Group II Oxide Grains: How Massive Are Their AGB Star Progenitors?

Sara Palmerini,Luciano Piersanti,Diego Vescovi,Maurizio Busso,Sergio Cristallo

doi:10.3390/universe7060175

Sara Palmerini, Luciano Piersanti + Show 3 more

Open Access

https://doi.org/10.3390/universe7060175

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Abstract

Presolar grains and their isotopic compositions provide valuable constraints to AGB star nucleosynthesis. However, there is a sample of O- and Al-rich dust, known as group 2 oxide grains, whose origin is difficult to address. On the one hand, the 17O/16O isotopic ratios shown by those grains are similar to the ones observed in low-mass red giant stars. On the other hand, their large 18O depletion and 26Al enrichment are challenging to account for. Two different classes of AGB stars have been proposed as progenitors of this kind of stellar dust: intermediate mass AGBs with hot bottom burning, or low mass AGBs where deep mixing is at play. Our models of low-mass AGB stars with a bottom-up deep mixing are shown to be likely progenitors of group 2 grains, reproducing together the 17O/16O, 18O/16O and 26Al/27Al values found in those grains and being less sensitive to nuclear physics inputs than our intermediate-mass models with hot bottom burning.

Highlights

The Asymptotic Giant Branch (AGB) is an advanced phase of stellar evolution experienced by low and intermediate mass objects [1,2,3,4]
Only recent condensation experiments confirmed that amorphous alumina can condense in circumstellar conditions [5], intermediate mass (IM) AGB stars have been considered to be the main source of alumina dust in the galaxy for many years, starting from the first work by Onaka et al [6], to the most recent ones Dell’Agli et al [7], Ventura et al [8]
Considering the new nuclear data that ahs become available in the last years, we present a comparison between low mass (LM)-AGBs and IM-AGBs nucleosynthesis computed with both nuclear data sets, in order to determine if such a comparison may help in discriminating between LM-AGBs and IM-AGBs as parent stars of presolar oxide grains of group

Summary

Introduction

The Asymptotic Giant Branch (AGB) is an advanced phase of stellar evolution experienced by low and intermediate mass objects [1,2,3,4]. Only recent condensation experiments confirmed that amorphous alumina can condense in circumstellar conditions [5], intermediate mass (IM) AGB stars have been considered to be the main source of alumina dust in the galaxy for many years, starting from the first work by Onaka et al [6], to the most recent ones Dell’Agli et al [7], Ventura et al [8] It is, reasonable to hypothesize that IM AGBs are the progenitors of the presolar oxide (mainly spinel MgAl2 O4 and corundum Al2 O3 ) and silicate grains (both amorphous or crystalline) found in pristine meteorites [9]. For both the nuclear physics inputs and the adopted stellar code (we use the FUNS code by Straniero et al [3]; see Section 4 for details)

Oxygen Isotopic Ratios Observed in the Spectra of Present-Day AGB Stars

Deep Mixing and the Formation of Oxide Grains in Low Mass AGB Stars

Hot Bottom Burning in Intermediate Mass Stars

Conclusions