2020A&A...633A..27O

Aims. Cometary globules, dense molecular gas structures exposed to UV radiation, are found inside HII regions. Understanding the nature and origin of these structures through a kinematic study of the molecular gas could be useful to advance in our knowledge of the interplay between radiation and molecular gas.
Methods. Using the Atacama Submillimeter Telescope Experiment (Chile), we carried out molecular observations toward two cometary globules (Sim129 and Sim130) in the HII region Sh2-236. We mapped two regions of about 1'x1' with the ¹²CO J=3-2 and HCO⁺ J=4-3 lines. Additionally, we carried out two single pointings with the C₂H N=4-3, HNC, and HCN J=4-3 transitions. The angular resolution was about 22''. We combined our molecular observations with public infrared and optical data to analyze the distribution and kinematics of the molecular gas.
Results. We find kinematic signatures of infalling gas in the ¹²CO J=3-2 and C₂H N=4-3 spectra toward Sim 129. We detect HCO⁺, HCN, and HNC J=4-3 only toward Sim 130. The HCN/HNC integrated ratio of about three found in Sim 130 suggests that the possible star-formation activity inside this globule has not yet ionized the gas. The location of the NVSS source 052255+33315, which peaks toward the brightest border of the globule, supports this scenario. The non-detection of these molecules toward Sim 129 could be due to the radiation field arising from the star-formation activity inside this globule. The ubiquitous presence of the C₂H molecule toward Sim 129 and Sim 130 evidences the action of the nearby O-B stars irradiating the external layer of both globules. Based on the mid-infrared 5.8µm emission, we identify two new structures: (1) a region of diffuse emission (R1) located, in projection, in front of the head of Sim 129 and (2) a pillar-like feature (P1) placed besides Sim 130. Based on the ¹²CO J=3-2 transition, we find molecular gas associated with Sim 129, Sim 130, R1, and P1 at radial velocities of -1.5, -11, +10, and +4km/s, respectively. Therefore, while Sim 129 and P1 are located at the far side of the shell, Sim 130 is placed at the near side, consistent with earlier results. Finally, the molecular gas related to R1 exhibits a radial velocity that differs in more than 11km/s with the radial velocity of S129, which suggests that while S129 is located at the far side of the expanding shell, R1 would be placed well beyond.