Follow-Up Observations of the Remarkable Double-Degenerate Binary LB 11146

Abstract

LB 11146 is a double-degenerate binary system consisting of two white dwarfs of very similar mass and surface temperature, but widely different magnetic field strengths. In an attempt to contrain the possible evolutionary tracks of the object, time-series H-alpha spectroscopy and the DA component was obtained with a time resolution as high as 11.5 minutes and total baseline of 16 days. Cross-correlation analysis reveals no evidence of variation to a limit of K <~ 20 km s^-1 on periods of ~1 hr, effectively ruling out the possibility of future orbit decay by gravitational radiation to a Type I supernova. However, both single-star and binary modes of past evolution are still viable. In addition, red spectroscopy was obtained to search for the highly displaced 2p0 to 3d-1 absorption component of H-alpha from the magnetic star, which is strong and nearly stationary at ~8500-8532A in fields of 350-1200 MG. The detection of a deep feature near this location coupled with an identification of a displaced component of Ly-alpha in the IUE spectrum of the object confirm that hydrogen is present in the atmosphere of the magnetic component. Spectral modeling suggests a polar strength of ~670 MG (for dipolar geometry), but no model obtains a completely satisfactory fit, and at least one additional atmospheric constituent (neutral helium?) is required. Based on these conclusions, the magnetic component is reclassified as DAXP. (SECTION: Stars)