The Young Cluster NGC 2362

Abstract

An Hα emission survey of the young cluster NGC 2362 resulted in the detection of 130 Hα emission stars in an 11' × 11' field approximately centered on the fourth magnitude O9 Ib multiple star τ CMa. The survey was carried out using the wide-field grism spectrograph on the University of Hawaii 2.2 m telescope and the Gemini Multi-Object Spectrograph (GMOS) on Gemini North. Deep optical VRCIC (to V ~ 23.0) and near-infrared (NIR) photometry (JHK) to K ~ 16 were obtained for several fields within the cluster. Spectra covering the 6000–8000 A region at a resolution of R ~ 3000 (adequate for the determination of Li I λ6708 line strengths) were also acquired for ~200 pre-main-sequence (PMS) candidates with GMOS. Ages and masses for the Hα emitters in NGC 2362 were inferred from the isochrones and evolutionary tracks of D'Antona & Mazzitelli, as well as those of Baraffe et al. An estimated cluster age of ~1.8 Myr follows from the models of D'Antona & Mazzitelli and 3.5–5.0 Myr from those of Baraffe et al. The fraction of the T Tauri star (TTS) population that is composed of weak-line emitters, f(WTTS), is 0.91, compared with 0.43 for the TTS population of NGC 2264. On the basis of W(Hα) alone, the fraction of TTSs still undergoing accretion is 5%–9%, comparable to the inner disk fraction determined from JHKL-band excesses by Haisch and coworkers (12%). Approximately 15% of the PMS sample in this study exhibits possible NIR excess, having EH-K > 0.1 mag. Given the lack of NIR excess and strong Hα emission from the majority of cluster members, it is inferred that the inner disk regions of the TTS population have dissipated significantly. The mean level of chromospheric activity among the WTTS population of NGC 2362 is log(LHα/Lbol) = -3.65, significantly greater than that of the low-mass population of the 600 Myr old Hyades cluster, log(LHα/Lbol) = -3.90. The total mass of the Hα emitters and the OB stellar population of NGC 2362 defines a lower limit for the cluster mass of ~300 M⊙. Allowance for A- and F-type stars still on the radiative track, multiplicity, outlying members, and the low-mass population lying below the completeness limit of the Hα emission survey increases this lower limit to well over 500 M⊙. The derived relaxation, disruption, and evaporation timescales for the cluster imply that NGC 2362 will likely survive beyond the age of the Pleiades, but statistics of galactic cluster lifetimes favor its disruption well before the age of the Hyades.