The chemical composition of the Solar System results from the mixing of material coming from several stars belonging to different populations and already evolved at the Sun birth, about 5 Gyr ago. Hence, all the elements observed in nature have been synthesized in stellar interior, with a very few exceptions, such as light elements (H, He and Li) produced during the Big Bang nucleosynthesis. Some of these elements have been produced during the hydrostatic quiescent phases characterizing the life of all stars; some others have been synthesized during the final explosive phase of massive stars; other elements are produced by Asymptotic Giant Branch (AGB) stars. Thermonuclear reaction of charge particles are responsible for the nucleosynthesis of isotopes lighter than iron (mass number A≤56), while more massive elements are produced via neutron capture processes.

The observed heavy elements distribution shows the existence of two main contributions associated to two different nucleosynthetic processes, the so called s (slow) and r (rapid) processes.

Figure 1
Figure 2

At the INAF-Astronomical Observatory of Abruzzo we investigate mainly the s-process nucleosynthesis occurring in low mass stars (1.5 < M/Mʘ < 3) during the Thermally Pulsing AGB phase (TP-AGB – see Figure 1). During this phase recurrent convective episodes usually addressed as Third Dredge-Up carry out up to the surface isotopes synthesized in the interior, thus allowing us to test the reliability of stellar models by means of spectral observation and abundance determination in laboratory. The main neutron source in AGB stars is the 13C(α,n)16O reaction; an additional contribution, even if marginal, comes from the 22Ne(α,n)25Mg. It has to be remarked that 22Ne forms directly during the convective pulse via the nuclear chain 14N(α,γ)18F(β–)18O(α,γ)22Ne and its abundance is fixed by the initial abundance of CNO isotopes.

 

On the other hand, the large amount 13C needed to reproduce the observed spectroscopic abundances of elements heavier than Iron requires that protons penetrates below the formal border of the convective envelope into the underlying radiative zone.

We treat such a proton inward penetration by introducing an exponentially decaying velocity profile at the inner border of the convective envelope, so that a 13C pocket consistently forms [1] (see Figure 2). Hence, we coupled our evolutionary models with a full nuclear network, from H to Bi, using the most up-to-date cross sections available in the literature [2]. As a consequence, we are able to determine the expected distribution of synthesized heavy elements as well as the amount of matter ejected from AGB stars into the interstellar medium [3]. All the results of our theoretical evaluations are available in the F.R.U.I.T.Y. database. Currently our database contains more than 100 fully evolutionary models of AGB stars with initial mass in the range 1.3-6 Mʘ and initial metallicity in the range -2.85≤ [Fe/H] ≤+0.15. For each model, we provide the pulse-by-pulse isotopic abundances at the surface and the total yields as well as all the physical properties which can be verified by means of observation (e.g. gravity and luminosity).

Figure 3

 

 

In particular, by comparing our theoretical models with observations, it has been possible to individuate some limitations of the convection-based scheme for the production of 13C.  For this reason we introduced the effects of rotation in our models and we investigated in detail its effects on the s-process nucleosynthesis [4]. In this way we demonstrated that the s-process elemental distributions does depend on the initial rotation velocity and by varying the latter we were able to explain a large part of the observed spread (see Figure 3).

 

 

 

Figure 4

 

An additional test of the reliability of our models is to compare our nucleosynthetic evaluations with the isotopic abundances derived from the pre-solar silicon carbide (SiC) grains. In fact, these small objects (as large as few micron) exhibit some isotopic anomalies whose origin has to be related to the different nucleosynthesis occurred in stars before the formation of the Solar System. We verified that rotation-induce mixing alone can not explain the observed isotopic anomalies. On the other hand, we found that the interplay between rotation and larger 13C pocket can explain the observed discrepancy [5,6] (see Figure 4).

 

References:

  • [1] Cristallo et al. 2009, Astrophysical Journal, 696, 797
  • [2] Straniero et al. 2006, Nuclear Physics A, 777, 311
  • [3] Cristallo et al. 2011, Astrophysical Journal Supplement, 197, 2
  • [4] Piersanti et al. 2013, Astrophysical Journal, 774, 98
  • [5] Cristallo et al. 2015, Astrophysical Journal, 801, 53
  • [6] Liu et al. 2015, Astrophysical Journal, 803, 12