Detection of H[TINF]2[/TINF] Emission from Mira B in Ultraviolet Spectra from the [ITAL]Hubble Space Telescope[/ITAL]

We present ultraviolet spectra of Mira's companion star from the SpaceTelescope Imaging Spectrograph (STIS) instrument on board the Hubble SpaceTelescope (HST). The companion is generally assumed to be a white dwarfsurrounded by an accretion disk fed by Mira's wind, which dominates the UVemission from the system. The STIS UV spectrum is dominated by numerous, narrowH2 lines fluoresced by H I Ly-alpha, which were not detected in any of thenumerous observations of Mira B by the International Ultraviolet Explorer(IUE). The high temperature lines detected by IUE (e.g., C IV 1550) still existin the STIS spectrum but with dramatically lower fluxes. The continuum fluxesin the STIS spectra are also much lower, being more than an order of magnitudelower than ever observed by IUE, and also an order of magnitude lower thanfluxes observed in more recent HST Faint Object Camera objective prism spectrafrom 1995. Thus, the accretion rate onto Mira B was apparently much lower whenSTIS observed the star, and this change altered the character of Mira B's UVspectrum.Comment: 12 pages, 4 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty; accepted by ApJ Letter