A test of star formation theory: The connection between rotation, accretion, and circumstellar disks among low-mass pre-main sequence stars.

摘要

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, presented in Stassun et al. (1999) and in Stassun et al. (2000), that challenge the current picture of disk-regulated PMS rotational evolution. We find no connection between stellar rotation period and near-IR diagnostics of circumstellar disks among a large sample of young PMS stars in the Orion Nebula Cluster (ONC). In addition, we do not find compelling agreement between our observations and the predictions of the disk-locking hypothesis when we consider in detail the evidence for disk material being present, and truncated near, the co-rotation radius for stars in our sample. We highlight our discovery of 1 Myr-old stars already rotating at breakup velocity-many of which do not possess disks. How do these stars deplete angular momentum prior to the main sequence? We suggest that the key to reconciling theory and observation may reside in the initial conditions: We find that the dispersion of rotation rates observed among low-mass ZAMS stars is already present at 1 Myr.; We also present models of the photopolarimetric variability arising from hot accretion spots on TTS, from Stassun & Wood (1999). The application of these models to further tests of magnetic disk-locking, and as tools for determining 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 to assist those wishing to develop such an outreach model themselves.