The Ionising Photon Budget of the Local Interstellar Cloud: Epsilon Eridani, Hot Stars and the Diffuse Background

The partial ionisation of the local cloud is a balance, and the balance can be run backwards: from the measured ionised fraction and the recombination coefficient, the photoionisation rate that the local radiation field must supply follows directly. The required rate is 8.1e-15 per second per hydrogen atom, corresponding to an ionisation timescale of 3.9e+06 years against a recombination timescale of 1.3e+06 years at the cloud electron density. Published radiation-field models, combining the nearby hot stars epsilon and beta Canis Majoris, other early-type stars, and the diffuse emission of the hot bubble itself, supply a rate of this order, which is why the observed ionisation fractions near one quarter for hydrogen are reproducible at all. The backward calculation is computed from stated inputs with the deposited script.