The Multiplicity of Pre–Main‐Sequence Stars in Southern Star‐forming Regions

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Abstract

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 km) multiplicity survey of premain-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 (151800 AU) and identies 25 companion stars of which nine are new detections. The com-0A .112A panion 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 & Zinneckers 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 151800 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).

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