Using CO line ratios to trace compressed areas in bubble N131

Abstract

Aims. N131 is a typical infrared dust bubble showing an expanding ring-like shell. We study the CO line ratios that can be used to trace the interaction in the expanding bubble. Methods. We carried out new CO (3–2) observations toward bubble N131 using the 15 m JCMT, and derived line ratios by combining these observations with our previous CO (2–1) and CO (1–0) data from IRAM 30 m observations. To trace the interaction between the molecular gas and the ionized gas in the HII region, we used RADEX to model the dependence of the CO line ratios on kinetic temperature and H2 volume density, and examined the abnormal line ratios based on other simulations. Results. We present CO (3–2), CO (2–1), and CO (1–0) integrated intensity maps convolved to the same angular resolution (22.5″). The three different CO transition maps show a similar morphology. The line ratios of WCO (3–2)/WCO (2–1) mostly range from 0.2 to 1.2 with a median of 0.54 ± 0.12, while the line ratios of WCO (2–1)/WCO (1–0) range from 0.5 to 1.6 with a median of 0.84 ± 0.15. The high CO line ratios WCO (3–2)/WCO (2–1) ≳ 0.8 and WCO (2–1)/WCO (1–0) ≳ 1.2 are beyond the threshold predicted by numerical simulations based on the assumed density-temperature structure for the inner rims of the ring-like shell, where the compressed areas are located in bubble N131. Conclusions. These high CO integrated intensity ratios, such as WCO (3–2)/WCO (2–1) ≳ 0.8 and WCO (2–1)/WCO (1–0) ≳ 1.2, can be used as a tracer of gas-compressed regions with a relatively high temperature and density. This further suggests that the non-Gaussian part of the line-ratio distribution can be used to trace the interaction between the molecular gas and the hot gas in the bubble.