Charles J. Lada

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 μm) imaging study of the nearby young embedded cluster NGC 2024. Two separate surveys of the cluster were acquired in order to obtain a census of the circumstellar disk fraction in the cluster. We detected 257 sources to the mL ≤ 12.0 completeness limit of our ~110 arcmin2 primary survey region. An additional 26 sources with 12.0 < L < 14.0 were detected in the deeper survey of the central ~6.25 arcmin2 region of the cluster. From an analysis of the JHKL colors of all sources in our largest area, we find an infrared excess fraction of ≥86% ± 8%. The JHKL colors suggest that the infrared excesses arise in circumstellar disks, indicating that the majority of the sources that formed in the NGC 2024 cluster are currently surrounded by, and likely formed with, circumstellar disks. The excess fractions remain very high, within the errors, even at the faintest L magnitudes from our deeper surveys, suggesting that disks form around the majority of the stars in very young clusters such as NGC 2024 independent of mass. From comparison with published JHKL observations of Taurus, we find the K-L excess fraction in NGC 2024 to be formally higher than in Taurus, although both fractions are quite high. Thus, existing L-band observations are consistent with a high initial incidence of circumstellar disks in both NGC 2024 and Taurus. Because NGC 2024 represents a region of much higher stellar density than Taurus, this suggests that disks may form around most of the YSOs in star-forming regions independent of environment. We find a relatively constant JHKL excess fraction with increasing cluster radius, indicating that the disk fraction is independent of location in the cluster. In contrast, the JHK excess fraction increases rapidly toward the central region of the cluster. The most likely cause for this increase is the contamination of the K-band measurements by bright nebulosity in the central regions of the cluster. This suggests that caution must be applied using only JHK-band observations to infer disk fractions in nebulous environments. Finally, we identify 45 candidate protostellar sources in the central regions of the cluster, and we find a lower limit on the protostellar phase of early stellar evolution in the NGC 2024 cluster of 0.4–1.4 × 105 yr, similar to that in Taurus.