Observational Properties of Protoplanetary Disk Gaps

Abstract

We study the effects of an annular gap induced by an embedded proto-planet on\ndisk scattered light images and the infrared spectral energy distribution. We\nfind that the outer edge of a gap is brighter in the scattered light images\nthan a similar location in a gap-free disk. The stellar radiation that would\nhave been scattered by material within in the gap is instead scattered by the\ndisk wall at the outer edge of the gap, producing a bright ring surrounding the\ndark gap in the images. Given sufficient resolution, such gaps can be detected\nby the presence of this bright ring in scattered light images. A gap in a disk\nalso changes the shape of the SED. Radiation that would have been absorbed by\nmaterial in the gap is instead reprocessed by the outer gap wall. This leads to\na decrease in the SED at wavelengths corresponding to the temperature at the\nradius of the missing gap material and a corresponding flux increase at longer\nwavelengths corresponding to the temperature of the outer wall. We note that,\nunlike an inner hole in the disk, the presence of an annular gap does not\nchange the bolometric IR flux; it simply redistributes the radiation,\npreviously produced by material within the gap, to longer wavelengths. This\nimplies that the changes in the SED generally will be smaller for gaps than\nholes. Although it will be difficult on the basis of the SED alone to\ndistinguish between the presence of a gap and other physical effects, the level\nof changes can be sufficiently large to be measurable with current instruments\n(e.g., Spitzer).\n