ALMA Observations of CO in Five Planetary Nebulae: Insights into Nebular Shaping

The J = 2→1 transition of CO near 230 GHz has been imaged in five bipolar/multipolar planetary nebulae (K3-45, K3-58, M1-7, M2-48, and M3-28) using ALMA with 1″ angular resolution. Single-dish data from the Arizona Radio Observatory Submillimeter Telescope (ARO SMT) were added to provide sensitivity on spatial scales up to 30″—essential for these data. The images show that the molecular gas resides primarily in the central regions of the nebulae and not in the bipolar flows of the optical images. CO appears to trace the remnant asymptotic giant branch (AGB) wind (V _exp ∼ 12–14 km s⁻¹) but has a higher velocity component (V > 16–23 km s⁻¹) that represents gas swept up in the outflows. The orientations, with respect to the plane of the sky, of the bipolar outflows are 45° ± 5, 65° ± 10, and 65° ± 5 for K3–45, K3-58, and M1-7, respectively, with the blueshifted lobes oriented to the NE, W, and NW. For M2-48 and M3-28, the flows lie in the plane of the sky, with the blueshifted lobes roughly lying E and N. Based on CO, velocities in the bipolar flows are estimated to be V _outflow > 18–31 km s⁻¹. Revised CO column densities are consistent with past measurements, except for M2-48, where the value increased to N _tot ∼ 9.3 × 10 16 cm⁻²; a value was also obtained for the first time for K3-58 (N _tot ∼ 5 × 10¹⁶ cm⁻²). Given the unusual ¹²C/¹³C ratios observed in two sources, the bipolar ejection may have an origin in runaway He-burning in the late AGB phase.