Asymptotic Giant Branch Stars and presolar grains

Maurizio Busso,Diego Vescovi,Sara Palmerini

doi:10.1051/epjconf/202022701002

Abstract

Starting from the recognition that radioactive isotopes were present alive in the Early Solar System, inducing composition anomalies from their decay, and through the discovery that other important anomalies affected also stable species, we shall discuss how the carriers of these abundance peculiarities were identified in very refractory pre-solar dust grains, formed in circumstellar environments. We shall outline how groups of such grains and subsequently in-dividual single crystals of C-rich or O-rich materials (like, e.g., SiC and Al2O3) could be analyzed, providing a new tool to verify the composition of stellar winds. This is so especially for AGB stars, which are the primary factories of dust in the Galaxy. For this reason, pristine meteorites open a crucial window on the details of nucleosynthesis processes occurring in such evolved red giants, for both intermediate-mass elements and rare heavy nuclei affected by slow neutron captures (the s-process).

Highlights

Along the past 60 years, starting with the identification in pristine meteorites of an excess in 129Xe from the in situ decay of 129I [1], we have known that the average Early Solar System (ESS) composition was contaminated through the addition of various materials coming from other stars and not previously homogenized in the Interstellar Medium (ISM) [2]
Discovery of anomalous abundances in certain isotopes that are daughters of radioactive nuclei involved several elements in the last decades of the 20th century [3, 4]. They led to the realization that the n-deficient isotope of Al, 26Al, had been present in the ESS in a huge and uniform concentration (26Al/27Al 5 · 10−5), sufficient to affect significantly, or even control, the heating of ancient materials, which induced the early differentiation of planetary embryos [5, 6]
The specific way in which such contaminations were possible was through the transport, across the ISM, of very refractory solid particles condensed around cool stars [11], capable of surviving the thermal stresses of many Galactic and Star-Formation processes

Summary

Introduction

Along the past 60 years, starting with the identification in pristine meteorites of an excess in 129Xe from the in situ decay of 129I [1], we have known that the average Early Solar System (ESS) composition was contaminated through the addition of various materials coming from other stars and not previously homogenized in the Interstellar Medium (ISM) [2]. The specific way in which such contaminations were possible was through the transport, across the ISM, of very refractory solid particles condensed around cool stars [11], capable of surviving the thermal stresses of many Galactic and Star-Formation processes Such refractory dust grains can be produced in the circumstellar envelopes of evolved stars and are. The identification and collection of samples enriched in such grains (especially SiC) were made possible by improved mass-spectrometry facilities and innovative separation techniques, made by sequential acid treatments of materials from pristine meteorites [13] Their analysis soon revealed that they were the carriers of a large excess in 22Ne, the so-called Ne-E(H) anomaly [14], which was subsequently recognized as being produced in the thermonuclear H- and He-shells of moderately massive stars (See Figure 1 and [15, 16]), during their so-called Asymptotic Giant Branch (AGB) phases. Two thermonuclear shells of He and H burn alternatively, while the intershell region is swept repeatedly by intermediate convective instabilities ( on called Intermediate Convective Shells, ICS)

An AGB origin for meteoritic anomalies

Deep mixing and s-processing as constrained by presolar grains