Velocity Structure and Molecular Formation in Polaris Molecular Cloud
Abstract
We present a wide-field (60 × 30) study of a dense region within the Polaris Flare, hereafter referred to as the `Polaris molecular cloud', using 12CO, 13CO, and C18O (J=1-0) observations at 20 resolution, obtained with the Nobeyama 45 m Radio Telescope. The analysis reveals molecular gas formation occurring at column densities up to 1021 cm-2, evidenced by an anti-correlation between Hi and CO distributions, indicating active atomic-to-molecular gas conversion. We found a threshold column density for molecular formation at 5×1020 cm-2, which is common among more evolved molecular clouds. The CO-to-H2 conversion factor, X CO, was found to be 0.7 × 1020 H2 cm-2 (K km s-1)-1, lower than the solar neighborhood average. Our chemical models estimate the cloud's age to be 105-106 years, suggesting an early stage of molecular cloud evolution. This interpretation is consistent with the observed low X CO factor. While virial analysis suggests that the entire cloud is gravitationally unbound, we identified several filamentary structures extending from the main cloud body. These filaments show systematic velocity gradients of 0.5-1.5 km s-1 pc-1, and analysis of the velocities shows that the molecular gas within them is falling toward the main cloud body, following a free-fall model. This suggests ongoing mass accumulation processes through the filaments, demonstrating that gravitational processes can be important even at column densities of 1021 cm-2.
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