Time-Variable Aluminum Absorption in the Polar AR Ursae Majoris, and an Updated Estimate for the Mass of the White Dwarf

TL;DR

This study observes time-variable aluminum absorption in AR Ursae Majoris, suggesting circumstellar material possibly from evaporating asteroids, and refines the white dwarf's mass estimate through spectral analysis.

Contribution

It reports the first detection of variable aluminum absorption in AR UMa and provides an updated mass estimate for its white dwarf using spectral decomposition.

Findings

Al I absorption doublet varies over time, indicating circumstellar origin.

White dwarf mass estimated between 0.91 and 1.24 solar masses.

Presence of rocky material may inform binary system evolution.

Abstract

We present spectra of the extreme polar AR Ursae Majoris (AR UMa) which display a clear Al I absorption doublet, alongside spectra taken less than a year earlier in which that feature is not present. Re-examination of earlier SDSS spectra indicates that the Al I absorption doublet was also present $\approx$8 years before our first non-detection. We conclude that this absorbing material is unlikely to be on the surface of either the white dwarf (WD) or the donor star. We suggest that this Al I absorption feature arises in circumstellar material, perhaps produced by the evaporation of asteroids as they approach the hot WD. The presence of any remaining reservoir of rocky material in AR UMa might help to constrain the prior evolution of this unusual binary system. We also apply spectral decomposition to find the stellar parameters of the M dwarf companion, and attempt to dynamically measure the mass of the WD in AR UMa by considering both the radial velocity curves of the H$_\beta$ emission line and the Na I absorption line. Thereby we infer a mass range for the WD in AR UMa of 0.91 $M_{\odot}$ $<$ $M_{\mathrm{WD}}$ $<$ 1.24 $M_{\odot}$.