  Theoretical models and spectroscopic observations of newborn stars suggest
that protoplantary disks have an inner "wall" at a distance set by the disk
interaction with the star. Around T Tauri stars, the size of this disk hole is
expected to be on a 0.1-AU scale that is unresolved by current adaptive optics
imaging, though some model-dependent constraints have been obtained by
near-infrared interferometry. Here we report the first measurement of the inner
disk wall around a solar-mass young stellar object, YLW 16B in the {\rho}
Ophiuchi star-forming region, by detecting the light travel time of the
variable radiation from the stellar surface to the disk. Consistent time lags
were detected on two nights, when the time series in H (1.6 {\mu}m) and K (2.2
{\mu}m) bands were synchronized while the 4.5 {\mu}m emission lagged by 74.5
+/- 3.2 seconds. Considering the nearly edge-on geometry of the disk, the inner
rim should be 0.084 AU from the protostar on average, with an error of order
0.01 AU. This size is likely larger than the range of magnetospheric
truncations, and consistent with an optically and geometrically thick disk
front at the dust sublimation radius at ~1500 K. The widths of the
cross-correlation functions between the data in different wavebands place
possible new constraints on the geometry of the disk.
