EVIDENCE FOR UNSEEN COMPANIONS AROUND T-TAURI STARS

The observed spectral energy distributions of HK Tau, T Tau, and RY Tau exhibit shallow (but significant) dips at mid-infrared wavelengths. This behavior can be explained by the existence of discrete gaps in their circumstellar disks since, if the temperature in the disks decreases monotonically outward, a gap would result in a range of "missing" temperatures. The gap centers for the three objects occur at radial distances of 0.5, 1.4, and 1.6 AU, respectively, while the corresponding ratios of outer to inner radii of the gaps are 6:1, 7:1, and 15:1, respectively. Larger mid-infrared dips are observed for SU Aur and GM Aur and are interpreted as correspondingly larger gaps, with almost complete clearing of the inner region of the disk in the latter case. The gaps in all cases are consistent with the tidal effects of either companion stars or planets. The dust temperatures at the outer gap boundaries for the five objects are 235 +/- 36 K. It may be significant that the mean value is near the condensation temperature of water under interstellar conditions, an important quantity in models of planet formation.