'Slow' and Fast Rotators among Hypervelocity Stars

TL;DR

This study measures the rotational velocities of two hypervelocity stars, revealing different origins and ejection mechanisms, and classifies their stellar types based on spectroscopic data.

Contribution

It provides the first detailed rotational velocity measurements of hypervelocity stars HVS7 and HVS8, suggesting different ejection scenarios and stellar classifications.

Findings

HVS7 has a slow rotation consistent with binary origin

HVS8 exhibits fast rotation typical of single B-type stars

HVS7 likely a blue horizontal branch star, HVS8 a main sequence star

Abstract

We measure the projected rotational velocities of the late B-type hypervelocity stars HVS7 and HVS8 from high resolution spectroscopy to be 60 +/- 17 km/s and 260 +/- 70 km/s. The 'slow' rotation of HVS7 is in principle consistent with having originated in a binary system, assuming a high inclination angle of the stellar rotation axis. However, the fast rotation of HVS8 is more typical of single B-type stars. HVS8 could have therefore been ejected by a mechanism other than that proposed by Hills. We also estimate the effective temperatures and surface gravities for HVS7 and HVS8 and obtain an additional measurement of their radial velocities. We find evidence in support of a blue horizontal branch nature for HVS7, and a main sequence nature for HVS8.