Infrared Signatures of Protoplanetary Disk Evolution

We investigate the observational signatures of a straightforward evolutionaryscenario for protoplanetary disks in which the disk mass of small (50 micron)particles decreases homologously with time, but the disk structure and stellarparameters do not change. Our goal is to identify optimal infrared spectralindicators of the existence of disks, their structure, and mass evolution thatmay be tested with the upcoming SIRTF mission. We present simulated spectralenergy distributions and colors over a wide range of masses. The SED is mostsensitive to disk mass in the far-IR and longer wavelengths, which is alreadyknown from millimeter and radio observations. As the disk mass decreases, theexcess emission of the disk over the stellar photosphere diminishes morerapidly at the longest rather than at short wavelengths. At near-infraredwavelengths, the disk remains optically thick to stellar radiation over a widerange of disk mass, resulting in a slower decline in the SED in this spectralregime. Therefore, near-IR excesses (K-L) provide a robust means of detectingdisks in star clusters down to 1E-7 solar masses, while the far-IR excessprobes the disk mass. Reducing the disk mass results in a clear progression incolor-color diagrams with low mass disks displaying the bluest colors. Weinterpret color-color diagrams for Taurus-Auriga sources in the context ofdecreasing disk mass.Comment: ApJ Accepte