INFRARED LUMINOSITY FUNCTIONS FOR THE YOUNG STELLAR POPULATION ASSOCIATED WITH THE L1641 MOLECULAR CLOUD

This contribution reports the results of an extensive near-infrared imaging survey of L1641, the nearest example of a giant molecular cloud. Our survey, which covers 0.77 deg2 (49 pc2) reaches 5 sigma limits at J (1.25 mum), H (1.65 mum), and K (2.2 mum) of 16.8, 15.8, 14.7 mag, reveals (1) a population of approximately 1500 stars (at 5 sigma) spread throughout the cloud (the distributed population); (2) seven small aggregates, each comprised of 10-50 stars whose typical projected surface densities (rho approximately 100 stars pc-2) exceed that of the distributed population by factors ranging from 4 to 9; and (3) a heretofore uncataloged, partially embedded dense cluster (rho approximately 100 stars pc-2) comprised of approximately 150 stars centered on the B4 V star HD 38023. In all cases, the stellar populations are dominated by solar-type PMS stars which appear to contain a mix of objects analogous to weak-line T Tauri stars (objects which lack circumstellar accretion disks) and classical T Tauri stars (stars surrounded by optically thick accretion disks). Analysis of the (J-H), (H-K) color-color diagram for all stars in our sample, and the (H-K), (R-I) color-color diagram for the optically visible subset of our sample, suggests that the seven aggregates contain a significantly higher proportion of stars (approximately 60%) surrounded by circumstellar accretion disks than do the cluster (approximately 40%) or the distributed populations (approximately 30%-36%). The difference in disk frequency among these populations apparently reflects a difference in age, as derived from comparison of reddening-corrected J-band luminosity functions with models calculated by assuming (1) that the stellar masses represented within all populations are drawn from a Scalo initial mass function, (2) that the stars comprising these populations follow the evolutionary tracks computed recently by D'Antona & Mazzitelli, and (3) that all stars in a given population are born simultaneously. The aggregates appear to have formed within t approximately 1 Myr of the current epoch, while the mean age of the distributed and cluster populations is estimated to be t approximately 5 Myr. The distributed population also appears to contain a component of stars with ages at least as great as t approximately 7 Myr. The apparent decrease in accretion disk frequency with increasing mean age is consistent with the hypothesis that most if not all solar-type stars are initially surrounded by disks, and that those disks evolve on time scales on the order of several million years. The observed shape of the J-band luminosity function for the L1641 South cluster also constrains allowable shapes for the stellar initial mass function. Comparison of its observed luminosity function with models computed with a variety of input initial mass functions, shows that the IMF must be closely similar to the Scalo IMF, which peaks near 0.3 M. and declines rapidly toward lower masses. Initial mass functions which are flat below 0.3 M., or rise toward lower masses are clearly excluded.