Searching for H$_{alpha}$ emitting sources around MWC758: SPHERE/ZIMPOL high-contrast imaging

MWC758 is a young star surrounded by a transitional disk. Recently, a protoplanet candidate has been detected around MWC758 through high-resolution $L$-band observations. The candidate is located inside the disk cavity at a separation of $sim$111 mas from the central star, and at an average position angle of $sim$165.5 degrees. We have performed simultaneous adaptive optics observations of MWC758 in the H$_{alpha}$ line and the adjacent continuum using SPHERE/ZIMPOL at the Very Large Telescope (VLT). We aim at detecting accreting protoplanet candidates through spectral angular differential imaging observations. The data analysis does not reveal any H$_{alpha}$ signal around the target. The derived contrast curve in the B_Ha filter allows us to derive a 5$sigma$ upper limit of $sim$7.6 mag at 111 mas, the separation of the previously detected planet candidate. This contrast translates into a H$_{alpha}$ line luminosity of $L_{rm H_{alpha}}lesssim$ 5$times$10$^{-5}$ $L_{odot}$ at 111 mas, and an accretion luminosity of $L_{acc} <$3.7$times$10$^{-4},L_{odot}$. For the predicted mass range of MWC758b, 0.5-5 $M_{rm Jup}$, this implies accretion rates of $dot M lesssim$ 3.4$times$(10$^{-8}$-10$^{-9}),M_{odot}/yr$, for an average planet radius of 1.1 $R_{rm Jup}$. Therefore, our estimates are consistent with the predictions of accreting circumplanetary accretion models for $R_{rm in} = 1 R_{rm Jup}$. In any case, the non-detection of any H$_{alpha}$ emitting source in the ZIMPOL images does not allow us to unveil the true nature of the $L$ detected source.