The Evolution of the Multiplicity of Embedded Protostars. I. Sample Properties and Binary Detections

We present the observational results of a near-infrared survey of a large sample of Class I protostars designed to determine the Class I binary separation distribution from ~100 AU to ~5000 AU. We have selected targets from a new sample of 267 nearby candidate Class I objects. This sample is well understood, consists of mostly Class I young stellar objects (YSOs) within 1 kpc, has targets selected from the whole sky, and is not biased by previous studies of star formation. We have observed 189 Class I YSOs north of δ = -40° at the H, K, and L' bands, with a median angular resolution of 0farcs33 at L'. We determine our detection limit for close binary companions by observing artificial binaries. We choose a contrast limit and an outer detection limit to minimize contamination and to ensure that a candidate companion is gravitationally bound. Our survey uses observations at the L' rather than the K band for the detection of binary companions since there is less scattered light and better seeing at L'. This paper presents the positions of our targets, the near-IR photometry of sources detected in our fields at L', as well as the observed properties of the 89 detected companions (73 of which are newly discovered). Although we have chosen contrast and separation limits to minimize contamination, we expect that there are about six stars identified as binary companions that are due to contamination. Finder charts at L' for each field are shown to facilitate future studies of these objects.

The Infrared Telescope Facility is operated by the University of Hawaii under Cooperative Agreement no. NCC 5-538 with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program. The United Kingdom Infrared Telescope is operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the U.K. Based in part on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.