The Discovery of Mg II λ4481 in the White Dwarf EG 102: Evidence for Ongoing Accretion

Recent optical spectra of the DA3 white dwarf EG 102 clearly show a feature due to Mg II λ4481. This is the first spectroscopic detection of Mg in a DA star at a temperature of 20,000 K. The presence of this ion in the stellar photosphere poses a severe dilemma for the theory of radiative levitation, which predicts a vanishingly small equilibrium abundance of Mg in a star such as EG 102. Ongoing accretion must to be the source of this ion. An accretion rate of 1.6 × 10^-16 M_☉ yr^-1 is implied by the short timescale (3.3 days) for the diffusive loss of Mg from the photosphere. Such a rate could be supplied by the interstellar medium, if EG 102 is embedded in a diffuse interstellar cloud. Alternately, EG 102 may possess a very low mass companion, which is transferring material onto the white dwarf. Finally, we use an IUE echelle spectrum to demonstrate the presence of photospheric Si in this star. We find respective abundances of log (Mg/H) = -5.3 and log (Si/H) = -7.0 for EG 102.