Gaia-VVVX search for M-type variable stars in young nearby open clusters

Context. While large samples of nearby M dwarfs are becoming available, it is usually difficult to estimate some intrinsic physical parameters for isolated field M dwarfs, especially their ages, radii, and masses. These parameters can be estimated through a comparison with model stellar isochrones and/or by studying eclipsing binary systems. Aims. Our goal is to gather and analyze a large sample of variable M dwarfs that are members of nearby young open clusters and have known reddenings, distances, ages, and metallicities. This distilled sample would be useful for the comparison with theoretical models and also for the identification of eclipsing binaries with substellar companions (brown dwarfs or giant planets) for future follow-up. Methods. We selected two dozen benchmark young and nearby open clusters (mostly with 40 < age < 132 Myr and 400 < D < 700 pc, respectively). We then combined the optical data from Gaia Data Release 3 (DR3) with near-IR JHKs−band photometry from the VISTA Variables in the Vía Láctea eXtended survey (VVVX) for these target clusters using the proper motions in order to select members within the cluster radii. Gaia and VVVX both provide a wide areal coverage that is ideally suited for mapping these nearby clusters, which are extended on the sky. We then produced optical and near-IR color–magnitude and color-color diagrams and applied appropriate color cuts to choose the M-type dwarfs at the end of the main sequence. The reddening and extinction for all clusters were estimated using the J − Ks color distributions of the cluster M dwarfs. We also compared them with the PARSEC theoretical isochrones adjusted to the respective cluster metallicities, ages, extinctions, and distances. Results. Finally, we selected variable M stars according to Gaia DR3 (using phot_variable_flag=VARIABLE) and present a catalog of 318 variable M-dwarf star members of two dozen young nearby Galactic open clusters. The cross-validation with the Gaia DR3 parallaxes supports the cluster membership for all these sources. We also present a few examples to validate the sample of variable cluster M dwarfs. Conclusions. This well-characterized catalog of M-dwarf members of benchmark open clusters not only constitutes a prime sample to search for transiting substellar companions, but also offers a training dataset for machine-learning applications aimed at selecting future similar targets.