A search of accreting protoplanets through high-angular resolution Hα observations

Protoplanets embedded in disks are expected to accrete material from their surrounding media. As a result of this process, they can emit in accretion tracers like the H alpha line. In this work, we show the result from SPHERE/ZIMPOL observations to detect accreting protoplanets around five stars with (pre-)transitional disks. They were obtained in the H_alpha line and the adjacent continuum, combining spectral and angular differential imaging techniques to increase the contrast in the innermost regions close to the star. We do not detect any point-like source around any of the stars. When we compare our detection limits with different planetary models, we estimate an average upper limit to the accretion luminosity of < 10^-4 Lsun at 200 mas, which is 2 orders of magnitude higher that that previously estimated from the extrapolation of the L__Halpha - L_acc stellar relationship. We explain the lack of protoplanet detections as a combination of different factors, like e.g. episodic accretion, extinction from the circumstellar disk, and/or a majority of low-mass, low-accreting planets. Space missions, like e.g. JWST, will help to shed light on the accretion process of planets at the earliest stages of their formation.