Light Axion Emission and the Formation of Merging Binary Black Holes

We study the impact of stellar cooling due to light axion emission on the formation and evolution of black hole binaries, via stable mass transfer and the common envelope scenario. We find that in the presence of light axion emission, no binary black hole mergers are formed with black holes in the lower mass gap ($\rm M_{\rm BH} < 4 M_\odot$) via the common envelope formation channel. In some systems, this happens because axions prevent Roche lobe overflow. In others, they prevent the common envelope from being ejected. Our results apply to axions with couplings $g_{a \gamma} \gtrsim 10^{-10}\, \rm GeV^{-1}$ (to photons) or $\alpha_{ae} \gtrsim 10^{-26}$ (to electrons) and masses $m_a \ll 10 \, \rm keV$. Light, weakly coupled particles may therefore apparently produce a mass gap $\rm 2 M_\odot < M_{\rm BH} < 4 M_\odot$ in the LIGO/Virgo/KAGRA data, when no mass gap is present in the stellar remnant population.