The intense production of silicates during the final AGB phases of intermediate mass stars

TL;DR

This study models silicate dust formation in intermediate-mass AGB stars, revealing their significant contribution to interstellar silicate enrichment and suggesting higher silicate yields than previously estimated.

Contribution

It provides new constraints on silicate production rates in AGB stars and revises the estimated yields to be about three times higher than earlier models.

Findings

Silicate production rates are tightly constrained during final AGB phases.

Mass-loss rates of 1-2×10^{-4} M_sun/yr are necessary to match observations.

Silicate yields are approximately three times higher than previous estimates.

Abstract

The formation of silicates in circumstellar envelopes of stars evolving through the AGB is still debated given the uncertainties affecting stellar evolution modelling, the description of the dust formation process, and the capability of silicate grains to accelerate stellar outflows via radiation pressure. We study the formation of dust in the winds of intermediate mass (M $\geq 4 M_{\odot}$) stars of solar metallicity while evolving through the AGB phase. We tested the different treatments of the mass-loss mechanism by this class of stars, with the aim of assessing their contribution to the general enrichment of silicates of the interstellar medium of galaxies. We consider a sub-sample of AGB stars, whose SED is characterised by deep absorption features at $10$ and $18\mu$m, which can be regarded as the class of stars providing the most relevant contribution to the silicates' production across the Universe. Results from stellar evolution and dust formation modelling were used to fit the observed SED and to reproduce, at the same time, the detected pulsation periods and the derived surface chemical composition. This analysis leads to the derivation of tight constraints on the silicates' production rates experienced by these sources during the final AGB stages. Two out of the four sources investigated are interpreted as stars currently undergoing HBB, evolving through phases close to the stage when the mass-loss rate is largest. The remaining two stars are likely evolving through the very final AGB phases, after HBB was turned off by the gradual consumption of the convective mantle. Mass-loss rates of the order of $1-2\times 10^{-4} M_{\odot}/$yr are required when looking for consistency with the observational evidence. These results indicate the need for a revision of the silicate yields by intermediate mass stars, which are found to be $\sim 3$ times higher than previously determined.