Constraints on the Stellar/Substellar Mass Function in the Inner Orion Nebula Cluster

We present the results of a 0.5-0.9" FWHM imaging survey at K (2.2 micron)and H (1.6 micron) covering 5.1' x 5.1' centered on Theta 1C Ori, the mostmassive star in the Orion Nebula Cluster (ONC). At the age and distance of thiscluster, and in the absence of extinction, the hydrogen burning limit (0.08 Mo)occurs at K~13.5 mag while an object of mass 0.02 Mo has K~16.2 mag. Ourphotometry is complete for source detection at the 7 sigma level to K~17.5 magand thus is sensitive to objects as low-mass as 0.02 Mo seen through visualextinction values as high as 10 magnitudes. We use the observed magnitudes,colors, and star counts to constrain the shape of the inner ONC stellar massfunction across the hydrogen burning limit. After determining the stellar ageand near-infrared excess properties of the optically visible stars in this sameinner ONC region, we present a new technique that incorporates thesedistributions when extracting the mass function from the observed density ofstars in the K-(H-K) diagram. We find that our data are inconsistent with amass function that rises across the stellar/sub-stellar boundary. Instead, wefind that the most likely form of the inner ONC mass function is one that risesto a peak around 0.15 Mo, and then declines across the hydrogen-burning limitwith slope N(log M) ~ M^(0.57+/-0.05). We emphasize that our conclusions applyto the inner 0.71 pc x 0.71 pc of the ONC only; they may not apply to the ONCas a whole where some evidence for general mass segregation has been found.