YSOVAR: Infrared Reverberation Mapping of a Protoplanetary Disk

Theoretical models and spectroscopic observations of protoplanetary disks suggest the presence of an inner disk wall, in which dust is depleted by thermal vaporization and/or magnetospheric accretion. The size of the inner evacuated region is expected to be on a sub-AU scale that is unresolved by current adaptive optics imaging, though some constraints can be obtained by interferometric observations. Here we report the first detection of an inner disk wall around a Class I YSO in ρ Ophiuchus, YLW 16B, by measuring the time lag between the near-infrared variations of partial protostellar radiation and the 4.5 μm response from the disk. The near-infrared time-series were obtained from four different ground-based telescopes, and the 4.5 μm time-series were simultaneously obtained with the Spitzer Space Telescope in staring mode. All times are corrected to barycentric dynamical time. Consistent and robust lags are detected in two nights with different data reduction methods. The time-series in H and K bands are synchronized while the 4.5 μm emission lags by ~60s, corresponding to a light travel time distance of ~0.2 AU. The accurate distance depends on the spectral contribution and geometry of the disk, which is difficult to ascertain due to the variability of the system.