New Detections of H 2 CO and c-C 3 H 2 in Planetary Nebulae: Broadening the Organic Inventory

Abstract Observations of H 2 CO and c-C 3 H 2 were conducted toward six planetary nebulae (PNs; M1-7, M1-8, M2-48, M4-9, NGC 6445, and NGC 6853) at 1, 2, and 3 mm using the Arizona Radio Observatory’s 12 m antenna and Submillimeter Telescope (SMT). H 2 CO was detected toward all six nebulae on the basis of multiple transitions: the J Ka,Kc = 2 1,2 → 1 1,1 , 2 0,2 → 1 0,1 , and 2 1,1 → 1 1,0 lines at 2 mm and the J Ka,Kc = 3 0,3 → 2 0,2 transition at 1 mm. The c-C 3 H 2 molecule was identified in three sources, M1-7, M4-9, and NGC 6445, from 2 and 3 mm transitions ( J Ka,Kc = 2 1,2 → 1 0,1 , 3 1,2 → 2 2,1 , and 4 1,4 → 3 0,3 ). These detections increase the number of PNs known to contain H 2 CO from five to 11, and for c-C 3 H 2 from four to seven. From a radiative transfer analysis, column densities of N tot (H 2 CO) ∼ 3.2 × 10 11 –1.2 × 10 13 cm −2 and N tot (c-C 3 H 2 ) ∼ 6.7 × 10 11 –6.3 × 10 12 cm −2 were determined. These values correspond to fractional abundances of f (H 2 CO/H 2 ) ∼ (0.3–2.4) × 10 −8 and f (c-C 3 H 2 /H 2 ) ∼ (1.6–6.6) × 10 −9 . M4-9 exhibits the highest abundance of each molecule. Based on all known identifications in PNs, H 2 CO appears to be present in both carbon- and oxygen-rich nebulae, whereas c-C 3 H 2 is associated with only carbon-rich types. This result appears to reflect the chemistry of the previous asymptotic giant branch phase and is consistent with chemical continuity throughout PN evolution. Within the small sample, both species show fairly constant abundances across a nebular age range of 700–12,000 yr, with no statistically significant decrease. Both H 2 CO and c-C 3 H 2 are commonly observed in diffuse clouds. Therefore, their ongoing identifications in PNs suggest chemical continuity from the late phases of stellar evolution into the formation of the diffuse interstellar medium.