Hidden in Plain Sight: A Double-lined White Dwarf Binary 26 pc Away and a Distant Cousin

TL;DR

This paper reports the discovery and analysis of two nearby double-lined white dwarf binaries, providing insights into their properties, orbital periods, and the population distribution of such systems, with implications for binary evolution models.

Contribution

It presents the first detailed spectroscopic analysis of two nearby SB2 white dwarf systems, including new synthetic spectra and population comparisons.

Findings

PG 1632+177 is a 2.05-day SB2 white dwarf binary, second closest known.

WD 1534+503 is an SB2 system with a 0.71-day orbital period.

The observed population aligns with binary evolution models predicting short and long period white dwarf binaries.

Abstract

We present high-resolution spectroscopy of two nearby white dwarfs with inconsistent spectroscopic and parallax distances. The first one, PG 1632+177, is a 13th magnitude white dwarf only 25.6 pc away. Previous spectroscopic observations failed to detect any radial velocity changes in this star. Here, we show that PG 1632+177 is a 2.05 d period double-lined spectroscopic binary (SB2) containing a low-mass He-core white dwarf with a more-massive, likely CO-core white dwarf companion. After L 870-2, PG 1632+177 becomes the second closest SB2 white dwarf currently known. Our second target, WD 1534+503, is also an SB2 system with an orbital period of 0.71 d. For each system, we constrain the atmospheric parameters of both components through a composite model-atmosphere analysis. We also present a new set of NLTE synthetic spectra appropriate for modeling high-resolution observations of cool white dwarfs, and show that NLTE effects in the core of the H$\alpha$ line increase with decreasing effective temperature. We discuss the orbital period and mass distribution of SB2 and eclipsing double white dwarfs with orbital constraints, and demonstrate that the observed population is consistent with the predicted period distribution from the binary population synthesis models. The latter predict more massive CO + CO white dwarf binaries at short ($<1$ d) periods, as well as binaries with several day orbital periods; such systems are still waiting to be discovered in large numbers.