A 10 micron search for truncated disks among pre-main-sequence stars with photometric rotation periods

We use mid-IR (primarily 10 μm) photometry as a diagnostic for the presence of disks with inner cavities among 32 pre-main-sequence stars in Orion and in Taurus-Auriga for which rotation periods are known and for which there is no evidence of inner disks at near-IR wavelengths. Disks with inner cavities are predicted by magnetic disk-locking models that seek to explain the regulation of angular momentum in T Tauri stars. Only three stars in our sample show evidence of excess mid-IR emission. Although these three stars may possess truncated disks consistent with magnetic disk-locking models, the remaining 29 stars in our sample do not. Apparently, stars lacking near-IR excesses in general do not possess truncated disks to which they are magnetically coupled. We discuss the implications of this result for the hypothesis of disk-regulated angular momentum. Evidently, young stars can exist as slow rotators without the aid of present disk locking, and there exist very young stars already rotating at nearly breakup velocity whose subsequent angular momentum evolution will not be regulated by disks. Moreover, we question whether disks, when present, truncate in the manner required by disk-locking scenarios. Finally, we discuss the need for rotational evolution models to take full account of the large dispersion of rotation rates present at 1 Myr; doing so may allow the models to explain the rotational evolution of low-mass pre-main-sequence stars in a way that does not depend on braking by disks.