On the need of the Light Elements Primary Process (LEPP)

TL;DR

This paper explores alternative explanations for the Galactic production of certain light elements and s-only isotopes, questioning the necessity of the LEPP process by analyzing stellar evolution models with various physical processes.

Contribution

It demonstrates that modifications in stellar physics and nucleosynthesis models can reconcile predictions with Solar System abundances, reducing the need for the LEPP process.

Findings

Standard models overpredict s-only element abundances at Solar System formation.

Rotation effects flatten the s-only distribution and increase heavy isotope efficiency.

Variations in convection and mass-loss prescriptions cause shifts in the s-only element distribution.

Abstract

Extant chemical evolution models underestimate the Galactic production of Sr, Y and Zr as well as the Solar System abundances of s-only isotopes with 90<A<130. To solve this problem, an additional (unknown) process has been invoked, the so-called LEPP (Light Element Primary Process). In this paper we investigate possible alternative solutions. Basing on Full Network Stellar evolutionary calculations, we investigate the effects on the Solar System s-only distribution induced by the inclusion of some commonly ignored physical processes (e.g. rotation) or by the variation of the treatment of convective overshoot, mass-loss and the efficiency of nuclear processes. Our main findings are: 1) at the epoch of the formation of the Solar System, our reference model produces super-solar abundances for the whole s-only distribution, even in the range 90<A<130; 2) within errors, the s-only distribution relative to 150Sm is flat; 3) the s-process contribution of the less massive AGB stars (M<1.5 M_SUN) as well as of the more massive ones (M>4.0 M_SUN) are negligible; 4) the inclusion of rotation implies a downward shift of the whole distribution with an higher efficiency for the heavy s-only isotopes, leading to a flatter s-only distribution; 5) different prescriptions on convection or mass-loss produce nearly rigid shifts of the whole distribution. In summary, a variation of the standard paradigm of AGB nucleosynthesis would allow to reconcile models predictions with Solar System s-only abundances. Nonetheless, the LEPP cannot be definitely ruled out, because of the uncertainties still affecting stellar and Galactic chemical evolution models.