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Abstract
To understand the physical process of star formation, we address three questions: (1) what are the critical indicators of evolutionary states of young stellar objects? (2) when and how are dense cores dissipated? and (3) how are young stars distributed in a single molecular cloud? We selected 224 IRAS sources from the IRAS coadded images in dark cloud L1641. The IRAS data analysis suggested that the 122 sources inside L1641 are probably young stellar objects, while the sources outside L1641 are probably infrared cirrus. In $\sp{13}$CO and HCO$\sp+$ millimeter observations, we found that most of the IRAS sources with CO outflows are associated with molecular dense cores, while most nonoutflow sources are not. This can be explained if dense cores are dissipated by the outflows before the driving sources become pre-main-sequence stars. Using near-IR array detectors, we imaged 59 IRAS sources in H and $K\sp\prime$, some of them also were imaged in narrow-band L and M. The most important result from the imaging is the detection of small groupings of 10-40 near-IR sources near 14 IRAS sources. These groupings, defined as stellar density enhancements (SDEs), are statistically significant compared to background stars. Most of the near-IR sources within the SDEs are low-mass pre-main-sequence stars physically associated with the IRAS sources. The SDEs probably result from pile-up of young stars due to continuous star formation within the dense cores, a phenomenon seen in the early stage of star formation. Our images showed that the young stars in L1641 can be characterized by a range of stellar densities, from a large cluster (the Trapezium Cluster) to the SDEs and to individual stars. The previously proposed cluster and isolated star formation modes are probably the high and low ends of the continuous distribution. With the near-IR imaging, we were able to identify the probable near-IR counterparts for 55 of the 59 IRAS sources. Forty-five of them are class I sources, and the other 10 sources are class II sources.
