High Spatial Resolution Imaging of Pre–Main‐Sequence Binary Stars: Resolving the Relationship between Disks and Close Companions

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Abstract
In order to study the passive and active effects of young close companion stars and to constrain possible multiple star formation mechanisms, we have carried out a high-resolution imaging program of six multiple T Tauri star systems using the Hubble Space Telescope from 0.3 to 0.9 μm and ground-based speckle imaging from 1 to 2 μm. Each system (GG Tau, UZ Tau, DF Tau, RW Aur, V773 Tau, and V410 Tau) has at least one binary pair with separation between 007 and 04 (10-50 AU). In addition to the previously known companions, a third component is discovered in the V410 Tau system. Many of the companion stars contribute significantly to the unresolved spectral energy distribution and thereby bias the inferred stellar and circumstellar properties. Age is the most systematically biased quantity; neglecting a companion causes an overestimate in age by a factor of ~2 typically and more than 10 in the extreme. Excess emission above the photospheric level is detected at ultraviolet (UV) and near-infrared (NIR) wavelengths for several of the components, which suggests that some close (10-50 AU) T Tauri binaries support at least a minimal circumstellar disk at an age of ~1 Myr. These inner disks not only provide a potential reservoir of material for planetary formation but also may play a key role in the evolution of the components' angular momenta. Possible multiple star formation scenarios are explored in light of the components' relative stellar and circumstellar properties for systems studied here as well as wider and closer systems whose properties are reported in the literature. In summary, the components (1) are coeval, (2) have secondary mass and mass ratio distributions that appear to be independent of both the primary stars' mass and the binary separation, and (3) have UV and NIR excess emission that is either similar or more dominant in the primary. These properties support a scenario in which binary stars form via core fragmentation.

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