Other
Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........9s&link_type=abstract
PhD Thesis, University of Wisconsin---Madison, USA, 2000.
Other
1
Scientific paper
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 today rely chiefly on magnetic disk-locking to successfully 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 recent observations (Stassun et al. 1999; Stassun et al. 2000) that challenge this picture of disk-regulated PMS rotational evolution. We present photometrically derived rotation periods for 254 stars in an area 40 × 80 arcmin centered on the Orion Nebula. We show that these stars are likely members of the young (~106 yr) Orion OBIc/d association. The rotation period distribution we determine, sensitive to periods 0.1 < Prot < 8 days, shows a sharp cutoff for periods Prot < 0.5 days, corresponding to breakup velocity for these stars; a population of stars rotating near breakup is already present at 1 Myr. Above 0.5 days the distribution is consistent with a uniform distribution; we do not find evidence for a ``gap" of periods at 4--5 days. We find signatures of active accretion among stars at all periods; active accretion does not occur preferentially among slow rotators in our sample. We find no correlation between rotation period and the presence of near-infrared signatures of circumstellar disks. We do not find compelling agreement between our observations and the requirements of the disk-locking hypothesis. We use near-IR photometry to argue that inner cavities in TTS disks are typically much smaller than allowed by theory for the regulation of stellar angular momentum. We further use mid-IR (primarily 10 microns) photometry to confirm that TTS lacking near-IR excesses do not harbor disks with large inner truncation radii. With a few exceptions, stars in our sample lacking near-IR excesses do not possess disks, truncated or otherwise. 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. We discuss the implications of our results for rotational evolution models of PMS stars. We call into question the initial conditions assumed by the models, which typically begin their calculations with slow rotators possessing a small dispersion of rotation rates, and which have thus relied upon magnetic disk-locking to explain the large dispersion of rotation rates in young ZAMS clusters. We find that TTS at 1 Myr in fact possess a dispersion of rotation rates that matches or even exceeds that observed among low-mass Pleiads. We thus advocate new model initial conditions---in which low-mass stars at 1 Myr possess a large dispersion of rotation rates---that may allow the models to explain the angular momentum evolution of low-mass PMS stars in a way that does not depend upon disk-regulated phenomena. We also present models of the photopolarimetric variability arising from hot accretion spots on TTS (Stassun & Wood 1999). The application of these models to further tests of magnetic disk-locking, and for constraining star/disk/spot parameters of TTS, is discussed. Finally, we describe in an Appendix a field-tested educational outreach program for minorities. A sample grant proposal and budget are provided for those wishing to replicate this E/PO model. http://www.astro.wisc.edu/~keivan/pubs.html
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