  An essential step to understanding protoplanetary evolution is the study of
disks that contain gaps or inner holes. The pretransitional disk around the
Herbig star HD 169142 exhibits multi-gap disk structure, differentiated gas and
dust distribution, planet candidates, and near-infrared fading in the past
decades, which make it a valuable target for a case study of disk evolution.
Using near-infrared interferometric observations with VLTI/PIONIER, we aim to
study the dust properties in the inner sub-au region of the disk in the years
2011-2013, when the object is already in its near-infrared faint state. We
first performed simple geometric modeling to characterize the size and shape of
the NIR-emitting region. We then performed Monte-Carlo radiative transfer
simulations on grids of models and compared the model predictions with the
interferometric and photometric observations. We find that the observations are
consistent with optically thin gray dust lying at Rin ~ 0.07 au, passively
heated to T ~ 1500 K. Models with sub-micron optically thin dust are excluded
because such dust will be heated to much higher temperatures at similar
distance. The observations can also be reproduced with a model consisting of
optically thick dust at Rin ~ 0.06 au, but this model is plausible only if
refractory dust species enduring ~2400 K exist in the inner disk.
