Abstract
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. We point 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.
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