The Evolution of Circumstellar Disks Surrounding Intermediate-mass Stars: IC 1805

We report the results of a study of the intermediate- and high-mass stars in the young, rich star-forming complex IC 1805, based on a combination of optical, near-infrared, and mid-infrared photometry, and classification spectra. These data provide the basis for characterizing the masses and ages for stars more massive than ~2 M_⊙ and enable a study of the frequency and character of circumstellar disks associated with intermediate- and high-mass stars. Optically thick accretion disks among stars with masses 2 < M/M_⊙ < 4 are rare (~2% of members) and absent among more massive stars. A larger fraction (~10%) of stars with masses 2 < M/M_⊙ < 4 appear to be surrounded by disks that have evolved from the initial optically thick accretion phase. We identify four classes of such disks. These classes are based on spectral energy distributions (SEDs) of excess emission above photospheric levels: disks that are (1) optically thin based on the magnitude of the observed excess emission from 2 to 24 μm, (2) optically thin in their inner regions (r < 20 AU) and optically thick in their outer regions, (3) exhibit empty inner regions (r < 10 AU) and optically thin emission in their outer regions, and (4) exhibit empty inner regions and optically thick outer regions. We discuss, and assess the merits and liabilities of, proposed explanations for disks exhibiting these SED types and suggest additional observations that would test these proposals.