Runaway blue main-sequence stars at high Galactic latitudes. Target selection with Gaia and spectroscopic identification

TL;DR

This study uses Gaia data and spectroscopy to identify and analyze runaway main-sequence stars at high Galactic latitudes, revealing their origins, velocities, and challenging existing ejection models.

Contribution

First identification of runaway MS stars at high Galactic latitudes using Gaia data combined with spectroscopic analysis, including detailed physical parameter determination.

Findings

12 runaway MS candidates ejected from the Galactic disc within 2-16.5 kpc.

Three candidates have ejection velocities over 450 km/s, challenging existing ejection theories.

Identification of 27 BHB candidates and rare late O/early B stars evolving towards white dwarfs.

Abstract

Motivated by the historical identification of runaway main-sequence (MS) stars of early spectral type at high Galactic latitudes, we test the capability of Gaia at identifying new such stars. We have selected ~2300 sources with Gaia magnitudes of GBP - GRP < 0.05, compatible with the colors of low-extinction MS stars earlier than mid-A spectral type, and obtained low-resolution optical spectroscopy for 48 such stars. By performing detailed photometric and spectroscopic analyses, we derive their atmospheric and physical parameters (effective temperature, surface gravity, radial velocity, interstellar reddening, spectrophotometric distance, mass, radius, luminosity, and age). The comparison between spectrophotometric and parallax-based distances enables us to disentangle the MS candidates from older blue horizontal branch (BHB) candidates. We identify 12 runaway MS candidates, with masses between 2 and 6 Msun. Their trajectories are traced back to the Galactic disc to identify their most recent Galactic plane crossings and the corresponding flight times. All 12 candidates are ejected from the Galactic disc within 2 to 16.5 kpc from the Galactic center and possess flight times that are shorter than their evolutionary ages, compatible with a runaway hypothesis. Three MS candidates have ejection velocities exceeding 450 km/s, thus, appear to challenge the canonical ejection scenarios for late B-type stars. The fastest star of our sample also has a non-negligible Galactic escape probability if its MS nature can be confirmed. We identify 27 BHB candidates, and the two hottest stars in our sample are rare late O and early B type stars of low mass evolving towards the white dwarf cooling sequence.