Doubly ionized aluminium - a diagnostic of cooling gas in the galactic corona

Even though it is thought to be abundant only in a very narrow temperature range, Al2+ has a large column density in the interstellar gas. This paper points out that charge transfer ionization of singly ionized aluminium is probably too slow to broaden the temperature range over which Al2+ is abundant in steady-state ionization equilibrium. Al2+ may exist in gas cooling from temperatures not exceeding about 50 000 K. If interstellar Al2+ in the halo forms in a galactic fountain, the fountain is unlikely to be thermally driven. (author)