M17 MIR: A Massive Star is Forming via Episodic Mass Accretion

TL;DR

This study uses ALMA data to reveal episodic, jet-like outflows from the massive protostar M17 MIR, indicating that massive star formation involves frequent accretion bursts driven by disk gravitational instability.

Contribution

It provides the first observational evidence linking episodic outflows to accretion bursts in a massive protostar, supporting the disk instability model for massive star formation.

Findings

Detection of collimated bipolar outflows with knots indicating episodic ejections.

Ejection velocities of approximately 421 km/s suggest recent accretion outbursts.

Multiple clustered accretion events over the past few hundred years support disk fragmentation models.

Abstract

We analyzed the Atacama Large Millimeter/submillimeter Array (ALMA) band 6 data for the outbursting massive protostar M17~MIR. The ALMA CO $J=2-1$ data reveal 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 $\sim421\,\mathrm{km\,s^{-1}}$ 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.