Lithium production in galactic flybys

Abstract

Unlike 7Li plateau which is expected but observed at a level that is difficult to explain, 6Li observations in low-metallicity halo stars also reveal a plateau which is completely unexpected within the standard picture of 6Li origin in galactic cosmic-ray (GCR) interactions. This indicates the existence of a new Li source, however all considered sources (e.g. supernovae, shocks due to structure-formation etc.) either completely fail to explain the observed abundance or require some fine-tuning. Here we present another possible source of lithium - cosmic rays that arise due to galactic flybys. Gravitational interaction between two galaxies flying by each other could result in large-scale tidal shocks that would give rise to a new cosmic-ray population - tidal cosmic rays. Such cosmic-ray population would increase the 6Li (and 7Li) abundance, but not be accompanied with increase in metallicity or gas content. The details, of course, depend on interacting galaxies. Our preliminary results presented here demonstrate that this could be an important source of lithium in some sites, such is the Small Magellanic Cloud which has experienced close encounters with the Large Magellanic Cloud and the Milky Way during its history.