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K. G. Stassun, R. D. Mathieu (Univ. Wisconsin), F. J. Vrba (USNO), T. Mazeh (Wise Observatory, Israel)
We present new N-band (10 \mum) photometry for a sample of young, low-mass PMS stars in the Orion Nebula Cluster (ONC) for which rotation periods are known and which do not show excess-emission signatures of disks in the near-IR. In the ONC and Tau-Aur regions, a large fraction (~40%) of low-mass stars with photometric rotation periods lack significant near-IR excesses. These stars may still have disks, truncated by stellar magnetospheres and to which the stellar rotation is thereby locked. Our aim is to use the longer wavelength 10 \mum emission as a definitive diagnostic for the presence of such disks.
Including a sample of Tau-Aur stars with existing data in the literature, our study sample consists of 32 stars that are roughly evenly divided between rapid rotators (P\rm rot < 4 days) and slow rotators (P\rm rot > 4 days). We find evidence for mid-IR excess emission in only 4 stars in our sample. While these four stars may possess truncated disks consistent with magnetic disk-locking models, the remaining 28 stars in our sample are evidently diskless. Apparently, stars lacking near-IR excesses in general do not possess disks, truncated or otherwise. This implies that a large fraction of slowly rotating T Tauri stars are not presently disk-locked, because they do not possess disks. Similarly, a large fraction of rapidly rotating stars, some of which rotate near breakup velocity, do not presently have disks that will affect their subsequent angular momentum evolution. Mechanisms other than magnetic disk-locking must be responsible for regulating the evolution of angular momentum between the 1 Myr age of these stars and the main sequence.
We acknowledge support from a Ford Foundation Minority Dissertation Fellowship, a grant from the Wisconsin Space Grant Consortium, and a grant from the National Science Foundation.
The author(s) of this abstract have provided an email address for comments about the abstract: keivan@astro.wisc.edu