2024ApJ...966..202Z

We perform a comprehensive CO study toward the Monoceros OB1 (Mon OB1) region based on the Milky Way Imaging Scroll Painting survey at an angular resolution of about 50″. The high-sensitivity data, together with the high dynamic range, show that molecular gas in the 8° × 4° region displays complicated hierarchical structures and various morphology (e.g., filamentary, cavity-like, shell-like, and other irregular structures). Based on Gaussian decomposition and clustering for ¹³CO data, a total of 263 ¹³CO structures are identified in the whole region, and 88% of raw data flux is recovered. The dense gas with relatively high column density from the integrated CO emission is mainly concentrated in the region where multiple ¹³CO structures are overlapped. Combining the results of 32 large ¹³CO structures with distances from Gaia DR3, we estimate an average distance of ${729}_{-45}^{+45}\,\mathrm{pc}$ for the giant molecular cloud (GMC) complex. The total mass of the GMC complex traced by ¹²CO, ¹³CO, and C¹⁸O is 1.1 × 10⁵M_⊙, 4.3 × 10⁴M_⊙, and 8.4 × 10³M_⊙, respectively. The dense gas fraction shows a clear difference between Mon OB1 GMC East (12.4%) and Mon OB1 GMC West (3.3%). Our results show that the dense gas environment is closely linked to the nearby star-forming regions. On the other hand, star-forming activities have a great influence on the physical properties of the surrounding molecular gas (larger velocity dispersion, higher temperatures, more complex velocity structures, etc.). We also discuss the distribution/kinematics of molecular gas associated with nearby star-forming activities.