An evidence-based assumption that helps to reduce the discrepancy between the observed and predicted ${}^7$Be abundances in novae

Abstract

Recent spectroscopic measurements of the equivalent widths of the resonant Be II doublet and Ca II K lines and their ratios in expanding nova ejecta indicate surprisingly high abundances of ${}^7$Be with a typical mass fraction $X_{\mathrm{obs}}{(}^7\mathrm{Be})=10^{-4}$. This is an order of magnitude larger than theoretically predicted values of $X_{\mathrm{theor}}{(}^7\mathrm{Be})\sim 10^{-5}$ for novae. We use an analytical solution of the ${}^7$Be production equations to demonstrate that $X_{\mathrm{theor}}{(}^7\mathrm{Be})$ is proportional to the ${}^4$He mass fraction $Y$ in the nova accreted envelope and then we perform computations of 1D hydrostatic evolution of the $1.15M_{\odot }$ CO nova model that confirm our conclusion based on the analytical solution. Our assumption of enhanced ${}^4$He abundances helps to reduce, although not completely eliminate, the discrepancy between $X_{\mathrm{obs}}{(}^7\mathrm{Be})$ and $X_{\mathrm{theor}}{(}^7\mathrm{Be})$. It is supported by UV, optical and IR spectroscopy data that reveal unusually high values of $Y$ in nova ejecta. We also show that a significantly increased abundance of ${}^3$He in nova accreted envelopes does not lead to higher values of $X_{\mathrm{theor}}{(}^7\mathrm{Be})$ because this assumption affects the evolution of nova models resulting in a decrease of both their peak temperatures and accreted masses and, as a consequence, in a reduced production of ${}^7$Be.

5 pages, 2 figures, accepted for publication in MNRAS Letters

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• Modules for Experiments in Stellar Astrophysics (MESA)
• Explosive lithium production in the classical nova V339 Del (Nova Delphini 2013)
• Highly Enriched 7Be in the ejecta of Nova Sagittarii 2015 No. 2 (V5668 Sgr) and the Galactic Lithium origin
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Cited by in corpus (1)

• The Gaia-ESO Survey: Galactic evolution of lithium from iDR6