Locating the Trailing Edge of the Circumbinary Ring in the KH 15D System

TL;DR

The paper reports the recent brightening of KH 15D, revealing direct light from star B, confirming models of a precessing, self-gravitating circumbinary ring, and highlighting the system's potential for detecting a young giant planet.

Contribution

It provides observational confirmation of the ring's precession and self-gravity, and demonstrates KH 15D's transition to a double-lined binary, aiding pre-main sequence studies.

Findings

Brightening reveals direct light from star B

Spectral class change from K6/K7 to ~K1

Supports the self-gravitating, precessing ring model

Abstract

Following two years of complete occultation of both stars by its opaque circumbinary ring, the binary T Tauri star KH 15D has abruptly brightened again during apastron phases, reaching I = 15 mag. Here, we show that the brightening is accompanied by a change in spectral class from K6/K7 (the spectral class of star A) to ~K1, and a bluing of the system in V-I by about 0.3 mag. A radial velocity measurement confirms that, at apastron, we are now seeing direct light from star B, which is more luminous and of earlier spectral class than star A. Evidently, the trailing edge of the occulting screen has just become tangent to one anse of star B's projected orbit. This confirms a prediction of the precession models, supports the view that the tilted ring is self-gravitating, and ushers in a new era of the system's evolution that should be accompanied by the same kind of dramatic phenomena observed from 1995-2009. It also promotes KH 15D from a single-lined to a double-lined eclipsing binary, greatly enhancing its value for testing pre-main sequence models. The results of our study strengthen the case for truncation of the outer ring at around 4 AU by a sub-stellar object such as an extremely young giant planet. The system is currently at an optimal configuration for detecting the putative planet and we urge expedient follow-up observations.