Fundamental parameters of hot subdwarf stars from the Tycho-Gaia sample
Final Report Abstract
The fundamental parameters of hot evolved stars, like masses and radii, are poorly constrained. Hot sub-dwarf stars (sdO/B) are compact helium stars and constitute an important population of hot evolved stars. They dominate old stellar populations in the ultraviolet and are key objects to study peculiar events in stellar evolution as diverse as the interaction of stars and planets or the progenitors of type Ia supernovae. With accurate distances from the Gaia mission available from 2016 we were planning to determine the fundamental parameters of those stars and study their formation. To achieve this, we have to improve the atmospheric models for those peculiar objects and minimize systematics using a homogeneous dataset of high quality. We therefore proposed for five nights with the IDS spectrograph to obtain high quality spectra of such a homogeneous sample of 68 bright sdO/B stars from the Tycho-2 catalogue. The UK time acquisition committee granted observing time for a project at the Observatorio del Roque de los Muchachos, La Palma, Spain. We submitted the proposal when I was still a Marie Curie fellow at the university of Warwick, UK. Shortly after the proposal deadline the PI started a permanent position at the University of Tübingen. We were granted 5 nights of observing time at the 2.5m Isaac Newton telescope from 21.9. to 25.9.2016 and direct project costs including travel and accommodation for two observers. The observing run was extremely successful. We observed uninterrupted for five nights under excellent conditions. In addition to our primary targets, we were able to observe several close binary stars, for which we could determine the orbital parameters. Largely based on these data, a master thesis was written and handed in 11.4.2018 by Maximilian Wolz from our collaborating group in Bamberg. Denny Hoyer, the PhD student who accommpanied the PI for the run, was trained in spectroscopic observations and very enthusiastic about this experience. He defended his PhD thesis in Tübingen successfully in August 2018 and started a job in the public service afterwards. The main purpose of this project was to determine the fundamental parameters of bright sdO/B stars combining the spectroscopic dataset with data from the Gaia mission. The first data release from the Gaia mission has been published in September 2016 by Gaia collaboration (2016). We expected about 200 bright sdO/Bs of all subtypes to be in this release. However, due to yet uncorrected chromatic effects, the bluest objects have been excluded and only approximately 60 stars of the hot sub-dwarf catalogue were part of the sample with preliminary parallaxes and proper motions, almost all of them composite sdB binaries, which were not part of this project. Due to this unexpected decision of the Gaia consortium, we had to postpone the start of the core Gaia project by 1.5 years until Gaia DR2 was released in April 2018. In addition to the high S/N spectra obtained with INT/IDS, we obtained similar datasets with VLT/XSHOOTER and CAHA2.2m/CAFOS of about > 100 hot sub-dwarf stars of all subtypes covering the full parameter space.
