KH 15D: Unraveling the Mystery of a Peculiar Winking Star

We present the results of a high-resolution spectroscopic monitoring program of the eclipsing pre-main-sequence star KH 15D that reveal it to be a single-line spectroscopic binary. We find that the radial velocity is variable with an observed range of 11.4 km/s. The best-fitting Keplerian model has an orbital period P = 48.38 days, which is nearly identical to the photometric period.

We also present a historical light curve from 1967--1982, based on our study of archival photographic plates from Asiago Observatory. During this time, the KH 15D system alternated periodically between bright and faint states, as observed today. However, the bright state was 0.9 mag brighter than the modern value, and the fractional variation between bright and faint states (Δ I = 0.7 mag) was smaller than observed today (3.5 mag).

Based on these results, we find the best explanation for the periodic dimming of KH 15D is that the binary motion carries the visible star alternately above and below the plane of a circumbinary disk, as recently proposed by Winn et al. (2004) and Chiang & Murray-Clay (2004).