A Multi-Method Age Determination for the Ursa Major Moving Group

TL;DR

UMa's age has been controversial, with estimates ranging from $200$ Myr to $1$ Gyr. This work constructs the largest Gaia DR3–based kinematic catalog of UMa members (1172 within 100 pc) and applies three independent ages—lithium EW, gyrochronology, and EVA—to identify a dominant coeval population. All methods converge on a robust age of $418^{+32}_{-34}$ Myr for the dominant population, with an approximate $41\%$ contamination among kinematic candidates. The resulting catalog and multi-diagnostic framework provide a solid benchmark for studying stellar rotation, lithium depletion, and early planetary system evolution at this epoch, and set the stage for spectroscopic membership confirmation and refined chemical mapping.

Abstract

The Ursa Major Moving Group (UMa) is one of the closest stellar associations, yet its age has remained controversial, with published estimates ranging from 200 Myr to 1 Gyr. We present a comprehensive age analysis using the largest sample of candidate UMa members to date. Using Gaia DR3, we identify 1172 stars within 100 pc of the Sun with 3D kinematic motions consistent with group membership. We determine the age of UMa's dominant population using three independent methods: lithium equivalent widths $(393.6^{+85.1}_{-80.9}\,\mathrm{Myr})$, gyrochronology $(428 \pm 93\,\mathrm{Myr})$, and photometric variability indicators $(449^{+114}_{-79}\,\mathrm{Myr})$. The three methods converge on a consistent age of $418^{+32}_{-34}\,\mathrm{Myr}$. While our kinematic selection includes field stars that share UMa's space motion but are not coeval members, the convergent age determinations clearly identify a dominant population that formed together approximately 400 Myr ago. These stars are important benchmarks for studies of stellar rotation, magnetic activity evolution, and lithium depletion. The presence of systems such as HD~63433, a young multiplanet host within the group, further illustrates the value of UMa as a laboratory for early planetary system evolution. Our expanded catalog of kinematic candidates lays the groundwork for spectroscopic membership confirmation, refined mapping of the group's structure and chemistry, and future investigations of both stellar and planetary evolution at this key epoch.

Figures (8)

• Figure 1: Color–magnitude diagram for 680 reddening- and extinction-corrected 3D kinematic UMa candidates and white dwarf proper-motion candidates. The white dwarfs are colored by their wdwarfdate age estimate Kiman2022. PARSEC v1.2s parsec2012 isochrones at 400 Myr (solid black line) and 650 Myr (dotted black line) are overplotted for comparison. Shaded background panels denote approximate spectral-type ranges, following the hexadecimal color scheme of Harre2021 for luminosity class V, subclass 5 stars.
• Figure 2: Comparison of UMa 3D kinematic candidates (blue points) with the full sample of Gaia stars within a 100 pc radius (grey points). Upper panel shows Galactocentric UVW velocities with density contour lines demonstrating the over-density around UMa kinematic members.
• Figure 3: EAGLES model output illustrating the lithium equivalent width measurements versus effective temperature for 14 UMa kinematic members. The grey shaded region shows the intrinsic dispersion in lithium equivalent width at the most probable ensemble age of $393.6 ^{+85.1}_{-80.9}$ Myr.
• Figure 4: Example TESS rotation period analysis for the target TIC 258674556. Top row: SPOC Sector 45 results, showing the Lomb–Scargle periodogram (left), the corresponding phase-folded light curve with the best-fit sinusoid (middle), and the normalized light curve with binned points overlaid (right). The dominant periodogram peak occurs at 4.34 days. Bottom row: SPOC Sector 46 results shown in the same format, with a dominant peak at 4.26 days.
• Figure 5: Top: Color–rotation diagram for UMa 3D kinematic candidate members with TESS-derived rotation periods. Yellow points denote sources included in our age analysis; the yellow star indicates HD 63433. Orange squares and circles indicate excluded stars (RUWE cuts and temperature/color cuts, respectively). Shaded sequences show the median $\pm$1 median absolute deviation envelopes for the Pleiades (120 Myr; Rebull16), Praesepe (670 Myr; Douglas19), and UMa. Middle and bottom: Adaptive clustering results for two- and three-cluster cases, respectively. The dominant 428 Myr population remains consistent despite changes in the older clustering.