Binary Stars in the Orion Trapezium Cluster Core

Abstract

We have obtained high angular resolution FWHM) near-infrared images of the central (0A .13 D40A ] 40A of the Trapezium cluster, using a speckle holography technique that we describe in detail. A search for close binary systems was made in (2.16 km) and H (1.65 km) mosaic images, in which 45 K s and 35 stars were detected, respectively. The sensitivity limits for stellar detections are and K s ^ 14.8 H ^ 14.9 over the whole mosaics and and H ^ 15.9 for those regions of the mosaics where K s ^ 16.0 most data were accumulated, thus potentially including objects with substellar masses down to D0.04 In total, four binary systems were identiÐed with projected linear separations in the range of M _. (63È225 AU). The resulting binary fraction for low-mass preÈmain-sequence stars is 0A .14È0A .5 5.9% ^ 4.0%. This fraction agrees well with the binary frequency observed for main-sequence Ðeld stars, but is lower by a factor of D3 than the fraction found from observations of young stars in Taurus-Auriga over the same range of separations. The di †erence in binary frequency between the core of the Trapezium cluster and the low-mass, low stellar density dark cloud Taurus-Auriga is established at a statistical signiÐcance level of 96% and suggests that binary frequencies are a †ected by the local star-forming environment. We show that the massive Trapezium star h1 Ori A has a nearby companion separated by (D90 AU). The location of this companion is coincident, within the positional uncer-D0A .2 tainties, with a nonthermal and variable VLA radio source, which was previously associated with h1 Ori A itself. We give H photometry for 32 stars, photometry for 43 stars, and present a color-magnitude K s diagram for the Trapezium core.