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