A Layered Debris Disk around M Star TWA 7 in Scattered Light

TL;DR

This study presents detailed multi-instrument observations of the layered debris disk around M star TWA 7, revealing a complex architecture with rings, a spiral, and a clump, and discusses implications for dust properties and potential planet detection.

Contribution

First detailed multi-wavelength scattered light imaging of TWA 7's debris disk, revealing its layered structure and dynamic features, advancing understanding of M star debris disks.

Findings

Discovery of a tertiary ring and a clump in the disk.

Identification of a spiral structure with possible Keplerian motion.

Evidence of small dust particles based on polarization and color analysis.

Abstract

We have obtained Hubble Space Telescope (HST) coronagraphic observations of the circumstellar disk around M star TWA 7 using the STIS instrument in visible light. Together with archival observations including HST/NICMOS using the F160W filter and Very Large Telescope/SPHERE at $H$-band in polarized light, we investigate the system in scattered light. By studying this nearly face-on system using geometric disk models and Henyey--Greenstein phase functions, we report new discovery of a tertiary ring and a clump. We identify a layered architecture: three rings, a spiral, and an ${\approx}150$ au$^2$ elliptical clump. The most extended ring peaks at $28$ au, and the other components are on its outskirts. Our point source detection limit calculations demonstrate the necessity of disk modeling in imaging fainter planets. Morphologically, we witness a clockwise spiral motion, and the motion pattern is consistent with both solid body and local Keplerian; we also observe underdensity regions for the secondary ring that might result from mean motion resonance or moving shadows: both call for re-observations to determine their nature. Comparing multi-instrument observations, we obtain blue STIS-NICMOS color, STIS-SPHERE radial distribution peak difference for the tertiary ring, and high SPHERE-NICMOS polarization fraction; these aspects indicate that TWA 7 could retain small dust particles. By viewing the debris disk around M star TWA 7 at a nearly face-on vantage point, our study allows for the understanding of such disks in scattered light in both system architecture and dust property.