Browsing by Author "Cauley, P. Wilson"
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Item Observational Tests of Magnetospheric Accretion Models in Young Stars(EDP Sciences, 2014) Johns-Krull, Christopher M.; Cauley, P. WilsonMagnetically controlled accretion of disk material onto the surface of Classical T Tauri stars is the dominant paradigm in our understanding of how these young stars interact with their surrounding disks. These stars provide a powerful test of magnetically controlled accretion models since all of the relevant parameters, including the magnetic field strength and geometry, are in principle measureable. Both the strength and the field geometry are key for understanding how these stars interact with their disks. This talk will focus on recent advances in magnetic field measurements on a large number of T Tauri stars, as well as very recent studies of the accretion rates onto a sample of young stars in NGC 2264 with known rotation periods. We discuss how these observations provide critical tests of magnetospheric accretion models which predict a rotational equilibrium is reached. We find good support for the model predictions once the complex geometry of the stellar magnetic field is taken into account. We will also explore how the observations of the accretion properties of the 2264 cluster stars can be used to test emerging ideas on how magnetic fields on young stars are generated and organized as a function of their internal structure (i.e. the presence of a radiative core). We do not find support for the hypothesis that large changes in the magentic field geometry occur when a radiative core appears in these young stars.Item TESTING DISK-LOCKING IN NGC 2264(The American Astronomical Society, 2012) Cauley, P. Wilson; Johns-Krull, Christopher M.; Hamilton, Catrina M.; Lockhart, KellyWe test analytic predictions from different models of magnetospheric accretion, which invoke disk-locking, using stellar and accretion parameters derived from models of low-resolution optical spectra of 36 T Tauri stars (TTSs) in NGC 2264 (age ∼3Myr). Little evidence is found for models that assume purely dipolar field geometries; however, strong support is found in the data for a modified version of the X-wind model which allows for non-dipolar field geometries. The trapped flux concept in the X-wind model is key to making the analytic predictions which appear supported in the data. By extension, our analysis provides support for the outflows predicted by the X-wind as these also originate in the trapped flux region. In addition, we find no support in the data for accretion-powered stellar winds from young stars. By comparing the analysis presented here of NGC 2264 with a similar analysis of stars in Taurus (age ∼1–2 Myr), we find evidence that the equilibrium interaction between the magnetic field and accretion disk in TTS systems evolves as the stars grow older, perhaps as the result of evolution of the stellar magnetic field geometry. We compare the accretion rates we derive with accretion rates based on U-band excess, finding good agreement. In addition, we use our accretion parameters to determine the relationship between accretion and Hβ luminosity, again finding good agreement with previously published results; however, we also find that care must be taken when applying this relationship due to strong chromospheric emission in young stars, which can lead to erroneous results in some cases.