Poster Presentation

Analysis of Asymmetric Structures in the IM Lup dusty Disk: Estimating the spiral pattern speed

Author(s): Yuto Hiraide (Ibaraki University), Masataka Aizawa (Ibaraki University),Munetake Momose (National Astronomical Observatory of Japan)

Presenter: Yuto Hiraide (Ibaraki Univercity)

Revealing the origin of spiral structures in protoplanetary disks is important for understanding disk evolution and planet formation.
Planet-disk interaction and gravitational instability (GI) are the two most widely discussed mechanisms for producing spiral structures.
In recent years, spiral pattern-speed measurements have attracted attention as a diagnostic of their origin: planet-driven spirals are expected to rotate approximately as a rigid pattern, whereas GI-driven spirals are expected to follow the local Keplerian rotation.

A previous study measured the pattern speed of the prominent m=2 spiral structure in the IM Lup dust disk using multi-epoch ALMA observations, and reported a result consistent with local Keplerian rotation, supporting a GI-driven origin.
However, spiral emission is relatively weak compared with the background disk emission, and precise extraction of the spiral structure requires accurate modeling of the underlying disk.
In this study, we reanalyze the multi-epoch ALMA data of IM Lup using the public code protomidpy, which directly infers a geometrically thin, axisymmetric background disk model from visibilities, and perform a pattern-speed analysis of the extracted spiral residuals with the updated disk geometry.

In contrast to the previous study, we find that the measured pattern speed tends to deviate from local Keplerian rotation.
By investigating the origin of this discrepancy, we find that the previous analysis used a disk geometry based on earlier geometric estimates, and that this geometry may have been biased by a few degrees.
We conclude that systematic residuals caused by this geometry bias distorted the measured spiral phase and apparently produced a local Keplerian pattern-speed signal.
This study not only provides a framework for spiral pattern-speed measurements but also demonstrates that the precise determination of the disk’s geometric parameters is of critical importance in this context. The application to other spirals will enable a systematic investigation of the origins of spiral structures in protoplanetary disks.