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

¹ INAF, Observatory of Rome, Via Frascati 33, 00077 Monte Porzio Catone, RM, Italy
e-mail: ester.marini@inaf.it
² School of Mathematical and Physical Sciences, Macquarie University, Sydney, NSW, Australia
³ Astronomy, Astrophysics and Astrophotonics Research Centre, Macquarie University, Sydney, NSW, Australia
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁵ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁷ Space Science Data Center, Via del Politecnico snc, 00133 Roma, Italy
⁸ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00100 Roma, Italy
⁹ Nordita, KTH Royal Institute of Technology and Stockholm University, Hannes Alfvéns väg 12, 10691 Stockholm, Sweden

Received: 18 November 2022
Accepted: 24 December 2022

Abstract

Context. The formation of silicates in circumstellar envelopes of stars evolving through the asymptotic giant branch (AGB) is still highly 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.

Aims. We study the formation of dust in the winds of intermediate mass (M ≥ 4 M_⊙) 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 and, on more general grounds, to the silicates budget of the Universe.

Methods. We consider a sub-sample of AGB stars, whose spectral energy distribution (SED) is characterised by deep absorption features at 10 μm and 18 μm, 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.

Results. Two out of the four sources investigated are interpreted as stars currently undergoing hot bottom burning (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 × 10⁻⁴ M_⊙ yr⁻¹ to 2 × 10⁻⁴ M_⊙ yr⁻¹ 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 ∼3 times higher than previously determined.