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Abstract Recently Gagné et al. suggested that young moving groups with similar kinematic properties could be part of larger dissolving structures. One example was IC 2602 as the core of a group of associations, including its corona (CIC 2602), Tucana-Horologium (THA), and parts of Theia 92. We explore this hypothesis by measuring the rotation periods of 953 objects selected using Gaia DR3 kinematics from IC 2602, CIC 2602, Theia 92, and a newly identified group of stars that bridge IC 2602 and THA. We use Transiting Exoplanet Survey Satellite (TESS) full frame images to measure new rotation periods and combine these with the rotation periods for THA from Popinchalk et al. to compare their rotation period distributions and other youth indicators where available to examine if the groups could be coeval. We find strong agreement between the rotation distributions of IC 2602, CIC 2602, and THA, suggesting a shared age of ∼40 Myr, and which in combination could serve as an example of a typical distribution at this age. Theia 92 does not agree at the same level, and we explore the potential kinematic reasons it does not match the rotation period distribution of the larger groups. Additionally, in our light curve analysis we identify ∼50 potential binaries, as well as four new M dwarf complex rotators that show major morphological changes between TESS cycles. Finally, using the amplitudes of the rotation periods we measured, we find strong agreement with the amplitude–age relation presented in Morris for our 40 Myr groups. 

Abstract Y dwarfs, the coolest known spectral class of brown dwarfs, overlap in mass and temperature with giant exoplanets, providing unique laboratories for studying low-temperature atmospheres. However, only a fraction of Y dwarf candidates have been spectroscopically confirmed. We present Keck/NIRES near-infrared spectroscopy of the nearby (d≈ 6–8 pc) brown dwarf CWISE J105512.11+544328.3. Although its near-infrared spectrum aligns best with the Y0 standard in theJband, no standard matches well across the fullYJHKwavelength range. The CWISE J105512.11+544328.3 NH₃-H= 0.427 ± 0.0012 and CH₄-J= 0.0385 ± 0.0007 absorption indices and absolute Spitzer [4.5] magnitude of 15.18 ± 0.22 are also indicative of an early-Y dwarf rather than a late-T dwarf. CWISE J105512.11+544328.3 additionally exhibits the bluest Spitzer [3.6]−[4.5] color among all spectroscopically confirmed Y dwarfs. Despite this anomalously blue Spitzer color given its low luminosity, CWISE J105512.11+544328.3 does not show other clear kinematic or spectral indications of low metallicity. Atmospheric model comparisons yield a log(g) ≤ 4.5 andT_eff≈ 500 ± 150 K for this source. We classify CWISE J105512.11+544328.3 as a Y0 (pec) dwarf, adding to the remarkable diversity of the Y-type population. JWST spectroscopy would be crucial to understanding the origin of this Y dwarf’s unusual preference for low-gravity models and blue 3–5μm color.