Binary Stars in the Orion Trapezium Cluster Core

Abstract

We have obtained high angular resolution (0.13" FWHM) near-infrared images of the central ~40" × 40" of the Trapezium cluster, using a speckle holography technique that we describe in detail. A search for close binary systems was made in K_s (2.16 μm) and H (1.65 μm) mosaic images, in which 45 and 35 stars were detected, respectively. The sensitivity limits for stellar detections are K_s ~= 14.8 and H ~= 14.9 over the whole mosaics and K_s ~= 16.0 and H ~= 15.9 for those regions of the mosaics where most data were accumulated, thus potentially including objects with substellar masses down to ~0.04 M_⊙. In total, four binary systems were identified with projected linear separations in the range of 0.14"-0.5" (63-225 AU). The resulting binary fraction for low-mass pre-main-sequence stars is 5.9% +/- 4.0%. This fraction agrees well with the binary frequency observed for main-sequence field stars, but is lower by a factor of ~3 than the fraction found from observations of young stars in Taurus-Auriga over the same range of separations. The difference 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 significance level of 96% and suggests that binary frequencies are affected by the local star-forming environment. We show that the massive Trapezium star θ¹ Ori A has a nearby companion separated by ~0.2" (~90 AU). The location of this companion is coincident, within the positional uncertainties, with a nonthermal and variable VLA radio source, which was previously associated with θ¹ Ori A itself. We give H photometry for 32 stars, K_s photometry for 43 stars, and present a color-magnitude diagram for the Trapezium core.