The Molecular Clouds associated with the H II Regions/Candidates between $l=207.7^\circ$ and $l=211.7^\circ$

TL;DR

This study uses large-scale CO surveys to analyze molecular clouds associated with H II regions between l=207.7° and 211.7°, revealing their physical properties, distributions, and potential excitation sources.

Contribution

It provides the first comprehensive survey of molecular clouds linked to specific H II regions in this sky area, detailing their physical characteristics and spatial structures.

Findings

Identified molecular clouds associated with ten H II regions/candidates.

Discovered that most H II regions and clouds are three-dimensional structures.

Found massive stars within several H II regions likely serve as excitation sources.

Abstract

Using the PMO-13.7 m millimeter telescope at Delingha in China, we have conducted a large-scale simultaneous survey of $^{12}$CO, $^{13}$CO, and C$^{18}$O $J=1-0$ emission toward the sky region centered at $l$=$209.7^\circ$, $b$=$-$2.25$^\circ$ with a coverage of $4.0^\circ \times 4.5^\circ$. The majority of the emission in the region comes from the clouds with velocities lying in the range from $-$3 km s$^{-1}$ to 55 km s$^{-1}$, at kinematic distances from 0.5 kpc to 7.0 kpc. The molecular clouds in the region are concentrated into three velocity ranges. The molecular clouds associated with the ten H II regions/candidates are identified and their physical properties are presented. Massive stars are found within Sh2-280, Sh2-282, Sh2-283, and BFS54, and we suggest them to be the candidate excitation sources of the H II regions. The distributions of excitation temperature and line width with the projected distance from the center of H II region/candidate suggest that the majority of the ten H II regions/candidates and their associated molecular gas are three-dimensional structures, rather than two-dimensional structures.