Sulfur- and silicon-bearing molecules in planetary nebulae: The case of M2-48

Molecular-line observations of the bipolar planetary nebula (PN) M2-48 have been conducted using the Sub-Millimeter Telescope and the 12 m antenna of the Arizona Radio Observatory at 1, 2, and 3 mm. M2-48 is estimated to be ∼4800 yr old, midway through the PN evolutionary track. SiO and SO₂were detected in this source - the first identification of either molecule in a PN. CN, HCN, HNC, CS, SO, HCO⁺, N₂H⁺, and several ¹³C isotopologues such as ¹³CN, H¹³CN, and H¹³CO⁺were also observed toward this object. A radiative transfer analysis of multiple SiO transitions indicates a gas kinetic temperature of T_K∼ 55 K and a density of n(H₂) ∼ 9 × 10⁵cm^-3in M2-48, in agreement with previous CS and CO modeling. After CO, CN, and SO were found to be the most prevalent molecules in this nebula, with fractional abundances, relative to H₂, of f ∼ 3.8 × 10^-7and 2.4 × 10^-7, respectively. SO₂and HCN are also abundant, with f ∼ 1.2 × 10^-7, indicating an [SO]/[SO₂] ratio of ∼2. Relatively high ion abundances were measured in M2-48 as well, with f ∼ 10^-7for both HCO⁺and N₂H⁺. An [HCN]/[HNC] ratio of ∼2 was determined, as typically observed in other PNe, independent of age. The high abundances of SO and SO₂, along with the presence of SiO with f ∼ 2.9 × 10^-8, suggest O/C > 1 in this source; furthermore, the prevalence of CN and N₂H⁺indicates nitrogen enrichment. The ¹²C/¹³C ratio of ∼3 in the nebula was also established. These factors indicate hot-bottom burning occurred in the progenitor star of M2-48, suggesting an initial mass > 4 M_⊙.