2019ApJ...873...14B

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, all available in the presolar grain database, to new CO nova models with white dwarf (WD) masses from 0.6 to 1.35 M_☉. The models were designed to match the Large Binocular Telescope high-dispersion spectra acquired for nova V5668 Sgr. 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 >7 x 10⁷ K are achieved during a thermonuclear runaway (TNR). The 0.8-1.35 M_☉ 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 1.35 M_☉ 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.