The Young Cluster NGC 2362

Abstract

An H{α} emission survey of the young cluster NGC 2362 resulted inthe detection of 130 H{α} emission stars in an11^{'}{\times}11^{'} field approximately centered on thefourth magnitude O9 Ib multiple star {τ} CMa. The survey was carriedout using the wide-field grism spectrograph on the University of Hawaii2.2 m telescope and the Gemini Multi-Object Spectrograph (GMOS) onGemini North. Deep optical VR_{C}I_{C} (to V~23.0) andnear-infrared (NIR) photometry (JHK) to K~16 were obtained for severalfields within the cluster. Spectra covering the 6000-8000 {Å} regionat a resolution of R~3000 (adequate for the determination of Li I{λ}6708 line strengths) were also acquired for ~200pre-main-sequence (PMS) candidates with GMOS. Ages and masses for theH{α} emitters in NGC 2362 were inferred from the isochrones andevolutionary tracks of D'Antona {\amp} Mazzitelli, as well as those ofBaraffe et al. An estimated cluster age of ~1.8 Myr follows from themodels of D'Antona {\amp} Mazzitelli and 3.5-5.0 Myr from those ofBaraffe et al. The fraction of the T Tauri star (TTS) population that iscomposed of weak-line emitters, f(WTTS), is 0.91, compared with 0.43 forthe TTS population of NGC 2264. On the basis of W(H{α}) alone, thefraction of TTSs still undergoing accretion is 5%-9%, comparable to theinner disk fraction determined from JHKL-band excesses by Haisch andcoworkers (12%). Approximately 15% of the PMS sample in this studyexhibits possible NIR excess, having E_{H-K}{\gt}0.1 mag. Giventhe lack of NIR excess and strong H{α} emission from the majority ofcluster members, it is inferred that the inner disk regions of the TTSpopulation have dissipated significantly. The mean level ofchromospheric activity among the WTTS population of NGC 2362 islog(L_{Halpha}/L_{bol})=-3.65, significantly greaterthan that of the low-mass population of the 600 Myr old Hyades cluster,log(L_{Halpha}/L_{bol})=-3.90. The total mass of theH{α} emitters and the OB stellar population of NGC 2362 defines alower limit for the cluster mass of ~300 M_{solar}. Allowancefor A- and F-type stars still on the radiative track, multiplicity,outlying members, and the low-mass population lying below thecompleteness limit of the H{α} emission survey increases this lowerlimit to well over 500 M_{solar}. The derived relaxation,disruption, and evaporation timescales for the cluster imply that NGC2362 will likely survive beyond the age of the Pleiades, but statisticsof galactic cluster lifetimes favor its disruption well before the ageof the Hyades.