M17 MIR: A Massive Star is Forming via Episodic Mass Accretion
The Astrophysical Journal Letters(2024)
摘要
We analyzed the ALMA band 6 data for the outbursting massive protostar
M17 MIR. The ALMA CO J=2-1 data reveals a collimated and bipolar north-south
outflow from M17 MIR. The blue-shifted outflow exhibits four CO knots (N1 to
N4) along the outflow axis, while the red-shifted outflow appears as a single
knot (S1). The extremely high velocity (EHV) emissions of N1 and S1 are
jet-like and contain sub-knots along the outflow axis. Assuming the nearest EHV
sub-knots trace the ejecta from the accretion outbursts in the past decades, a
tangential ejection velocity of ∼421 km s^-1 is derived for
M17 MIR. Assuming the same velocity, the dynamical times of the multiple
ejecta, traced by the four blue-shifted CO knots, range from 20 to 364 years.
The four blue-shifted CO knots imply four clustered accretion outbursts with a
duration of tens of years in the past few hundred years. The intervals between
the four clustered accretion outbursts are also about tens of years. These
properties of the four clustered accretion outbursts are in line with the disk
gravitational instability and fragmentation model. The episodic accretion
history of M17 MIR traced by episodic outflow suggests that a massive star can
form from a lower-mass protostar via frequent episodic accretion events
triggered by disk gravitational instability and fragmentation. The first
detection of the knotty outflow from an outbursting massive protostar suggests
that mass ejections accompanied with accretion events could serve as an
effective diagnostic tool for the episodic accretion histories of massive
protostars.
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关键词
Star formation,Stellar accretion,Stellar jets
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