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Unresolved triple systems in the aged open cluster Trumpler 19: Discovery, confirmation, and implications

This study utilizes multi-wavelength surveys and radial velocity measurements to identify and confirm a population of unresolved triple and quadruple systems in the aged open cluster Trumpler 19, demonstrating their survival over nearly 4 billion years and revealing a higher multiplicity fraction compared to the similarly aged cluster M 67.

Original authors: D. Minniti, R. K. Saito, V. D. Ivanov, C. O. Obasi, P. W. Lucas, J. G. Fernández-Trincado, J. Alonso-García, E. R. Garro, J. Corral-Santana, A. Luna, Z. Guo, R. Kurtev, J. Borissova, V. Fermiano, J. O

Published 2026-09-09✓ Author reviewed ⓘ

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CC BY 4.0

Original authors: D. Minniti, R. K. Saito, V. D. Ivanov, C. O. Obasi, P. W. Lucas, J. G. Fernández-Trincado, J. Alonso-García, E. R. Garro, J. Corral-Santana, A. Luna, Z. Guo, R. Kurtev, J. Borissova, V. Fermiano, J. Osses, C. Morris, B. Dias, C. Cáceres, S. Saroon, J. B. Pullen, P. Rodriguez, M. Lad, S. Federle, I. Petralia, D. Bhadrakumar, P. Esteves, M. G. Navarro, T. Palma, L. Baravalle, D. Galdeano, M. V. Alonso, J. L. Nilo Castellon, A. N. Chené

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Stars are rarely solitary. In the neighborhood of our Sun, most stars are found in pairs, and many are part of even larger families containing three, four, or more members. These groups, known as multiple systems, are crucial for understanding how stars are born, how they weigh themselves, and how they evolve over billions of years. However, spotting these families beyond our immediate cosmic backyard is incredibly difficult. When stars are too far away, they blur together into a single point of light, making it impossible to tell if a bright dot is a single star or a crowded family hiding in plain sight. To solve this, astronomers often turn to star clusters. These are dense groups of stars born from the same cloud of gas at the same time and distance. Because every star in a cluster shares the same age and chemical makeup, they act as a controlled laboratory. If a star in the cluster appears unusually bright, it is likely not because it is a different type of star, but because it is actually two or more stars shining together.

A team of astronomers has now used this laboratory approach to uncover a hidden population of complex star families in an ancient open cluster called Trumpler 19. Located about 2,300 pc away in the plane of our galaxy, this cluster is roughly 3.7 billion years old. By combining data from several powerful sky surveys, the researchers mapped the brightness and color of thousands of stars in the cluster. They knew that a pair of identical stars would appear about three-quarters of a magnitude brighter than a single star of the same type. But they were looking for something even brighter: stars that were so luminous they must be triplets or quadruplets, systems where three or four stars are locked together. The team identified a distinct sequence of these overly bright stars sitting well above the normal line of single stars in their charts. To confirm that these were indeed members of the cluster and not just random background stars, they used a large telescope equipped with a spectrograph to measure the speed at which these stars were moving. The results showed that the bright, multi-star candidates were moving at the same speed as the rest of the cluster, proving they belong there.

The discovery is significant because it shows that these complex families can survive for a very long time. While triple and quadruple systems have been seen in young clusters, finding them in a cluster as old as Trumpler 19 suggests they are remarkably stable. The researchers found that the fraction of these multiple systems in Trumpler 19 is comparable to what we see in the solar neighborhood, though the local sample includes fainter stars that are harder to see from this distance. Interestingly, Trumpler 19 contains nearly three times as many unresolved triple and quadruple systems as another famous, similarly aged cluster called M 67. This difference is puzzling because both clusters are about the same age and have similar total masses. It raises the question of whether Trumpler 19 was born with more of these families, or if M 67 somehow managed to break them apart over time. The study rules out several other explanations for the extra brightness, such as measurement errors, variable stars that pulse in brightness, or the presence of dusty disks that young stars often have. The data points clearly to these being genuine, long-lived families of stars.

This work highlights the power of modern sky surveys to reveal the hidden architecture of the galaxy. By using precise measurements of star positions and movements, combined with deep infrared imaging, the team was able to distinguish between single stars and complex families that were previously indistinguishable. The findings suggest that the formation and survival of multiple star systems may vary significantly from one cluster to another, even when those clusters look very similar on the surface. Future observations with even more powerful telescopes will be needed to see if this pattern holds true in other clusters and to understand exactly why some clusters seem to preserve these complex families while others do not. For now, the discovery in Trumpler 19 provides a rare glimpse into the long-term stability of the most crowded stellar families in the universe.

Technical Summary: Unresolved Triple Systems in the Aged Open Cluster Trumpler 19

Problem Statement
Identifying and characterizing unresolved triple and higher-order multiple star systems beyond the solar neighborhood presents significant observational challenges. While binary sequences in star clusters are well-documented, stars located significantly above the equal-mass binary sequence in color-magnitude diagrams (CMDs) are often discarded as field contaminants or photometric outliers. However, theoretical considerations suggest that unresolved triples and quadruples should occupy a distinct region above the binary sequence. The difficulty lies in distinguishing these systems from field stars, variable stars, or blending artifacts, particularly in the crowded and reddened Galactic plane where open clusters reside.

Methodology
The study focuses on the aged open cluster Trumpler 19 ( $t \approx 3.7$ Gyr, $D \approx 2.3$ kpc), utilizing a multi-survey approach to isolate a clean sample of cluster members and identify high-order multiples:

Membership Selection: The authors combined astrometry from Gaia DR3 with photometry from the VVVX (near-IR $JHK_s$ ) and DECaPS2 (optical $grizY$) surveys. Members were selected based on proper motion ( $\pm 1.0$ mas yr $^{-1}$ of the cluster mean) and parallax ( $0.365 \lesssim \varpi \lesssim 0.515$ mas). A Gaussian Mixture Model (GMM) was applied to positions, proper motions, and parallaxes to calculate membership probabilities.
Candidate Selection: The team constructed optical and near-IR CMDs to identify stars located $> 0.75$ mag above the cluster main sequence (MS). This threshold targets unresolved equal-mass binaries; stars significantly brighter than this limit (e.g., $> 1.19$ mag above the MS) were flagged as candidates for unresolved triples and quadruples.
Hypothesis Elimination: Potential false positives were systematically ruled out:
- Photometric scatter: Rejected due to magnitude differences exceeding measurement errors ( $\sigma < 0.02$ mag).
- Variability: Cross-referenced with Gaia, VVVX, and WISE multi-epoch data; most candidates were non-variable.
- Blending/Distance: Gaia parallaxes and proper motions eliminated foreground/background blends.
- Circumstellar disks: Discarded as the cluster age ( $\sim 4$ Gyr) precludes the presence of such disks.
Spectroscopic Confirmation: To independently verify membership and rule out field interlopers, near-IR spectroscopy was obtained using the KMOS spectrograph on the VLT (H-band, $R=4000$ ) as part of the VVVX-GalCen survey. Radial velocities (RVs) were measured via cross-correlation against a standard globular cluster template.

Key Contributions and Results

Discovery of a Triple Sequence: The study reports the first identification of a distinct sequence of unresolved multiple systems (triples and quadruples) in an aged open cluster. The authors identified 25 candidate triple/quadruple systems and 390 unresolved binaries.
Spectroscopic Confirmation: KMOS observations provided RVs for 22 candidate systems. The resulting velocity distribution showed a small dispersion ( $\sigma = 6 \pm 1$ km s $^{-1}$ ) consistent with the cluster mean ( $RV = -25.65 \pm 1.75$ km s $^{-1}$ ), confirming cluster membership for the majority of the candidates.
Multiplicity Fractions:
- The fraction of unresolved triples and higher-order systems in Trumpler 19 is measured at $THF = 0.084 \pm 0.010$ .
- This is approximately three times higher than the fraction found in the similarly aged and metal-rich open cluster M 67 ( $THF = 0.029 \pm 0.010$ ).
- When compared to the solar neighborhood, the fraction of multiple stars in Trumpler 19 is comparable, though the authors note the local sample probes deeper down the main sequence (lower masses) than the Trumpler 19 sample at 2.3 kpc.
Survival of Complex Systems: The detection of these systems in a 4 Gyr-old cluster demonstrates that unresolved triple and quadruple systems can survive dynamical evolution within a cluster environment for gigayears.

Significance and Claims
The paper concludes that the discovery of unresolved triple systems in Trumpler 19 challenges the assumption that such complex hierarchical systems are easily disrupted in dense cluster environments over long timescales. The authors highlight a discrepancy between Trumpler 19 and M 67: despite having similar ages, metallicities, and present-day masses, Trumpler 19 retains a significantly higher population of unresolved multiples.

The authors posit that this difference may be either a primordial feature (Trumpler 19 formed with more multiples) or a result of dynamical evolution (M 67 destroyed multiples more efficiently). They emphasize that current synthetic CMD codes (e.g., SIESTA) must be updated to include unresolved triples and quadruples to accurately model cluster populations. The study underscores the necessity of combining Gaia astrometry, deep near-IR photometry (VVVX), and spectroscopic follow-up to fully census multiplicity in star clusters, paving the way for future investigations using Gaia DR4, LSST, and the Roman Space Telescope.

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