Discovery of a Debris Disk Around TWA 20

TL;DR

This study reports the JWST/NIRCam coronagraphic discovery of a debris disk around the nearby M3 star TWA 20, demonstrating JWST's high sensitivity to cool disks around low-luminosity hosts. Using reference-star differential imaging and model-constrained PSF subtraction, the team forward-modeled the disk with a GRaTer-based structure to derive $r_0\approx64.7$ AU, $i\approx70^{\circ}$, and $\mathrm{PA}\approx-133^{\circ}$, while identifying a nebulous non-axisymmetric feature. No companions are detected down to approximately $0.3\,M_J$ at 80 AU in F444W and $2\,M_J$ in F200W, setting meaningful mass-sensitivity constraints and implying no Jupiter-mass perturber beyond $\sim48$ AU. The results place TWA 20 among a growing sample of M-dwarf debris disks and underscore JWST's pivotal role in expanding our census of planetary-system architectures around low-mass stars, including potential insights into grain properties and disk dynamics.

Abstract

We report the discovery of a debris disk surrounding the M3 star, TWA 20, revealed by JWST coronagraphic observations using the Near-infrared Camera (NIRCam). With reference-star differential imaging (RDI), we resolve the disk in scattered light in the F200W filter at a high signal-to-noise ratio and in the F444W filter at a low signal-to-noise ratio. The disk morphology and orientation are characterized via a forward modeling approach, where we determine a radius of 64.7-6.5+6.2 AU and an inclination of 70.1-3.3+2.5 deg. Utilizing our forward model, we improve the fidelity of the debris disk image using model-constrained RDI (MCRDI). The newly discovered disk is one of only 6 disks detected in scattered light that orbit M dwarf stars; it is the third largest of the 6 resolved M dwarf disks and orbits the third faintest host star. The detection of this disk exemplifies the sensitivity of JWST to debris disks around low-luminosity host stars, which have historically been difficult to detect because these disks are cool and dim. We identify a nebulous structure that cannot be explained by an axisymmetric disk. A search for companions in the TWA 20 system yields no candidates.

Figures (11)

• Figure 1: Final MCRDI data reductions for TWA 20 in the F200W (left) and F444W (right) filters. The images are rotated North up, and the $+$ represents the star location. Gaussian smoothing is applied to each image. Each tick represents 1$\hbox{$^{\prime\prime}$}$ ($\sim$80 au). The flux is log scaled in these images.
• Figure 2: Optimized analytical disk model (left) and the optimized convolved disk model (right) constructed via forward modeling the data in the F200W filter.
• Figure 3: Comparison of the forwarded modeled disk and MCRDI image in each filter. Residuals in the F200W filter show some slight over-subtraction, indicating potential over-fitting of the disk to the data. Given how faint the disk is in the F444W image, it is unclear how good of a fit the model is to the data. The smoothness of the model compared to the data is simply because the model is constructed (and displayed) without any noise, while the data has various noise sources.
• Figure 4: Left: Disk-subtracted RDI image of the TWA 20 system in the F444W filter. Each off-axis point source is identified by a circle and a label. Middle: Disk-subtracted RDI image of the TWA 20 system in the F444W filter zoomed in to the F200W FOV. Right: Disk-subtracted RDI image of the TWA 20 system in the F200W filter.
• Figure 5: Color-magnitude diagram depicting the off-axis point source identified in both filters as well as the expected color-magnitude relationship for Jupiter-mass planets of a similar age to the TW Hydrae association (computed with SPECIES with planetary isochrone data from linder2019evolutionaryphillips2020new). The point source clearly does not follow this trend, indicating it is likely a background source.