Contributed Talk - Splinter HotStars

Thursday, 15 September 2022, 16:55   (SFG 1030 / virtual HotStars)

Kinematic analysis of faint blue stars in the MMT HVS survey

S. Weich (1), S. Kreuzer (1), U. Heber (1), M. Dorsch(1), A. Irrgang(1), R. Raddi(2), D. Koester(3)
(1) Dr. Karl Remeis-Observatory and ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nuremberg (FAU), Sternwartstr. 7, 96049 Bamberg, Germany 2) Universitat Politcnica de Catalunya, Departament de Fsica, c/ Esteve Terrades 5, 08860 Castelldefels, Spain 3) Institut f¨ur Theoretische Physik und Astrophysik, Christian-Albrechts-Universit¨at, Kiel 24118, Germany

The old stellar population of the Galactic halo provides a huge amount of mass probes for investigating the structure, the history and the shape of the gravitational potential of the Milky Way, especially its dark matter halo. Therefore, a spectroscopic survey at the Multiple-Mirror-Telescope (MMT) recorded an unbiased sample of faint blue halo stars, based on strict color and magnitude constraints. Kreuzer (2021) revisited the MMT sample by complementing the spectroscopic data with Gaia DR2 astrometry and carried out a comprehensive analysis of atmospheric, stellar and kinematical properties. Since Gaia EDR3 has been published in the meantime, providing improved proper motions, the kinematic analysis of the MMT sample is redone for the most abundant stellar classes of the sample, which are blue horizontal branch (BHB) stars, blue stragglers (BSs) and white dwarfs (WDs). Galactic space velocity components (U,V,W) are derived, based on spectrophotometric distances and radial velocities from Kreuzer (2021) and the new proper motions. Galactic trajectories are computed, in order to assign the stars to the Galactic populations of the thin and thick disk, as well as the halo. To this end, a new classification scheme is developed, which makes use of six kinematic diagrams, beginning with the U − V , W − V and W − U diagrams. Properties of the orbits, i.e. their eccentricities (e), their Z components of the angular momentum (L_z) and their maximum extensions above or below the Galactic plane (z_max), complete the set of classification tools. The results are compared to the predictions from a Galactic model for each diagram in terms of population contours. A system of population scores finally allows the assignment of individual stars to a Galactic population. Accordingly, the white dwarf sample is dominated by the thin disk population and consist of 79% thin disk stars, 19% thick disk stars and 2% halo stars. This was expected, because of the proximity of most WDs. The halo candidates are confirmed by the shape of their orbits. The BHB star and BS samples, however, are strongly dominated by the halo population, because of the large average distances (even beyond 100 kpc) of the stars. For both classes more than 98% belong to the halo population. Six BHB stars and two BSs are identified to be possibly unbound to the Galaxy, as their boundness probability is below 5%. A particularly enigmatic object is the blue straggler J0946+4256, because it is traveling towards the Galactic plane rather than receding, which suggests that it comes from the outskirts of the Galactic halo or from even further away towards us. An alternative scenario, based on binary star evolution, is discussed to unravel the nature of J0946+4256. Follow-up observations are required to confirm or falsify its nature.