What Gaia Might Be Missing: Searching for Hidden Stars in the NGC 3532 Star Cluster
The Gaia space observatory has transformed how astronomers study the Milky Way, especially in mapping out open star clusters—collections of stars that move together through space after forming from the same cloud of gas. However, even this high-precision mission has blind spots. In their recent paper, D. I. Tagaev and A. F. Seleznev investigated how many stars in the open cluster NGC 3532 might be missing from standard datasets because of what are known as "poor astrometric solutions" in Gaia’s third data release (DR3). These “poor” solutions occur when Gaia cannot reliably measure a star’s motion or distance, often because the star is part of a binary or multiple system that confuses its instruments.
Identifying the Cluster with Reliable Data
The researchers first focused on stars that had reliable astrometric data—specifically, stars with five- or six-parameter solutions in Gaia DR3, which include accurate positions, parallaxes, and proper motions. By examining stars in a 12-degree square around the center of NGC 3532 and narrowing down based on their motion and distance ranges, they were able to estimate the cluster’s extent. They found that the cluster has a radius of 178 ± 3 arcminutes and includes approximately 2,200 stars with good Gaia data, confirming previous results from other studies while slightly expanding the known population.
The Hidden Half: Stars with Poor Astrometric Solutions
Next, Tagaev and Seleznev turned their attention to stars Gaia had trouble measuring—those with only two-parameter solutions, high RUWE values (greater than 1.4), or large relative parallax errors. These stars are often excluded from studies to avoid contamination by unrelated background stars, but doing so may leave out real cluster members. To identify possible missed members, the authors used a technique called the Hess diagram—a probability map of the color-magnitude diagram (CMD)—to highlight stars whose brightness and color match those of known NGC 3532 members.
New Candidates and Revised Star Counts
By comparing roughly 1.5 million stars in the surrounding sky to the Hess diagram, the researchers found over 30,000 stars that matched the expected color and brightness of cluster members. After narrowing this list to those within the cluster's likely boundaries, they identified about 14,800 candidates with poor astrometric data. Using statistical methods, they estimated that 2,150 ± 230 of these stars are likely true members of NGC 3532—a number nearly equal to the previously known 2,200 members. This means that traditional methods may be missing as much as half of the cluster’s population.
The Role of Binary and Multiple Star Systems
One reason many of these stars were missed is that Gaia’s model assumes stars move as single points across the sky. This assumption breaks down for binary or multiple star systems, where the presence of a companion causes shifts in the star’s apparent position. This issue shows up as a high RUWE value in Gaia data. The study found that many of the newly identified stars had high RUWE values, suggesting they might indeed be binaries. Including these stars changes our picture of the cluster’s makeup and highlights the importance of accounting for unresolved systems.
Brightness Distribution and Impacts on the Luminosity Function
Adding the newly identified stars also changed the cluster’s luminosity function—the number of stars at different brightness levels. In particular, the number of stars between magnitudes 14 and 18 increased significantly. This matches expectations, as Gaia's astrometric measurements become less reliable for fainter stars. The difference between the original and revised luminosity functions highlights how crucial it is to consider poor-solution stars when trying to understand a cluster’s full population.
Conclusion: A Cautionary Tale for Star Cluster Studies
Tagaev and Seleznev’s study reveals that traditional selection methods, which rely only on clean and reliable Gaia data, may leave out nearly half of a cluster’s true members. Many of these missing stars are faint or part of binary systems, making them harder to analyze. By using a color-magnitude-based approach and statistical tools, the authors provide a fuller picture of NGC 3532 and suggest that similar strategies could improve our understanding of other clusters. This research reminds us that sometimes, what we don’t see is just as important as what we do.
Source: Tagaev
