

• DFG Programme: Research Grants