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Abstract
Young binary stars offer unique advantages as laboratories for the study of binary formation and early the evolution of stars in general. The recent development of high-resolution imaging techniques for the near-infrared has enabled rapid progress to be made in the study of these objects. It is possible for the first time to examine directly the components of numerous pre-main sequence binary stars whose separation is $\sbsp{\sim}{<}$100 AU, using infrared brightness ratios, knowledge of the spectral type of the primary, and broadband photometry to estimate the masses, ages, and luminosities of the components. Because 100 AU is the fiducial size of a circumstellar disk, it is likely that there are strong interactions between the stellar components and the disks in systems within reach of the new techniques.
This thesis is an investigation into the properties of the stellar and disk components of young binary stars, with an emphasis on the diagnostics of the effects of star-disk interactions. It begins with an overview of the workings and capabilities of speckle interferometry, which is the most important high-resolution imaging technique for the study of young stars in the near-infrared. Observational studies are presented for four young multiple-star systems whose properties appear to be inconsistent with the predictions of simple theories of stellar and binary formation. Following this is a survey of the properties of the numerous low-mass pre-main sequence (T Tauri) binaries in the Taurus star-forming region whose broadband spectra, from the visual region through the far-infrared, can be modeled as emission from stellar photospheres surrounded by optically-thick disks. Finally, there is a discussion of the properties of a group of six T Tauri stars with "infrared companions" which exhibit near-infrared color temperatures too low to be understood in terms of these simple models.
Young binaries are found to exhibit a rich variety of phenomena related to circumstellar disks, including rapid disk accretion, heavily-extincted 'infrared' companions, and infrared nebulae. Active disks around one or both stars in a binary may be responsible for many of the phenomena characteristic of these young systems.