Variable Dynamics in the Inner Disk of HD 135344B Revealed with Multi-epoch Scattered Light Imaging*

We present multi-epoch Very Large Telescope/Spectro-PolarimetricHigh-contrast Exoplanet REsearch (VLT/SPHERE) observations of theprotoplanetary disk around HD 135344B (SAO 206462). The J-band scatteredlight imagery reveal, with high spatial resolution (˜41 mas, 6.4au), the disk surface beyond ˜20 au. Temporal variations areidentified in the azimuthal brightness distributions of all epochs,presumably related to the asymmetrically shading dust distribution inthe inner disk. These shadows manifest themselves as narrow lanes, castby localized density enhancements, and broader features which possiblytrace the larger scale dynamics of the inner disk. We acquired visibleand near-infrared photometry which shows variations up to 10% in the JHKbands, possibly correlated with the presence of the shadows. Analysis ofarchival Very Large Telescope Interferometer/Precision Integrated-OpticsNear-infrared Imaging ExpeRiment (VLTI/PIONIER) H-band visibilitiesconstrain the orientation of the inner disk toI=18\buildrel{\circ}\over{.} {2}-4.1+3.4 and{PA}=57\buildrel{\circ}\over{.} 3+/- 5\buildrel{\circ}\over{.} 7,consistent with an alignment with the outer disk or a minor disk warp ofseveral degrees. The latter scenario could explain the broad,quasi-stationary shadowing in north-northwest direction in case theinclination of the outer disk is slightly larger. The correlationbetween the shadowing and the near-infrared excess is quantified with agrid of radiative transfer models. The variability of the scatteredlight contrast requires extended variations in the inner disk atmosphere(H/r≲ 0.2). Possible mechanisms that may cause asymmetricvariations in the optical depth ({{Δ }}τ ≲ 1) through theatmosphere of the inner disk include turbulent fluctuations,planetesimal collisions, or a dusty disk wind, possibly enhanced by aminor disk warp. A fine temporal sampling is required to followday-to-day changes of the shadow patterns which may be a face-on variantof the UX Orionis phenomenon.