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K.G. Stassun (University of Wisconsin)
Circumstellar disks have come to be seen as dominant players in the rotational evolution of low-mass stars during the pre-main-sequence (PMS) phase. In fact, most rotational evolution models rely on magnetic disk-locking to connect the rotational properties of T Tauri stars (TTS) to those of zero-age main sequence (ZAMS) stars. The principal aim of this dissertation is to summarize our recent observations that challenge this picture of disk-regulated PMS rotational evolution.
As the foundation for our investigation, we present rotation periods for 254 PMS stars in Orion. We find that a population of stars rotating near breakup velocity is already present at 1 Myr. We do not find evidence for a bimodal distribution of rotation periods. We find no correlation between rotation period and the presence of near-IR signatures of disks.
We do not find compelling agreement between our observations and the requirements of the disk-locking hypothesis. Near-IR photometry suggests that inner cavities in TTS disks are typically much smaller than allowed by theory for the regulation of stellar angular momentum. Furthermore, we use 10 \mum photometry to confirm that TTS lacking near-IR excesses are in fact diskless, and do not possess disks with large inner cavities. Evidently, many young stars can exist as slow rotators without the aid of present disk-locking, and there exist very young stars already rotating near breakup velocity whose subsequent angular momentum evolution will not be regulated by disks.
Finally, we discuss the need for rotational evolution models to take full account of the large dispersion of rotation rates present at 1 Myr, which may allow the models to explain the rotational evolution of low-mass pre-main sequence stars in a way that does not depend upon braking by disks.
This dissertation was funded by fellowships from the National Science Foundation, the National Academy of Sciences, and the University of Wisconsin. Additional support was provided by the Wisconsin Space Grant Consortium, Sigma Xi, and NOAO.
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