NEAR-INFRARED IMAGES OF IC-348 AND THE LUMINOSITY FUNCTIONS OF YOUNG EMBEDDED STAR-CLUSTERS

We present the results of a sensitive near-infrared (JHK) imaging survey of the young cluster IC 348 and a nearby control field. From comparison of the cluster and control field observations we estimate that 380 sources, the majority of the stars observed in the cluster field, are members of the cluster. The spatial density of these stars is found to be significantly larger than that typical of classical open clusters but comparable to that which characterizes young embedded clusters such as NGC 2024 and the Trapezium. Overall, we find the surface density distribution of stars in IC 348 to be centrally concentrated and to decrease inversely with distance from the inner (r≈0.1 pc) to the outer (r≈1.0 pc) regions of the cluster. In detail the stellar surface density distribution of this cluster exhibits significant structure. Roughly half the stars are contained within a central subcluster with a radius of 0.5 pc. Outside this half-mass radius, we identify eight small subclusters which contain 10-20 stars and have radii 0.1-0.2 pc in extent. We construct the K luminosity function (KLF) for IC 348 and find it to increase with magnitude in a nonlinear, power-law fashion in the range 8≤mK≤11 mag. The measured slope (0.40) of the power-law portion of the IC 348 KLF is very similar to the slopes (0.37-0.38) determined for the KLFs of four young embedded clusters in Orion. The IC 348 KLF departs from a power-law shape at mK>11 magnitudes, and appears to decrease at magnitudes (mK≈14) near the completeness limit of the survey. We construct evolutionary models for the near-infrared luminosity functions of young (τcl≤107 yr) star clusters containing pre-main-sequence stars. We find that the KLFs of very young synthetic clusters evolve in a systematic and predictable manner as the clusters age. For a fixed IMF, the shape of a cluster KLF depends primarily on the duration of the star formation (τsf) and the age (τcl) of the cluster. In general we find that the luminosity functions of young clusters broaden with age. For coeval models (i.e., τsf≪τcl) the slopes of the power-law portion of the KLFs exhibit significant variation with time, while models with continuous star formation (i.e., τsf≈τcl) maintain more or less constant slopes as they age. The observed similarity between the KLF slopes of numerous embedded clusters and those of the models suggests that uniform, continuous star formation may be characteristic of the star formation histories of many embedded clusters. From comparison of our models with our observations of IC 348 we conclude that star formation in IC 348 has been a continuous process over the last 5-7×106 yr and that the overall rate of star formation and the rate of star formation as a function of mass has been constant over the cluster lifetime. From a comparative analysis of published observations of the Trapezium cluster with our models and observations of IC 348, we find that the underlying mass function of both clusters is similar to the IMF for field stars down to the hydrogen burning limit with little evidence for a significant population of single, lower mass objects (brown dwarfs). In addition we also find that despite the similarities in their mass functions, stellar densities, and sizes, IC 348 and the Trapezium have been characterized by significantly different rates of star formation over their lifetimes. The rate of star formation in the younger Trapezium cluster has been a factor of 20 greater than that in IC 348. Finally, analysis of the JHK colors of the stars in IC 348 reveals that ∼20% of the cluster sources are infrared excess sources. The size of the population of infrared excess sources coupled with the age of the cluster suggests a lifetime of 2-3×106 yr for the (protoplanetary) disk phase of early stellar evolution, consistent with previous estimates.