The T Tauri Star Population of the Young Cluster NGC 2264

An Hα emission survey of the young cluster NGC 2264 in the Mon OB1 association resulted in the detection of 490 Hα emission stars in a 25' × 40' field approximately centered between the O7 V multiple star S Mon and the Cone Nebula. The survey was carried out with the wide-field grism spectrograph (WFGS) on the University of Hawaii 2.2 m telescope on Mauna Kea. X-ray observations made with the European Photon Imaging Camera on board the European Space Agency's XMM-Newton satellite observatory will be discussed in a subsequent paper. Optical (BVR_CI_C) photometry was obtained for selected fields to supplement similar data from the literature. Spectra covering the 6000–8000 Å region at a resolution of R ∼ 3000 (adequate for the determination of Li I λ6708 line strengths) were obtained for 150 Hα and X-ray emission sources with the Gemini Multi-Object Spectrograph. Near-infrared spectra (1–2.5 μm) of a number of T Tauri stars (TTSs), X-ray sources, and LHα25 (W90) were also obtained using SpeX on the Infrared Telescope Facility. Ages and masses for the Hα emitters were inferred from the isochrones and evolutionary tracks of D'Antona & Mazzitelli. The median age for the TTS population is about 1.1 Myr, but a considerable dispersion, from 0.1 to 5 Myr, exists for individual objects. Several fields in the cluster were observed with the WFGS on more than one occasion, permitting an examination of Hα variability over long baselines in time. About 90% of the classical T Tauri stars (CTTSs) showed changes in W(Hα) of at least 10%, while 57% varied at levels of 50% or more. No evidence was found for a significant pool of dormant Hα emitters. Summing the masses of the TTSs and the OB stellar population of NGC 2264, a lower limit for the total stellar mass content of the cluster is about 430 M_⊙. This is less than 1% of the total mass of the atomic and molecular gas believed to be associated with NGC 2264. Evidence for hierarchical structure within the cluster is suggested by the spatial distribution of TTSs. Four concentrations of Hα emitters are evident: two near S Mon and two near the Cone Nebula. The median age of the TTSs in the immediate vicinity of S Mon was found to be greater than that of the TTSs near Allen's infrared source (IRS-1), but a significant dispersion is present. From the rotational data of Lamm et al. and Makidon et al., 241 of the TTSs are periodic variables, 150 weak-line T Tauri stars (WTTSs) and 91 CTTSs, while 123 stars are irregular variables (30 WTTSs and 93 CTTSs). A weak-to-moderate positive correlation is found between H - K color and P_rot for the CTTSs, in the sense that stars having longer periods tend to have larger H - K colors. A similar positive correlation is found between L_Hα and P_rot among the CTTSs. No statistically significant correlation is found between P_rot and theoretical age or between P_rot and L_X. Other topics discussed include the fraction of Hα emitters that are WTTSs, f(WTTS) = N(WTTS)/N(TTS), for clusters of different ages; the relative detectability of Hα emission using WFGS and narrowband filter imaging techniques; and the correlation of W(Li I) with T_e, age, H - K color, and W(Hα).