Gas and dust from extremely metal-poor AGB stars

¹ INAF, Observatory of Rome, Via Frascati 33, 00077 Monte Porzio Catone (RM), Italy
e-mail: paolo.ventura@inaf.it
² INAF, Astrophysics and Space Science Observatory, Via Piero Gobetti 93/3, 40129 Bologna, Italy
³ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00100 Roma, Italy
⁴ Kavli Institute for the Physics and Mathematics of the Universe, Todai Institutes for Advanced Study, the University of Tokyo, Kashiwa, 277-8583 (Kavli IPMU, WPI), Japan
⁵ INFN. Sezione di Perugia, via A. Pascoli s/n, 06125 Perugia, Italy
⁶ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁷ Dipartimento di Fisica e Astronomia ‘Galileo Galilei’, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁸ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
⁹ ELTE Eötvös Loránd University, Institute of Physics, Pázmány Péter sétány 1/A, Budapest 1117, Hungary
¹⁰ School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
¹¹ Nordita, KTH Royal Institute of Technology and Stockholm University Hannes Alfvens väg 12, 114 21 Stockholm, Sweden
¹² Instituto de Astrofisica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
¹³ Departamento de Astrofisica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain

Received: 7 April 2021
Accepted: 9 August 2021

Abstract

Context. The study of stars that evolve through the asymptotic giant branch (AGB) proves crucial in several astrophysical contexts because these objects provide important feedback to the host system in terms of the gas that is poured into the interstellar medium after being exposed to contamination from nucleosynthesis processes, and in terms of the dust that forms in their wind. Most of the studies conducted so far have been focused on AGB stars with solar and sub-solar chemical composition, whereas the extremely metal-poor domain has been poorly explored.

Aims. We study the evolution of extremely metal-poor AGB stars with metallicities down to [Fe/H] = −5 to understand the main evolutionary properties and the efficiency of the processes able to alter their surface chemical composition, and to determine the gas and dust yields.

Methods. We calculated two sets of evolutionary sequences of stars in the 1−7.5  M_⊙ mass range that evolved from the pre-main sequence to the end of the AGB phase. To explore the extremely metal-poor chemistries, we adopted the metallicities Z = 3 × 10⁻⁵ and Z = 3 × 10⁻⁷, which correspond to [Fe/H] = −3 and [Fe/H] = −5, respectively. The results from stellar evolution modelling were used to calculate the yields of the individual chemical species. We also modelled dust formation in the wind to determine the dust produced by these objects.

Results. The evolution of AGB stars in the extremely metal-poor domain we explored proves highly sensitive to the initial mass of the star. M ≤ 2  M_⊙ stars experience several third-dredge-up events, which favour the gradual surface enrichment of ¹²C and the formation of significant quantities of carbonaceous dust, ∼0.01  M_⊙. The ¹³C and nitrogen yields are found to be significantly smaller than in previous explorations of low-mass metal-poor AGB stars because the proton ingestion episodes experienced during the initial AGB phases are weaker. M ≥ 5  M_⊙ stars experience hot bottom burning, and their surface chemistry reflects the equilibria of a very advanced proton-capture nucleosynthesis; little dust production takes place in their wind. Intermediate-mass stars experience both third dredge-up and hot bottom burning: they prove efficient producers of nitrogen, which is formed by proton captures on ¹²C nuclei of primary origin dredged up from the internal regions.