Context :

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 15m JCMT , and derived line ratios by combining these observations with our previous CO ( 2 - 1 ) and CO ( 1 - 0 ) data from IRAM 30m 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 H _ { 2 } 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 ^ { \prime \prime } ) .
The three different CO transition maps show a similar morphology .
The line ratios of W _ { CO ( 3 - 2 ) } / W _ { CO ( 2 - 1 ) } mostly range from 0.2 to 1.2 with a median of 0.54 \pm 0.12 , while the line ratios of W _ { CO ( 2 - 1 ) } / W _ { CO ( 1 - 0 ) } range from 0.5 to 1.6 with a median of 0.84 \pm 0.15 .
The high CO line ratios W _ { CO ( 3 - 2 ) } / W _ { CO ( 2 - 1 ) } \gtrsim 0.8 and W _ { CO ( 2 - 1 ) } / W _ { 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 the ring-like shell , where the compressed areas are located in bubble N131 .

Conclusions : These high CO integrated intensity ratios , such as W _ { CO ( 3 - 2 ) } / W _ { CO ( 2 - 1 ) } \gtrsim 0.8 and W _ { CO ( 2 - 1 ) } / W _ { CO ( 1 - 0 ) } \gtrsim 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 .