===================================================================== ==> In this proposal, more time is being requested for SVWB.1YSOSED ===================================================================== Observations of the spectral energy distributions (SEDs) and higher resolution spectra of circumstellar disks will be taken to investigate the physical properties of the disks. The principal goals of the proposal are: 1. To determine if current theories of disk structure and heating properly describe the spectral energy distributions between 5 and 200 microns. 2. To search for evolutionary changes in the disk properties brought about by the formation of planetary systems, gaps and holes in the disks, for example, and the dissipation of the disks. 3. To resolve five outstanding questions about disk properties which can only be assessed through high resolution observations: a. how are the disks heated, b. are the vertical temperature gradients in the disks positive (heating from above) or negative (heating from within by accretion), c. what are the sizes of the disks, d. what are the far infrared optical depths of the disks, and e. are there gaps in the disk structure which can be identified through the SEDs? The current proposal is an extension of an ongoing ISO investigation targetting the same questions. The ongoing investigation derives its source sample entirely from the Chameleon dark cloud, the one available from the fall launch. This proposal extends the investigation to the better studied Taurus dark cloud.