Model atmosphere analysis of the weakly magnetic DZ white dwarf G165-7

A reanalysis of the strongly metal-blanketed DZ white dwarf G165-7 is presented. An improved grid of model atmospheres and synthetic spectra is used to analyze BVRI, JHK, and ugriz photometric observations as well as a high-quality Sloan Digital Sky Survey spectrum covering the energy distribution from 3600 to 9000 Å. The detection of splitting in several lines of Ca, Na, and Fe, suggesting a magnetic field of B_s ∼ 650 kG, is confirmed by spectro-polarimetric observations that reveal as much as ±7.5% circular polarization in many of the absorption lines, most notably Na, Mg, and Fe. Our combined photometric and spectroscopic fit yields T _eff = 6440 K, log g = 7.99, log (H/He) = -3.0, and log (Ca/He) = -8.1. The other heavy elements have solar ratios with respect to Ca, with the exception of Na and Cr, which had to be reduced by a factor of 2 and 3, respectively. A crude polarization model based on the observed local spectral flux gradient yields a longitudinal field of 165 kG, consistent with the mean surface field inferred from the Zeeman splitting. The inclusion of this weak magnetic field in our synthetic spectrum calculations, even in an approximate fashion, is shown to improve our fit significantly.