Molecular-Line Observations of the Remnant AGB Envelopes Around Planetary Nebulae

Abstract

We present recent results from a “search and mapping” program of molecular line emission (mainly CO) from remnant AGB envelopes around planetary nebulae (PNe), using the SEST (La Silla, Chile). New detections in CO J=2−1 include NGC2899 (0.02K), NGC6369 (0.14K) & NGC7009 (0.08K). In many of the detected PNe, notably NGC3132, IC4406, NGC6302, M1-16, and CPD-56°8032, the molecular envelopes contain 2 kinematically distinct outflows. Mapping of the strongest of these shows (1) that the fast (e.g. V_exp≳40–60 km s⁻¹ in NGC3132, IC4406) outflows have bipolar spatial structure, and (2) there exists an equatorial density enhancement in the slower, more massive [· (M_⊙yr⁻¹)>5 10⁻⁶(NGC3132), >2 10⁻⁵(IC4406)] outflows, which presumably collimates the fast outflow (e.g. Sahai et al. 1990, A & A, 234, L1; Sahai et al. 1992, A & A, 251, 560). The fractional CO abundance in the envelope, f(CO), is probably rather low (<10⁻⁴), as a result of photodissociation by the stellar and interstellar UV radiation [e.g. f(CO)≲10⁻⁵ in IC4406]. HCN, HCO⁺, and ¹³CO have also been detected in several PNe, and sensitive upper limits set on CS, C¹⁸O, & C¹⁷O (in M1-16), and SO (in NGC3132). Some results are tabulated below, and calculations to estimate the molecular masses, mass-loss rates and molecular abundances are in progress.