The Secret Lives of Open Clusters: a Multiwavelength Examination of Three Open Clusters

TL;DR

This paper presents a multiwavelength census of three nearby open clusters—IC 2602, NGC 2632, and M67—to explore how high-energy source populations evolve with cluster age. By integrating X-ray data from eROSITA, Chandra, and XMM-Newton with radio observations from EMU and the VLA, and anchoring memberships with Gaia, the authors classify X-ray sources and correlate them with optical variability to probe dynamical evolution and binary activity. The results reveal substantial X-ray populations in IC 2602 and NGC 2632, and a rich, centrally concentrated X-ray/radio set in M67, including WOCS 3012/S1077, whose nature remains ambiguous between a quiescent black hole and a chromospherically active binary. The study demonstrates that the X-ray and variability content of open clusters changes with age, underscoring the need for broader, uniform multiwavelength surveys to calibrate dynamical models and constrain compact-object populations in clusters.

Abstract

Star clusters are well known for their dynamical interactions, an outcome of their high stellar densities; in this paper we use multiwavelength observations to search for the unique outcomes of these interactions in three nearby Galactic open clusters: IC 2602 (30 Myr), NGC 2632 (750 Myr) and M67 (4 Gyr). We compared X-ray observations from all-sky surveys like eROSITA, plus archival observations from Chandra X-ray Observatory, survey radio observations from ASKAP's Evolutionary Map of the Universe survey plus archival VLA observations, in conjunction with new cluster catalogs with Gaia. From X-ray, we found 77 X-ray sources likely associated with IC 2602, 31 X-ray sources in NGC 2632, and 31 near M67's central regions. We were further able to classify these X-ray sources based on their optical variability and any radio emission. Three IC 2602 X-ray sources had radio counterparts, which are likely all chromospherically active binary stars. We also identified luminous radio and X-ray variability from a spectroscopic triple system in M67, WOCS 3012/S1077, which is either consistent with a quiescent black hole binary, or due to an active binary stellar system. A recent population study of optical variables by Anderson & Hunt 2025 shows that the population of optical variables in open clusters clearly changes over cluster age; this pilot study gives evidence that the X-ray population also changes with time, and demonstrates the need for a broader multiwavelength study of Galactic open clusters.

Figures (8)

• Figure 1: CMD for IC 2602. Black x's are X-rays from eROSITA, green triangles are $\delta$ Scuti/$\gamma$ Doradus/SX Pheonicis, purple pentagons are ACV systems, red triangles are RS Canum Venaticorum, teal squares are eclipsing binaries and orange points are solar-like variability.
• Figure 2: X-ray luminosity (eROSITA band) versus absolute G magnitude for X-ray sources in IC 2602. Sources with variability flags from 2023AA...674A..13E are labeled in orange pentagons, with periodic sources marked with pink diamonds. Three sources (yellow triangles) had radio emission associated with them, but were not flagged as variable by 2023AA...674A..13E.
• Figure 3: CMD for NGC 2632. Black x's are X-rays from eROSITA, green diamonds are $\delta$ Scuti/$\gamma$ Doradus/SX Pheonicis, purple pentagons are ACV stars, red triangles are RS Canum Venaticorum, teal squares are eclipsing binaries, and orange points are solar-like variability.
• Figure 4: X-ray luminosity (0.5-2.3 keV) versus absolute G magnitude for X-ray sources in NGC 2632. Variable sources are denoted with orange pentagons, and periodic variable sources are marked with pink diamonds.
• Figure 5: CMD for M67. X-rays from Chandra are denoted in x's, and X-rays from XMM-Newton are denoted by purple plusses. Different Gaia variability classifications are shown, a green diamond for pulsating systems $\delta$ Scuti/$\gamma$ Doradus/SX Pheonicis, teal squares for eclipsing binaries (which are also X-ray sources), red triangles for RS Canum Venaticorum (which are also X-ray sources), and orange dots for solar-like variability.