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In the last few years multiwavelength observations have revealed the ubiquity of gaps/rings in circumstellar discs. Here we report the first ALMA observations of HD 92945 at 0.86 mm, that reveal a gap at about 73 ± 3 au within a broad disc of planetesimals that extends from 50 to 140 au. We find that the gap is 20 +10 −8 au wide. If cleared by a planet in situ, this planet must be less massive than 0.6 M Jup , or even lower if the gap was cleared by a planet that formed early in the protoplanetary disc and prevented planetesimal formation at that radius. By comparing opposite sides of the disc we also find that the disc could be asymmetric. Motivated by the asymmetry and the fact that planets might be more frequent closer to the star in exoplanetary systems, we show that the gap and asymmetry could be produced by two planets interior to the disc through secular resonances. These planets excite the eccentricity of bodies at specific disc locations, opening radial gaps in the planetesimal distribution. New observations are necessary to confirm if the disc is truly asymmetric, thus favouring the secular resonance model, or if the apparent asymmetry is due to a background galaxy, favouring the in-situ planet scenario. Finally, we also report the non-detection of CO and HCN gas confirming that no primordial gas is present. The CO and HCN non-detections are consistent with the destruction of volatile-rich Solar System-like comets.

We compile a sample of 341 binary and multiple star systems with the aim of searching for and characterising Kuiper belt-like debris discs. The sample is assembled by combining several smaller samples studied in previously published work with targets from two unpublished Herschel surveys. We find that 38 systems show excess emission at 70 or 100 µm suggestive of a debris disc. While nine of the discs appear to be unstable to perturbations from their host binary based on a simple analysis of their inferred radii, we argue that the evidence for genuine instability is not strong, primarily because of uncertainty in the true disc radii, uncertainty in the boundaries of the unstable regions, and orbital projection effects. The binary separation distributions of the disc-bearing and disc-free systems are different at a confidence level of 99.4%, indicating that binary separation strongly influences the presence of detectable levels of debris. No discs are detected for separations between ∼25 and 135 au; this is likely a result of binaries whose separations are comparable with typical disc radii clearing out their primordial circumstellar or circumbinary material via dynamical perturbations. The disc detection rate is 19 +5 −3 % for binaries wider than 135 au, similar to published results for single stars. Only 8 +2 −1 % of systems with separations below 25 au host a detectable disc, which may suggest that planetesimal formation is inhibited in binaries closer than a few tens of au, similar to the conclusions of studies of known planet-hosting binaries.

The young A0V star HR 4796A is host to a bright and narrow ring of dust, thought to originate in collisions between planetesimals within a belt analogous to the Solar System's Edgeworth-Kuiper belt. Here we present high spatial resolution 880µm continuum images from the Atacama Large Millimeter Array. The 80au radius dust ring is resolved radially with a characteristic width of 10au, consistent with the narrow profile seen in scattered light. Our modelling consistently finds that the disk is also vertically resolved with a similar extent. However, this extent is less than the beam size, and a disk that is dynamically very cold (i.e. vertically thin) provides a better theoretical explanation for the narrow scattered light profile, so we remain cautious about this conclusion. We do not detect 12 CO J=3-2 emission, concluding that unless the disk is dynamically cold the CO+CO 2 ice content of the planetesimals is of order a few percent or less. We consider the range of semi-major axes and masses of an interior planet supposed to cause the ring's eccentricity, finding that such a planet should be more massive than Neptune and orbit beyond 40au. Independent of our ALMA observations, we note a conflict between mid-IR pericenter-glow and scattered light imaging interpretations, concluding that models where the spatial dust density and grain size vary around the ring should be explored.

We investigate the evolution on secular time-scales of a radially extended debris disc under the influence of a system of two coplanar planets interior to the disc, showing that the secular resonances of the system can produce a depleted region in the disc by exciting the eccentricities of planetesimals. Using Laplace-Lagrange theory, we consider how the two exterior secular resonance locations, time-scales and widths depend on the masses, semi-major axes and eccentricities of the planets. In particular, we find that unless the resonances are very close to each other, one of them is very narrow and therefore unimportant for determining the observable structure of the disc. We apply these considerations to the debris disc of HD 107146, identifying combinations of the parameters of a possible unobserved two-planet system that could configure the secular resonances appropriately to reproduce the depletion observed in the disc. By performing N -body simulations of such systems, we find that planetesimal eccentricities do indeed become large near the theoretical secular resonance locations. The N -body output is postprocessed to set the initial surface density profile of the disc, and to include the possible effects of collisional depletion. We find that it is possible to obtain a double-ringed disc in these simulations but not an axisymmetric one, with the inner ring having an offset whose magnitude depends on the eccentricities of the planets, and the outer ring showing spiral structure.