Giant Elephant Trunks from Giant Molecular Clouds

TL;DR

This paper reports the discovery of giant elephant trunk-like molecular gas structures, significantly larger than typical ETs, in star-forming regions, and discusses their physical properties, origins, and potential as stellar cluster birth sites.

Contribution

The study introduces the identification and analysis of giant elephant trunk (GET) structures, extending the known size range of ETs and exploring their formation mechanisms and role in star formation.

Findings

GETs are 20-50 pc long, much larger than typical ETs.

GETs have cometary structures aligned with the galactic plane.

Molecular masses of GET heads suggest gravitational stability.

Abstract

We report the discovery of large elephant trunk (ET)-like objects, named giant elephant trunk (GET), of molecular gas in star forming complexes in the Scutum and Norma arms using the $^{12}$CO(J=1-0)-line survey data with the Nobeyama 45-m telescope. In comparison with the CO maps of ETs in M16 as derived from the same data, we discuss physical properties of the GETs. Their lengths are $\sim 20$ to 50 pc, an order of magnitude larger than ETs. GETs show a cometary structure coherently aligned parallel to the galactic plane, and emerge from bow-shaped concave surface of giant molecular clouds (GMC) facing the HII regions, and point down-stream of the gas flow in the spiral arms. The molecular masses of the head clumps are $\sim 10^3 -10^4 M_\odot$, about 3 to 4 times the virial masses, indicating that the clumps are gravitationally stable. Jeans masses calculated for the derived density and assumed kinetic temperature are commonly sub-solar. We suggest that the GET heads are possible birth sites of stellar clusters, similarly to ETs' globules, but at much greater scale. We discuss the origin of the GETs by Rayleigh-Taylor instability due to deceleration of GMCs by low density gas stagnated in the galactic shock waves as well as by pressure of the HII regions.