A Mid-Infrared Study of the Young Stellar Population in the NGC 2024 Cluster

We present the results of the first broadband N (10.8 μm) survey of the NGC 2024 cluster. The mid-infrared data were combined with our previously published JHKL photometry to construct spectral energy distributions for all detected sources. The main scientific goals were to investigate the nature of the young stellar objects (YSOs) in the cluster and to examine the efficiency of detecting circumstellar disk sources from near-infrared JHKL color-color diagrams. Out of 59 sources surveyed having K-band (2.2 μm) magnitudes m_K ≤ 10.5, we detected 35 (∼59%) at 10 μm. Combining these detections and upper limits for the nondetections with existing JHKL data, we identify one Class I, six flat-spectrum, 28 Class II, and five Class III sources. We find a circumstellar disk fraction for NGC 2024 of ∼85% ± 15%, which confirms earlier published suggestions that the majority, if not all, of the stars in the NGC 2024 cluster formed with disks, and these disks still exist at the present time. In addition, all but one of the disk sources identified in our survey lie in the infrared-excess region of the JHKL color-color diagram for the NGC 2024 cluster. This demonstrates that JHKL color-color diagrams are extremely efficient in identifying YSOs with circumstellar disks. Of the 14 sources in our survey with K-L colors suggestive of protostellar objects, ∼29% are protostellar in nature, while ∼7% are true Class I sources. This may be due to extinction producing very red K-L colors in Class II YSOs, thus making them appear similar in color to protostars. This suggests that caution must be applied when estimating the sizes and lifetimes of protostellar populations within star-forming regions based on K-L colors alone. A comparison of the ratio of the number of Class I and flat-spectrum sources to the number of Class II and III sources in NGC 2024, ρ Oph, and Taurus-Auriga indicates that NGC 2024 and ρ Oph have similar ages, while Taurus-Auriga is an older region of star formation, consistent with published T Tauri star ages in each region. Finally, we calculate the luminosities of the Class II sources in NGC 2024, ρ Oph, and Taurus and discuss the results.