Apocenter pileup and arcs: A narrow dust ring around HD 129590

Abstract

Context. Observations of debris disks have significantly improved over the past decades, both in terms of sensitivity and spatial resolution. At near-infrared wavelengths, new observing strategies and post-processing algorithms allow us to drastically improve the final images and reveal faint structures in the disks. These structures inform us about the properties and spatial distribution of the small dust particles. Aims. We present new H-band observations of the disk around the solar-type star HD 129590, which display an intriguing arc-like structure in total intensity observations but not in total polarimetry, and we propose an explanation for the origin of this arc. Methods. Assuming geometric parameters for the birth ring of planetesimals, our model provides the positions of millions of particles of different sizes to compute scattered light images. The code can either produce images over the full size distribution or over several smaller intervals of grain sizes. Results. We demonstrate that if the grain size distribution is truncated or strongly peaks at a size larger than the radiation pressure blow-out size, we are able to produce an arc quite similar to the one detected in the observations. If the birth ring is radially narrow, given that particles of a given size have similar eccentricities, they will have their apocenters at the same distance from the star. Since this is where the particles spend most of their time, an “apocenter pileup” occurs that can look like a ring. Due to more efficient forward scattering, this arc only appears in total intensity observations and remains undetected in polarimetric data, which is in good agreement with our observations. Conclusions. For the secondary ring to be detected, sharp variations either in the grain size distribution or for the scattering efficiencies Qsca (or a combination of both) are required. We show that a wavy size distribution and a size-dependent phase function can strengthen the apocenter pileup. Overall, such arcs are rarely detected in other systems, which can mainly be explained by the fact that most parent belts are usually broad.