Applications of Photospheric Spot Temperature Models to the Study of Stellar Angular Momentum Evolution in the Orion Nebula Cluster
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2004
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Swarthmore College. Dept. of Physics & Astronomy
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Thesis (B.A.)
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Abstract
We apply a simple photospheric spot temperature model to photometric variability measurements of
T Tauri stars in the Trapezium region of the Orion Nebula Cluster. Our aim is to search for the
relationship, if any, between spot temperatures and stellar rotation periods to better understand the
relationship between accretion and angular momentum regulation in T Tauri stars. Current magnetic
disk-locking models of young stars ascribe spot temperatures hotter than the photosphere to active
accretion from a circumstellar disk. If accretion acts to brake stellar rotation, spot temperatures hotter
than the photosphere should be more prevalent among slow rotators. From the variability amplitudes
at four wavelengths (B, V, R, I), we determine spot temperatures and the areal coverage of the spot on
the stellar surface. The results of our model show that we can unambiguously distinguish spots hotter
than the photosphere from spots cooler than the photosphere for most stars. Comparisons between spot
temperatures and previously determined rotation periods reveal no significant correlation between spot
temperatures and stellar rotation. Comparisons between spot temperatures and measurements of nearIR
excess, Δ(I-K), reveal possible problems with the accuracy of Δ(I-K) as an indicator of circumstellar
disk presence. We confirm this problem by further comparing spot temperatures to Ca II equivalent
widths and comparing Δ(I-K) measurements with Ca II equivalent widths corresponding to evidence for
accretion from a circumstellar disk. We propose that future studies of the angular momentum evolution
of T Tauri stars take considerable care in distinguishing stars with circumstellar disks and possible
magnetic disk locking from stars without circumstellar disks or magnetic disk locking. Finally, we argue
that our spot temperature and rotation period comparisons support new theories of angular momentum
evolution in T Tauri stars that relate the angular momentum evolution of pre-main-sequence stars to the
core-envelope decoupling that occurs during the convective-radiative transition.