2025ApJ...984...76T

Studying the atomic-to-molecular transition is essential for understanding the evolution of the interstellar medium. The linear edge of the Taurus molecular cloud, clearly identified in the ¹³CO(1-0) intensity map, serves as an ideal site for investigating this transition. Utilizing the Arizona Radio Observatory Submillimeter Telescope, we obtained mapping observations of CO(2-1), ¹³CO(2-1), and CO(3-2) across this linear edge. The intensity ratio between CO(2-1) and ¹³CO(2-1) indicates a lower limit of the ¹²C/¹³C ratio of 54 ± 17. Based on multitransition observations of CO and ¹³CO, we performed a Markov Chain Monte Carlo fit of the physical properties across this edge using non-local thermodynamic equilibrium analysis with the RADEX code, based on the large velocity gradient assumption. The number density profile exhibits a pronounced jump coinciding with the H₂ infrared emission peak. The cold H I gas within the molecular cloud, manifested as H I narrow self-absorption features, is detected along the cloud edge. Our quantitative comparison with numerical simulations provides tentative evidence that shocks induced by colliding gas flows may contribute to the atomic-to-molecular phase transition observed along the linear edge.