M17 MIR: A massive protostar with multiple accretion Outbursts
  We report the discovery of a massive protostar M17~MIR embedded in a hot
molecular core in M17. The multiwavelength data obtained during 1993--2019 show
significant mid-IR (MIR) variations, which can be split into three stages: the
decreasing phase during 1993.03--mid-2004, the quiescent phase from mid-2004 to
mid-2010, and the rebrightening phase from mid-2010 until now. The variation of
the 22\,GHz H$_2$O maser emission, together with the MIR variation, indicates
an enhanced disk accretion rate onto M17~MIR during the decreasing and
rebrightening phases. Radiative transfer modeling of the spectral energy
distributions of M17~MIR in the 2005 epoch (quiescent) and 2017 epoch
(accretion outburst) constrains the basic stellar parameters of M17~MIR, which
is an intermediate-mass protostar (M~5.4 Msun) with accretion rate ~1.1x10^-5
Msun in the 2005 epoch and ~1.7x10^-3 Msun/yr in the 2017 epoch. The enhanced
accretion rate during outburst induces the luminosity outburst $\Delta
L\approx7600 $Lsun. In the accretion outburst, a larger stellar radius is
required to produce accretion rate consistent with the value estimated from the
kinematics of water masers. M17 MIR shows two accretion outbursts ($\Delta
t\sim 9-20$ yr) with outburst magnitudes of 2 mag, separated by a 6 yr
quiescent phase. The accretion outbusrt occupies 83\% of the time over 26 yr.
The accretion rate in outburst is variable with amplitude much lower than the
contrast between quiescent and outburst phases. The extreme youth of M17 MIR
suggests that minor accretion bursts are frequent in the earliest stages of
massive star formation.
