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Imaging the signatures of planet formation using spectro-interferometry

Final Report Summary - DISKIMAGING (Imaging the signatures of planet formation using spectro-interferometry)

The disks around young stars play a key role in the star formation process and provide also the stage where planets form. In this project, we employ three innovative and highly complementary techniques that allow us to study different spatial regions of these disks: (a) PIONIER long-baseline interferometry constrains the distribution of hot dust in the inner disk regions, (b) SPHERE polarimetric imaging traces the intermediate/outer disk in scattered light and allows us to constrain the dust grain composition, and (c) Aperture masking interferometry enables us to search for thermal emission of low-mass companions in the disk. Combining these techniques allows us to relate the outer disk properties with the disk geometry in the inner regions.

We combined all three techniques in a comprehensive study on the Herbig Ae star MWC614 (Kluska, Kraus, et al., ApJ submitted). This object exhibits the most extended near-infrared-emitting region in the sample of 51 young stars surveyed with PIONIER, which makes it an ideal target to study the nature of the extended emission found around many Herbig stars. The physical origin of this extended flux is still unknown, but might correspond to a scattered light halo, the presence of very small particles, or complex extended disk structures at a few tens of astronomical units (AU). We aimed to determine the physical origin of this extended flux by tracing both the scattered light with SPHERE and the thermal emission from the disk with interferometry. Our data reveals an optically-thick outer disk truncated at 12 AU that was also detected in scattered light with SPHERE. The near-infrared emission, usually associated with the dust sublimation region, is very extended (about 10 AU, 30 times larger than the theoretical sublimation radius) and has a high temperature (T=1800 K). We find that quantum-heated dust grains could explain the extended near-infrared emission and the measured temperature distribution. Our observations confirm the peculiar state of this object where a companion could recently have opened the gap and where the inner disk has already been accreted onto the star, exposing small particles inside the gap to direct UV stellar radiation.

Aperture masking interferometry provides an effective method to search for planetary-mass companions on scales of tens to hundreds of AU. As part of our aperture masking survey, we searched for companions around eight young stars. This resulted in the detection of asymmetries in three objects (DM Tau, LkHa330, TW Hya) that are consistent with companion detections (Willson, Kraus et al. 2016, A&A 595, A9). Our aperture masking observations on the Herbig star V1247 Orionis revealed an asymmetry that moved by more than 45 degrees within 2 years, consistent with a companion on a 6 AU orbit (Willson, Kraus, et al. A&A, submitted). We also studied systematically how the disk emission affects the aperture masking observables and can in some cases mimic companion detections. Finally, we applied and optimised our image reconstruction techniques in order to retrieve direct images from aperture masking data.

The CIG grant played an important role for my successful integration at the University of Exeter, following my two postdoctoral positions in the United States. It enabled my team to acquire important expertise in reducing and interpreting extreme-AO polarimetry data and enabled us to attend conferences and triggered new collaborations related to aperture masking interferometry.

The results of the CIG project are also disseminated through the project website: