The first catalog of candidate white dwarf-main sequence binaries in open star clusters: A new window into common envelope evolution

TL;DR

This paper presents the first catalog of candidate white dwarf-main sequence binaries in open star clusters, using Gaia data and machine learning, to improve understanding of common envelope evolution.

Contribution

It introduces a systematic method combining Gaia, multi-band photometry, and machine learning to identify post-CE binary candidates in star clusters, providing new observational benchmarks.

Findings

Identified 52 high-probability WD-MS binary candidates in 38 clusters.

Follow-up spectroscopy confirmed some systems with hot companions and variability.

Some candidates show spatial offsets, suggesting natal kicks.

Abstract

Close binary systems are the progenitors to both Type Ia supernovae and the compact object mergers that can be detected via gravitational waves. To achieve a binary with a small radial separation, it is believed that the system likely undergoes common envelope (CE) evolution. Despite its importance, CE evolution may be one of the largest uncertainties in binary evolution due to a combination of computational challenges and a lack of observed benchmarks where both the post-CE and pre-CE conditions are known. Identifying post-CE systems in star clusters can partially circumvent this second issue by providing an independent age constraint on the system. For the first time, we conduct a systematic search for white dwarf (WD) and main-sequence (MS) binary systems in 299 Milky Way open star clusters. Coupling Gaia DR3 photometry and kinematics with multi-band photometry from Pan-STARRS1 and 2MASS, we apply a machine learning based approach and find 52 high-probability candidates in 38 open clusters. For a subset of our systems, we present follow-up spectroscopy from the Gemini and Lick Observatories and archival light curves from TESS, Kepler/K2 and the Zwicky Transient Facility. Examples of M-dwarfs with hot companions are spectroscopically observed, along with regular system variability. While the kinematics of our candidates are consistent with their host clusters, some systems have spatial positions offset relative to their hosts, potentially indicative of natal kicks. Ultimately, this catalogue is a first step to obtaining a set of observational benchmarks to better link post-CE systems to their pre-CE progenitors.