Ultraviolet and X-ray detection of the 56 Peg system (K0 II p+WD) : evidence for accretion of a cool stellar wind onto a white dwarf.

Abstract

IUE spectra of the slowly rotating mild barium star 56 Peg (HD 218356; K0 IIp) show excess continuum emission from 1300 to 2000 Å, a broad Lyα absorption feature, and emission lines usually associated with a l0⁴-(2 × l0⁵) K plasma. The best fit blackbody curve to the dereddened continuum gives a temperature of 32000 ± 4000 K and a radius for the object of (2.7 ± 1.0) × l0⁹ cm, consistent with that of a white dwarf. Einstein IPC observations of this system yield L_x ≈ 3 × 1O³¹ ergs s^-1, which is as bright as the RS CVn binary systems. The X-rays can be fitted to a bremsstrahlung spectrum with kT = 0.45 ± 0.3 keV, or a blackbody spectrum with kT ≈ 0.2 keV. Since bright X-ray and high temperature emission lines are unusual for single stars in this region of the H-R diagram, we do not believe that the 56 Peg primary has a hot corona and transition region. Instead, we propose that the observed X-ray luminosity is due to accretion onto the white dwarf of ∼0.1% of the wind from the primary, which we assume has a reasonable mass loss rate of 2 × 10^-7 to 4 × l0^-9 M_sun yr^-1. The ultraviolet emission lines likely result from reprocessed X-radiation absorbed by the wind. The Mg II K line exhibits a time-varying emission core, that may be explained by ionization of Mg⁺ in the wind by X-rays from the white dwarf.