Spiral Structure and Differential Dust Size Distribution in the LkHa 330 Disk

TL;DR

This study combines near-infrared polarimetric and millimeter interferometric observations to reveal spiral arms and asymmetric dust structures in the LkHa 330 disk, suggesting the presence of an unseen planet and differential dust grain growth.

Contribution

It provides new multi-wavelength imaging evidence of spiral arms and asymmetric dust distribution, indicating complex dust evolution and potential planet formation processes in the disk.

Findings

Detection of spiral arms in H-band polarimetric images.

Identification of asymmetric dust structures in millimeter continuum.

Evidence of differential dust grain sizes across the disk.

Abstract

Dust trapping accelerates the coagulation of dust particles, and thus it represents an initial step toward the formation of planetesimals. We report $H$-band (1.6 um) linear polarimetric observations and 0.87 mm interferometric continuum observations toward a transitional disk around LkHa 330. As results, a pair of spiral arms were detected in the $H$-band emission and an asymmetric (potentially arm-like) structure was detected in the 0.87 mm continuum emission. We discuss the origin of the spiral arm and the asymmetric structure, and suggest that a massive unseen planet is the most plausible explanation. The possibility of dust trapping and grain growth causing the asymmetric structure was also investigated through the opacity index (beta) by plotting the observed SED slope between 0.87 mm from our SMA observation and 1.3 mm from literature. The results imply that grains are indistinguishable from ISM-like dust in the east side ($beta = 2.0 pm 0.5$), but much smaller in the west side $beta = 0.7^{+0.5}_{-0.4}$, indicating differential dust size distribution between the two sides of the disk. Combining the results of near-infrared and submillimeter observations, we conjecture that the spiral arms exist at the upper surface and an asymmetric structure resides in the disk interior. Future observations at centimeter wavelengths and differential polarization imaging in other bands (Y to K) with extreme AO imagers are required to understand how large dust grains form and to further explore the dust distribution in the disk.