The fastest hot subdwarfs revisited

TL;DR

This study revisits the fastest hot subdwarfs, especially US 708, using spectral analysis and Gaia data, confirming most are bound to the Galaxy except US 708, which is unbound and hyper-velocity.

Contribution

It provides a detailed spectral and kinematic analysis of hot subdwarfs, refining their classification and confirming the hyper-velocity nature of US 708 with new data.

Findings

US 708 is confirmed as an unbound hyper-velocity star.

Most hot subdwarfs are bound to the Galaxy, except US 708.

The subdwarf population shows a bimodal distribution in the Kiel diagram.

Abstract

Hyper-velocity stars (HVS) are enigmatic objects because they are travelling so fast that they escape from the Galaxy. Among hot subdwarfs, only one such star is known, the He-sdO US 708. The Hyper-MUCHFUSS collaboration provided additional HVS candidates. Here we revisit the fastest candidates including US 708 by analysing optical spectra and spectral energy distributions using a new grid of tailored model atmospheres and report preliminary results. The sample is dominated by H-rich subdwarfs and their distribution in the Kiel diagram appears to be bimodal for the sdB stars but otherwise fits canonical evolutionary models well. Gaia proper motion measurements allowed a precise kinematic investigation to be made. It turns out that all previously proposed HVS candidates are actually bound to the Galaxy, except US 708. The original candidate sample turns out to belong to an extreme halo population. The scarcity of available observations of US 708 calls for space-based UV and IR photometry as well as high precision radial velocity measurements.