Maxime Roumesy

Context. The debris disk surrounding HR 4796 is a bright, narrow ring observed at multiple wavelengths, both in scattered light and thermal emission. The optical properties of the dust particles (scattering phase function, SPF; degree of linear polarization, DoLP; reflectance) that can be retrieved through scattered light imaging are linked to the physicochemical properties of the dust particles (size, shape, composition, etc.), but the retrieving process is challenging. Among debris disks, HR 4796 presents several peculiarities, in particular unusually high DoLP values at small scattering angles. Aims. This study is aimed at improving the constraints on the physicochemical properties of the dust particles orbiting in the disk. Methods. We used multiwavelengths scattered light observations of HR 4796, obtained with SPHERE/IRDIS and SPHERE/ZIMPOL. We forward modeled those observations using a novel joint parametric approach to constrain the morphology, the scattering phase function and the degree of linear polarization of the disk. These observations are then compared to laboratory measurements, previous observations of HR 4796 and of some Solar System asteroids and comets. Results. We present new observational data in scattered light of HR 4796, for λ = 0.63 μm, λ = 0.79 μm, and λ =1.25 μm. We modeled the disk at each of these wavelengths and found geometric parameters consistent with previous results. We obtained the parametrized SPF and DoLP over the whole range of scattering angles (13°-167°) and extracted the DoLP at 90° (without any parametrization) as well as the spectral reflectance from the visible to near-infrared (NIR) wavelength range. We compared the reflectance and polarimetric properties of HR 4796 to those measured on a laboratory dust sample and obtained from previous results on HR 4796. We find that the sample providing the best match to these properties appears to be large (from a few micrometers to 100 µm) iron sulfides particles, such as pyrrhotite and troilite. Such iron sulfides are among the opaque minerals (Fe _x S _y , FeNi, etc.) that contribute to the low albedo of some small bodies of the Solar System, such as B-, C-, D, P-type asteroids, comets, and so on. Conclusions. We confirm that the DoLP of HR4796 peaks at high values (>45%) and for small scattering angles (<55°). At a 90° scattering angle, we observe a red spectral slope in the visible and NIR wavelength range, as well as a blue polarimetric slope. These results are compatible with large (a few micrometers to 100 µm) absorbents being the main scatterers in the disk. We show that the DoLP of HR 4796 is notably different from that of Solar System comets and seems closer to that of some more processed near-Earth orbit asteroids, possibly indicating devolatilization and/or melting induced by space weathering on the dust particles of HR 4796.

We present near- and mid-infrared broadband imaging observations of the edge-on protoplanetary disk 2MASS J16281370-2431391, a.k.a. the “Flying Saucer,” obtained with NIRCam and MIRI on board JWST. These data are combined with archival optical and near-IR broadband imaging from Hubble Space Telescope (HST), millimeter dust continuum emission from Atacama Large Millimeter/submillimeter Array, and optical lightcurves from the K2 and TESS for a comprehensive multiwavelength study. The JWST observations reveal a reversal of the flux ratio between the two reflection nebulae at near-infrared wavelengths (2–3 μ m), which can be explained by a misaligned inner disk. For the first time, JWST reveals the bright lining along the disk edge at 2 and 3 μ m, arising from forward-scattering of background photons by dust grains near the τ ≈ 1 layer. By combining the dust lane thickness measured with HST and JWST, we find the dust grains up to at least ∼3 μ m are vertically mixed throughout the disk and have undergone significant growth. In addition, a comparison of earlier HST/NICMOS J - and K -band images with those from JWST/NIRCam reveals variability in the scattered-light morphology, as well as measurable proper motion of the system with respect to the background stars, confirming that the Flying Saucer is comoving with the young star population of ρ Ophiuchus.

Context. The evolution of protoplanetary disks, especially in the early stages of planetary formation, as dust grows, is the cornerstone of the birth of planets. The mechanisms involved in the growth of sub-micrometric dust grains into planetesimals within a very short time frame are a challenging field of study, while the initial conditions remain relatively undefined. Aims. One of the main challenges is to unambiguously identify the dust properties within the disk, and our goal is to break this barrier by investigating the light scattered by dust particles lying on the protoplanetary disk surface from many recent promising observations. Methods. In this study, we used a set of 30 polarized light images composed of new VLT/SPHERE observations to examine the light scattered by dust grains. For each ring-shaped system, we used the new DRAGyS tool to estimate the disk geometry using the substructures visible on the surface and to extract the limb-brightening-corrected scattering phase function, which encodes the dust grains’ physical properties. Finally, we compared our results with the AggScatVIR database of numerical scattering phase functions of nonspherical dust. Results. We combined our measurement of disk geometry to estimate an average disk flaring of about 1.357. We note some general trends of dust populations in our results. First, we recovered the two categories of scattering phase functions based on their shape, as determined in previous studies. Category I is monotonically decreasing and can be explained by fractal organic aggregates with small monomers of 100nm, or compact aggregates with medium porosity and big monomers of 400nm. Category II is defined by a bellshaped scattering phase function and can be explained by sub-micrometric irregular grains or compact aggregates with low porosity. This statistical study offers general trends about dust populations, but the degeneracy is too strong to apply this method to a unique disk analysis. Conclusions. The extracted scattering phase function from the protoplanetary disk surface is a promising measurement for constraining the properties of dust within the disk. However, their interpretation should be combined with other observations. Multiwavelength investigation, i.e., combining SPFs with disk color measurements, is a promising option.

Context. The early stages of planet formation, involving dust grain growth and planetesimals formation, remain shrouded in mystery. The analysis of the scattering phase function (SPF) measured in disks surrounding young stars holds great potential for revealing crucial information about dust grain properties. Given the rapidly increasing number of high-quality datasets available, an efficient method of extracting the SPF is required. Aims. We developed DRAGyS (Disk Ring Adjusted Geometry yields Scattering phase function), a tool designed for the quick and comprehensive analysis of protoplanetary disks in which gaps and rings are present. DRAGyS directly estimates the disk geometry and extracts the total and polarized SPF from scattered light images, without requiring any radiative transfer modeling, a limitation of previous efforts. Methods. Key disk parameters – the inclination, position angle, and aspect ratio – are obtained by fitting ellipses to the disk intensity peaks from the ring surface, assuming that the disks are circular. We validated the method using simulated disk images and then applied it to archival polarized-intensity images of nine images for six protoplanetary disks. DRAGyS also provides a method of correcting for the effect of limb brightening on the SPF. Results. DRAGyS recovers well the injected geometry and the SPF from synthetic images where the parameters are known. When compared to previously published results extracted from images without taking into account limb brightening, DRAGyS yields similar results for the inclination, disk position angle, and SPF. We show that the effect of limb brightening on the SPF is significant, with consequences for the inference of dust properties. Conclusions. DRAGyS takes advantage of a fast and purely geometrical approach to estimate ringed-disk geometries. It allows for the efficient extraction of the SPF either globally or by sectors, allowing it to deal with disk asymmetries. By bypassing the need for a full modeling of the disk geometry before SPF extraction, DRAGyS is well suited for studying large samples of disk images.

We present near- and mid-infrared (IR) broadband imaging observations of the edge-on protoplanetary disk around HH 30 with the James Webb Space Telescope/Near Infrared Camera and the Mid-Infrared Instrument (MIRI). We combine these observations with archival optical/near-IR scattered light images obtained with the Hubble Space Telescope and a millimeter-wavelength dust continuum image obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) with the highest spatial resolution ever obtained for this target. Our multiwavelength images clearly reveal the vertical and radial segregation of micron-sized and submillimeter-sized grains in the disk. In the near- and mid-IR, the images capture not only bireflection nebulae separated by a dark lane but also diverse dynamical processes occurring in the HH 30 disk, such as spiral- and tail-like structures, a conical outflow, and a collimated jet. In contrast, the ALMA image reveals a flat dust disk in the disk midplane. By performing radiative transfer simulations, we show that grains of about 3 μm in radius or larger are fully vertically mixed to explain the observed mid-IR scattered light flux and its morphology, whereas millimeter-sized grains are settled into a layer with a scale height of ≳1 au at 100 au from the central star. We also find a tension in the disk inclination angle inferred from optical/near-IR and millimeter observations, with the latter being closer to exactly edge-on. Finally, we report the first detection of the proper motion of an emission knot associated with the mid-IR collimated jet detected by combining two epochs of our MIRI 12.8 μm observations.

Context. Observations of highly inclined protoplanetary disks provide a different point of view, in particular, they provide a more direct access to the vertical disk structure when compared to less steeply inclined more pole-on disks. Aims. PDS 453 is a rare highly inclined disk where the stellar photosphere is seen at grazing incidence on the disk surface. Our goal is take advantage of this geometry to constrain the structure and composition of this protoplanetary disk. In particular, it shows a 3.1 µm water-ice band in absorption that can be uniquely related to the disk. Methods. We observed the system in polarized intensity with the VLT/SPHERE instrument, as well as in polarized light and total intensity using the HST/NICMOS camera. Infrared archival photometry and a spectrum showing the water-ice band were used to model the spectral energy distribution under the Mie scattering theory. Based on these data, we fit a model using the radiative transfer code MCFOST to retrieve the geometry and dust and ice content of the disk. Results. PDS 453 has the typical morphology of a highly inclined system with two reflection nebulae in which the disk partially attenuates the stellar light. The upper nebula is brighter than the lower nebula and shows a curved surface brightness profile in polarized intensity. This indicates a ring-like structure. With an inclination of 80° estimated from models, the line of sight crosses the disk surface, and a combination of absorption and scattering by ice-rich dust grains produces the water-ice band. Conclusions. PDS 453 is seen at high inclination and is composed of a mixture of silicate dust and water ice. The radial structure of the disk includes a significant jump in density and scale height at a radius of 70 au that produces a ring-like image. The depth of the 3.1 µm water-ice band depends on the amount of water ice, until it saturates when the optical thickness along the line of sight becomes too large. Therefore, quantifying the exact amount of water from absorption bands in edge-on disks requires a detailed analysis of the disk structure and tailored radiative transfer modeling. Further observations with JWST and ALMA will allow us to refine our understanding of the structure and content of this interesting system.