The low mass IMF in young open clusters.

Abstract

We present the results of the investigation of the Initial Mass Function at the end of the Main Sequence in young open clusters. We find that over a large range in age and environment the IMFs are similar to each other, and to recent determinations of the field star IMF. We have obtained V, I, and K band photometry of fields in the three relatively unembedded open clusters. The photometry reaches down to various masses in ech cluster: 0.08Mʘ for Praesepe, 0.04Mʘ for the Pleiades, and 0.15Mʘ for NGC 7160. We compare the methods for estimating the masses of young, embedded stars developed by Comeron et al. (1993) and by Strom et al. (1995) and show them to be in good agreement. Spectra in the 2 11m region of six low mass objects from Comeron et al. (1993) are also in agreement with the mass estimates using these methods. The spectrum of a brown dwarf candidate is used to place an upper limit on its mass of 60% of the minimum required for hydrogen burning. The IMFs from these four clusters plus NGC 2024 (Cameron et al. (1995)) are shown to be in agreement with each other. The composite MF can be fitted with a power law between 0.04 and 0.5Mʘ with a slope of -0.75 ± 0.3. There is no evidence for a cutoff at the bottom of the main sequence (0.08Mʘ); brown dwarfs appear to be abundant in open clusters. However, the slope of the MF is well above the value of ≲ -2 required for very low mass stars and brown dwarfs to contribute a significant portion of the mass of open clusters. The composite cluster MF also is in agreement with recent determinations of the field star IMF for stellar masses. The field star data do not extend into the brown dwarf range; however, if we extrapolate in accordance with the cluster MF, we conclude that brown dwarfs probably do not contribute significantly to the dark matter.