Systematic Multi-Epoch Monitoring of LkCa 15: Dynamic Dust Structures on Solar-System Scales

TL;DR

This study uses high-resolution infrared imaging from 2014-2020 to monitor LkCa 15, revealing dynamic disk substructures and challenging the interpretation of protoplanets, suggesting a highly variable and complex circumstellar environment.

Contribution

It provides the first systematic multi-epoch infrared monitoring of LkCa 15, demonstrating that observed signals are due to dynamic disk features rather than orbiting protoplanets.

Findings

Infrared sources lack coherent orbital motion expected of protoplanets.

Disk substructures are dynamic and move through the forward-scattering side.

Hα detection is consistent with variable disk scattering, not protoplanets.

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 (one of which was simultaneously seen at H$\alpha$). We use high-angular-resolution infrared imaging from 2014-2020 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 multi-epoch observations suggest the presence of complex and dynamic substructures moving through the forward-scattering side of the disk at $\sim20$ AU, or quickly-varying shadowing by closer-in material. We explore whether the previous H$\alpha$ detection of one candidate would be inconsistent with this scenario, and in the process develop an analytical signal-to-noise penalty for H$\alpha$ excesses detected near forward-scattered light. Under these new noise considerations, the H$\alpha$ detection is not strongly inconsistent with forward scattering, making the dynamic LkCa 15 disk a natural explanation for both the infrared and H$\alpha$ data.