ALMA Detection of Dust Trapping around Lagrangian Points in the LkCa 15 Disk

We present deep high-resolution (similar to 50 mas, 8 au) Atacama Large Millimeter/submillimeter Array (ALMA) 0.88 and 1.3 mm continuum observations of the LkCa 15 disk. The emission morphology shows an inner cavity and three dust rings at both wavelengths, but with slightly narrower rings at the longer wavelength. Along a faint ring at 42 au, we identify two excess emission features at similar to 10 sigma significance at both wavelengths: one as an unresolved clump and the other as an extended arc, separated by roughly 120 degrees in azimuth. The clump is unlikely to be a circumplanetary disk (CPD) as the emission peak shifts between the two wavelengths even after accounting for orbital motion. Instead, the morphology of the 42 au ring strongly resembles the characteristic horseshoe orbit produced in planet-disk interaction models, where the clump and the arc trace dust accumulation around Lagrangian points L-4 and L-5, respectively. The shape of the 42 au ring, dust trapping in the outer adjacent ring, and the coincidence of the horseshoe ring location with a gap in near-IR scattered light, are all consistent with the scenario of planet sculpting, with the planet likely having a mass between those of Neptune and Saturn. We do not detect pointlike emission associated with a CPD around the putative planet location (0.'' 27 in projected separation from the central star at a position angle of similar to 60 degrees), with upper limits of 70 and 33 mu Jy at 0.88 and 1.3 mm, respectively, corresponding to dust mass upper limits of 0.02-0.03 M-circle plus.