Lithium Synthesis in Microquasar Accretion

We study the synthesis of lithium isotopes in the hot tori formed around stellar mass black holes by accretion of the companion star. We find that sizable amounts of both stable isotopes ${}^6\mathrm{Li}$ and ${}^7\mathrm{Li}$ can be produced, the exact figures varying with the characteristics of the torus and reaching as much as ${10}^{-2}{M}_{\odot }$ for each isotope. This mass output is enough to contaminate the entire Galaxy at a level comparable with the original, pregalactic amount of lithium and to overcome other sources such as cosmic-ray spallation or stellar nucleosynthesis.

Figure 1

The mass of ${}^7\mathrm{Li}$ (plus ${}^7\mathrm{Be}$) and ${}^6\mathrm{Li}$ synthesized in a hot torus around a black hole of $5M_{\odot }$ (left) and $30M_{\odot }$ (right). The shadowed area encompasses the Eddington upper limit on the accretion rate $\dot{M}$ assuming accretion efficiencies $0.06\le ϵ\le 0.42$.

Figure 2

The mass of ${}^7\mathrm{Li}$ (plus ${}^7\mathrm{Be}$) and ${}^6\mathrm{Li}$ synthesized as a function of $M_{\mathrm{BH}}$ for fixed $\alpha$ and $\dot{m}$. Here we take $ϵ=0.06$.