The "Nessie" Nebula: Cluster Formation in a Filamentary Infrared Dark Cloud

TL;DR

The Nessie Nebula, a long filamentary IRDC, exhibits dense core formation consistent with gravitational instability theories, suggesting such filaments are key sites for high-mass star and cluster formation.

Contribution

This study provides observational evidence linking filamentary IRDCs and core spacing to the sausage instability theory, highlighting a mechanism for high-mass star formation.

Findings

Core spacing matches theoretical predictions of sausage instability.

Nessie Nebula is a coherent, single filamentary cloud.

Dense cores are regularly spaced along the filament.

Abstract

The "Nessie" Nebula is a filamentary infrared dark cloud (IRDC) with a large aspect ratio of over 150:1 (1.5 degrees x 0.01 degrees, or 80 pc x 0.5 pc at a kinematic distance of 3.1 kpc). Maps of HNC (1-0) emission, a tracer of dense molecular gas, made with the Australia Telescope National Facility Mopra telescope, show an excellent morphological match to the mid-IR extinction. Moreover, because the molecular line emission from the entire nebula has the same radial velocity to within +/- 3.4 km/s, the nebula is a single, coherent cloud and not the chance alignment of multiple unrelated clouds along the line of sight. The Nessie Nebula contains a number of compact, dense molecular cores which have a characteristic projected spacing of ~ 4.5 pc along the filament. The theory of gravitationally bound gaseous cylinders predicts the existence of such cores, which, due to the "sausage" or "varicose" fluid instability, fragment from the cylinder at a characteristic length scale. If turbulent pressure dominates over thermal pressure in Nessie, then the observed core spacing matches theoretical predictions. We speculate that the formation of high-mass stars and massive star clusters arises from the fragmentation of filamentary IRDCs caused by the "sausage" fluid instability that leads to the formation of massive, dense molecular cores. The filamentary molecular gas clouds often found near high-mass star-forming regions (e.g., Orion, NGC 6334, etc.) may represent a later stage of IRDC evolution.