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

TL;DR

This study uses high-resolution ALMA observations to detect dust trapping at Lagrangian points in the LkCa 15 disk, providing evidence for planet-disk interactions and constraining the properties of potential planets.

Contribution

First high-resolution ALMA imaging revealing dust trapping at Lagrangian points, supporting planet-induced structures in the LkCa 15 disk.

Findings

Detection of a horseshoe-shaped dust ring at 42 au with excess features at L4 and L5.

No direct detection of a circumplanetary disk, with stringent upper limits on dust mass.

Evidence consistent with a Neptune- to Saturn-mass planet shaping the disk.

Abstract

We present deep high-resolution ($\sim$50 mas, 8 au) 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 $\sim$10$\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 point-like emission associated with a CPD around the putative planet location ($0.''27$ in projected separation from the central star at a position angle of $\sim$60\degr), 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 $M_{\oplus}$.