  The mechanisms of planet formation are still under debate. We know little
about how planets form, even if more than 4000 exoplanets have been detected to
date. Recent investigations target the cot of newly born planets: the
protoplanetary disk. At the first stages of their life, exoplanets still
accrete material from the gas-rich disk in which they are embedded.
Transitional disks are indeed disks that show peculiarities, such as gaps,
spiral arms, and rings, which can be connected to the presence of substellar
companions. To investigate what is responsible for these features, we selected
all the known transitional disks in the solar neighborhood (<200 pc) that are
visible from the southern hemisphere. We conducted a survey of 11 transitional
disks (TDs) with the SPHERE instrument at the VLT. This is the largest Halpha
survey that has been conducted so far to look for protoplanets. The
observations were performed with the Halpha filter of ZIMPOL in order to target
protoplanets that are still in the accretion stage. All the selected targets
are very young stars, less than 20 Myr, and show low extinction in the visible.
We reduced the ZIMPOL pupil stabilized data by applying the method of the
angular spectral differential imaging (ASDI), which combines both techniques.
The datacubes are composed of the CntHalpha and the narrow band filter Halpha,
which are taken simultaneously to permit the suppression of the speckle
pattern. The principal component analysis (PCA) method was employed for the
reduction of the data. For each dataset, we derived the 5sigma contrast limit
and converted it in upper limits on the accretion luminosity. We do not detect
any new accreting substellar companions around the targeted transition disks
down to an average contrast of 12 magnitudes at 0.2 arcsec from the central
star (continues in the manuscript).
