Giuseppe Lodato

Context . Brown dwarfs are common as free-floating objects but rare as close companions to Sun-like stars, a disparity known as the “brown-dwarf desert.” Host-star metallicity can constrain whether these companions form mainly through metal-sensitive core accretion or through less metal-dependent disk or cloud fragmentation. Aims . We extend our homogeneous spectroscopic analysis of directly imaged planet hosts into the brown-dwarf regime and compare their metallicities with those of planet hosts and close-in brown-dwarf hosts. Methods . We compiled 54 unique directly imaged brown-dwarf systems selected over an inclusive 13–80 M _Jup interval and projected separations from about 5 au to several thousand au. Objects near the model-dependent 70–75 M _Jup hydrogen-burning boundary may, however, be very low-mass stars. For 31 hosts with archival high-resolution spectra, we derived atmospheric parameters and metallicities using Bayesian spectral synthesis. Literature companion masses and projected separations are heterogeneous and often model-dependent, and are used only for demographic context. Galactic velocities were calculated for 46 hosts solely to characterize the youth-biased imaging sample. Results . The host stars have a broadly solar metallicity distribution, with a median [Fe/H] of +0.06 dex and a median absolute deviation of 0.11 dex, and show no strong metal-rich bias. No statistically significant metallicity difference is detected between the directly imaged lower- and higher-mass subsamples at the current sample size. The hosts are kinematically cold, as expected from the youth-biased selection of direct-imaging surveys. Conclusions . The absence of a strong metal-rich bias suggests that classical core accretion does not dominate the wide-orbit brown-dwarf population. Disk instability and cloud fragmentation remain plausible, but the current sample and heterogeneous companion properties do not permit object-by-object discrimination between these channels. The comparisons involving the smallest samples remain exploratory.

Constraining the total mass of protoplanetary disks is crucial for determining the amount of material available for planet formation. Yet providing accurate measurements of the disk mass is challenging. Investigating the gas dynamics provides a powerful tracer-independent method to precisely characterize disk masses. By fitting the velocity rotation curves of different molecular tracers with an accurate model including the thermal stratification and self-gravity of the disk, we constrained the stellar masses, disk masses, and scale radii for the disks around HD 97048 and WaOph 6. We obtained M _⋆ = 2.226 _−0.049 ^+0.054 M _⊙ , M _d = 0.3 _−0.061 ^+0.055 M _⊙ , and R _c = 172 ₋₁₄ ⁺²⁴ au for HD 97048 and M _⋆ = 0.956 _−0.006 ^+0.006 M _⊙ , M _d = 0.21 _−0.038 ^+0.045 M _⊙ , and R _c = 647 ₋₁₅₅ ⁺¹⁹³ au for WaOph 6. We also measured the corresponding gas-to-dust and disk-to-star mass ratios. We efficiently extended the dynamical method to characterize embedded sources exhibiting features of absorption, for which a careful analysis is required to avoid biases in the retrieved velocity profiles. We prove the importance of including a beam smearing correction to the curves. If not included, this observational effect can systematically bias the velocity profiles, thus altering the disk mass estimates up to ≈45%. We provide comprehensive estimates of the systematic uncertainties on the best-fit parameters by bootstrapping over both the retrieved geometry and 2D thermal structure of the two disks. The overall uncertainty on the disk masses is ≈20%. Finally, we investigated the connection between disk stability and the appearance of spiral morphologies in the millimeter continuum emission by comparing the Toomre parameter of all dynamically weighed disks to date, demonstrating that disks with millimeter-dust spirals have systematically lower values of Q .

Kinematic perturbations observed with the Atacama Large Millimeter/submillimeter Array in CO line emission provide evidence for a population of embedded giant protoplanets shaping the structure of protoplanetary disks. We present JWST/MIRI F1140C ( λ = 11.3 μ m) coronagraphic observations of five protoplanetary disks—HD 163296, RXJ 1615.3–3255, RXJ 1842.9–3532, SY Cha, and LkCa 15—with the goal of directly detecting candidate protoplanets orbiting at ≳70 au that were previously inferred from gas kinematics. The data were analyzed using a bespoke methodology that combines subtraction of a reference point-spread function with forward modeling of partially resolved inner disk emission, which otherwise dominates the diffraction pattern in the images. This approach improves the sensitivity to young companions at small separations. No point source consistent with an embedded protoplanet is detected in any of the systems. Instead, in three systems we detect extended emission at 11.3 μ m tracing the outer disk out to radii comparable to those probed by CO. Injection tests indicate upper mass limits of roughly 3–20 M _J at separations of a few hundred astronomical units, assuming no additional thermal contribution from the circumplanetary environment. Even with space-based observations, these limits remain mostly above the ∼1–5 M _J masses inferred from disk kinematics, largely due to the limitations imposed by emission (and/or scattered light) contributions from both the inner disk and outer disk. These observations highlight the challenges of observing protoplanets embedded in their forming environment at large separation with JWST/MIRI. Lessons learned can inform future studies with the Extremely Large Telescope, which will probe separations where the occurrence rate of gas giants is expected to be higher.

Context . Orion is the closest region hosting active star formation and young OBA stars. Accurately determining the far-ultraviolet (FUV) flux at its stellar population is essential to connect stellar and protoplanetary disc properties to the environment. Aims . We (1) accurately estimated the FUV flux and its distribution at a numerous stellar population of Orion by statistically accounting for the uncertainty in parallax measurements, and (2) investigated the relation between stellar accretion and external FUV radiation field by comparing observations and disc evolution models. Methods . We selected a large stellar population in Orion (within a 6° radius of the Orion Nebula Cluster core), assigned sub-cluster memberships, and used the two-dimensional sub-cluster geometry to infer three-dimensional separations from OBA stars and compute the FUV flux (and its uncertainty) at each stellar position. We studied the accretion luminosities ( L _acc ) inferred from H _α emission in Gaia XP spectra of Orion sources and determined their detection fraction as a function of age and FUV flux. We compared the results with population synthesis models of viscous discs experiencing external photoevaporation. Results . We provided a publicly available table of FUV fluxes at ~8600 stars in Orion. Most of this stellar population is weakly FUV irradiated, 10 ⁴ G ₀ . Gaia -based L _acc decreases with age, and H _α detection fraction declines more rapidly in regions with strong FUV fluxes (≳10 ² G ₀ ) than in regions exposed to weaker FUV fluxes (≲10 ² G ₀ ), broadly consistent with the model. This result may suggest that external photoevaporation efficiently depletes strongly FUV irradiated accretion discs, but it is not sufficient to reliably confirm this conclusion. Conclusions . The tools we provided for accurately computing FUV fluxes at the Orion stellar population will be essential for targeting sources in future observations aimed at assessing the role of external photoevaporation on protoplanetary disc. Our study highlights the need for additional measurements of stellar and disc properties across the Orion population, covering the FUV flux range 1–10 ⁵ G ₀ .

The presence of asymmetries and substructures in protoplanetary disks, revealed by both dust and gas emission, highlights the potential interplay and the broader connection between chemistry and dynamics in disk evolution. We explore multiple relationships using the nonparametric Kendall- τ correlation to examine formaldehyde (H ₂ CO) emission with relation to stellar and disk properties for a subset of disks from the exoALMA sample. We also retrieve the H ₂ CO column density and excitation temperature using four transitions, measured in radial bins of 100 au, and quantify the level of asymmetry in the resolved peak intensity of the H ₂ CO emission. From our correlation analysis, we find no correlations with sufficient statistical significance. However, we identify tentative relationships that can be tested with larger samples. In particular, we report a proposed correlation (2.1 σ ) between stellar effective temperature and the formaldehyde excitation conditions, suggesting that, to first order, the central star dominates the nature of the H ₂ CO emission over possible dynamical asymmetries traced by dust. Although a correlation with the stellar luminosity was also expected, a larger sample is required to confirm or refute this trend. A possible correlation with spectral type, together with the broad range of H ₂ CO excitation temperatures within the inner 100 au of the studied disks, hint at possible multiple chemical formation pathways for H ₂ CO, including both gas-phase reactions and ice-surface chemistry on dust grains.

The exoALMA large program provided an unprecedented view of the morphologies and kinematics of 15 circumstellar disks, offering a biased but homogenous and well-characterized sample for population-level analysis. Continuum observations revealed numerous dust substructures, known to be potential signatures of embedded planets. We analyze the observed dust morphologies with the simulation-based inference tool DBNets2.0, assuming these are due to embedded planets at fixed locations, to infer the system’s properties. We estimate the putative planet mass, the disk α viscosity, scale height, and dust Stokes number that would reproduce 19 substructures in 13 of the 15 exoALMA disks. We compare our results with literature estimates derived with different methods, and find good agreement in most cases. We further explore the implications of the inferred disk properties for accretion, showing that for the Herbig stars in our sample, the implied viscous accretion timescales are too long to account for their observed stellar accretion rates. Regarding planet migration, our results favor inward migration, with only three putative planets expected to migrate outward. Finally, we check for correlations of the inferred disk and planet properties with the disks’ gas-to-dust mass ratio, nonaxisymmetry index, and masses of the gas, dust, and host stars, finding no remarkable trends.

Protoplanetary disks are the birthplaces of planetary systems, and deviations from Keplerian rotation imprinted in disk gas kinematics serve as key tracers of physical processes and the presence of protoplanets within disks. Using the CO ( J = 3–2) data from the exoALMA Large Program, encompassing 15 disks, we constructed two-dimensional (2D) maps of centroid velocity, line width, and peak intensity, and extracted non-Keplerian deviations by subtracting smooth Keplerian models. This Letter provides the first systematic and uniform overview of 2D gas substructures across the entire exoALMA sample. We find that all targets exhibit large-scale deviations from smooth Keplerian disks, displaying a variety of morphologies, including spiral-like structures, arc- or ring-like features, and patterns indicative of variations in the emitting surface height. Nonaxisymmetric spiral-arm features are detected or suggested in five disks (CQ Tau, MWC 758, HD 135344B, HD 34282, and SY Cha), and are preferentially found in Herbig Ae/Fe systems. In contrast, some other sources (J1852, PDS 66, and V4046 Sgr), despite exhibiting noticeable deviations, appear to be dynamically quieter. This 2D atlas suggests that kinematic substructures are ubiquitous in large (≳100 au) protoplanetary disks with ages of a few million years, based on the observations obtained with sufficient sensitivity at a moderate-to-high spatial resolution of ∼20 au and high-velocity resolution of ∼0.1 km s ⁻¹ .

Vertical gas flows—such as winds and meridional circulations—are natural outcomes of protoplanetary disk processes and play a critical role in the earliest stages of planet formation. We analyze the vertical gas motions in 14 disks, as part of the exoALMA Large Program, focusing on the ¹² CO J = 3–2 and ¹³ CO J = 3–2 emission lines. Using discminer to model the Keplerian velocity field, we extract line-of-sight velocity residuals and measure the radial and vertical components of the gas motion. Vertical motions are detected in most disks. Two types of patterns emerge: (1) oscillatory up/down flows, likely linked to instabilities; and (2) transitions from downward to upward motions that we interpret as the bases of disk winds. In most cases, the velocity amplitudes are of a few tens of m s ⁻¹ . However, two disks, MWC758 and CQ Tau, show two spiral velocity features in their residual maps, red- and blueshifted, which we interpret as vertical velocities reaching up to ∼350 m s ⁻¹ (∼0.7 c _s ), consistent with the gas motion in eccentric disks. Fast upward motions (up to 500 m s ⁻¹ ; ∼1.8 c _s ) are also detected in the outer disk of MWC758. Synthetic observations from (magneto)hydrodynamic simulations validate the reliability of our method. Although strong molecular winds appear to be relatively rare in ¹² CO and ¹³ CO, our study shows that, when traced by deep high-spectral-resolution line data, protoplanetary disks exhibit ubiquitous vertical flows. However, their overall velocity structure is highly complex, preventing the identification of a coherent dominant physical mechanism driving the vertical motions across all disks, thus requiring further theoretical investigation.

The exoALMA Large Program has revealed a wealth of substructures in the dust and molecular line emission of several protoplanetary discs, suggesting that planet formation may unfold within highly dynamic environments. Using synthetic observations of planet–disk interactions and disk instabilities, we demonstrate how the origin of these substructures can be investigated through a tomographic study of molecular lines, extending the scope of the analysis beyond line-centroid kinematics alone. Our results indicate that with only a few hours of ALMA integration at moderate angular resolution ( 0 . ″ 15 − 0 . ″ 30 ), it is possible to identify the key signatures driven by planets more massive than 0.1% of the stellar mass. These signatures manifest not only as deviations from Keplerian motion but also as localized line broadening, enabling accurate constraints on the orbital radius and azimuthal location of the planets. We further show that a diagnostic based on line skewness in spectrally resolved observations can help distinguish between planetary and instability-driven signatures, owing to the distinct degrees of velocity coherence associated with each mechanism. Finally, we apply this tomographic analysis to exoALMA CO line data for the disks of HD 135344B and MWC 758. In HD 135344B, we identify strongly localized velocity and line-width perturbations, suggesting the possibility of three massive planets embedded in the disk: one at R = 95 au, exterior to the continuum substructures, and two within dust gaps at R = 41 au and R = 73 au. For MWC 758, the dominance of vertical-velocity spirals over localized signatures is consistent with predictions from models of moderate disk eccentricities or warps, potentially induced by a substellar companion in the inner regions of the system.

Context . The evolution of protoplanetary discs is a function of their internal processes and of the environment in which the discs are located. It is unclear if angular momentum is mainly removed viscously or by magnetic winds or by a combination of the two. While external photoevaporation is expected to severely influence disc evolution and eventually dispersal, there are observational limitations towards highly irradiated discs. Consequently, the interplay between these ingredients and their combined effects on the gas and dust distributions within the disc are poorly understood. Aims . We investigate, for the first time, the evolution of both the gaseous and solid components of viscous, MHD-wind, or hybrid discs, in combination with external far-ultraviolet (FUV) driven mass loss. We tested which combinations of parameters may protect discs from the external irradiation, allowing the solid component to live long enough to allow planet formation to succeed. Methods . We ran a suite of 1D simulations of smooth discs with varying initial sizes, different levels of viscous and MHD-wind stresses (modelled via an α parametrisation), and strengths of the external FUV environment. We then tracked disc properties such as their radii, various lifetime diagnostics, and the amount of dust removed by the photoevaporative wind, as a function of the underlying parameters. Results . We find that the biggest role in determining the fate of discs is played by a combination of a disc’s ability to spread radially outwards and the strength of FUV-driven erosion. While MHD wind-driven discs experience less FUV erosion due to the lack of spread, they do not live for longer amounts of time compared to viscously evolving discs, especially at low-to-moderate FUV fluxes, while higher fluxes (≳100 G ₀ ) yield disc lifetimes that are rather insensitive to the disc’s angular momentum transport mechanism. Specifically, for the solid component, the biggest role is played by a combination of inward drift and removal by FUV winds. This points to the importance of other physical ingredients, such as disc substructures, even in highly irradiated disc regions, to retain solids.