9457458 Ghez The study of star formation is often motivated by the desire to explain the formation and evolution of the solar system. Thus, the current paradigm for star formation is centered on the production of a single star that, at a young age, is surrounded by a disk of gas and dust, roughly a few hundred astronomical units (AUs) in size, from which planets may form. However, recent surveys, carried out by Dr. Ghez and others, of the star forming regions of Taurus and Ophiuchus in our Galaxy have led to the conclusion that most, if not all, young low mass stars have companion stars. Furthermore, the majority of these binary stars are separated by distances that are smaller than the size typically assumed for a circumstellar disk. At the distance to Taurus and Ophiuchus, the nearest sites of star formation in our Galaxy (Distance = 150 parsec), the angular extent of a circumstellar disk is expected to be roughly 1-2 seconds of arc. Unfortunately, due to turbulence in the atmosphere, the angular resolution of traditional imaging techniques is limited to 1 second of arc, a factor of 10-30 times worse than the theoretical resolution of a 4-m telescope. Thus, to study these perplexing and yet prevalent close binary stars, the high resolution imaging technique of speckle imaging will be used. The current research effort is focussed on studying the effect of these close binary stars on the formation and evolution of circumstellar disks. The presence of a circumstellar disk and its properties are primarily inferred from the measured spectral energy distribution (SED) of T Tauri stars. In particular, it is the excesses of radiation at ultra-violet and infrared wavelengths that have been attributed to disk processes. More recently, increased efforts in modeling the effects have resulted in the interpretation of dips in the SED as evidence for planetary formation. However, the SED's for close binary stars have so far been the result of observing the co mbined light from both components. In one approach to answering questions regarding the consequence of close companion stars for circumstellar disks, Dr. Ghez is measuring the SED, from optical to infrared wavelengths, of each individual component within these close binary star systems using high spatial resolution imaging techniques. This will allow a determination of (1), how much of the previously observed excess can be attributed to the presence of a secondary source as opposed to a circumstellar disk (2), if the excess remains, whether both components or only one shows evidence for a circumstellar disk and (3), whether or not the dips in the SED are created by the presence of two stars with different apparent temperatures. In a second approach, Dr. Ghez will also compare the binary star frequency of the classical T Tauri stars (CTTS), which are thought to have an inner accretion disk, with that of the weak-lined T Tauri stars (WTTS), which are thought to lack an inner accretion disk. In their initial work, Dr. Ghez and her collaborators observed a distinction at the 3 sigma level; at separations less than 35 AU binary stars are more frequent among the WTTS than the CTTS. These observations suggest that close companions significantly interfere with the disk accretion process associated with many T Tauri stars. This result is an exciting possibility but requires follow up observations of more T Tauri stars to gain statistical significance. This will be accomplished by (1), expanding the sample of observed T Tauri stars to additional star forming regions and to fainter systems with the use of a more sensitive camera and (2), eventually using larger telescopes (Diameter > 5-m, such as the Keck telescope) to probe this relationship further. If Dr. Ghez' hypothesis is correct, then the difference between the CTTS and WTTS binary star populations should increase at smaller separations. This award is to recognize an outstanding young faculty member in s cience and engineering. The award will enhance the career of the faculty member by providing flexible support for research and educational activities. Cooperation with industry and institutions that support research and education is encouraged.
