SULFUR- AND SILICON-BEARING MOLECULES IN PLANETARY NEBULAE: THE CASE OF M2-48

Abstract

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 SO2 were 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 × 105 cm–3 in 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 × 10–7 and 2.4 × 10–7, respectively. SO2 and 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 ~ 10–7 for 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 ☉.