Using CO line ratios to trace compressed areas in bubble N131

TL;DR

This study uses CO line ratios from multiple transitions to identify and trace compressed, high-temperature, and high-density regions in the expanding infrared dust bubble N131, revealing interaction zones between molecular and ionized gas.

Contribution

It demonstrates that high CO line ratios effectively trace compressed regions in bubble N131, providing a new method to study gas interactions in such environments.

Findings

High CO line ratios indicate compressed, heated regions in N131.

Line ratios beyond simulation thresholds mark interaction zones.

CO line ratios can serve as tracers for gas compression and interaction.

Abstract

N131 is a typical infrared dust bubble showing an expanding ringlike shell. We study what kinds of CO line ratios can be used to trace the interaction in the expanding bubble. We carry out new $\rm CO\,(3-2)$ observations towards bubble N131 using the JCMT 15-m telescope, and derive line ratios by combining with our previous $\rm CO\,(2-1)$ and $\rm CO\,(1-0)$ data from the IRAM 30-m telescope observations. To trace the interaction between the molecular gas and the ionized gas in the HII region, we use RADEX to model the dependence of CO line ratios on kinetic temperature and H$_2$ volume density, and examine the abnormal line ratios based on other simulations. We present $\rm CO\,(3-2)$, $\rm CO\,(2-1)$, and $\rm CO\,(1-0)$ integrated intensity maps convolved to the same angular resolution (22.5$"$). The three different CO transition maps show apparently similar morphology. The line ratios of $W_{\rm CO\,(3-2)}$/$W_{\rm CO\,(2-1)}$ mostly range from 0.2 to 1.2 with a median of $0.54\pm0.12$, while the line ratios of $W_{\rm CO\,(2-1)}$/$W_{\rm CO\,(1-0)}$ range from 0.5 to 1.6 with a median of $0.84\pm0.15$. The high CO line ratios $W_{\rm CO\,(3-2)}$/$W_{\rm CO\,(2-1)}\gtrsim 0.8 $ and $W_{\rm CO\,(2-1)}$/$W_{\rm CO\,(1-0)}\gtrsim 1.2$ are beyond the threshold predicted by numerical simulations based on the assumed density-temperature structure for the inner rims of ringlike shell, where are the compressed areas in bubble N131. These high CO integrated intensity ratios, such as $W_{\rm CO\,(3-2)}$/$W_{\rm CO\,(2-1)}\gtrsim0.8$ and $W_{\rm CO\,(2-1)}$/$W_{\rm CO\,(1-0)}\gtrsim1.2$, 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.