Molecular Hydrogen Emission Lines in Far Ultraviolet Spectroscopic Explorer Observations of Mira B^*

We present new Far Ultraviolet Spectroscopic Explorer (FUSE) observations of Mira A's wind-accreting companion star, Mira B. We find that the strongest lines in the FUSE spectrum are H₂ lines fluoresced by H I Lyα. A previously analyzed Hubble Space Telescope (HST) spectrum also shows numerous Lyα-fluoresced H₂ lines. The HST lines are all Lyman band lines, while the FUSE H₂ lines are mostly Werner band lines, many of them never before identified in an astrophysical spectrum. We combine the FUSE and HST data to refine estimates of the physical properties of the emitting H₂ gas. We find that the emission can be reproduced by an H₂ layer with a temperature and column density of T = 3900 K and log N(H₂) = 17.1, respectively. Another similarity between the HST and FUSE data, besides the prevalence of H₂ emission, is the surprising weakness of the continuum and high-temperature emission lines, suggesting that accretion onto Mira B has weakened dramatically. The UV fluxes observed by HST on 1999 August 2 were previously reported to be over an order of magnitude lower than those observed by HST and the International Ultraviolet Explorer (IUE) from 1979-1995. Analysis of the FUSE data reveals that Mira B was still in a similarly low state on 2001 November 22.