The Montreal Open Clusters and Associations (MOCA) Database: A Census of Nearby Associations, Open Clusters, and Young Substellar Objects within 500 pc of the Sun

TL;DR

The Montreal Open Clusters and Associations (MOCA) database provides a public, queryable census of nearby young stellar associations, open clusters, and substellar objects within 500 pc. It consolidates memberships and literature measurements from Gaia-driven surveys and numerous catalogs, delivering derived quantities such as distances, space velocities, and fundamental ages using reproducible, hash-driven recalculations. A core advance is the updated BANYAN Σ model, built on a 6D XYZUVW Gaussian mixture framework with extreme deconvolution, enabling identification of 11,535 new candidate members and a refined mapping of association structures. The MOCA platform integrates a rich substellar and exoplanet inventory, including 134 age-calibrated exoplanet systems and 455 substellar candidates, providing a powerful resource for age-dating, population studies, and targeted follow-up with current and upcoming observatories.

Abstract

We present the Montreal Open Clusters and Associations database (MOCAdb), a public MySQL database with a Python interface. MOCAdb provides a census of memberships for 10259 associations and open clusters within 500 pc of the Sun, with a comprehensive compilation of literature measurements such as spectral types, kinematics, rotation periods, activity indices, spectral indices, and photometry. All known substellar objects are cataloged in MOCAdb, along with 2943 public spectra, to enable the characterization of substellar association members. MOCAdb also features periodically updated calculations such as Galactic UVW space velocities. We use this compilation to construct mappings between independent association definitions, and to update the BANYAN $Σ$ membership classification tool, which now includes 8125 associations. The BANYAN $Σ$ model construction is improved to account for heterogeneous and correlated errors and to capture complex association shapes using Gaussian mixture models. Combined with Gaia DR3, this enabled us to identify 11535 yet unrecognized candidate members of young associations within 500 pc, mostly M dwarfs. Our results corroborate a recent observation that systematics up to $\approx$4 km/s remain in Gaia DR3 radial velocities for A-type stars. We present an updated census of age-calibrated exoplanets and substellar objects: 134 age-calibrated exoplanet systems (plus 121 TESS exoplanet candidates), 99 of which did not appear to have known memberships so far, and 455 substellar (L0 or later) candidate members of young associations, 196 of which appear newly recognized. We bring the total of candidate isolated planetary-mass objects to 101, 53 of which are newly recognized candidate members.

Figures (26)

• Figure 1: Flow chart of update algorithm in MOCAdb. Black arrows represent dependencies for calculations or data transfer. Astrophysical objects are mostly inserted into MOCAdb starting from the data_memberships table using the designations from the original paper. They are then cross-matched using CDS to compile all of their designations -- the main designation is added to the moca_objects table with a new unique moca_oid integer identifier assigned by the database. These moca_oid identifiers are propagated back to the data_memberships table, and all alternate designations are ported to the table mechanics_all_designations. All designations relevant to one of the known online databases (such as Gaia DR3) are then used to download the relevant rows from the corresponding database into the corresponding cat_ table, and MySQL procedures then port several observable quantities (such as radial velocities and proper motions) in the corresponding data_ table in MOCAdb. These newly available observables trigger changes in the MD5 hashes used for calculations, and trigger a recalculation of the relevant properties for the moca_oid objects which gained new data. See Section \ref{['sec:dbs']} for more detail.
• Figure 2: Gaia DR3 vbroad and $v\sin i$ measurements based on two distinct pipelines, compared with literature $v\sin i$ measurements. The Gaia DR3 vbroad measurements are representative of literature values above 12 km s$^{-1}$ and the Gaia DR3 $v\sin i$ measurements are representative of literature values above 20 km s$^{-1}$. We have therefore chosen these respective thresholds for their inclusion in MOCAdb as a valid value for $v\sin i$. See Section \ref{['sec:vsinibias']} for more details.
• Figure 3: Upper panels: Gaia DR3 vbroad errors compared with the error-normalized residuals for stars with a literature $v\sin i$ measurement (left). If the measurement errors of the Gaia DR3 vbroad and the literature $v\sin i$ are realistic, normalized residuals should cluster around 1.0 (horizontal, red dashed line), which is not the case, indicating that an extra error term should be added in quadrature to the Gaia DR3 vbroad errors. Once an extra error is added (5 km s$^{-1}$ for stars with $T_{\rm eff}$ in the range 4,000--7,000 K, or 14.5 km s$^{-1}$ for other stars), the normalized residuals (right) behave as expected. The selection threshold based on the raw Gaia DR3 vbroad errors (5 km s$^{-1}$) for MOCAdb inclusion is also displayed with a vertical blue line. Bottom panels: idem but for the Gaia DR3 $v\sin i$ measurements determined by the Gaia DR3 Apsis pipeline, which warranted the inclusion of a 26 km s$^{-1}$ error term in quadrature. See Section \ref{['sec:vsinibias']} for more details.
• Figure 4: Galactic positions of individual stars in MOCAdb, color-coded by their association age. This figure shows how different age populations are not homogeneously distributed spatially, and how the nearby youngest stars are distributed along a plane that is slanted with respect to $Z = 0$. The MOCAdb census is mostly complete at distances up to 500 pc, marked within the dashed line.
• Figure 5: The best-fitting spectral type to Gaia DR3 sequence (red line) for stars younger than 150 Myr (black circles) and older than 500 Myr (green x symbols), as described in Section \ref{['sec:cmds']}. We observe no significant systematic differences between younger and older stars in this particular parameter space, which allows us to assign color-based spectral type estimates independent of stellar ages.