The multiplicity of pre-main-sequence stars in southern star-forming regions

High-resolution studies of young stars in the star-forming regions of Taurus and Ophiuchus have revealed a large population of multiple star systems. To test how applicable this earlier result is for other star-forming regions, we have carried out a K-band (2.2 μm) multiplicity survey of pre-main-sequence stars located in the dark cloud complexes Chameleon, Lupus, and Corona Australis. This survey, which was conducted with both speckle- and direct-imaging techniques, covers a binary star separation range of 0″.1-12″ (15-1800 AU) and identifies 25 companion stars of which nine are new detections. The companion star fraction over the separation range covered by this survey is estimated to be 0.52 ± 0.11, in agreement with Taurus (0.58 ± 0.08) and Ophiuchus (0.50 ±0.12). A comparison of the direct-imaging portion of this survey with Reipurth & Zinnecker's optical multiplicity study reveals that 4% of the overlap sample have "infrared companions," companions too red to be detected at optical wavelengths. This suggests that infrared surveys will systematically measure a slightly higher companion star fraction compared with optical surveys. The result of combining all K-band surveys of dark cloud complexes, which cover the separation range 15-1800 AU, shows a factor of 2 excess of the companion star fraction for young stars compared with that for the solar-type stars in the solar neighborhood (0.54 ± 0.06 vs. 0.26 ± 0.04).