The Frequency and Effect of Young Companion Stars

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Session 99 -- Star Formation
Invited presentation, Wednesday, 11, 1995, 3:40pm

[99.01] The Frequency and Effect of Young Companion Stars

A. Ghez (UCLA)

Our current paradigm for star formation is centered on the formation of a single star that, at a young age, is surrounded by a disk of gas and dust, roughly a few hundred AU in size, from which planets may form. However the results of recent surveys carried out with a variety of techniques (spectroscopy, lunar occultation, speckle imaging, and direct imaging) in a number of different star forming regions suggest that most, if not all, T Tauri stars have companions. In the separation range 3 - 200 AU the observed pre-main sequence binary star frequency is a factor of 2-4 greater than that of the solar-type main sequence stars. The prevalence of multiple star systems at the T Tauri stage of evolution, roughly a few million years, suggests the star formation processes produce primarily multiple star systems.

Companion stars both passively and actively affect T Tauri star models. A nearby source when unaccounted for in photometric measurements can bias determinations of T Tauri star properties, such as age. They can also affect the circumstellar disk properties derived from modeling spectral energy distributions. Given that the majority of the observed close binary star systems have separations smaller than the size typically assumed for a circumstellar disk, these sources are expected to play an important role in the formation and evolution of a circumstellar disk. The hypothesis of a destructive companion star-disk interaction is supported by observations of lower disk masses for close binary star systems compared to single or wide (larger than a few 100 AU) binary star systems, as well as evidence of disk clearing due to a close companion in the individual cases of a number of stars. Rapid erosion of a circumstellar disk as a result of these interactions could seriously compromise the potential for planetary formation in such close binary star systems.

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