Star Formation Environments and the Distribution of Binary Separations

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Abstract
We have carried out K-band speckle observations of a sample of 114 X-ray-selected weak-line T Tauri stars in the nearby Scorpius-Centaurus OB association. We find that for binary T Tauri stars closely associated with the early-type stars in Upper Scorpius, which is the youngest subgroup of the OB association, the peak in the distribution of binary separations is at 90 AU. For binary T Tauri stars located in the direction of an older subgroup but not closely associated with early-type stars, the peak in the distribution is at 215 AU. A Kolmogorov-Smirnov test indicates at a significance level of 98% that the two binary populations do not result from the same distribution. Apparently, the same physical conditions that facilitate the formation of massive stars also facilitate the formation of closer binaries among low-mass stars, whereas physical conditions unfavorable for the formation of massive stars lead to the formation of wider binaries among low-mass stars. The outcome of the binary formation process might be related to the internal turbulence and the angular momentum of molecular cloud cores, to the magnetic field, to the initial temperature within a cloud, or—most likely—to a combination of all of these. We conclude that the distribution of binary separations is not a universal quantity and that the broad distribution of binary separations observed among main-sequence stars can be explained by a superposition of more peaked binary distributions resulting from various star-forming environments. The overall binary frequency among pre-main-sequence stars in individual star-forming regions is not necessarily higher than that among main-sequence stars.
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