Crowning the Queen: Membership, Age, Rotation, and Activity for the Open Cluster Coma Berenices

TL;DR

This study establishes a Gaia-based membership catalog for the nearby open cluster Coma Berenices, identifying 291 members (262 high-confidence, 29 candidate) and 9 non-members, enabling a precise CMD, metallicity, and reddening assessment. The cluster is found to be solar metallicity with Av ≈ 0, and an isochrone age of 675 ± 100 Myr, corroborated by independent white-dwarf dating (~560 ± 40 Myr). By combining literature rotation periods with new measurements from TESS and ZTF, the authors derive rotation periods for 161 members (137 new), extending the rotational census into late-type M dwarfs and computing Rossby numbers using τ from empirical relations. Chromospheric Hα and coronal X-ray data, drawn from extensive optical spectroscopy and serendipitous X-ray detections, reveal that Coma Ber’s activity versus rotation roughly matches that of Praesepe and Hyades at this age, with metallicity differences influencing rotation distributions in color space but not activity due to magnetic saturation. The findings solidify Coma Ber as a benchmark for age-rotation-activity studies, illustrating how saturation can mask metallicity-driven differences in stellar magnetic evolution while still allowing rotation-based gyrochronology to be effective near solar metallicity.

Abstract

Despite being only 85 pc away, the open cluster Coma Berenices (Coma Ber) has not been extensively studied. This is due in part to its sparseness and low proper motion, which together made Coma Ber's membership challenging to establish. Gaia data for $\approx$400 previously cataloged candidate cluster stars allowed us to identify $\approx$300 as members. With [Fe/H] measurements for nine members, we found that Coma Ber has a solar metallicity, and then fit isochrones to its color--magnitude diagram to determine that it is 675$\pm$100 Myr old. With photometry obtained by the Transiting Exoplanet Survey Satellite (TESS) and Zwicky Transient Facility (ZTF), we measured rotation periods for 137 of Coma Ber's low-mass stars, increasing the sample of members with measured periods by a factor of six, and extending the rotational census for the cluster from its late F stars through to its fully convective M dwarfs. By measuring the equivalent width of the H$α$ line for $\approx$250 stars and collecting X-ray detections for $\approx$100 ($\approx$85% and $\approx$33% of the cluster's members, respectively), we characterized magnetic activity in Coma Ber and examined the dependence of chromospheric and of coronal activity on rotation in these stars. Despite having a metallicity that is 0.2 dex below that of their coeval cousins in Praesepe and the Hyades, low-mass stars in Coma Ber seem to follow a similar rotation--activity relation. In detail, however, there are differences that may provide further insight into the impact of metallicity on this still poorly understood relation.

Figures (24)

• Figure 1: CMD for the $\approx$400 candidate members in our initial Coma Ber catalog, a merger of pre-existing catalogs for which we updated the astrometry and photometry to the Gaia DR3 values. Spectral types are indicated along the top axis for reference. The cyan stars appear in more than one of these catalogs. As discussed in the text, this CMD shows important contamination from background giants lacking parallaxes that were included in the kraus2007b catalog. These stars are identified as non-members in Figure \ref{['fig:parallax']}.
• Figure 2: Flowchart illustrating our approach to classifying the 379 stars in our initial Coma Ber catalog. We first followed the path indicated by the thick arrows, which allowed us to define a velocity core of 140 high-confidence members for the cluster, before considering the membership status of stars either lacking RVs or whose RVs have large errors ($>$5 km s$^{-1}$).
• Figure 3: An illustration of the discriminating power of Gaia data: DR3 parallax distribution for the 379 stars in our initial Coma Ber catalog. Stars to the left of the $\varpi$ = 8 mas line are likely non-members, with the clump with $\varpi\ {\hbox{$\buildrel <\over\sim$}}\ 2$ mas likely to be background giants that contaminated the kraus2007b catalog because of the cluster's relatively low proper motion compared to other field stars
• Figure 4: U, V, and W velocities for the 181 stars with $\varpi$$>$ 8 mas and Gaia RV measurements in our initial catalog. The gold stars are the 140 stars that form the cluster's velocity core and are classified as high-confidence members, while the nine light blue points are UVW outliers and therefore candidate non-members. We used the proper motions of the 140 high-confidence members to select members from the 119 stars with $\varpi$$>$ 8 mas but lacking RVs, and from the 24 stars having RVs with RV$_{\rm err} > 5$ km s$^{-1}$ (see Figure \ref{['fig:pms']}).
• Figure 5: Top---Positions for the 140 stars confirmed as high-confidence members of Coma Ber based on their UVW velocities (see Figure \ref{['fig:uvw']}). We used this distribution to test the membership of the 143 stars with $\varpi$$>$ 8 mas but lacking or having unreliable RVs in our sample. Bottom---Positions for those 143 stars. The 122 stars whose positions and proper motions all fell within the range defined by the astrometry for the UVW high-confidence members were also classified as high-confidence members and are shown as gold stars. The 21 purple points are positional and/or proper-motion outliers, and are classified as candidate members until their RVs can be obtained.