A Decade of MWC 758 Disk Images: Where Are the Spiral-Arm-Driving Planets?

Large-scale spiral arms have been revealed in scattered light images of a few protoplanetary disks. Theoretical models suggest that such arms may be driven by and co-rotate with giant planets, which has called for remarkable observational efforts to look for them. By examining the rotation of the spiral arms for the MWC 758 system over a 10-yr timescale, we are able to provide dynamical constraints on the locations of their perturbers. We present reprocessed Hubble Space Telescope (HST)/NICMOS F110W observations of the target in 2005, and the new Keck/NIRC2 $L'$-band observations in 2017. MWC~758's two well-known spiral arms are revealed in the NICMOS archive at the earliest observational epoch. With additional Very Large Telescope (VLT)/SPHERE data, our joint analysis leads to a pattern speed of ${0.6^\circ}^{+3.3^\circ}_{-0.6^\circ}\, \mathrm{yr}^{-1}$ at $3\sigma$ for the two major spiral arms. If the two arms are induced by a perturber on a near-circular orbit, its best fit orbit is at $89$ au ($0.59"$), with a $3\sigma$ lower limit of 30 au ($0.20"$). This finding is consistent with the simulation prediction of the location of an arm-driving planet for the two major arms in the system.