Millimeter observations of CS, HCO⁺, and CO toward five planetary nebulae: following molecular abundances with nebular age

Millimeter and sub-millimeter observations of CO, CS, and HCO⁺ have been conducted toward five planetary nebulae (PNe: K4-47, NGC 6537 (Red Spider), M2-48, NGC 6720 (Ring), and NGC 6853 (Dumbbell)), spanning an age range of 900-10,000 yr, using the Sub-Millimeter Telescope and the 12 m antenna of the Arizona Radio Observatory. The J = 5 → 4, J = 3 → 2, and J = 2 → 1 transitions of CS at 245, 147, and 98 GHz, as well as the J = 3 → 2 and J = 1 → 0 lines of HCO⁺ at 268 and 89 GHz, were detected toward each source. At least three rotational transitions of CO have also been observed, including the J = 6 → 5 and J = 4 → 3 lines at 691 and 461 GHz. CS had not been definitively identified previously in PNe, and new detections of HCO⁺ were made in four of the five nebulae. From a radiative transfer analysis of the CO and CS data, kinetic temperatures of T _K ∼ 10-80 K and gas densities of n(H₂) ∼ 0.1-1 × 10⁶ cm^–3 were determined for the molecular material in these sources. Column densities for CO, CS, and HCO⁺ were N _tot ∼ 0.2-5 × 10¹⁶ cm^–2, N _tot ∼ 0.4-9 × 10¹² cm^–2, and N _tot ∼ 0.3-5 × 10¹² cm^–2, respectively, with fractional abundances, relative to H₂, of f ∼ 0.4-2 × 10^–4, f ∼ 1-4 × 10^–8, and f ∼ 1 × 10^–8, with the exception of M2-48, which had f(HCO⁺) ∼ 10^–7. Overall, the molecular abundances do not significantly vary over a duration of 10,000 yr, in contrast to predictions of chemical models. The abundances reflect the remnant asymptotic giant branch shell material, coupled with photochemistry in the early PN phase. These observations also suggest that PNe eject substantial amounts of molecular material into the diffuse interstellar medium.