Imaging diagnostics for transitional discs

Abstract

Transitional discs are a special type of protoplanetary disc, where planet formation is thought to be taking place. These objects feature characteristic inner cavities and/or gaps of a few tens of AUs in sub-millimetre images of the disc. This signature suggests a localised depletion of matter in the disc that could be caused by planet formation processes. However, recent observations have revealed differences in the structures imaged at different wavelengths in some of these discs. In this paper, we aim to explain these observational differences using self-consistent physical 2D hydrodynamical and dust evolution models of these objects, assuming their morphology is indeed generated by the presence of a planet. We use these models to derive the distribution of gas and dust in a theoretical planet-hosting disc for various planet masses and orbital separations. We then simulate observations of the emitted and scattered light from these models with Very Large Telescope (VLT)/SPHERE-ZIMPOL, Subaru/HiCIAO, VLT/VISIR, and ALMA. We do this by first computing the full resolution images of the models at different wavelengths and then simulating the observations while accounting for the characteristics of each particular instrument. The presence of the planet generates pressure bumps in the gas distribution of the disc, whose characteristics strongly depend on the planet mass and position. These bumps cause large grains to accumulate, while small grains are allowed into inner regions. This spatial differentiation of the grain sizes explains the differences in the observations, since different wavelengths and observing techniques trace different parts of the dust size distribution. Based on this effect, we conclude that the combination of visible/near-infrared polarimetric and sub-mm images is the best strategy to constrain the properties of the unseen planet responsible for the disc structure.

Variability and dust filtration in the transition disk J160421.7-213028 observed in optical scattered light

“…de Juan Ovelar et al (2013) predicted the radial profile of emission at different wavelengths after performing radiative transfer together with hydrodynamical and dust-evolution modelling and combined with instrument simulations (including ZIMPOL and ALMA). A large radial segregation between the inner edge of the annulus or "wall" observed with ZIMPOL polarimetric images (defined as the radial location where the flux has increased by half from the minimum in the gap and the peak of the annulus) and the peak of emission at submillimetre (850 µm) was predicted for different planet masses and locations (Fig.…”

“…Garufi et al 2013), is a natural consequence of filtration effects caused by particle traps (e.g. Rice et al 2006;Zhu et al 2012;de Juan Ovelar et al 2013). One way to form a particle trap is planet-disk interaction: at the outer edge of a planetary gap, a region with positive pressure gradient can stop the fast inward migration of large dust particles (e.g.…”

Shadows cast on the transition disk of HD 135344B

“…Garufi et al (2013) carried out VLT/NACO polarimetric imaging observations in the H and K s -bands which resolved a 28 au cavity. The outer radius of the scattered light cavity is approximately 20 au smaller than the (sub-)millimeter cavity which can be explained by spatial segregation of micronsized and mm-sized dust grains (Garufi et al 2013) which is a natural outcome of particle trapping by a pressure maximum, for example due to planet-disk interactions (e.g., Rice et al 2006;Pinilla et al 2012;Zhu et al 2012;de Juan Ovelar et al 2013). More recently, angular differential imaging (ADI) observations with the Gemini Planet Imager (GPI) were presented by Wahhaj et al (2015) who recovered the spiral arms and new streamer-like features.…”

“…Spatial segregation of different dust grain sizes is expected to occur around a planet-induced gap edge (Zhu et al 2012;de Juan Ovelar et al 2013). The resolved millimeter cavity of the HD 135344B disk is indeed larger than the scattered light cavity as a result of dust filtration (Garufi et al 2013).…”

The long-term evolution of photoevaporating transition discs with giant planets

“…For example Zhu et al (2012) needed to invoke grain growth in the inner disc to remove these small grains which would otherwise produce too much (unobserved) emission in the MIR. Indeed, de Juan Ovelar et al (2013) presented models including also the effect of grain coagulation, that in some cases can successfully clear the inner disc of the small dust. Recently, another scenario was proposed by Owen (2014), who investigated the effect of radiation pressure from the accretion luminosity on the planet on the dust grains.…”