The remarkable molecular content of the red spider nebula (NGC 6537)

Millimeter and sub-millimeter molecular-line observations of planetary nebula (PN) NGC 6537 (Red Spider) have been carried out using the Sub-Millimeter Telescope and the 12 m antenna of the Arizona Radio Observatory in the frequency range 86-692 GHz. CN, HCN, HNC, CCH, CS, SO, H₂CO, HCO ⁺ and N₂H⁺, along with the J = 3 → 2 and 6 → 5 lines of CO and those of several isotopologues, were detected toward the Red Spider, estimated to be ∼1600 yr old. This extremely high excitation PN evidently fosters a rich molecular environment. The presence of CS and SO suggest that sulfur may be sequestered in molecular form in such nebulae. A radiative transfer analysis of the CO and CS spectra indicate a kinetic temperature of T _K ∼ 60-80 K and gas densities of n(H ₂) ∼ 1-8 × 10⁵ cm^-3 in NGC 6537. Column densities of the molecules in the nebula and their fractional abundances relative to H₂ ranged from N _tot ∼ 10¹⁶ cm^-2 and f ∼ 10^-4 for CO, to ∼7 × 10 ¹¹ cm^-2 and f ∼ 8 × 10^-9 for the least abundant species, N₂H⁺. For SO and CS, N _tot ∼ 2 × 10¹² cm^-2 and 10 ¹³ cm^-2, respectively, with f ∼ 10^-7 and 2 × 10^-8. It was also found that HCN/HNC 2. A low ¹²C/¹³C ratio of ∼4 was measured, indicative of hot-bottom burning. These results, coupled with past observations, suggest that molecular abundances in PNe are governed principally by the physical and chemical properties of the individual object and its progenitor star, rather than nebular age.