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Imaging the Dynamical Imprints of Planet Formation in Protoplanetary Discs

Periodic Reporting for period 3 - ImagePlanetFormDiscs (Imaging the Dynamical Imprints of Planet Formation in Protoplanetary Discs)

Reporting period: 2018-07-01 to 2019-12-31

Planet formation is a complex process that takes place in the discs around young stars. The dominant fraction of the planet population is believed to form in the inner few astronomical units of these discs. Therefore, it is essential to study the physical processes that take place in these inner disc regions to unveil the initial conditions of planet formation. Studying protoplanetary disc structure and evolution will also advance our understanding of how Earth formed and how it developed the conditions for harboring life, addressing one of the oldest questions of mankind. Detailed imaging of the inner disc environment might also reveal planets that are currently in the process of formation and that could be detectable either through the emission from circumplanetary accretion processes, or through the gravitational influence that these planets exert on the disc.

Observational studies of planet formation in protoplanetary discs are primarily limited by the achieved angular resolution that is set by the telescope diameter. Accordingly, most studies of protoplanetary discs could only investigate the outer disc regions, on scales of tens to hundreds of astronomical units. Infrared interferometry offers an elegant way to overcome this resolution barrier by coherently combining the light from separate smaller telescopes that can be spread over hundreds of metres, thereby providing the first direct view into the innermost astronomical unit of protoplanetary discs. The key requirement for obtaining direct images with infrared interferometers is the number of telescopes that are combined, which has so far been limited to 4 telescopes for protoplanetary disc observations. The primary objective of the ERC Starting Grant is to push this barrier by equipping the MIRC beam combiner at the CHARA telescope array with an innovative ultra-low read-noise detector system that will permit us to obtain first 6-telescope interferometric observations of low- and intermediate-mass young stars. Increasing from 4 telescopes to 6 telescopes provides 3.5-times more observables per measurements, while the CHARA array will also provide us about 2.5-times longer baselines than what was achieved in earlier observations. This should enable us to obtain an image in a single night of observing and to study also the time evolution of any resolved structures.

We will use the observational capabilities that will result from our CHARA instrumentation work in order to search for possibly planet-induced structures in the inner regions of protoplanetary discs and to image their temporal evolution. We will combine interferometric data obtained over a wide wavelength range in order to characterise the resolved structures. Finally, we will search for the signatures of the planets themselves by imaging in accretion-tracing spectral lines.
In the initial 30 months of the project, we successfully commissioned the ultra-fast, ultra-low read-noise infrared detector system at the CHARA array. Some optical components still need to be replaced before we will be able to achieve full performance.

Besides pushing this instrumentation work ahead, we started implementing the software tools that are necessary in order to interpret our observations in an optimal way. These software tools enable us to model multi-wavelength interferometric data and to extract quantitative physical parameters such as the dust density and 3-dimensional disc structure. Our team is currently applying these models to fit CHARA 3-telescope data from three protoplanetary discs, which should result soon in a first publication using this tool.

We imaged the disc around the star V1247 Orionis at multiple wavelengths and find an intriguing crescent-shaped asymmetry that might constitute a dust-trapping vortex. We find strong evidence that the dust trap might be triggered by an embedded (yet undiscovered) planet (Kraus et al. 2017, ApJ 848, L11). Dust traps are believed to provide the environment for dust grains to grow from micrometer-size to planetesimal size, marking one of the earliest stages of planet formation.

In another study, we discovered with the VLTI array a young high-mass multiple system and resolved the circumstellar discs around both stars (Kraus et al. 2017, ApJ 835, L5). We could also measure the individual accretion contributions from both stars using an approach that we plan to employ now to search for the accretion signatures of low-mass companions (possibly planets) embedded in protoplanetary discs.
Our instrumentation work pushes the state-of-the-art in interferometric imaging at infrared wavelengths. We expect to achieve at least an order-of-magnitude improvement in sensitivity (compared to the original MIRC instrument), which should enable us to conduct the first 6-telescope interferometric imaging observations on protoplanetary discs. These images (and complementary images obtained with VLTI and ALMA) will provide new insights in the physics that governs the inner regions of protoplanetary discs. We might detect possibly planet-induced asymmetric structures and will attempt to trace their orbital motion using multi-epoch imaging.

One of our recent studies has demonstrated the feasibility of using spectro-interferometry to reveal sites of active accretion in discs. We will use that approach to search for young planets. Whether these accretion signatures might be detectable depends on various parameters, such as the circumplanetary accretion geometry, which is still unknown.
V1247 Orionis: ALMA image, illustration, and SPH simulation (Kraus et al. 2017, ApJ 848, L11)