Right Ascension ct
Figure 4.12 Stars and gas in NGC 2264. The solid contour represents the boundary of emission in this halo population either migrated from the present-day Chamaeleon region or else formed out of molecular gas that has long since vanished.
The fourth HR diagram of Figure 4.9 shows NGC 2264, a populous grouping of stars in Monoceros. In both evolution and morphology, this region represents a transition from a fully embedded to an open cluster. Several hundred optically visible members range in mass from the O7 star S Mon to very late-type T Tauri stars. The system lies at a distance of 800 pc, too far for complete X-ray identification of weak-lined stars. As seen in Figure 4.12, most visible members are crowded into the southern portion of a large molecular cloud. This cloud in turn is part of the Mon OB1 complex, whose boundary we have also included in Figure 1.1. Judging from the slight color excesses observed, the visible cluster sits just in front of the cloud, while a host of embedded infrared stars - some driving vigorous molecular outflows - extend further behind. The cloud itself, some 25 pc long and with a mass of 3 x 104 Mq, conveniently blocks background starlight at optical wavelengths, facilitating study of the cluster members.
The HR diagram of NGC 2264 again shows numerous stars close to the birthline, including now the most massive pre-main-sequence objects near 10 Mq. In addition, there is a clearly defined main sequence, but only down to 3 Mq, corresponding to a spectral type of A0. Less massive stars, with their slower contraction rates, have not yet had time to reach the ZAMS. His-
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