Constraints on Metal Oxide and Metal Hydroxide Abundances in the Winds of AGB Stars: Potential Detection of FeO in R Dor

Using the Atacama Large Millimeter/submillimeter Array (ALMA), we observed the stellar wind of two oxygen-rich asymptotic giant branch stars, IK Tau and R Dor, between 335 and 362 GHz. One aim was to detect metal oxides and metal hydroxides (AlO, AlOH, FeO, MgO, and MgOH), some of which are thought to be direct precursors of dust nucleation and growth. We report on the potential first detection of FeO (v = 0, Ω = 4, J = 11–10) in R Dor (mass-loss rate M• ~ 1 × 10−7 M ⊙ yr‾¹). The presence of FeO in IK Tau (M• ~ 5 × 10‾⁶ M ⊙ yr‾¹) cannot be confirmed, due to a blend with ²⁹SiS, a molecule that is absent in R Dor. The detection of AlO in R Dor and of AlOH in IK Tau was reported earlier by Decin et al. All other metal oxides and hydroxides, as well as MgS, remain undetected. We derive a column density N(FeO) of 1.1 ± 0.9 × 10¹⁵ cm‾² in R Dor, or a fractional abundance [FeO/H] ~ 1.5 × 10‾⁸ accounting for non-local thermodynamic equilibrium effects. The derived fractional abundance [FeO/H] is a factor ~20 larger than conventional gas-phase chemical-kinetic predictions. This discrepancy may be partly accounted for by the role of vibrationally excited OH in oxidizing Fe, or it may be evidence for other currently unrecognized chemical pathways producing FeO. Assuming a constant fractional abundance w.r.t. H₂, the upper limits for the other metals are [MgO/H₂] < 5.5 × 10‾¹⁰ (R Dor) and <7 × 10‾¹¹ (IK Tau), [MgOH/H₂] < 9 × 10‾⁹ (R Dor) and <1 × 10‾⁹ (IK Tau), [CaO/H₂] < 2.5 × 10‾⁹ (R Dor) and <1 × 10‾¹⁰ (IK Tau), [CaOH/H₂] < 6.5 × 10‾⁹ (R Dor) and <9 × 10‾¹⁰ (IK Tau), and [MgS/H₂] < 4.5 × 10‾¹⁰ (R Dor) and <6 × 10‾¹¹ (IK Tau). The retrieved upper-limit abundances for these latter molecules are in accord with the chemical model predictions.