The ultraviolet spectrum of the companion of Mira (omicron Ceti) : observational evidence for a disk formed by wind accretion.

Abstract

The authors present extensive IUE observations of the companion of Mira (ο Cet B). It is shown that the UV continuum observed with IUE is exclusively due to Mira B at Mira A minimum. The UV continuum appears to vary erratically with a total range of ±30% within days or weeks. The white dwarf companion of Mira A itself is not seen and must be cooler than 14,000K. The observations imply the existence around Mira B of a thin, rotating disk with a typical radius of r ≈ 10¹¹cm. The disk thickness D is determined as 8×10⁹ > D > 2.4×10⁸cm. The disk rotational velocity is seen to increase from 135 km s^-1 in the cool outer layers to 1000 km s^-1 in the innermost region. The accretion rate onto the WD is estimated as 0.5×10¹⁷g s^-1 which yields an accretion luminosity of 5×10³³ergs s^-1, sufficient to account for the estimated Mira B luminosity.