Runaway blue main-sequence stars at high Galactic latitudes

Abstract

Context. The ESA Gaia mission is a remarkable tool for stellar population analysis through its accurate Hertzsprung-Russell diagram. Its precise astrometry has propelled detailed kinematic studies of the Milky Way and the identification of high-velocity outliers. Aims. 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. Methods. We have selected ≈ 2300 sources with Gaia magnitudes of G BP − G RP ≤ 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. Results. We identify 12 runaway MS candidates, with masses between 2 and 6 M. 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 −1 , 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. Conclusions. The combination of Gaia parallaxes and proper motions can lead to the efficient selection of runaway blue MS candidates up to 10 kpc away from the Sun. High resolution spectra are needed to confirm the MS status, via precise measurements of projected rotational velocities and chemical compositions.

Blue extreme disk-runaway stars with Gaia EDR3

“…Increasingly more early-type main-sequence (MS) stars in the Galactic halo have been reported to move at very high space velocities (see, e.g., Irrgang et al 2019;Koposov et al 2020;Raddi et al 2021, and references therein), and some may even exceed their local Galactic escape velocity (Brown 2015;Boubert et al 2018, and references therein). These stars are believed to have been born in the star-forming regions of the Milky Way, that is, inside the Galactic disk, and then been ejected from their birth environment.…”

Comparing hypervelocity star populations from the Large Magellanic Cloud and the Milky Way

“…• Following the philosophy of the MMT Hypervelocity Star Survey (Brown et al 2006(Brown et al , 2007(Brown et al , 2009a(Brown et al , 2012, we select only early-type stars at significant Galactic latitudes, |b| > 15 • (see also Raddi et al 2021). Given the lack of ongoing star formation in the stellar halo, early-type stars at faint magnitudes should not be seen at high latitudes and significant distances unless they were ejected there from their place of birth.…”

Mass Transfer as an Explanation for the Lifetime-Travel Time Discrepancy in IT Librae

“…Martin argued that the differences between the two timescales might be avoided with a more complete consideration of the uncertainties. Indeed, a recent discovery and analysis of 12 new runaway MS stars (Raddi et al 2021) shows they all have evolutionary lifetimes longer than their time-of-flight, as is expected for normal runaway stars.…”