The ALMA Survey of 70 $\mu \rm m$ Dark High-mass Clumps in Early Stages (ASHES). II: Molecular Outflows in the Extreme Early Stages of Protocluster Formation

TL;DR

This study investigates molecular outflows in extremely early high-mass star formation stages, revealing episodic outflows, increasing outflow timescales with core mass, and insights into accretion processes and turbulence in protoclusters.

Contribution

It provides the first evidence of episodic outflows at the earliest stages of high-mass star formation, highlighting the evolution of outflow properties and accretion in dense cores.

Findings

Episodic outflows are present in early protostellar stages.

Outflow timescales increase with core mass.

Outflow energy is insufficient to sustain turbulence.

Abstract

We present a study of outflows at extremely early stages of high-mass star formation obtained from the ALMA Survey of 70 $\mu \rm m$ dark High-mass clumps in Early Stages (ASHES). Twelve massive 3.6$-$70 $\mu \rm m$ dark prestellar clump candidates were observed with the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 6. Forty-three outflows are identified toward 41 out of 301 dense cores using the CO and SiO emission lines, yielding a detection rate of 14%. We discover 6 episodic molecular outflows associated with low- to high-mass cores, indicating that episodic outflows (and therefore episodic accretion) begin at extremely early stages of protostellar evolution for a range of core masses. The time span between consecutive ejection events is much smaller than those found in more evolved stages, which indicates that the ejection episodicity timescale is likely not constant over time. The estimated outflow dynamical timescale appears to increase with core masses, which likely indicates that more massive cores have longer accretion timescales than less massive cores. The lower accretion rates in these 70 $\mu \rm m$ dark objects compared to the more evolved protostars indicate that the accretion rates increase with time. The total outflow energy rate is smaller than the turbulent energy dissipation rate, which suggests that outflow induced turbulence cannot sustain the internal clump turbulence at the current epoch. We often detect thermal SiO emission within these 70 $\mu \rm m$ dark clumps that is unrelated to CO outflows. This SiO emission could be produced by collisions, intersection flows, undetected protostars, or other motions.