Origin of a massive hyper-runaway subgiant star LAMOST-HVS1 -- implication from Gaia and follow-up spectroscopy

TL;DR

This paper identifies LAMOST-HVS1 as a massive hyper-runaway star ejected from the Milky Way's inner disk, likely due to dynamical interactions in a dense star cluster involving an intermediate-mass black hole or very massive stars.

Contribution

It provides the first detailed analysis of LAMOST-HVS1's origin, ejection mechanism, and possible natal environment using Gaia data and spectroscopy.

Findings

LAMOST-HVS1 is an 8.3 Msun hyper-runaway star ejected 33 Myr ago.

Ejection velocity of 568 km/s suggests dynamical ejection, not supernova.

Potential ejection agents include intermediate-mass black holes or very massive stars.

Abstract

We report that LAMOST-HVS1 is a massive hyper-runaway subgiant star with mass of 8.3 Msun and super-Solar metallicity, ejected from the inner stellar disk of the Milky Way $\sim$ 33 Myr ago with the intrinsic ejection velocity of $568^{+19}_{-17}$ km/s (corrected for the streaming motion of the disk), based on the proper motion data from Gaia Data Release 2 (DR2) and high-resolution spectroscopy. The extremely large ejection velocity indicates that this star was not ejected by the supernova explosion of the binary companion. Rather, it was probably ejected by a 3- or 4-body dynamical interaction with more massive objects in a high-density environment. Such a high-density environment may be attained at the core region of a young massive cluster with mass of $\gtrsim 10^4$ Msun. The ejection agent that took part in the ejection of LAMOST-HVS1 may well be an intermediate mass black hole ($\gtrsim$ 100 Msun), a very massive star ($\gtrsim$ 100 Msun), or multiple ordinary massive stars ($\gtrsim$ 30 Msun). Based on the flight time and the ejection location of LAMOST-HVS1, we argue that its ejection agent or its natal star cluster is currently located near the Norma spiral arm. The natal star cluster of LAMOST-HVS1 may be an undiscovered young massive cluster near the Norma spiral arm.