Gas and dust from extremely metal-poor AGB stars

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.

CHEMOUT: CHEMical complexity in star-forming regions of the OUTer Galaxy

“…2). The decrease in the 14 N/ 15 N ratio in the outer Galaxy, on the other hand, is dictated by the nucleosynthesis prescriptions for single low-and intermediatemass stars: Romano et al (2019) show that a decreasing trend has to be expected in the outer Galaxy when adopting the stellar yields of Ventura et al (2013Ventura et al ( , 2014Ventura et al ( , 2018Ventura et al ( , 2020Ventura et al ( , 2021 due to the strong metal dependence of the 14 N yield in this case. Their predictions are confirmed by the observations presented in this work.…”

“…The adopted nucleosynthesis prescriptions follow Romano et al (2019Romano et al ( , 2021, with some updates (Romano et al, in prep.). For single low-and intermediate-mass stars (1 ≤ M/M ⊙ < 9), we adopted the yields of Ventura et al (2013Ventura et al ( , 2014Ventura et al ( , 2018Ventura et al ( , 2020Ventura et al ( , 2021 that cover all metallicity regimes, from the ultra metal-poor to the super-solar. The adopted yields include the effects of dredge ups, hot bottom burning, and mass loss, as well as a proper treatment of the super-AGB phase for the most massive stars.…”

Nucleosynthetic yields of intermediate-mass primordial to extremely metal-poor stars

“…We compared our results with V21 (Ventura et al 2021), a recent study reporting on the evolution and nucleosynthesis in low-and intermediate-mass stars of Z ∼ 10 −5 and Z ∼ 10 −7 . This allowed us to further explore the critical effects of uncertain input physics.…”

“…We note that 5 M ⊙ models of Z = ×10 −8 and Z = 10 −10 are not shown because they do not follow a standard TP-AGB evolution (see main text for details). For the sake of comparison, Z = 10 −5 models from GP21 with different wind prescriptions (Bloecker 1995, B95, in solid andVassiliadis &Wood 1993, VW93 in dotted lines), and the models from Ventura et al (2021) are also included (V21, dashed lines). We note that, for the latter, maximum (instead of average) values of λ are shown.…”

The evolution of CNO elements in galaxies

“…Truran and Cameron (1971) first noticed that some primary 14 N production from intermediate-mass stars with initial mass in the range 2-4 M , resulting from convective mixing of material from the helium-burning region to the hydrogen-1 Following Maeder (1992), the element yield p j of a star of initial mass m is the newly formed mass of element j that is ejected in the ISM during the whole stellar lifetime normalized to the initial mass of the star. Nowadays, the term yield is more often employed to denote the unnormalized ejected mass, m new j ¼ mp j , in solar mass units (Karakas and Lattanzio 2007;Cristallo et al 2011;Limongi and Chieffi 2018;Ventura et al 2020Ventura et al , 2021Cinquegrana and Karakas 2022, among others). The yield may be positive or negative, depending on whether element j is produced or destroyed in the star.…”