Condensation of SiC Stardust in CO Nova Outbursts

TL;DR

This paper compares isotopic compositions of presolar SiC grains with new CO nova models, providing evidence that some grains originate from pure CO nova ejecta and suggesting inhomogeneous nova winds.

Contribution

It introduces detailed CO nova models matching isotopic data of presolar grains, highlighting the potential for pure nova ejecta in meteorites and inhomogeneous nova wind formation.

Findings

Best-fit models involve 25% WD core mixing after TNR onset.

Some presolar grains match 50% WD core and 50% solar material models.

Evidence of pure CO nova ejecta material in meteorites.

Abstract

This study on presolar grains compares high-precision isotopic compositions of individual SiC grains with low 12C/13C ratios, low 14N/15N ratios, large 30Si excesses and high 26Al/27Al ratios, available in the presolar grain database, to new CO nova models with white dwarf (WD) masses from 0.6-1.35M_sol. The models were designed to match the Large Binocular Telescope (LBT) high dispersion spectra acquired for nova V5668 Sgr (Starrfield et al. 2018). These CO nova models provide elemental abundances up to calcium and include mixing of WD material into the accreted material in a binary star system under several scenarios, including one where mixing occurs only after temperatures >7x10^7K are achieved during a thermonuclear runaway (TNR). The 0.8-1.35M_sol simulations where 25% of the WD core matter mixes with 75% of the accreted material (assumed solar) from its binary companion after the TNR has begun, provide the best fits to the measured isotopic data in four presolar grains. One grain matches the 50% accreted 50% solar 1.35M_sol simulation. For these five presolar grains, less than 25% of solar system material is required to be mixed with the CO nova ejecta to account for the grains' compositions. Thus, our study reports evidence of pure CO nova ejecta material in meteorites. Finally, we speculate that SiC grains can form in the winds of cool and dense CO novae, where the criterion C>O may not be locally imposed, and thus nova winds can be chemically inhomogeneous.