A near-infrared L-band survey of the young embedded cluster NGC 2024

We present the results of the first sensitive L-band (3.4 mu m) imaging stu dy of the nearby young embedded cluster NGC 2024. Two separate surveys of t he cluster were acquired in order to obtain a census of the circumstellar d isk fraction in the cluster. We detected 257 sources to the m(L) less than or equal to 12.0 completeness limit of our similar to 110 arcmin(2) primary survey region. An additional 26 sources with 12.0 < L < 14.0 were detected in the deeper survey of the central similar to 6.25 arcmin(2) region of th e cluster. From an analysis of the JHKL colors of all sources in our larges t area, we find an infrared excess fraction of greater than or equal to 86% +/- 8 %. The JHKL colors suggest that the infrared excesses arise in circu mstellar disks, indicating that the majority of the sources that formed in the NGC 2024 cluster are currently surrounded by, and likely formed with, c ircumstellar disks. The excess fractions remain very high, within the error s, even at the faintest L magnitudes from our deeper surveys, suggesting th at disks form around the majority of the stars in very young clusters such as NGC 2024 independent of mass. From comparison with published JHKL observ ations of Taurus, we find the K-L excess fraction in NGC 2024 to be formall y higher than in Taurus, although both fractions are quite high. Thus, exis ting L-band observations are consistent with a high initial incidence of ci rcumstellar disks in both NGC 2024 and Taurus. Because NGC 2024 represents a region of much higher stellar density than Taurus, this suggests that dis ks may form around most of the YSOs in star-forming regions independent of environment. We find a relatively constant JHKL excess fraction with increa sing cluster radius, indicating that the disk fraction is independent of lo cation in the cluster. In contrast, the JHK excess fraction increases rapid ly toward the central region of the cluster. The most likely cause for this increase is the contamination of the K-band measurements by bright nebulos ity in the central regions of the cluster. This suggests that caution must be applied using only JHK-band observations to infer disk fractions in nebu lous environments. Finally, we identify 45 candidate protostellar sources i n the central regions of the cluster, and we find a lower limit on the prot ostellar phase of early stellar evolution in the NGC 2024 cluster of 0.4-1. 4 x 10(5) yr, similar to that in Taurus.