Search for protoplanets with the FIRST instrument – FIRST

Only a few young giant planets have been detected so far, although they are essential to understand the mechanisms of the planet formation. Planets under formation, the so-called protoplanets, are still in the process of accreting matter, and, in consequence, show a strong emission in the Halpha line (656nm). The detection of a signal at this wavelength is thus the signature of the presence of a protoplanet, and allows the characterization of the accretion rate, depending on the mass of the planet. The planets detected around PDS70 have been unambiguously identified as protoplanets thanks to this particular feature in the Halpha line region, while the hypothesis of a clump of dust has been formally discarded. Besides, this effect is especially interesting from an observational point of view, since the contrast between the star and its companion is consequently reduced, making their direct detection easier at this wavelength.
With the present project, I propose to use optical interferometry to detect new accreting protoplanets, otherwise inaccessible with current imagers, and to characterize their accretion rate. The FIRST instrument (Fibered Imager foR a Single Telescope) is based on the pupil remapping technique and offers unique capabilities to detect and spectrally characterize companions at very short angular separations. Installed on the extreme adaptive optics system of the Subaru Telescope SCExAO, the angular resolution of FIRST reaches 10 mas in the visible, that is 1.5 astronomical units (AU) at 150pc. While this angular resolution capability has already been proven, with this project, I want to push the detection limits of the FIRST instrument in terms of sensitivity and dynamic range, to enable the detection of new protoplanets. I thus plan to upgrade the instrument, in particular by optimizing the photonic chip, enhancing the wavefront sensing capability and developing a new spectrograph to resolve the Halpha line. Besides, I propose to implement a self-calibrated phase measurement technique, consisting in subtracting the phase measured in the continuum to the phase measured at the Halpha line (differential phase), that will allow to reach highly accurate phase measurements, and thus increase the contrast performance.
In the second step of the project, I will conduct a survey of several tens of young stars in the Taurus Molecular Cloud, located at a relatively short distance from Earth (140pc) and encompassing numerous young stars. The enhanced version of FIRST, with a sensitivity goal of Rmag=12 and a contrast goal of 10^-3 at 5 sigma (that is 2.10^-4 at 1 sigma), will allow to probe an unexplored area of the parameter space, between 1 and 10 AU. According to the latest estimations of the giant planet distribution as a function of their orbital distance, this region is of critical interest, because the probability for the presence of the giant planets is highest around the snowline, i.e. 1-2 AU. With the help of the targeted detection limits, I expect our observation program to lead to the detection of 2-3 new protoplanets in a sample of 50 stars, which would double the current number of known protoplanets. Their discovery and the characterization of their accretion rate will be a considerable addition to further constrain the models of planet formation.