New Constraints on the Major Neutron Source in Low-mass AGB Stars

Abstract

We compare updated Torino postprocessing asymptotic giant branch (AGB) nucleosynthesis model calculations with isotopic compositions of mainstream SiC dust grains from low-mass AGB stars. Based on the data-model comparison, we provide new constraints on the major neutron source, 13 C(α,n) 16 O in the He-intershell, for the s-process. We show that the literature Ni, Sr, and Ba grain data can only be consistently explained by the Torino model calculations that adopt the recently proposed magnetic-buoyancy-induced 13 C-pocket. This observation provides strong support to the suggestion of deep mixing of H into the Heintershell at low 13 C concentrations as a result of efficient transport of H through magnetic tubes. The Astrophysical Journal 2The main s-process 1 occurs in thermally pulsing asymptotic giant branch (AGB) stars, the final evolutionary stage of low-to-intermediate mass stars. Figure 1 illustrates the evolution of a 3 M ¤ , 1.5 Z ¤ 2 AGB star predicted by FRUITY model calculations (Cristallo et al. 2011). The star can be divided into three main regions: an innermost zone corresponding to the partially degenerate CO core, inside the magenta line; an external zone corresponding to the H-rich convective envelope, outside the black line; and a thin region (10 −2 −10 −3 M⊙) between the magenta and black lines, called the He-intershell. The red and blue lines show where maximum energy release from H-burning and within the He-intershell takes place, respectively. The blue line shows a plateau during each interpulse period, where the main energy release in the He-intershell is from the 13 C(α,n) 16 O reaction. The base of the H-rich envelope is radiative between the black and red lines. Hydrogen burning is activated for most of the time, on the top of the growing inert He-intershell. The He-intershell is heated and compressed until the temperature and density at its bottom are high enough to trigger thermonuclear runaways by He-burning (Thermal Pulse, TP, Iben & Renzini 1983). Because of the surplus of energy locally released, the whole He-intershell becomes unstable and convects. As a consequence of the energy release and expansion of the He-shell, the overlying layers, including the H-shell, are pushed to larger radii (e.g., numbered TPs in Fig. 1). At the quenching of a TP, the He-shell returns to radiative conditions; conversion of a significant fraction of the 4 He in the He-shell into 12 C is responsible for the jumps in size of the CO core (magenta line) after each TP. After a few hundred years, if the H-shell has been sufficiently lifted by the TP, the convective envelope penetrates the underlying region and brings newly synthesized materials to the surface. This process is called third dredge-up (TDU). Consequently, the convective envelope becomes more and more C-rich. As soon as the temperature becomes sufficiently high, H burning is reactivated in the H-shell. The time elapsed from the maximum penetration of the convective envelope and the hydrogen reignition is a few thousand years. An AGB star s...

Additional fluorine abundance determinations in evolved stars

“…This agrees well with theoretical studies aiming at fitting isotopic composition of pre-solar SiC grains (e.g. Liu et al 2018). Here we have expanded the set of theoretical models and have compared them to the newly available observations.…”

Presolar Silicon Carbide Grains of Types Y and Z: Their Molybdenum Isotopic Compositions and Stellar Origins

“…To be consistent with our previous study (Liu et al 2018c), FRUITY Torino models shown in this study adopted (1) the lower limit of the 22 Ne(α,n) 25 Mg rate given by Jaeger et al (2001) and Longland et al (2012); and (2) the 13 C pocket profile reported by Trippella et al (2016). We adopted the 22 Ne(α,n) 25 Mg rate at its lower limit in this study to investigate its minimum effect in low-metallicity models.…”

“…and 1.5 Z ! have been recently presented by Liu et al (2018c). In the present study, we extend the metallicity down to 0.07 Z !…”

“…A detailed comparison of the Trippella pocket and previously adopted 13 C-pockets (Liu et al 2014a(Liu et al ,b, 2015 is given in Fig. 2 of Liu et al (2018c). We adopted Maxwellian-averaged neutron capture cross section (MACS) values reported in both KADoNiS v0.3 and KADoNiS v1.0 databases 2 (Dillmann et al 2006(Dillmann et al , 2014 for model calculations.…”

“…The strong effect of T MAX , however, can be used to examine the validity of each model in explaining the Mo isotope data, as shown in this study. Finally, we want to stress here that Mo isotopes need to be considered in future studies for grain-model comparison with AGB models using other codes, e.g., the Monash (Karakas & Lattanzio 2014) and NuGrid (Pignatari et al 2016;Battino et al, 2016) codes, because they are sensitive to T MAX and need to be used to constrain the maximally allowed stellar mass, which is still a matter of debate (e.g., Lugaro et al 2018, Liu et al 2018c.…”