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

ABSTRACT We have performed mapping observations of 42 AGB stars, of which 23 and 14 stars were detected in $^{12}$CO and $^{13}$CO ($J = 1 \rightarrow 0$) emission, respectively, using 45 m telescope of Nobeyama Radio Observatory. Our aim is to reveal distributions of extended cold gas in circumstellar envelopes and test a possibility of detecting shell structures with high $^{12}$CO/$^{13}$CO ratios caused by thermal pulses and third dredge-ups. By comparing our maps with those of $^{12}$CO ($J = 3 \rightarrow 2$) emission obtained with James Clerk Maxwell Telescope at the same angular resolution, we found that envelopes of $^{12}$CO ($J = 1 \rightarrow 0$) emission has at least twice the sizes of $^{12}$CO ($J = 3 \rightarrow 2$) emission. Among 14 stars detected in both $^{12}$CO and $^{13}$CO emission, nine stars show radial variations of $^{12}$CO/$^{13}$CO ratio along radial direction in their envelopes, which is likely caused by selective photodissociation by interstellar ultraviolet radiation, not thermal pulses or third dredge ups. Based on radiative transfer simulations, we conclude that optical depths are small (typically $\tau \lesssim 0.5$), ruling out the possibility that the absence of detected shells in our data is due to optical depth. This implies no signatures of thermal pulses or third dredge ups in the last $\sim$1500–13 000 yr. We further confirmed aspherical envelope morphologies of o Cet, WX Psc, and RAFGL 618, which were likely formed by bipolar outflows and photodissociation. The elongated morphology of SV Peg that we newly found is subject to study.