An Extended Model of KH 15D: Determining the Structure of the Circumbinary Ring After the Reemergence of Star B

Abstract

The periodic eclipses of the T Tauri binary KH 15D have been attributed to occultations by a circumbinary ring inclined to the binary plane. Due to the advantageous orientation of KH 15D, we have the opportunity to learn more about the relationship between binary T Tauri stars and their circumbinary disks. One process we can observe is dust settling and material accretion onto the binary stars. While we expect the disk’s gas is heated by the T Tauri stars and flare outwards, we find that the abnormal light curves are caused by a sharp occulting edge. This implies that all of the dust should have settled to the midplane of the disk from gravity, despite the hot surrounding gas. Using the model by Winn et al. (2006) as a starting point, we first test to determine whether or not this model is compatible with newly available data. Now that Star B has fully emerged from behind the occulting screen, we can finally determine the location of the trailing edge of the projected screen. Afterwards, we optimize our model with a finite screen width to the observed photometric data and radial velocities from 1995 to 2017 to provide a possible best-fit model. We find that our screen projection has a finite width of 0.218 AU. We confirm that the premise of the sharp-edged occulting screen and halo models by Winn et al. (2006) accurately predicts the timing and shape of eclipses. Ultimately, we hope to understand the three-dimensional structure of the circumbinary ring and its orbital dynamics with the central binary. We explore a theory which establishes a ring made up of massless particles which precess nodally about the binary. We find that, based on our model, the circumbinary disk has an average radius of 3.19 AU, an inclination of 91.382 degrees with respect to the orbital plane, and a precession rate of −8.02 ∗ 10−4 degrees/day. We confirm that the ring is misaligned with the orbital plane and precesses in a retrograde motion. The current literature on misaligned disks suggests that these disks must be vertically warped to maintain the nonzero inclination, but we have yet to detect a misaligned circumbinary planet. This existence implies that there are possibly other circumbinary disks and planets which orbit at a nonzero inclination and orbit in retrograde motion. We realize that we likely do not present a unique solution to the structure of KH 15D. By providing a possible best-fit solution to our model, we hope that future studies can build on this model to gain more insights on this interesting system.