Poster Presentation Astronomical Society of Australia Annual Scientific Meeting including HWWS 2013

Decoding the Message from Silicon in Stardust Mainstream Silicon Carbide Grains (#242)

Karen M Lewis 1 , Maria Lugaro 2 , Brad K Gibson 2 3 , Kate Pilkington 2 3
  1. Tokyo Institute of Technology, Ookayama, Japan
  2. Monash Centre for Astrophysics (MoCA) , Monash University, Melbourne, Victoria, Australia
  3. Jeremiah Horrocks Institute, University of Central Lancashire , Preston, UK

SiC mainstream grains are presolar grains believed to form in the envelopes of carbon rich asymptotic giant branch (AGB) stars with masses between 1.5 and 3 solar masses.  These grains represent a conundrum as the 29Si and 30Si abundances indicate that they formed in stars of super-solar metallicity, before the solar system formed.   To shed light on this problem, we use silicon isotopic abundances to derive an age metallicity relation for the stars believed to have produced the SiC mainstream grains.  For 2732 mainstream SiC grains listed in the Presolar Grain Database, we use the 29Si abundances with the latest GCE models to derive [Fe/H], and 30Si abundances along with the models of Zinner et al. (2006) to determine an approximate birth age for the AGB star in which the grain formed.  Comparing our age metallicity relation with observational relationships derived for stars in the solar neighbourhood, we find that the spread of [Fe/H] is in agreement, but the mean [Fe/H] in our relation is higher by 0.2 dex.  We propose that this difference is because stars with higher [Fe/H] produce more dust and are consequently over-represented in our age metallicity diagram, a finding consistent with other works in the literature. This result offers a solution for the long-standing problem of silicon in stardust SiC grains, confirms the necessity of coupling chemistry and dynamics in simulations of the chemical evolution of our Galaxy, and constrains the modelling of dust condensation in stellar winds as a function of the metallicity.