Yields of AGB and SAGB models with chemistry of low- and high-metallicity globular clusters

Abstract

We present yields from stars of mass in the range M ⊙ M 8M ⊙ of metallicities Z = 3 × 10 −4 and Z = 8 × 10 −3 , thus encompassing the chemistry of low-and high-Z Globular Clusters. The yields are based on full evolutionary computations, following the evolution of the stars from the pre-Main Sequence through the Asymptotic Giant Branch phase, until the external envelope is lost.Independently of metallicity, stars with M < 3M ⊙ are dominated by Third Dredge-Up, thus ejecting into their surroundings gas enriched in carbon and nitrogen. Conversely, Hot Bottom Burning is the main responsible for the modification of the surface chemistry of more massive stars, whose mass exceeds 3M ⊙ : their gas shows traces of proton-capture nucleosynthesis.The extent of Hot Bottom Burning turns out to be strongly dependent on metallicity. Models with Z = 8×10 −3 achieve a modest depletion of oxygen, barely reaching −0.3 dex, and do not activate the Mg-Al chain. Low-Z models with Z = 3 × 10 −4 achieve a strong nucleosynthesis at the bottom of the envelope, with a strong destruction of the surface oxygen and magnesium; the most extreme chemistry is reached for models of mass ∼ 6M ⊙ , where δ[O/Fe]∼ −1.2 and δ[Mg/Fe]∼ −0.6. Sodium is found to be produced in modest quantities at these low Z's, because the initial increase due to the combined effect of the second dredge-up and of 22 Ne burning is compensated by the later destruction via proton capture. A great increase by a factor ∼ 10 in the aluminium content of the envelope is also expected. These results can be used to understand the role played by intermediate mass stars in the self-enrichment scenario of globular clusters: the results from spectroscopic investigations of stars belonging to the second generation of clusters with different metallicity will be used as an indirect test of the reliability of the present yields.The treatment of mass loss and convection are confirmed as the main uncertainties affecting the results obtained in the context of the modeling of the thermal pulses phase. An indirect proof of this comes from the comparison with other investigations in the literature, based on a different prescription for the efficiency of convection in transporting energy and using a different recipe to determine the mass loss rate.

Globular cluster formation with multiple stellar populations from hierarchical star cluster complexes

“…yield table provided by Ventura et al (2013). However, Naenhanced stars do not always show O-depletion, if they are formed from AGB ejecta (See Fig.…”

“…However, Naenhanced stars do not always show O-depletion, if they are formed from AGB ejecta (See Fig. 6 in Ventura et al 2013). …”

The planetary nebulae and H II regions in NGC 6822 revisited. Clues to AGB nucleosynthesis

“…We consider that Fig. 4 but showing the predictions of stellar evolution models for Z = 0.001 (green squares) by Ventura et al (2013Ventura et al ( , 2014b and Z = 0.004 (red triangles) by Ventura et al (2014a, V14). See text for discussion.…”

Evolution of long-lived globular cluster stars

“…In the case of the AGB scenario, the quantity of helium released by the polluters is set during the second dredgeup episode on the early AGB; it amounts to a maximum of ∼0.36−0.38 in mass fraction, independently of the initial mass of the AGB progenitor (e.g., Doherty et al 2014). On the other hand, the sodium content of the ejecta of the polluter results from the competition between hot-bottom-burning and third dredgeup episodes that may occur during the later thermal-pulse AGB (TP-AGB) phase; therefore, it strongly depends on the initial mass of the polluter (Forestini & Charbonnel 1997;Siess 2007;D'Ercole et al 2010;Ventura et al 2013). Consequently, 2P stars spanning a large range of Na abundances are all expected to be born with very similar helium contents (maximum of 0.36−0.38 in mass fraction if no dilution with the ISM matter is taken into account compared with ∼0.248 for 1P stars; see Sect.…”

“…Different scenarios of secondary star formation invoke different types of 1G polluters, namely fast rotating massive stars (FRMS) with initial masses above 25 M (Maeder & Meynet 2006;Prantzos & Charbonnel 2006;Decressin et al 2007a,b;Krause et al 2013), massive asymptotic giant branch (AGB) stars with initial masses between ∼6 and 11 M (Ventura et al 2001(Ventura et al , 2013D'Ercole et al 2010D'Ercole et al , 2011D'Ercole et al , 2012Ventura & D'Antona 2011), and supermassive stars with initial masses around 10 4 M (Denissenkov & Hartwick 2014). In some cases the possible contribution of massive binary stars A&A 578, A117 (2015) (de Mink et al 2009;Izzard et al 2013), of FRMS paired with AGB stars (Sills & Glebbeek 2010), and of FRMS paired with high-mass interactive binaries (Bastian et al 2013b;) are also considered.…”