Searching for Hα emitting sources around MWC 758 SPHERE/ZIMPOL high-contrast imaging

Abstract

Context. MWC 758 is a young star surrounded by a transitional disk. The disk shows an inner cavity and spiral arms that could be caused by the presence of protoplanets. Recently, a protoplanet candidate has been detected around MWC 758 through high-resolution L'-band observations. The candidate is located inside the disk cavity at a separation of similar to 111 mas from the central star, and at an average position angle of similar to 165.5 degrees.

Aims. We aim at detecting accreting protoplanet candidates within the disk of MWC 758 through angular spectral differential imaging (ASDI) observations in the optical regime. In particular, we explore the emission at the position of the detected planet candidate. Methods. We have performed simultaneous adaptive optics observations in the H-alpha line and the adjacent continuum using SPHERE/ZIMPOL at the Very Large Telescope (VET).

Results. 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 similar to 7.6 mag at 111 mas, the separation of the previously detected planet candidate. This contrast translates into a H-alpha line luminosity of LH alpha less than or similar to 5x10(-5) L-circle dot, at 111 mas. Assuming that LH alpha scales with L-acc as in classical T Tauri stars (CTTSs) as a first approximation, we can estimate an accretion luminosity of L-acc < 3.7 x 10(-4) L-circle dot, for the protoplanet candidate. For the predicted mass range of MWC 758b, 0.5-5 M-Jup, this implies accretion rates smaller than M < 3.4 x (10(-8)-10(-9)) M-circle dot yr(-1), for an average planet radius of 1.1 R-Jup. Therefore, our estimates are consistent with the predictions of accreting circumplanetary accretion models for R-in = 1R(jup). The ZIMPOL line luminosity is consistent with the H-alpha upper limit predicted by these models for truncation radii less than or similar to 3.2 R-Jup.

Conclusions. The non-detection of any H-alpha emitting source in the ZIMPOL images does not allow us to unveil the nature of the detected source. Either it is a protoplanet candidate or a disk asymmetry.