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

Understand

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.