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

Abstract

MWC758 is a young star surrounded by a transitional disk. Recently, a protoplanet candidate has been detected around MWC758 through high-resolution $L^{\prime }$-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 $L_{{\mathrm{H}}_{\alpha }}\lesssim$ 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 $M_{\mathrm{Jup}}$, 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 $R_{\mathrm{Jup}}$. Therefore, our estimates are consistent with the predictions of accreting circumplanetary accretion models for $R_{\mathrm{in}}=1R_{\mathrm{Jup}}$. 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 $L^{\prime }$ detected source.

arXiv comment5 pages, accepted for publication in A&A letters

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