ar X iv : a st ro - p h / 04 03 09 9 v 2 9 M ay 2 00 4 KH 15 D : A Spectroscopic Binary 1

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 best-fit Keplerian model has a period P = 48.38 days, which is nearly identical to the photometric period. Thus, we find the best explanation for the periodic dimming of KH 15D is that the binary motion carries the currently visible star alternately above and below the edge of an obscuring cloud. The data are consistent with the models involving an inclined circumstellar disk, as recently proposed by Winn et al. (2004) and Chiang & Murray-Clay (2004). We show that the mass ratio expected from models of PMS evolution, together with the mass constraints for the visible star, restrict the orbital eccentricity to 0.68 ≤ e ≤ 0.80 and the mass function to 0.125 ≤ FM/ sin 3 i ≤ 0.5 M⊙. Subject Headings: stars: pre–main-sequence — stars: individual (KH 15D) — binaries: spectroscopic — techniques: radial velocities — circumstellar matter Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the University of California and the California Institute of Technology; Las Campanas Observatory of the Carnegie Institution with the Magellan II Clay telescope ; and McDonald Observatory of the University of Texas at Austin. Department of Astronomy, University of California, Mail Code 3411, Berkeley, CA 94720 Visiting astronomer, McDonald Observatory which is operated by The University of Texas at Austin. Department of Physics and Astronomy, Rice University, Houston, TX 77005. Van Vleck Observatory, Wesleyan University, Middletown, CT 06459