Program: GN-2009A-Q-62

Transition objects are pre-main-sequence stars with optically thin inner disks and optically thick outer disks. Different mechanisms have been proposed to explain the inner opacity holes of transition disks: planet formation, grain growth, photoevaporation, tidal truncation in close binaries. These mechanisms, all relevant to disk evolution in general, can be distinguished when disk masses, accretion rates, and multiplicity information are available. This proposal is part of an international collaboration, lead by P.I. L. Cieza, aiming to collect such information for a large sample of Spitzer discovered transition disks in order to (i) establish the relative importance of the mechanisms potentially responsible for their inner holes and (ii) identify systems with strong evidence for ongoing planet formation to be followed-up with Herschel and ALMA. We have searched the catalogs of the Cores to Disks, Gould Belt, and Taurus Spitzer Legacy Projects and have selected a total of 174 transition disks associated with the Perseus, Taurus, Auriga, Serpens, Ophuichus, Scorpius, and Lupus molecular clouds. In order to obtain multiplicity information, accretion rates, and disk masses for our sample, we propose the following set of observations: Laser-Guide-Star Adaptive Optics observations with Gemini North of our 80 northern targets to search for companions down to 0.05''-0.06'' projected separations, corresponding to 7-15 AU at the distance of our objects. These observations will be complemented by VLT observations of the 94 targets with dec < -5. Echelle Optical Spectroscopy with CFHT/Espadons of northern targets (with the exception of faint Sepens objects, which RA is likely to be oversubscribed by a large Espadons program) to estimate accretion rates from the velocity dispersion of the H_alpha line. These observations will also be the first epoch of a radial velocity search for companions at smaller separations than what is reachable by AO imaging. These observations will be complemented by Magellan observations of the southern targets and the fainter Serpens objects. 1.3 mm observations with the SMA}} to measure the mass of 30 transition disks in Ophiuchus (dec ~ -25 deg). We have already submitted a proposal to observe with the IRAM-30 m telescope all the targets that can be reached from Spain (i.e., dec > -15 deg). Objects with dec < -30 will be observed from Chile with APEX. In the near future, Herschel and ALMA studies of systems that are actively forming planets will revolutionize the field of planet formation. The project proposed herein will place our group in a front line position to compete for their observing time. Our team, despite being relatively small, has all the necessary expertise and resources needed to carry out this project (e.g., we have first hand access to the Spitzer catalogs of nearby star forming regions and host country access to all the telescopes in Chile as well as to the IRAM-30 m telescope).