Johan Olofsson

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.

Context. Some debris discs contain cold molecular gas. The origin of this gas is one of the key questions in our understanding of the evolution of planetary systems. The effect of gas on the smallest dust grains, which are expected to be unbound due to radiation pressure, is not well understood. The unbound grains contribute to the brightness of scattered light and thermal emission from the disc halo at short wavelengths. Aims. We explored the conditions under which gas can prevent or slow down the escape of unbound grains and increase their number density in the disc halo sufficiently to affect its brightness and reveal properties of the gas. Methods. We numerically integrated the equations of motion of dust grains released on unbound orbits in optically thin, gaseous debris discs, taking into account stellar gravity, radiation pressure, Poynting-Robertson drag, and gas drag. Results. We find that gas drag can significantly slow down the escape of unbound grains only for very high gas masses, such as those expected if the gas is remnant protoplanetary gas and not of secondary origin. We also present an analytical model for estimating the density of unbound grains in gaseous discs. Conclusions. If a difference in the abundance of unbound grains between gaseous and non-gaseous debris discs were observed, it would imply that the gas in debris discs is protoplanetary. Currently, there is no evidence of such a difference, suggesting that the gas detected in debris discs is either secondary or strongly depleted protoplanetary gas.

Context. The young hierarchical quadruple system HD 98800 is composed of the tight binaries AaAb and BaBb on a wide, highly inclined outer orbit. The B subsystem hosts a circumbinary disc in a polar configuration, and the geometry of the system offers a rare opportunity to observe the passage of the disc around BaBb in front of AaAb. Aims. We update the overall orbital solution to offer precise time windows of the main occultation features by combining the orbit based on the most recent knowledge of the disc structure. Methods. We combined new and published radial velocity measurements and multi-wavelength astrometry for the outer AB orbit and the two inner subsystems, AaAb and BaBb, in a joint orbital fit. Results. The revised solution is consistent with previous dynamical masses and improves the outer orbit, reducing the uncertainties in the period and periastron epoch by about a factor of two. It also narrows the predicted crossing-phase windows to 5–15 days at the 1 σ level, improving the timing of ingress, egress, and the first cavity-crossing predictions. Conclusions. These results provide a more accurate timing framework for future observations of the occultation, although the predicted epochs remain model-dependent because of uncertainties in the disc structure.

Context . The dust observed in debris disks is the result of a collisional cascade initiated from approximately kilometer-sized parent bodies. Using near-infrared to submillimeter observations, we can probe particle sizes spanning 2-3 orders of magnitude, and with sufficient angular resolution we can follow the dynamics of these dust particles. Observations taken as part of the ALMA survey to Resolve exoKuiper belt Substructures (ARKS) program allowed for a detailed comparison with near-infrared scattered light observations, at an unprecedented resolution. Aims . The comparison between the two wavelength regimes reveals that for most gas-bearing debris disks, the distribution of small dust grains peaks outside the distribution of large dust grains. In this paper, we investigate whether gas-dust interactions can explain such radial offsets. Methods . We performed numerical simulations that account for the effects of radiation pressure, gas drag, and collisions, and computed surface brightness profiles at several wavelengths to assess which parameters drive these radial offsets. We explored several families of models, varying the gas mass, disk optical depth, dust size distribution, and radiation pressure strength. Results . We find that while higher gas masses lead to more efficient outward radial drift, the resulting radial offset strongly depends on the optical depth of the disk, as the drift efficiency directly competes with the particles’ collisional lifetime. We also find that increasing the relative number of micron-sized dust grains usually yields a larger radial offset between scattered light and millimeter observations. Finally, we show that mid-infrared observations can complement near-infrared and submillimeter images, and we discuss the formation of secondary rings at near-infrared wavelengths. Conclusions . The angular resolution achieved by the ARKS program has opened a new avenue for studying the dynamics of dust particles in debris disks, revealing unexpected differences between the appearance of the disks scattered light and thermal emission. We show that gas-dust interactions can explain the observed radial offsets and provide pointers as to which parameters have the most significant impact.

Context. Scattering phase functions (SPFs) derived from resolved scattered-light images of debris discs are widely used to infer dust grain properties, often through parametric descriptions such as the Henyey-Greenstein (HG) phase function. However, the extent to which the inferred scattering behaviour reflects the intrinsic dust properties, rather than projection effects, disc geometry, or methodological choices, remains uncertain. Aims. We investigate how reliably scattering phase functions and HG asymmetry parameters can be recovered from scattered-light images and identify the regimes in which geometric and methodological effects introduce significant biases. Methods. For the first time, we performed a systematic test of the reliability of SPF determinations using a physically motivated forward-modelling framework that combines dust-scattering calculations, size-dependent grain dynamics, and ray-tracing imaging to generate synthetic total-intensity scattered-light observations. Because the intrinsic SPFs are known a priori, surface-brightness phase functions extracted from the images can be directly compared to the input scattering behaviour. We explored a grid of grain size distributions, disc inclinations, and opening angles, and we fitted two-component HG functions to evaluate how reliably the forwardscattering parameter g ₁ traces the underlying dust properties. Results. Even under idealised conditions with perfect knowledge of the disc geometry, the extracted phase functions can differ substantially from the intrinsic SPFs. First, the limited range of accessible scattering angles plays a dominant role, as the smallest angles, where the strongest forward-scattering peaks occur, are generally unobservable. As a result, the apparent scattering behaviour does not vary monotonically with grain size: Large grains may not appear as forward-scattering, while grains comparable to the observing wavelength can appear comparatively more anisotropic. Second, projection effects, line-of-sight mixing, and SPF-extraction choices further modify the recovered phase functions. Consequently, the fitted HG asymmetry parameter g ₁ depends strongly on viewing geometry and methodology and does not uniquely trace grain properties. Conclusions. Scattering phase functions and HG asymmetry parameters derived from scattered-light images should be interpreted as effective observation-dependent quantities rather than direct proxies of dust properties. Robust interpretation of debris-disc images therefore requires forward modelling that accounts for projection effects, limited scattering-angle coverage, and observational biases.

Context . Debris discs are populated by belts of planetesimals, whose structure carries dynamical imprints of planets and the formation and evolutionary history of the planetary system. The relatively faint emission of debris discs has previously made it challenging to obtain a large sample of high-resolution ALMA images to characterise their substructures. Aims . The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) was recently completed to cover the lack of high-resolution observations and to investigate the prevalence of substructures such as radial gaps and rings in a sample of 24 debris discs. This study characterises the radial structure of debris discs in the ARKS programme. Methods . We modelled all discs with a range of non-parametric and parametric approaches, including those that deconvolve and deproject the image or fit the visibilities directly, in order to identify and quantify the disc substructures. Results . Across the sample we find that of the 24 discs, 5 host multiple rings, 7 are single rings that display halos or additional low-amplitude rings, and 12 are single rings with at most tentative evidence of additional substructures. The fractional ring widths that we measured are significantly narrower than previously derived values, and they follow a distribution similar to the fractional widths of individual rings resolved in protoplanetary discs. However, there exists a population of rings in debris discs that are significantly wider than those in protoplanetary discs. We also find that discs with steep inner edges consistent with planet sculpting tend to be found at smaller (Conclusions . While our findings suggest that it may be possible for some debris discs to inherit their structures directly from pro-toplanetary discs, there exists a sizeable population of broad debris discs that cannot be explained in this way. Assuming that the distribution of millimetre dust reflects the distribution of planetesimals, mechanisms that cause rings in protoplanetary discs to migrate or debris discs to broaden soon after formation may be at play, possibly mediated by planetary migration or scattering.

Context . Debris discs are analogues to our own Kuiper belt around main-sequence stars and are therefore referred to as exoKuiper belts. They have been resolved at high angular resolution at wavelengths spanning the optical/near-infrared to the submillimetre-millimetre regime. Short wavelengths can probe the light scattered by such discs, which is dominated by micron-sized dust particles, while millimetre wavelengths can probe the thermal emission of millimetre-sized particles. Determining differences in the dust distribution between millimetre- and micron-sized dust is fundamental to revealing the dynamical processes affecting the dust in debris discs. Aims . We aim to compare the scattered light from the discs of the ‘ALMA survey to Resolve exoKuiper belt Substructures’ (ARKS) with the thermal emission probed by ALMA. We focus on the radial distribution of the dust, and we also put constraints on the presence of giant planets in those systems. Methods . We used high-contrast scattered light observations obtained with VLT/SPHERE, GPI, and the HST to uniformly study the dust distribution in those systems and compare it to the dust distribution extracted from the ALMA observations carried out in the course of the ARKS project. We also set constraints on the presence of planets by using these high-contrast images combined with exoplanet evolutionary models. Results . Fifteen of the 24 discs comprising the ARKS sample are detected in scattered light, with TYC 9340-437-1 being imaged for the first time at near-infrared wavelengths. For six of those 15 discs, the dust surface density seen in scattered light peaks farther out compared to that observed with ALMA. These six discs except one are known to also host cold CO gas. Conversely, the systems without significant offsets are not known to host gas, except one. Moreover, with our scattered light near-infrared images, we achieve typical sensitivities to planets from 1 to 10 M _Jup beyond 10 to 20 au, depending on the system age and distance. Conclusions . This observational study suggests that the presence of gas in debris discs may affect the small and large grains differently, pushing the small dust to greater distances where the gas is less abundant.

Context . ExoKuiper belts around young A-type stars often host CO gas, whose origin is still unclear. The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) includes six of these gas-bearing belts, to characterise their dust and gas distributions and investigate the gas origin. Aims . As part of ARKS, we observed the gas-rich system HD 121617 with a 0 _⋅ ^′′ 12 (14 au) resolution and discovered an arc of enhanced dust density. In this paper, we analyse in detail the dust and gas distributions and the gas kinematics of this system. Methods . We extracted radial and azimuthal profiles of the dust (in the millimetre and near-infrared) and gas emission ( ¹² CO and ¹³ CO) from reconstructed images. To constrain the morphology of the arc, we fitted an asymmetric model to the dust emission. To characterise the gas kinematics, we fitted a Keplerian model to the velocity map and extracted the gas azimuthal velocity profile by deprojecting the data. Results . We find that the dust arc is narrow (1–5 au wide at a radius of 75 au), azimuthally extended with a full width at half maximum of ~90°, and asymmetric; the emission is more azimuthally compact in the direction of the system’s rotation, and represents 13% of the total dust mass (0.2 M _⊕ ). From analysis of the scattered light and CO images, we conclude that the arc is much less pronounced or absent for small grains and gas. Finally, we find strong non-Keplerian azimuthal velocities at the inner and outer wings of the ring, as was expected due to strong pressure gradients. Conclusions . The dust arc resembles the asymmetries found in protoplanetary discs, often interpreted as the result of dust trapping in vortices. If the gas disc mass is high enough (≳20 M _⊕ , requiring a primordial gas origin), both the radial confinement of the ring and the azimuthal arc may result from dust grains responding to gas drag. Alternatively, it could result from planet-disc interactions via mean motion resonances. Further studies should test these hypotheses and may provide a dynamical gas mass estimate in this CO-rich exoKuiper belt.

Context . Debris disks – collisionally sustained belts of dust and sometimes gas around main sequence stars – are remnants of planet formation processes and are found in systems ≳10 Myr old. Millimeter-wavelength observations are particularly important, as the grains probed by these observations are not strongly affected by radiation pressure and stellar winds, allowing them to probe the dynamics of large bodies producing dust. The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) is analyzing high-resolution observations of 24 debris disks to enable the characterization of debris disk substructures across a large sample for the first time. Aims . For the most highly inclined disks, it is possible to recover the vertical structure of the disk. We aim to model and analyze the most highly inclined systems in the ARKS sample in order to uniformly extract the vertical dust distributions for a sample of well-resolved debris disks. Methods . We employed both parametric and nonparametric methods to constrain the vertical dust distributions for the most highly inclined ARKS targets. Results . We find a broad range of aspect ratios, revealing a wide diversity in vertical structure, with a range of best-fit parametric values of 0.0026 ≤ h _HWHM ≤ 0.193 and a median best-fit value of h _HWHM = 0.021. The results obtained by nonparametric modeling are generally consistent with the parametric modeling results. We find that five of the 13 disks are consistent with having total disk masses less than that of Neptune (17 M _⊕ ), assuming stirring by internal processes (self-stirring and collisional and frictional damping). Furthermore, most systems show a significant preference for a Lorentzian vertical profile rather than a Gaussian.

Context . Dusty discs detected around main-sequence stars are thought to be signs of planetesimal belts in which the dust distribution is shaped by collisional and dynamical processes, including interactions with gas if present. The debris disc around the young A-type star HD 131835 is composed of two dust rings at ∼65 au and ∼100 au, a third unconstrained innermost component, and a gaseous component centred at ∼65 au. New ALMA observations show that the inner of the two dust rings is brighter than the outer one, in contrast with previous observations in scattered light. Aims . We explore two scenarios that could explain these observations: the two dust rings might represent distinct planetesimal belts with different collisional properties, or only the inner ring might contain planetesimals while the outer ring consists entirely of dust that has migrated outwards due to gas drag. Methods . To explore the first scenario, we employed a state-of-the-art collisional evolution code. To test the second scenario, we used a simple dynamical model of dust grain evolution in an optically thin gaseous disc. In each case we identified the parameters of the planetesimal and the gaseous disc that best reproduce the observational constraints. Results . Collisional models of two planetesimal belts cannot fully reproduce the observations by only varying their dynamical excitation, and matching the data through a different material strength requires an extreme difference in dust composition. The gas-driven scenario can reproduce the location of the outer ring and the brightness ratio of the two rings from scattered light observations, but the resulting outer ring is too faint overall in both scattered light and sub-millimetre emission. Conclusions . The dust rings in HD 131835 could be produced from two planetesimal belts, although how these belts would attain the required extremely different properties needs to be explained. The dust-gas interaction is a plausible alternative explanation and deserves further study using a more comprehensive model.