Limits on Eclipses of the Pre-Main-Sequence Star KH 15D in the First Half of the 20th Century

Over the last decade, the pre-main sequence star KH 15D has exhibitedperiodic eclipses that are surprisingly deep (~3 mag) and long-lasting (~40% ofthe 48.4-day period). The cause of the eclipses is unknown, but it could be afeature in a nearly edge-on protoplanetary disk. Here we report on an analysisof archival photographs of KH 15D from the Harvard College Observatory platecollection, most of which were taken during the years 1913-1951. During thistime range, the data are consistent with no eclipses; the duty cycle of 1 mageclipses was less than 20%. The decadal timescale of this change in eclipsebehavior is compatible with the expected timescale of protoplanet/diskinteractions. Archival images from more recent epochs should reveal the onsetof the eclipses.Comment: ApJ Letters, in press [10pp, 4 figs