Study of Molecular Clouds associated with HII Regions

TL;DR

This study investigates molecular clouds associated with HII regions, identifying two types based on size-line width relations, and explores their properties and implications for star formation processes.

Contribution

It classifies molecular cloud clumps into two categories based on size-line width relations and links these to star formation activity and cloud evolution.

Findings

Type I clumps show a power law size-line width relation with larger indices.

Type II clumps lack this relation and exhibit active star formation signs.

Internal turbulence sources may influence molecular cloud dynamics.

Abstract

The properties of molecular clouds associated with 10 HII regions were studied using CO observations. We identified 142 dense clumps within our sample and found that our sources are divided into two categories: those with clumps that show a power law size-line width relation (Type I) and those which do not show any relation (Type II). The clumps in the Type I sources have larger power law indices than found in previous studies. The clumps in the Type II sources have larger line widths and show evidence (such as outflows) of current active star formation within the clump. We suggest that the lack of a size-line width relation is a sign of current active star formation. Massive clumps seem to have similar masses calculated by different methods but lower mass clumps have larger virial masses compared to velocity integrated and LTE mass. We found no relation between mass distribution of the clumps and distance from the H~II region ionization front, but a weak decrease of the excitation temperature with increasing distance from the ionized gas. The clumps in collected shells around the H~II regions have slightly larger line widths but no relation was found between line width and distance from the H~II region, which probably indicates that the internal dynamics of the clumps are not affected by the ionized gas. Internal sources of turbulence, such as outflows and stellar winds from young proto-stars may have a more important role on the molecular gas dynamics. We suggest that large line width and larger size-line width power law indices are therefore the initial characteristics of clumps in massive star forming clouds and that some may evolve into objects similar to our Type II sources, where local "second generation" stars are forming and eliminating the size-line-width relation.