The Inner Astronomical Unit of Protoplanetary Disks

Discs around young stars play a central role in the star formation process and provide the stage where planet formation is taking place. A particularly important region in this context is the dust sublimation rim, where the disc reaches temperatures of 1500K and dust grains are destroyed by the strong stellar heating. The sudden change in gas-to-dust ratio is expected to create a pressure bump that might trigger grain growth and planet formation in this region. Early studies envisioned the dust sublimation rim as a simple vertical disc truncation, while more recent simulations with realistic dust properties point towards a more complex, vertically and radially extended rim structure. There are also indications for dynamical structural changes taking place at the inner rim, although it is unclear whether these are triggered by disc instabilities or whether they indicate the gravitational influence of forming planets in the region.
The best constraints on the inner disc structure are provided by infrared interferometric telescope arrays, such as the VLT Interferometer and the CHARA array. These arrays achieve an unprecedented, milliarcsecond angular resolution by combining the light of telescopes separated hundreds of metres apart. The Exeter interferometry group is leading an instrumentation project that aims to revolutionise these inner disc studies by enabling the first 6-telescope observations of protoplanetary discs with baselines up to 330 metres. For this purpose, we will equip the MIRC instrument at the CHARA array in California with a new ultra-low read-noise detector and other optical components that will provide a large advancement both in angular resolution and imaging efficiency compared to other facilities.
The aim of this PhD project is to employ these substantial new observational opportunities in order to study the structure of the dust rim and to search for density asymmetries that might indicate the presence of planets in the disc. These asymmetric structures should orbit the star on timescales of months, which we will confirm with multi-epoch observations. We will combine multi-wavelength VLTI and CHARA data and model it with the Exeter radiative transfer code TORUS in order to deduce the temperature and density structure in the inner disc regions. This will allow us to address fundamental open questions about inner disc physics and the early stages of planet formation.