Infrared Signatures of Protoplanetary Disk Evolution

We investigate the observational signatures of a straightforward evolutionary scenario for protoplanetary disks, in which the disk mass of small (≲50 μm) particles decreases homologously with time, but the disk structure and stellar parameters do not change. Our goal is to identify optimal infrared spectral indicators of the existence of disks, their structure, and mass evolution that may be tested with the upcoming SIRTF mission. We present simulated spectral energy distributions (SEDs) and colors over a wide range of masses, 10-8 M☉ ≤ Mdisk ≤ 10-1 M☉. Our Monte Carlo radiative equilibrium techniques enable us to explore the wide range of optical depths of these disks and incorporate multiple, anisotropic dust scattering. The SED is most sensitive to disk mass in the far-IR and longer wavelengths, as is already known from millimeter and radio observations. As the disk mass decreases, the excess emission of the disk over the stellar photosphere diminishes more rapidly at the longest than at short wavelengths. At near-infrared wavelengths, the disk remains optically thick to stellar radiation over a wide range of disk masses, resulting in a slower decline in the SED in this spectral regime. Therefore, near-IR excesses (K-L) provide a robust means of detecting disks in star clusters down to Mdisk ~ 10-7 M☉, while the far-IR excess probes the disk mass, the caveat being that large inner-disk holes can decrease the near-IR disk emission. Various other disk parameters (outer radius, flaring, and dust size distribution) alter the SED quantitatively, but do not change our general conclusions on the evolution of SEDs and colors with the mass of small particles in the disk. Reducing the disk mass results in a clear progression in color-color diagrams, with low-mass disks displaying the bluest colors. We interpret color-color diagrams for Taurus-Auriga sources in the context of decreasing disk mass. Different viewing angles yield degeneracies in the color-mass relationship, but highly inclined disks are very faint and red and are readily identified in color-magnitude diagrams.