SDSS J074511.56+194926.5: Discovery of a Metal-rich and Tidally Distorted Extremely Low Mass White Dwarf

We present the discovery of an unusual, tidally distorted extremely low mass white dwarf (WD) with nearly solar metallicity. Radial velocity measurements confirm that this is a compact binary with an orbital period of 2.6975 hr and a velocity semi-amplitude of K = 108.7 km s–1. Analysis of the hydrogen Balmer lines yields an effective temperature of T eff = 8380 K and a surface gravity of log g = 6.21 that in turn indicate a mass of M = 0.16 M ☉ and a cooling age of 4.2 Gyr. In addition, a detailed analysis of the observed metal lines yields abundances of log (Mg/H) = –3.90, log (Ca/H) = –5.80, log (Ti/H) = –6.10, log (Cr/H) = –5.60, and log (Fe/H) = –4.50, similar to the sun. We see no evidence of a debris disk from which these metals would be accreted, though the possibility cannot entirely be ruled out. Other potential mechanisms to explain the presence of heavy elements are discussed. Finally, we expect this system to ultimately undergo unstable mass transfer and merge to form a ~0.3-0.6 M ☉ WD in a few Gyr.