Systematic Multiepoch Monitoring of LkCa 15: Dynamic Dust Structures on Solar System Scales

Abstract

We present the highest-angular-resolution infrared monitoring of LkCa 15, a young solar analog hosting a transition disk. This system has been the subject of a number of direct-imaging studies from the millimeter through the optical, which have revealed multiple protoplanetary disk rings as well as three orbiting protoplanet candidates detected in infrared continuum emission (one of which was simultaneously seen at Hα). We use high-angular-resolution infrared imaging from 2014 to 2020 to systematically monitor these infrared signals and determine their physical origin. We find that three self-luminous protoplanets cannot explain the positional evolution of the infrared sources since the longer time baseline images lack the coherent orbital motion that would be expected for companions. However, the data still strongly prefer a time-variable morphology that cannot be reproduced by static scattered-light disk models. The multiepoch observations suggest the presence of complex and dynamic substructures moving through the forward-scattering side of the disk at ∼20 au or quickly varying shadowing by closer-in material. We explore whether the previous Hα detection of one candidate would be inconsistent with this scenario and in the process develop an analytical signal-to-noise penalty for Hα excesses detected near forward-scattered light. Under these new noise considerations, the Hα detection is not strongly inconsistent with forward scattering, making the dynamic LkCa 15 disk a natural explanation for both the infrared and Hα data.