
 Light echoes of debris disks around active stars can reveal disk structure and composition even when disks are not spatially resolved. Unfortunately, distinguishing reflected light from quiescent starlight and unexpected post-peak flare structure is challenging, especially for edge-on geometries where the time delay between observed flare photons and light scattered from the near side of the disk is short. Here, we take advantage of the fact that scattered light from a dusty disk is polarized, depending on the location of the scattering site and the orientation of the disk relative to a distant observer. Filtering reflected light into its polarized components allows echoes to stand out in predictable ways. We test this idea with a simple model for a disk around an active M dwarf. Our results demonstrate that the use of polarimetric data of flaring stars can significantly enhance echo signals relative to starlight and yield more robust and accurate fits to disk parameters compared to analyses based on the total intensity alone.

Polarization of Circumstellar Debris Disk Light Echoes

A significant fraction of debris discs consist of a bright ring beyond which extends a wide halo. Such a halo should be made of small grains produced in the ring of parent bodies (PB) and pushed on high-e orbits by radiation pressure. It has been shown that, under several simplifying assumptions, the surface brightness (SB) of this halo should radially decrease as $r^{-3.5}$ in scattered light. We aim to revisit the halo phenomenon and focus on two so far unexplored issues: 1) How the unavoidable presence of small unbound grains, non-isotropic scattering phase functions (SPF) and finite instrument resolution affect scattered light SB profiles, and 2) How the halo phenomenon manifests itself at longer wavelengths. We find that unbound grains account for a significant fraction of the halo's luminosity in scattered light, and can significantly flatten the SB radial profile. Realistic size-dependent SPFs also have an effect, resulting here again in shallower SB profiles. For edge-on discs, non-resolving the vertical profile can also flatten the projected SB. We show that roughly half of the observationally-derived halo profiles found in the literature are compatible with our new results, and that roughly half of the remaining systems are probably shaped by additional processes. We also propose that, in future observational studies, the characteristics of PB belt and halos should be fitted separately. In thermal emission, wide halos should remain detectable up to the far-IR and, with the exception of the $\sim 8-15\mu$m domain, the halo accounts for more than half of the system's total flux up to $\lambda\sim80-90\mu$m. The halo's contribution strongly decreases in the sub-mm to mm but still represents a few percents of the system's luminosity at $\lambda\sim 1$mm. For unresolved systems, the presence of a halo can also affect the determination of the disc's radius from its SED.

A reinvestigation of debris disc halos

Context: Circumstellar debris disks provide insight into the formation and early evolution of planetary systems. Resolved belts in particular help to locate planetesimals in exosystems, and can hint at the presence of disk-sculpting exoplanets. Aims: We study the circumstellar environment of HD 112810 (HIP 63439), a mid-F type star in the Sco-Cen association with a significant infrared excess indicating the presence of a circumstellar debris disk. Methods: We collected five high-contrast observations of HD 112810 with VLT/SPHERE. We identified a debris disk in scattered light, and found that the debris signature is robust over a number of epochs and a variety of reduction techniques. We modelled the disk, accounting for self-subtraction and assuming that it is optically thin. Results: We find a single-belt debris disk, with a radius of 118$\pm$9au and an inclination angle of ${75.7}^{+1.1}_{-1.3}$$\deg$. This is in good agreement with the constraints from SED modelling and from a partially-resolved ALMA image of the system. No planets are detected, though planets below the detection limit ($\sim$2.6M$_\textrm{J}$ at a projected separation of 118au) could be present and could have contributed to sculpting the ring of debris. Conclusions: HD 112810 adds to the growing inventory of debris disks imaged in scattered light. The disk is faint, but the radius and the inclination of the disk are promising for follow-up studies of the dust properties.

The first scattered light images of HD 112810, a faint debris disk in the Sco-Cen association

Debris disks are the signposts of collisionally eroding planetesimal circumstellar belts, whose study can put important constraints on the structure of extrasolar planetary systems. The best constraints on the morphology of disks are often obtained from spatially resolved observations in scattered light. Here, we investigate the young (~16 Myr) bright gas-rich debris disk around HD121617. We use new scattered-light observations with VLT/SPHERE to characterize the morphology and the dust properties of this disk. From these properties we can then derive constraints on the physical and dynamical environment of this system, for which significant amounts of gas have been detected. The disk morphology is constrained by linear-polarimetric observations in the J band. Based on our modeling results and archival photometry, we also model the SED to put constraints on the total dust mass and the dust size distribution. We explore different scenarios that could explain these new constraints. We present the first resolved image in scattered light of the debris disk HD121617. We fit the morphology of the disk, finding a semi-major axis of 78.3$\pm$0.2 au, an inclination of 43.1$\pm$0.2{\deg} and a position angle of the major axis with respect to north, of 239.8$\pm$0.3{\deg}, compatible with the previous continuum and CO detection with ALMA. Our analysis shows that the disk has a very sharp inner edge, possibly sculpted by a yet-undetected planet or gas drag. While less sharp, its outer edge is steeper than expected for unperturbed disks, which could also be due to a planet or gas drag, but future observations probing the system farther from the main belt would help explore this further. The SED analysis leads to a dust mass of 0.21$\pm$0.02 M$_{\oplus}$ and a minimum grain size of 0.87$\pm$0.12 $\mu$m, smaller than the blowout size by radiation pressure, which is not unexpected for very bright col...

Morphology of the gas-rich debris disk around HD121617 with SPHERE observations in polarized light

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Morphology of the gas-rich debris disk around HD121617 with SPHERE observations in polarized light