Institut de Planétologie et d'Astrophysique de Grenoble

In the past few years, ALMA unveiled a variety of substructures (rings, spirals, crescents) in the continuum emission of most protoplanetary disks imaged at high spatial resolution. While the majority of disks presents axisymmetric ring-like structures in the dust brightness distribution, some sources display asymmetric morphologies (blobs, crescents) that have been often associated to vortices and/or mechanisms generated by the presence of one or more embedded planets. In this brief research report we present the analysis of the arc structure observed in the dust continuum emission of the disk around HD 163296, using high resolution ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m1"> <mml:mrow> <mml:mo>∼</mml:mo> </mml:mrow> </mml:math> 8 au) matched continuum data from ALMA at four wavelengths. We characterize in detail the arc structures and present a kinematic signature observed in the CS(3–2) emission at the same location. Our results indicate that the crescent is caused by differential dust trapping in a local pressure maxima, for which plausible mechanisms can be the presence of a vortex or trapping in a Lagrangian point of the planet-star system.

Context . Studying young proto-planetary discs is essential for understanding planet formation, but traditional angular differential imaging introduces self-subtraction artefacts that make their small-scale structure difficult to interpret. We present high-resolution total- and polarised-intensity K s -band images of the LkCa 15 system that are free of such artefacts. Aims . LkCa 15 is a young proto-planetary system with a ~160 au disc and previous claims of two protoplanet candidates at 15 and 18 au. We aim to analyse the LkCa 15 proto-planetary disc using high-contrast imaging to search for super-Jupiter planets beyond 20 au and to characterise the dust distribution and grain composition. Methods . We used near-simultaneous reference-star differential imaging (RDI, ‘star-hopping’) to obtain self-subtraction-free K s -band images beyond 0.1″. We first modelled the K s -band total- and polarised-intensity images together with ALMA submillimetre continuum maps using RADMC-3D and a two grain-size (micron and millimetre) compact olivine model. Residual mismatches in the near-IR then motivated us to extract the scattering phase function, S ( θ ), and polarised fraction, P ( θ ), from the SPHERE data and compare them with aggregate-scattering models, which pointed to porous CAHP grains in the surface layer and led us to recompute the NIR scattered-light models with CAHP. Results . Our initial two grain-size (micron and millimetre) olivine model roughly reproduces the observed NIR and ALMA disc morphology, with a flared micron surface layer from ~25-85 au ( H / R ~ 0.08 at 50 au; surface gap ~35-40 au) and a millimetre mid-plane ring from ~55-130 au with a gap at ~75-100 au, for i ~ 50° and PA ~ 61°. The near-IR data, however, are less forward-scattering than the model. From the phase functions, we find that S ( θ ) rises by ~5× from θ ~ 90° to θ ~ 35°, while P ( θ ) shows a broad sub-Rayleigh peak with P max ~ 0.35 near θ ~ 90°. These analyses disfavour compact olivine Mie spheres and are better matched by porous aggregates (CAHP-128-100 nm), so we recomputed the NIR scattered-light models with CAHP-128-100 nm grains in the surface layer (retaining compact millimetre grains for the ALMA continuum), which improves the match to the K s -band morphology and polarisation. From the number ratio between the 12 μm and 2 mm grains, we inferred a size-distribution slope of ζ ~ −2.3. Although no new candidate planets were detected, we estimated upper mass limits: beyond 200 au, planets more massive than ~1.5 M J are unlikely, while in the inner disc planets up to ~3.6 M J could remain undetected. Conclusions . The star-hopping RDI data, together with phase-function diagnostics and RADMC-3D modelling with compact olivine and porous CAHP grains, allow us to reproduce the main observed features of the LkCa 15 system. The number ratio between the 12 μm and 2 mm olivine grains further shows that micron-sized grains are under-abundant relative to size distributions in the ISM or debris discs, providing new insights into grain growth and dust dynamics in gas-rich proto-planetary discs.

Accretion processes in the planetary-mass regime remain poorly constrained, yet they strongly influence planet formation, evolution, and the composition of circumplanetary disks (CPDs). We investigate the resolved Balmer hydrogen emission-line profiles and their variability in the ~13Mjup, 30-45 Myr-old companion Delorme to constrain the underlying accretion mechanisms. Using VLT/UVES, we obtained 31 new epochs of high-resolution optical spectra (330-680 nm, R = 50,000), probing variability from hours to years. We analyze the shape and flux variability of hydrogen emission lines and compare them to two proposed origins: magnetospheric accretion funnels and localized accretion shocks. We detect Balmer lines from Halpha to H10 (6564-3799 AA) and a UV continuum excess, both indicative of ongoing accretion. All features are variable. The hydrogen lines decompose into two static components that vary only in flux. The broader velocity component correlates strongly with the UV excess and is qualitatively consistent with magnetospheric funnel models, but not with shock models. This component dominates the shape variability. The narrower component, which correlates less with the UV excess, is better matched by shock-emission models and drives most of the flux variability. Line fluxes show low variability on hour timescales but up to ~100% over weeks, similar to T Tauri stars. Our findings support magnetospheric accretion as the origin of the broad component. The narrow component may arise from accretion shocks or chromospheric activity. Higher-cadence observations could reveal rotational modulations and help constrain the object's rotation period and accretion geometry.

Context . Medium-resolution integral field spectrographs (IFS), such as the Multi-Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT), are equipped to detect the emission lines (e.g., H α , H β ) of faint accreting companions when associated with dedicated stellar halo subtraction methods. We recently proposed a new approach based on polynomial modulations of a stellar spectrum estimate across the field of view, with orthogonal polynomials and lines masking. This new technique is designed to better preserve both continuum and emission lines of accreting companions. Aims . We seek to highlight and quantify analytically and on real data the benefits of this new approach over the one classically used, particularly with regard to distortions of the extracted spectra. We also examine both operating regimes. Methods . We carried out analytical calculations based on simple toy models of spectra to identify and quantify the main theoretical problems of the state-of-the-art technique, the proposed corrections of our new method, and the remaining limitations of the latter. Simulations of the most extreme situations identified were used to highlight these problems and corrections. Archival VLT/MUSE data of the young PDS70 and HTLup systems were used to vet the detection and characterization capabilities using on-sky observations. New images of the YSES1 planetary system were used to further illustrate the gains. Results . We show that the state-of-the-art stellar halo subtraction method, based on low-pass filtering, can lead to the self-subtraction of the emission lines and modify the neighboring continua, depending on the line contrast to neighboring continuum contrast ratios. We show that the proposed technique corrects these characterization problems, while maintaining the same detection capabilities. The two protoplanets PDS70 b and c were detected with 5σ significance. The H α line estimate of the HTLup B stellar companion was improved by ~30% for the integrated flux and by ~8% for the 10%-width. As for YSES1 b, we found it uniquely displays a combination of H α , H β , CaII H&amp;K triplet, and HeI lines in emission that can be attributed to accretion and/or chromospheric activity. We derived an accretion rate at H α of ~1.45 × 10 −9 M Jup year with our new method, versus ~1.11 x 10 −9 M Jup year with the reference method, namely, ~30% less. These new results are compatible with values derived for other companions in this mass range. We note that YSES1 c was not detected in our observations. Conclusions . The proposed subtraction method better preserves the spectral information, notably the emission line fluxes and profiles, while achieving similar detection performance. Based on a linear and parametric approach, it can be extended and/or combined with additional faint signal search algorithms.

Abstract During the NASA Perseverance rover's exploration of the Jezero crater floor, coatings were commonly observed on rocks. These features may record past water‐rock‐atmosphere interactions on the crater floor, and understanding their origin is important for constraining the timing of potential water activity and habitability at Jezero. Here, we characterize the morphologic, chemical, and spectral properties of the crater floor rock coatings using color images, visible/near‐infrared reflectance spectra, and chemical data from the Mastcam‐Z and SuperCam instruments. We show that coatings are common and compositionally similar across the crater floor, and consistent with a mixture of dust, fine regolith, sulfates, and ferric oxides indurated as a result of one or more episodes of widespread surface alteration. All coatings exhibit a similar smooth homogenous surface with variable thickness, color, and spatial extent on rocks, likely reflecting variable oxidation and erosional expressions related to formation and/or exposure age. Coatings unconformably overlie eroded natural rock surfaces, suggesting relatively late deposition that may represent one of the last alteration episodes on the Jezero crater floor. While more common at Jezero, these coatings may be consistent with rock coatings previously observed in situ at other landing sites and may be related to duricrust formation, suggesting a global alteration process on Mars that is not unique to Jezero. The Perseverance rover likely sampled these rock coatings on the crater floor and the results from this study could provide important context for future investigations by the Mars Sample Return mission aimed at constraining the geologic and alteration history of Jezero crater.

Abstract HD 138965 is a young A type star and member of the nearby young Argus association. This star is surrounded by a broad, bright debris disc with two temperature components that was spatially resolved at far-infrared wavelengths by Herschel. Here we present ALMA millimetre-wavelength imaging of the cool outer belt. These reveal its radial extent to be 150$^{+10}_{-7}$ au with a width (σ) of 49$^{+7}_{-6}$ au (ΔR/R = 0.77), at a moderate inclination of 49.○9$^{+3.3}_{-3.7}$. Due to the limited angular resolution, signal-to-noise, and inclination we have no constraint on the disc’s vertical scale height. We modelled the disc emission with both gravitational and radiation forces acting on the dust grains. As the inner belt has not been spatially resolved, we fixed its radius and width prior to modelling the outer belt. We find astronomical silicate is the best fit for the dust composition. However, we could not reject possible scenarios where there are at least 10 % water-ice inclusions. Combining the spatially resolved imaging by ALMA with non-detection at optical wavelengths by HST, we obtain a limit on the scattering albedo ω ≤ 0.09 for the debris dust in the outer belt. Analysis of the outer belt’s architecture in conjunction with simple stirring models places a mass limit of 2.3 ± 0.4 MJup on a companion interior to the belt (a ≤ 78 au), a factor of two improvement over constraints from high contrast imaging.

Context. Ultra-carbonaceous Antarctic micrometeorites (UCAMMs) are carbon-rich micrometeorites. Their organic matter formed in a N-rich environment, possibly via the galactic cosmic ray (GCR) irradiation of ice mantles at the surfaces of small bodies. Nanoscale secondary ion mass spectrometry (NanoSIMS) revealed that UCAMMs exhibit large H, C, and N isotopic heterogeneities at the micron scale. Aims. We aim to investigate the transfer of H, C, and N isotopic heterogeneity from an ice mixture to its ion-irradiation-induced organic residue. The goal of this work is to understand the formation of H, C, and N bulk- and micron-scale isotopic heterogeneities in the organic matter of UCAMMs. Methods. We performed irradiation experiments of isotopically heterogeneous ice films at 10 K, with swift heavy ions to model the irradiation of isotopically heterogeneous icy surfaces via GCR. The irradiated ice mixtures were subsequently annealed to room temperature, which led to the formation of refractory organic residues. NanoSIMS imagery was performed on the organic residues to identify micron-scale H, C, and N isotopic heterogeneity. The ice films consisted of N2-CH4 (9:1) and NH3-CH4 (9:1) ices that contain a thin layer of D-, 13C-, and 15N-labeled species. Results. The irradiation-induced organic residues exhibit micron-scale isotopic anomalies. The transfer of isotopic anomalies from the ice to the organic residue appears to depend on the chemical composition of the ice film. Conclusions. These experiments show it is possible to transfer isotopic heterogeneity that is initially present in cometary ices to refractory organic residues that formed by heavy ion irradiation. Gaseous reservoirs of volatile species with highly fractionated isotopic compositions are predicted to exist in the early Solar System and should coexist under the form of ice mantles at the surface of small bodies at large heliocentric distances. The study of organics in UCAMMs can thus shed light on the composition of N- and C-rich ices at the surface of outer Solar System small icy bodies.

In this contribution, we present a theoretical investigation of the reaction involving atomic sulfur in its first electronically excited state, 1D, and H2O on an ice-surface model. This study is motivated by the work of Giustini et al. (ACS Earth Space Chem., 2024, 8, 2318), which indicated a strong effect of the presence of four additional water molecules in the S(1D) + H2O reaction compared to the pure gas-phase case. Our simulation treats the long-range interactions (H-bonds and dispersion forces) with the ice water molecules in a much more realistic way being based on the use of a cluster of 18 water molecules, thus overcoming the limits of the small cluster used by Giustini et al. According to our results, S(1D) reacts via two possible reaction mechanisms: (1) addition to the O atom of a water molecule with the formation of H2OS or (2) insertion into one of the O-H bonds of a water molecule with the formation of HOSH. Both H2OS and HOSH are stabilized on ice by energy dissipation rather than isomerizing or dissociating into two products as seen in the gas-phase reaction. The interaction with surrounding water molecules affects the entire reaction pathway by stabilizing intermediate species, reducing some barriers, and impeding the only two-product open channel of the gas-phase reaction. S(1D) can be produced by UV-induced photodissociation of various precursor molecules on the surface of interstellar or cometary ice or by other high-energy processes induced by electrons or cosmic rays also in the ice bulk. Therefore, our results can be of help in elucidating the mysterious sulfur chemistry occurring in the icy mantles of interstellar grains or in cometary nuclei. Furthermore, this study demonstrates that the product branching ratios of gas-phase reactions should not be uncritically used in modeling interstellar ice chemistry.