The Nearby Evolved Stars Survey. IV. Mapping cold gas in the circumstellar envelopes of evolved stars with ¹²CO, ¹³CO (<i>J </i>= 1 → 0) emission

We have performed mapping radio observations of 42 AGB stars of which 23 and 14 stars were detected in ¹²CO, ¹³CO (J = 1 → 0) emission lines using the 45 m radio telescope of Nobeyama Radio Observatory, aiming to reveal the distributions of extended cold gas in circumstellar envelopes of these stars. By comparing these emission maps with those of the ¹²CO (J = 3 → 2) line obtained with the James Clerk Maxwell Telescope at the same angular resolution, we found that the envelope of the ¹²CO (J = 1 → 0) emission has at least double the size of that seen in the ¹²CO (J = 3 → 2) emission in all the stars. We also found that the ¹²CO/¹³CO line intensity ratio changes radially in the envelopes of eight stars. This suggests the effect of selective photodissociation by interstellar ultraviolet radiation. Through constructing a toy model of the envelope to solve radiative transfer, we demonstrated that all stars except two carbon stars are optically thin in the ¹²CO (J = 1 → 0) line. Thus, there is no evidence for a thermal pulse or the third dredge up in the material ejected in the last 1 000 to 3 000 yr for those optically-thin envelopes. Furthermore, we confirmed aspherical envelope morphologies, such as o Cet and WX Psc, which were formed by bipolar outflow. However, the origins of the aspherical envelopes in SV Peg and RAFGL 618 remain unknown.