Detection of $^7$Be II in the Small Magellanic Cloud

TL;DR

This study reports the first detection of $^7$Be II in novae outside the Milky Way, confirming that low-metallicity environments like the SMC can produce lithium via thermonuclear reactions, impacting our understanding of cosmic lithium origins.

Contribution

It provides the first extragalactic detection of $^7$Be II in novae, demonstrating lithium production in low-metallicity environments and offering insights into galactic chemical evolution.

Findings

$^7$Be II detected in SMC novae, first outside the Galaxy.

Novae in the SMC produce less lithium than Galactic novae.

Estimated lithium yield per nova suggests significant contribution to SMC's lithium abundance.

Abstract

We analyse high resolution spectra of two classical novae that exploded in the Small Magellanic Cloud. $^7$Be II resonance transitions are detected in both ASASSN-19qv and ASASSN-20ni novae. This is the first detection outside the Galaxy and confirms that thermo-nuclear runaway reactions, leading to the $^7$Be formation, are effective also in the low metallicity regime, characteristic of the SMC. Derived yields are of N($^7$Be=$^7$Li)/N(H) = (5.3 $\pm$ 0.2) $\times$ 10$^{-6}$ which are a factor 4 lower than the typical values of the Galaxy. Inspection of two historical novae in the Large Magellanic Cloud observed with IUE in 1991 and 1992 showed also the possible presence of $^7$Be and similar yields. For an ejecta of $M_{H,ej} =$ 10$^{-5}$ M$_{\odot}$, the amount of $^7$Li produced is of $M_{^7 Li} = (3.7 \pm 0.6) \times 10^{-10}$ M$_{\odot}$ per nova event. Detailed chemical evolutionary model for the SMC shows that novae could have made an amount of lithium in the SMC corresponding to a fractional abundance of A(Li) $\approx$ 2.6. Therefore, it is argued that a comparison with the abundance of Li in the SMC, as measured by its interstellar medium, could effectively constrain the amount of the initial abundance of primordial Li, which is currently controversial.