On the origins of Hypervelocity stars as revealed by large-scale Galactic surveys

Qing-Zheng Li, Yang Huang, Xiao-Bo Dong, Hua-Wei Zhang, Timothy C. Beers, Zhen Yuan

Submitted on 10 July 2022


We assemble a large sample of 13,863 high-velocity stars (HiVels) with total velocity VGSR300kms1, selected from the RAVE DR5, SDSS DR12, LAMOST DR8, APOGEE DR16, GALAH DR2, and Gaia EDR3. In this HiVel sample, 43 sources are hypervelocity stars (HVSs) that have VGSR exceeding their local escape velocities, Vesc, 32 of which are discovered for the first time. Interestingly, all the HVSs are metal-poor and late-type, significantly different from the previous HVSs in the literature, which are largely massive, metal-rich early-type stars, identified originally by extreme radial velocity alone. This finding suggests that our newly found HVSs are ejected by different mechanisms from the previous population. To investigate their origins, for the 571 extreme HiVel stars with VGSR0.8Vesc in our sample, we reconstruct their backward-integrated trajectories in the Galactic potential. According to the orbital analysis, no HVSs are found to be definitely ejected from the Galactic center (GC), while 8 late-type metal-poor HiVels are found to have a closest distance to the GC within 1 kpc. Intriguingly, 15 HiVels (including 2 HVSs) are found from their backward-integrated trajectories to have experienced a close encounter with the Sagittarius dwarf spheroidal galaxy (Sgr dSph), suggesting that they originated from this dSph. This hypothesis is supported by an analysis of the [α/Fe]--[Fe/H] diagram. From a preliminary analysis of all the HiVels in our sample, we propose a general picture: Star ejection from Galactic subsystems such as dwarf galaxies and globular clusters, either via tidal stripping or even the Hills mechanism, can be an important channel to produce HiVels/HVSs, particularly the metal-poor late-type halo population.


Comment: 15 pages, 12 figures and 4 tables; submitted to AAS journal

Subject: Astrophysics - Astrophysics of Galaxies