Constraints on the stellar/substellar mass function in the inner Orion Nebula Cluster

We present the results of a 0." 15-0." 9 FWHM imaging survey at K (2.2 mu m ) and H (1.6 Crm) covering similar to 5.'1 x 5.'1 centered on theta(1)C Ori , the most massive star in the Orion Nebula Cluster (ONC). At the age and d istance of this cluster, and in the absence of extinction, the hydrogen-bur ning limit (0.08 M.) occurs at K approximate to 13.5 mag, while an object o f mass 0.02 M. has K approximate to 16.2 mag. Our photometry is complete fo r source detection at the 7 sigma level to K approximate to 17.5 mag and th us is sensitive to objects as low-mass as 0.02 M. seen through visual extin ction values as high as 10 mag. We use the observed magnitudes, colors, and star counts to constrain the shape of the inner ONC stellar mass function across the hydrogen-burning limit. After determining the stellar age and ne ar-infrared excess properties of the optically visible stars in this same i nner ONC region, we present a new technique that incorporates these distrib utions when extracting the mass function from the observed density of stars in the K-(U-K) diagram. We find that our data are inconsistent with a mass function that rises across the stellar/ substellar boundary. Instead, we f ind that the most likely form of the inner ONC mass function is one that ri ses to a peak around 0.15 M., and then declines across the hydrogen-burning limit with slope N(log M) proportional to M-0.57. We emphasize that our co nclusions apply to the inner 0.71 pc x 0.71 pc of the ONC only; they may no t apply to the ONC as a whole where some evidence for general mass segregat ion has been found.