For more than 50 years, the rate and distribution of positron (anti-electron) annihilation in the Milky Way has puzzled astronomers. Positrons with low (~MeV) initial energies are predominantly produced by beta+ unstable radioactive elements. However, the observed rate of positron annihilation in the Milky Way far exceeds the predicted rate of positron production by ‘conventional sources’ such as core-collapse and Type Ia supernovae. Moreover, the observed distribution of positron annihilation in the Milky Way appears to trace the older stellar populations of the Galaxy (the Galactic bulge and - according to most recent analysis - a thick, truncated disk), while radioactive material is predominantly synthesised in the thin disk of the Galaxy where star formation occurs. Over the past few years, Fiona has worked on constraining scenarios for positron injection and transport in energetic outflows of the Milky Way, investigated positron annihilation sites in the Milky Way beyond annihilation with electrons bound to hydrogen and helium atoms, as well as investigating in detail the possibility that positrons are born in the ejecta of subluminous thermonuclear supernovae that concentrate in old stellar populations. Fiona will present highlights of this theoretically-focussed work in her talk, along with emphasis on the value of positron astrophysics for other areas of astronomy including galactic chemical evolution.
Fiona is a Cluster guest. Local host: Roland Diehl (MPE)