Constraints on metal oxide and metal hydroxide abundances in the winds of AGB stars - Potential detection of FeO in R Dor

TL;DR

This study uses ALMA observations to detect and analyze metal oxides and hydroxides in the winds of AGB stars, reporting a potential first detection of FeO in R Dor and comparing observed abundances with chemical models.

Contribution

It reports the potential first detection of FeO in R Dor's stellar wind and compares observed abundances with chemical predictions, highlighting discrepancies and possible chemical pathways.

Findings

Potential first detection of FeO in R Dor.

FeO abundance is about 20 times higher than predicted by models.

Upper limits for other metal oxides are consistent with chemical models.

Abstract

Using ALMA, we observed the stellar wind of two oxygen-rich Asymptotic Giant Branch (AGB) 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, 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, Omega=4, J=11-10) in RDor (mass-loss rate, Mdot, ~1e-7 Msun/yr). The presence of FeO in IK Tau (Mdot~5e-6 Msun/yr) 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. (2017). All other metal oxides and hydroxides, as well as MgS, remain undetected. We derive a column density N(FeO) of 1.1+/-0.9e15 cm^{-2} in R Dor, or a fractional abundance [FeO/H]~1.5e-8 accounting for non-LTE 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 may be evidence for other currently unrecognised chemical pathways producing FeO. Assuming a constant fractional abundance w.r.t. H_2, the upper limits for the other metals are [MgO/H_2] <5.5e-10 (R Dor) and <7e-11 (IK Tau), [MgOH/H_2] <9e-9 (R Dor) and <1e-9 (IK Tau), [CaO/H_2] <2.5e-9 (R Dor) and <1e-10 (IK Tau), [CaOH/H_2] <6.5e-9 (R Dor) and <9e-10 (IK Tau), and [MgS/H_$] <4.5e-10 (R Dor) and <6e-11 (IK Tau). The retrieved upper limit abundances for these latter molecules are in accord with the chemical model predictions.