The low-mass initial mass function in the Orion nebula cluster based on HST/NICMOS III imaging

We present deep HST/NICMOS Camera 3 F110W and F160W imaging of a 26' x 33', corresponding to 3.1 pc x 3.8 pc, non-contiguous field towards the Orion nebula cluster (ONC). The main aim is to determine the ratio of low-mass stars to brown dwarfs for the cluster as a function of radius out to a radial distance of 1.5 pc. The sensitivity of the data outside the nebulous central region is F160W <= 21.0 mag, significantly deeper than previous studies of the region over a comparable area. We create an extinction limited sample and determine the ratio of low-mass stars (0.08-1 M-circle dot) to brown dwarfs (0.02-0.08 M-circle dot and 0.03-0.08 M-circle dot) for the cluster as a whole and for several annuli. The ratio found for the cluster within a radius of 1.5 pc is R-02 = N(0.08-1 M-circle dot)/N(0.02-0.08 M-circle dot) = 1.7 +/- 0.2, and R-03 = N(0.08-1 M-circle dot)/N(0.03-0.08 M-circle dot) = 2.4 +/- 0.2, after correcting for field stars. The ratio for the central 0.3 pc x 0.3 pc region down to 0.03 M-circle dot was previously found to be R-03 = 3.3(-0.7)(+0.8), suggesting the low-mass content of the cluster is mass segregated. We discuss the implications of a gradient in the ratio of stars to brown dwarfs in the ONC in the context of previous measurements of the cluster and for other nearby star forming regions. We further discuss the current evidence for variations in the low-mass IMF and primordial mass segregation.