A Test of Spectroscopic Age Estimates of White Dwarfs using Wide WD+WD Binaries

TL;DR

This study tests the accuracy of spectroscopic age estimates of white dwarfs in wide binaries, finding that photometric methods often outperform spectroscopic ones and highlighting the importance of proper spectral identification.

Contribution

It provides a comprehensive comparison of spectroscopic and photometric age estimates for white dwarfs in wide binaries, revealing limitations of spectroscopic fits and emphasizing the need for spectral identification.

Findings

Spectroscopic fits do not significantly improve age agreement.

Photometric parameters yield better age agreement than spectroscopic ones.

Approximately 20% of wide WD+WD binaries show signs of mergers.

Abstract

White dwarf stars have been used for decades as precise and accurate age indicators. This work presents a test of the reliability of white dwarf total ages when spectroscopic observations are available. We conduct follow-up spectroscopy of 148 individual white dwarfs in widely separated double-white-dwarf (WD+WD) binaries. We supplement the sample with 264 previously published white dwarf spectra, as well as 1292 high-confidence white dwarf spectral types inferred from their Gaia XP spectra. We find that spectroscopic fits to optical spectra do not provide noticeable improvement to the age agreement among white dwarfs in wide WD+WD binaries. The median age agreement is $\approx$$1.5\sigma$ for both photometrically and spectroscopically determined total ages, for pairs of white dwarfs with each having a total age uncertaintiy $<$ 20\%. For DA white dwarfs, we further find that photometrically determined atmospheric parameters from spectral energy distribution fitting give better total age agreement ($1.0\sigma$, 0.2 Gyr, or 14\% of the binary's average total age) compared to spectroscopically determined parameters from Balmer-line fits (agreement of $1.5\sigma$, 0.3 Gyr, or 28\% of binary's average total age). We find further evidence of a significant merger fraction among wide WD+WD binaries: across multiple spectroscopically identified samples, roughly 20\% are inconsistent with a monotonically increasing initial-final mass relation. We recommend the acquisition of an identification spectrum to ensure the correct atmospheric models are used in photometric fits in order to determine the most accurate total age of a white dwarf star.