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Imaging the Dynamical Imprints of Planet Formation in Protoplanetary Discs

Periodic Reporting for period 4 - ImagePlanetFormDiscs (Imaging the Dynamical Imprints of Planet Formation in Protoplanetary Discs)

Reporting period: 2020-01-01 to 2021-04-30

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 also advances our understanding of how Earth formed and how it developed the conditions for harboring life, addressing one of the oldest questions of humankind. Detailed imaging is also the key for detecting 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 offers us 2.5-times longer baselines than what was achieved in earlier observations. This enables us to obtain an image in a single night of observing and to study also the time evolution of any resolved structures.

We use interferometric observations to search for possibly planet-induced structures in the inner regions of protoplanetary discs. We combine interferometric data obtained over a wide wavelength range in order to characterise the resolved structures. Finally, we use interferometric observations in spectral lines to separate accretion onto the star and on putative planets.
We successfully commissioned an ultra-fast, ultra-low read-noise infrared detector system at the CHARA array and optimised the sensitivity of our MIRC-X instrument. This has allowed us to image protoplanetary discs with unprecedented angular resolution (about 1 milliarcsecond) and with unprecedented efficiency (35 observables per 6-telescope measurement, compared to 10 observables per 4-telescope measurement). The CHARA observations were complemented with data from the VLT Interferometer and the ALMA array. This multi-wavelength approach allowed us to determine the geometry and physical conditions near the dust sublimation rim in protoplanetary (where dust grains start to sublimate due to stellar irradiation) as well as in the intermediate and outer disc regions. For instance, for the young triple star systems GW Orionis, VLTI+CHARA allowed us to measure the stellar orbits and inner disc emission, while ALMA+SPHERE showed misaligned rings and a warped disc surface on larger scales. We could link the detected disc asymmetries to the torque excerted by the stars, which revealed the actions of the disc tearing effect and raised the exciting possibility that planets might be forming in the strongly misaligned, massive dust rings around GW Orionis. We detected a crescent-shaped asymmetry in V1247 Orionis that might be triggered by an embedded (yet undiscovered) planet. We find that gas and dust is lifted out of the disc plane in the inner regions, both through spectro-interferometry spectral line observations, and CHARA continuum imaging. Using a dust-band observing mode with our CHARA instrument allowed us directly measure the disc temperature profile down to a few stellar radii. We modelled the in inner region near the dust sublimation rim with radiation transfer simulations and constrained the dust minerology in this region.
We carried out the first 6-telescope interferometric imaging observations on protoplanetary discs. Our CHARA, VLTI and ALMA observations provided new insights on the physics that governs the inner regions of protoplanetary discs, including on the 'disc tearing' effect, disc winds, and dust trapping. Our instrumentation work pushed the state-of-the-art in interferometric imaging at infrared wavelengths, namely by achiving 3 magnitudes better sensitivity than the precursor instrument and by opening the 1-1.4 micrometer wavelength range for 6-telescope interferometry. The instrument enables the highest-resolution imaging of any infrared instrument world-wide and is offered to other users in the international astronomical community since late-2017 on an open-access basis. This work points the way for future beam combination instruments operating at these wavelengths. Also, we demonstrated the feasibility of using spectro-interferometry to reveal and charactise accretion onto close-in companions, although possible 'noise' contributions from stellar rotation, magnetospheric accretion, and disc winds need to be taken in account.