Formation of an O-Star Cluster by Hierarchical Accretion in G20.08-0.14 N

TL;DR

This study reveals hierarchical accretion flows across multiple scales in the G20.08-0.14 N star-forming region, demonstrating how large-scale gas inflow feeds smaller, dense cores, leading to massive star cluster formation.

Contribution

It provides detailed multi-scale observations of accretion flows and ionized gas dynamics, highlighting the hierarchical and continuous nature of mass assembly in a massive star-forming region.

Findings

Hierarchical accretion flows from 0.5 pc to 0.05 pc scales.

Alignment of rotation axes across scales.

Detection of a new NH3 (3,3) maser.

Abstract

Spectral line and continuum observations of the ionized and molecular gas in G20.08-0.14 N explore the dynamics of accretion over a range of spatial scales in this massive star-forming region. Very Large Array observations of NH_3 at 4'' angular resolution show a large-scale (0.5 pc) molecular accretion flow around and into a star cluster with three small, bright HII regions. Higher resolution (0.4'') observations with the Submillimeter Array in hot core molecules (CH_3CN, OCS, and SO_2) and the VLA in NH_3, show that the two brightest and smallest HII regions are themselves surrounded by smaller scale (0.05 pc) accretion flows. The axes of rotation of the large- and small-scale flows are aligned, and the timescale for the contraction of the cloud is short enough, 0.1 Myr, for the large-scale accretion flow to deliver significant mass to the smaller scales within the star formation timescale. The flow structure appears to be continuous and hierarchical from larger to smaller scales. Millimeter radio recombination line (RRL) observations at 0.4" angular resolution indicate rotation and outflow of the ionized gas within the brightest HII region (A). The broad recombination lines and a continuum spectral energy distribution (SED) that rises continuously from cm to mm wavelengths, are both characteristic of the class of HII regions known as "broad recombination line objects". The SED indicates a density gradient inside this HII region, and the RRLs suggest supersonic flows. These observations are consistent with photoevaporation of the inner part of the rotationally flattened molecular accretion flow. We also report the serendipitous detection of a new NH_3 (3,3) maser.