Gaps and Rings: A Near-Universal Trait of Extended Protoplanetary Discs

TL;DR

This work targets the long-standing question of whether extended protoplanetary discs universally host substructures. Using ~0.12" ALMA Band 6 imaging of 26 extended discs within 200 pc, the authors model the dust continuum in the visibility domain with Frankenstein and Galario, deriving ring/shoulder configurations and disc sizes. They find that ~91% of extended discs show substructures, rising to ~0.98% after correcting for high-inclination systems, indicating a near-universal presence of rings, gaps, or shoulders in extended discs. The results imply dust traps, possibly carved by giant planets, are a common feature shaping disc morphologies and have important implications for dust evolution and planet formation theories.

Abstract

Substructures such as rings, gaps, and cavities are commonly observed in protoplanetary discs and are thought to play a key role in dust evolution and planet formation. However, a fraction of the extended discs (68% dust radii > 30 AU) in nearby star-forming regions remain unresolved, leaving their substructure content uncertain and thereby limiting our understanding of dust evolution and the initial conditions for planet formation across the full disc population. We aim to investigate the presence of substructures in previously unresolved, extended discs to assess whether all extended protoplanetary discs in the Solar neighbourhood exhibit substructures. We present new high-resolution ($\sim$0.12") ALMA Band 6 continuum observations at 1.33 mm of 26 previously unresolved, extended discs within 200 pc, completing the high-resolution sample of extended discs in Taurus, Ophiuchus, Chamaeleon, Lupus, Upper Scorpius, Upper Centaurus-Lupus and Lower Centaurus-Crux. We analyse radial intensity profiles using Frankenstein and Galario to detect substructures. Seventeen discs show clear substructures, while nine appear compact and structureless, smooth or ambiguous due to inclination or possible binarity/late-stage infall. We detect $^{12}$CO J=2-1 emission in 15 discs, with extended emission in four. Combined with literature data, our complete sample of 730 protoplanetary discs reveals that nearly all extended discs exhibit substructures, $\sim$91% detected in the full sample, and up to $\sim$98% when correcting for high-inclination systems where substructures may be hidden. Substructures are a near-universal feature of extended protoplanetary discs. They are more commonly detected in larger, massive discs and around higher-mass stars, and structured discs retain their dust mass over time. This supports the scenario in which dust traps, possibly induced by giant planets, shape disc morphologies.

Figures (14)

• Figure 1: Gallery of the 1.3 mm cleaned continuum emission images for the full sample. All images were cleaned using a Briggs weighting scheme with a robust value of +0.5, except for SSTc2d J163952.9-241931 and SSTc2d J162718.4-243915 (robust = 0.0), and SSTc2d J162823.3-242241 and 2MASS J11094742-7726290 (robust = -0.5). RA and Dec offsets from the disc centre are shown in arcseconds on the x- and y-axes, respectively. To emphasise the weak outer emission of a few discs, an asinh scaling function was applied. Each panel spans $2.0\text{"}\times2.0\text{"}$, with white ellipses in the lower left corner indicating the beam sizes. The colour scale represents the intensity in Jy/beam. The final disc classifications are marked in the lower right corner: TD (transition disc), RD (ring disc), SD (shoulder disc), HID (highly-inclined disc), CD (compact disc), ED (extended disc). For discs with names starting with SSTc2d, 2MASS, or RX, the names in the upper left corners are abbreviated to the first part after "J" (e.g., SSTc2d J162823.3-242241 is labelled as J162823.3). The white star in 2MASS J11095340-7634255 marks the current position of the central star.
• Figure 2: Azimuthally averaged radial brightness profiles for all 26 discs (grey), overlaid with the best-fit model profiles (red) for the 25 modelled discs. Ring and shoulder locations in the models are marked with R and S, respectively. The corresponding physical radii in AU are indicated in green above each panel. For discs with names starting with SSTc2d, 2MASS, or RX, the names in the upper right corners are abbreviated to the first part after "J" (e.g., SSTc2d J162823.3-242241 is labelled as J162823.3).
• Figure 3: Ring and shoulder widths as a function of their radial locations. Ring substructures are shown in blue, and shoulder substructures are shown in purple.
• Figure 4: Gallery of the high-resolution $^{12}$CO J=2-1 moment maps for 2MASS J11095340-7634255, constructed from emission exceeding the 3$\sigma$ threshold. The top row shows a $20.0\text{"}\times20.0\text{"}$ field of view, while the bottom row provides a zoomed-in view of a $4.6\text{"}\times4.6\text{"}$ region around the disc. The moment 0 maps (left column) display integrated intensity in Jy/beam km/s, and the moment 1 maps (right column) show velocity in km/s. Axes indicate RA and Dec offsets from the disc centre in arcseconds. The white ellipse in the moment 0 maps represents the beam size. The black star in the zoomed-in moment 1 map marks the current position of the central star.
• Figure 5: Dust disc size as a function of dust disc mass for all detected discs in the complete sample. Transition discs are shown in red, ring discs in blue, shoulder discs in purple, highly-inclined discs in orange, extended discs in green, and compact discs with no detected substructures in black. Circles represent detections; upper limits are indicated by triangles.