2025ApJ...994...81G

Molecular observations were conducted of the protoplanetary nebula (PPN) Frosty Leo (IRAS 09371+1212) using the 12 m antenna and the Submillimeter Telescope of the Arizona Radio Observatory at 1-3 mm. In addition to CO, multiple transitions were detected of the sulfur-containing molecules CS (J = 2 → 1, 3 → 2, 5 → 4), SO (N_J = 2₃ → 1₂, 3₃ → 2₂, 3₄ → 2₃, 4₃ → 3₂, 4₄ → 3₃, 5₆ → 4₅, 6₅ → 5₄), SO₂ (J_Ka,Kc = 3_1,3 → 2_0,2, 5_1,5 → 4_0,4, 4_2,2 → 4_1,3, 3_2,2 → 3_1,3, 5_2,4 → 5_1,5, 7_1,7 → 6_0,6), and H₂S, ${{{\rm{H}}}_{2}}^{33}{\rm{S}}$, and ${{{\rm{H}}}_{2}}^{34}{\rm{S}}$ (J_Ka,Kc = 1_1,0 → 1_0,1). The J = 2 → 1 and 3 → 2 lines of SiO were also measured, but the carbon-bearing species HCN and HCO⁺ were not detected despite intensive searches. Aside from SO, these identifications are new for this source. The observed line profiles appear broad but mostly symmetric, consistent with an origin in the so-called central ring. From a radiative transfer analysis, fractional abundances, relative to H₂, were determined. H₂S was found to be the most abundant S-bearing species, with f ∼ 1.4 × 10^–6, followed by SO (f ∼ 2.1 × 10^–7) and SO₂ (f ∼ 8.6 × 10^–8). CS and SiO had lower values of f ∼ 2.2-2.9 × 10^–8. From H₂S, unusually low sulfur isotope ratios were determined, with ³²S/³³S = 21.6 ± 7.4 and ³²S/³⁴S = 6.6 ± 1.3. The striking absence of HCN and HCO⁺, commonly found in O-rich environments, and the prevalence of S-bearing molecules, suggests this PPN has a unique oxygen-driven chemistry and may explain the strong crystalline ice features observed.