A rotation-based census of blue lurker candidates in open clusters

TL;DR

Blue lurkers are rejuvenated main-sequence stars that masquerade as normal MS stars in star clusters. The authors construct a rotation- and SED-based approach to identify BL candidates, synthesizing rotation data from Kepler, K2, TESS, and spectroscopic indicators with multi-band photometry. They identify 97 new BL candidates across 35 open clusters, for a total of 133 BL candidates when including literature identifications, and argue that the true BL population in clusters exceeds $2000$ due to an estimated completeness of only $<$ $3\%$. The work highlights thousands of BLs potentially hidden in clusters, emphasizes the need for spectroscopic follow-up and UV observations to confirm mass-transfer histories, and underscores the importance of BLs for understanding binary evolution and stellar populations.

Abstract

Blue lurkers (BLs) are rejuvenated main-sequence stars hidden among normal main-sequence stars on color-magnitude diagrams of star clusters. In comparison, the blue straggler stars, formed via similar mass transfers or mergers, occupy a distinct space in the color-magnitude diagrams. We compile a list of BL candidates in open clusters using available rotation catalogs. BLs can be identified using either unusually faster rotation compared to similar mass stars, which is a signature of recent accretion, or the presence of a companion, which can only be formed by mass donation, e.g., an extremely low mass white dwarf. Here, we searched for fast-rotating stars on the main sequence of open clusters using Kepler, TESS, and spectroscopic rotation indicators, such as rotation periods and $v\sin i$ measurements. We identified 97 new BL candidates across 35 open clusters, almost tripling the previously known sample of 36. Based on the estimated completeness of $\approx$3\%, thousands of BLs are likely hidden within the cluster population. Detailed spectroscopic and time-series analyses will be essential to confirm their mass-transfer histories.

Figures (5)

• Figure 1: Diagnostic plots for NGC 6811. (a) The apparent Gaia CMD colored according to the rotation period Van2025ApJ...986...59V. (b) Distribution of $P_{rotation}$ and Gaia$G$ magnitude colored according to the temperature (if unavailable, the stars are shown as gray dots). (c) Distribution of $P_{rotation}$ and temperature colored according to the G magnitude. The BL candidates (black squares) and the automatically detected outliers (red crosses) are highlighted.
• Figure 2: (a) Absolute Gaia CMD of the 133 BL candidates. (b) Distribution of the number of BLs and the cluster mass. The points are colored according to cluster age. The dashed line showing the upper limit on the number of BSSs for given cluster mass is taken from Jadhav2021MNRAS.507.1699J. The 4 BLs in the globular cluster NGC 362 were assigned an age of 13 Gyr in panel (a) for visual convenience.
• Figure 3: Distribution of the rotation periods and Gaia$G$ magnitudes for clusters analyzed with light curves. The stars are colored by their temperature (if unavailable, the stars are shown as gray dots). The BL candidates are highlighted by the black squares.
• Figure 4: Distribution of the $v\sin i$ and Gaia$G$ magnitudes for clusters analyzed with spectroscopy. The stars are colored by their temperature (if unavailable, the stars are shown as gray dots). The BL candidates are highlighted by the black squares.
• Figure 5: SEDs of stars with significant excess flux in at least 3 UV filters. The UV (magenta) and optical--IR fluxes (orange) are shown using the corresponding flux errors. The best-fitted Kurucz--UVBLUE model is shown in gray.