Detection of H-2 emission from Mira B in ultraviolet spectra from the Hubble Space Telescope

We present ultraviolet spectra of Mira's companion star from the Space Tele scope Imaging Spectrograph (STIS) instrument on board the Hubble Space Tele scope (HST). The companion is generally assumed to be a white dwarf surroun ded by an accretion disk fed by Mira's wind, which dominates the UV emissio n from the system. The STIS UV spectrum is dominated by numerous, narrow H- 2 lines fluoresced by H I Ly alpha, which were not detected in any of the n umerous observations of Mira B by the International Ultraviolet Explorer (I UE). The high-temperature lines detected by IUE (e.g., C IV lambda 1550) st ill exist in the STIS spectrum but with dramatically lower fluxes. The cont inuum fluxes in the STIS spectra are also much lower, being more than an or der of magnitude lower than ever observed by IUE and also an order of magni tude lower than fluxes observed in more recent HST Faint Object Camera obje ctive prism spectra from 1995. Thus, the accretion rate onto Mira B was app arently much lower when STIS observed the star, and this change altered the character of Mira B's UV spectrum.