New Insights into the Problem of the Surface Gravity Distribution of Cool DA White Dwarfs

TL;DR

This paper investigates the high surface gravity measurements in cool DA white dwarfs, ruling out helium contamination as a cause and highlighting discrepancies between spectroscopic and photometric methods.

Contribution

It provides new helium abundance constraints, extends models with improved Stark profiles, and clarifies the high-log g problem's relation to spectroscopic analysis.

Findings

Helium contamination is not responsible for high gravities in cool DA white dwarfs.

Improved Stark profiles do not resolve the high gravity issue.

Photometric masses align with expectations, unlike spectroscopic measurements.

Abstract

We review at length the longstanding problem in the spectroscopic analysis of cool hydrogen-line (DA) white dwarfs (Teff < 13,000 K) where gravities are significantly higher than those found in hotter DA stars. The first solution that has been proposed for this problem is a mild and systematic helium contamination from convective mixing that would mimic the high gravities. We constrain this scenario by determining the helium abundances in six cool DA white dwarfs using high-resolution spectra from the Keck I 10-m telescope. We obtain no detections, with upper limits as low as He/H = 0.04 in some cases. This allows us to put this scenario to rest for good. We also extend our model grid to lower temperatures using improved Stark profiles with non-ideal gas effects from Tremblay & Bergeron and find that the gravity distribution of cool objects remains suspiciously high. Finally, we find that photometric masses are, on average, in agreement with expected values, and that the high-log g problem is so far unique to the spectroscopic approach.