The T Tauri Star Population of the Young Cluster NGC 2264

Abstract

An H emission survey of the young cluster NGC 2264 in the Mon OB1 association resulted in the detection of 490 H emission stars in a 25 0 ; 40 0 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 C I C ) photometry was obtained for selected fields to supplement similar data from the literature. Spectra covering the 6000-8000 8 region at a resolution of R $ 3000 (adequate for the determination of Li i k6708 line strengths) were obtained for 150 H and X-ray emission sources with the Gemini Multi-Object Spectrograph. Nearinfrared spectra (1-2.5 m) of a number of T Tauri stars (TTSs), X-ray sources, and LH25 (W90) were also obtained using SpeX on the Infrared Telescope Facility. Ages and masses for the H 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 H variability over long baselines in time. About 90% of the classical T Tauri stars (CTTSs) showed changes in W(H) of at least 10%, while 57% varied at levels of 50% or more. No evidence was found for a significant pool of dormant H 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 H 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 À K color and P rot for the CTTSs, in the sense that stars having longer periods tend to have larger H À K colors. A similar positive correlation is found between L H 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 H emitters that are WTTSs, f (WTTS) ¼ N (WTTS)=N (TTS), for clusters of different ages; the relative detectability of H emissio...

Stellar models simulating the disk-locking mechanism and the evolutionary history of the Orion Nebula cluster and NGC 2264

“…To relate this Hα index to some physical property, Lamm (2003) and Lamm et al (2004) (2003), for ctts and wtts the Hα-index values agree well with the W λ (Hα) measurements by Rebull et al (2002) and Dahm & Simon (2005 Unfortunately, the only observational disk indicator for which we have measurements for all stars is Δ[R c − Hα], and this indicator is inconclusive for more than half of our sample stars, classifying 210 objects as intermediate cases. To distinguish these stars as objects with and without disks, we also used the α IRAC index reported by Teixeira et al (2012).…”

“…These IRAC data were obtained in two epochs, March and October 2004, and can help us to distinguish the stars with a disk from diskless objects. In addition, we investigated W λ (Hα), U − V and near-infrared excesses (I − K and H − K) by Rebull et al (2002), near-infrared excess and W λ (Hα) by Dahm & Simon (2005), mid-infrared excess by Cieza & Baliber (2007), and uband exccess by Venuti et al (2014). For disk identification purposes, we used the following values to indicate the presence of a disk: .15, [3.8 − 8.0] ≥ 0.7, and α IRAC > −2.56.…”

“…We can also compare the Hα index with other disk indicators, such as U − V, I − K, H − K excesses (Rebull et al 2002) and the [3.6−8.0] excess (Dahm & Simon 2005). In the ctts group, 43 have measurements of another disk indicator, which for 23 stars (53.5%) indicate the presence of disks.…”

“…Makidon et al (2004) used these same disk indicators to search for eventual correlations with period of NGC 2246 stars, but found no conclusive evidence that slower rotating stars have disk indicators, or that faster rotators are less likely to have disk indicators. Dahm & Simon (2005) analyzed the NGC 2264 rotational data from Lamm et al (2004) and Makidon et al (2004) and found a weak-to-moderate positive correlation between H − K color and rotation periods for the ctts, in the sense that longer period objects tend to have larger H − K colors. They found a similar positive correlation between L Hα , Hα luminosity, and rotation period among the ctts.…”

Spitzerobservations of NGC 2264: the nature of the disk population

“…The most recent distance measurement to the cluster places it at 913 ± 110 pc (Baxter et al 2009) (the value used in this paper); previous estimates of the cluster distance had varied between 700 pc and 800 pc. The oldest members of the cluster are ∼5 Myr old (Dahm & Simon 2005), although there is significant observational evidence of ongoing star formation such as CO outflows (e.g. Margulis et al 1988), Herbig-Haro objects (Reipurth et al 2004), and luminous far-infrared sources detected by the Infrared Astronomical Satellite (Margulis et al 1989).…”