Spectropolarimetry and Modeling of the Eclipsing T Tauri Star Kh 15d

KH 15D is a strongly variable T Tauri star in the young star cluster NGC 2264 that shows a decrease in flux of 3.5 mag lasting for 18 days and repeating every 48 days. The eclipsing material is likely due to orbiting dust or rocky bodies in a partial ring or warped disk that periodically occults the star. We measured the polarized spectrum in and out of eclipse at the Keck and Palomar observatories. Outside of the eclipse, the star exhibited low polarization consistent with zero. During eclipse, the polarization increased dramatically to 2% across the optical spectrum, while the spectrum had the same continuum shape as outside of eclipse and exhibited emission lines of much larger equivalent width, as previously seen. From the data, we conclude that (1) the scattering region is uneclipsed, (2) the scattering is nearly achromatic, and (3) the star is likely completely eclipsed so that the flux during eclipse is entirely due to scattered light, a conclusion also argued for by the shape of the ingress and egress. We argue that the scattering is not due to electrons but may be due to large dust grains of size 10 m, similar to the interplanetary grains that scatter the zodiacal light. We construct a warped-disk model with an extended dusty atmosphere that reproduces the main features of the light curve, namely, (1) a gradual decrease before ingress due to extinction in the atmosphere (similar for egress), (2) a sharper decrease within ingress due to the optically thick base of the atmosphere, and (3) a polarized flux during eclipse that is 0.1% of the total flux outside of eclipse, which requires no fine-tuning of the model. The inclination of the warp is set by the duration of ingress and egress assuming that the warp is located at the Keplerian radius, and the inclination of the observer is then determined by the duration of the eclipse. Subject headings: eclipses — polarization — stars: individual (KH 15D) — stars: pre–main-sequence — stars: variables: other On-line material: color figures, mpeg animation