From dusty filaments to massive stars: The case of NGC 7538 S

TL;DR

This study uses high-resolution submillimeter observations to analyze the structure of NGC 7538 S, revealing multiple protostellar sources and suggesting it is a fragmenting filament forming several B-type stars, rather than a single massive disk.

Contribution

It provides high-resolution evidence that NGC 7538 S is a fragmenting filament with multiple protostars, challenging the previous view of it as a single massive circumstellar disk.

Findings

NGC 7538 S contains five compact sources with associated outflows.

The large structure is likely a filament fragmenting into multiple star-forming cores.

Thermal pressure alone may not support the filament, indicating possible collapse.

Abstract

We report on high-sensitivity and high-angular resolution archival Submillimeter Array (SMA) observations of the large ($\sim$15000 AU) putative circumstellar disk associated with the O-type protostar NGC 7538 S. Observations of the continuum resolve this putative circumstellar disk into five compact sources, with sizes $\sim$ 3000 AU and masses $\sim10 M_\odot$. This confirm the results of recent millimeter observations made with CARMA/BIMA towards this object. However, we find that from most of these compact sources eject collimated bipolar outflows, revealed by our silicon monoxide (SiO {\it J}=5-4) observations and confirm that these sources have a (proto)stellar nature. All outflows are perpendicular to the large and rotating dusty structure. We propose therefore that, rather than being a single massive circumstellar disk, NGC 7538 S could be instead a large and massive contracting or rotating filament that is fragmenting at scales of 0.1 to 0.01 pc to form several B-type stars, via the standard process involving outflows and disks. As in recent high spatial resolution studies of dusty filaments, our observations also suggest that thermal pressure does not seem to be sufficient to support the filament, so that either additional support needs to be invoked, or else the filament must be in the process of collapsing. An SPH numerical simulation of the formation of a molecular cloud by converging warm neutral medium flows produces contracting filaments whose dimensions and spacings between the stars forming within them, as well as their column densities, strongly resemble those observed in the filament reported here.