Sierk E. van Terwisga

Context. A star’s radiation field is the main energy source of a planet-forming disk. Stellar photons – from X-rays to optical – are reprocessed in the inner protoplanetary disk, and this process enables mid-infrared (mid-IR) diagnostics of the disk’s physical conditions. Most pre-main-sequence stars accrete at moderate rates and emit X-rays efficiently. Such activity is expected to drive stochastic variability in the stellar spectrum. It is not clear to what extent such variability affects the thermochemical structure of protoplanetary disks. Aims. We examine the effect of routine accretion variability and X-ray flares on the thermochemical structure and mid-IR line emission spectra of protoplanetary disks. Methods. We combined multi-epoch contemporaneous X-ray and NUV/optical observations of the DQ Tau binary with JWST/MIRI spectra of its circumbinary disk. We interpreted the data and assessed the effect of a varying stellar spectrum on the disk using a forward thermochemical model illustrative of the system. Results. Synthetic mid-IR fluxes of CO, CO ₂ , HCN, and H ₂ O are systematically stronger at periastron than at apastron due to a fourfold increase in accretion luminosity. Variations in the integrated fluxes lie within a factor of two between epochs and are in reasonable agreement with the JWST/MIRI spectra of DQ Tau. Routine stellar variability induces only modest changes in the disk’s thermochemical structure, yet it can still shift the snowline outward by 17% relative to its apastron position. Because the JWST observations did not coincide with X-ray flares, we relied on simulations to analyze these phenomena. We find that moderate-intensity flares leave most mid-IR molecular emission unchanged. In contrast, X-ray flares strongly enhance the abundances and mid-IR luminosities of H I , Ar II , and Ne II on the disk surface. Conclusions. Our results imply that the Spitzer /JWST line variability observed in non-outbursting systems is consistent with moderately variable stellar spectra driven by accretion and magnetospheric activity.

Aims. We present the largest homogeneous VLT/X-shooter study of accretion in young stellar objects spanning the entire Orion A complex. Our sample includes 91 pre-main-sequence stars hosting a protoplanetary disk according to Spitzer photometry, selected to span a wide range of optical colors, 34 of which have complementary ALMA dust mass measurements. Methods. We derived the stellar and accretion properties using a self-consistent multicomponent fitting procedure. Our sample encompasses spectral types from K3 down to M5, corresponding to stellar masses within ∼0.8−0.1 M _⊙ . Results. The accretion-stellar luminosity ( L _acc − L _★ ) and the scaling relations of the mass-accretion rate to stellar mass ( Ṁ _acc − M _★ ) in Orion A are consistent with those of other star-forming regions that span a range of stellar densities, far-ultraviolet (FUV) irradiation fields, and ages, with all regions occupying the same locus in parameter space. For the 34 sources with complementary dust mass measurements, we present the first investigation of the Ṁ _acc and disk mass ( M _disk ) correlation in Orion A, recovering a spread consistent with that reported for other regions. Although our sample spans nearly five orders of magnitude in local FUV field strength, we found no statistically significant correlation between Ṁ _acc and FUV irradiation. However, by combining our sample with literature measurements in Lupus, the Orion nebula cluster, and σ Orionis, we found a tentative population-level decrease in the inferred disk lifetime t _disk = M _disk / Ṁ _acc toward stronger FUV environments, but the large intrinsic scatter and limited sample sizes at intermediate- and high-FUV fields prevent firm conclusions. Conclusions. The homogeneous Ṁ _acc measurements reported here provide a solid foundation for future studies of accretion and disk evolution in the diverse environments of Orion A and other star-forming regions.

Small planets are extremely common in the Galaxy, including planets with masses and radii between those of Earth and Neptune. Characterising the mass of such planets requires ultra-precise radial velocities. The ESPRESSO spectrograph was designed and built for this purpose. We present an overview of the ESPRESSO Guaranteed Time Observations transit follow-up sub-programme, which aimed to confirm and characterise small transiting planet candidates from the K2 and TESS space missions. We analysed the global stellar and planetary properties of the sample of 65 planets in 30 systems characterised by this sub-programme. This includes six systems presented in this paper, for which we either obtain only the upper mass limits or provide updates to previously published parameters. We also placed this sample in the context of the overall population of precisely characterised small planets. After separating the population into insolation regimes, we find a tentative mass threshold at for the rocky to volatile-rich composition transition in the medium-insolation regime, along with a population of likely stripped massive rocky planets in the high-insolation regime. Likewise, we find a correlation between planet mass and stellar metallicity, with more massive planets hosted by more metal-rich stars. We also explored the radius valley, finding that planets below the gap have a tighter mass distribution. We compared planetary masses with typical protoplanetary disk masses, drawing tentative conclusions about likely formation conditions. We also discuss the impact of our observing strategy on our results. The ESPRESSO transit follow-up sub-programme has been highly productive in characterising a diverse population of small planets, which has enabled us to identify population-level features. Likewise, the lessons learned from this sub-programme will be valuable for PLATO follow-up planning.

High-mass star-forming regions (SFRs), dominated by intense far-ultraviolet (FUV) radiation fields (∼10 ⁵ G ₀ ) from nearby O stars, are the birthplaces of most planetary systems. While our understanding of protoplanetary disks (PPDs) and their evolution has primarily come from observations of isolated low-mass stars in the solar neighborhood, recent JWST observations of distant high-mass SFRs have provided new insights into the inner regions of the disks surrounding young stars. The impact of strong FUV irradiation on the structure and chemistry of PPDs remains poorly constrained. Here we use the thermochemical code ProDiMo to model how disk size and polycyclic aromatic hydrocarbon (PAH) abundance influence the infrared spectra of PPDs in high-mass SFRs. Our models explore taper radii from 5 to 70 au and PAH abundances from 10 ⁻⁴ to 10 ⁻² relative to the interstellar medium, under both strong external irradiation (10 ⁵ G ₀ ) and isolated conditions ( G ₀ ). We find that line-to-continuum ratios and integrated fluxes of key molecules, such as HCN and C ₂ H ₂ , decrease by up to 90% in the most compact, irradiated disks, indicating a strong dependence on disk truncation. PAH abundance primarily affects the continuum and PAH feature strengths in highly irradiated models. These results provide a framework for interpreting JWST spectra of PPDs in high-mass SFRs and underscore the critical roles of disk size and external FUV irradiation in shaping disk chemistry and evolution.

Context. Multiple photometric studies have reported the presence of seemingly older accreting pre-main-sequence (PMS) stars in optical colour-magnitude diagrams (CMDs). These sources appear bluer than the majority of cluster members, leading to older isochronal age estimates. Aims. We investigated this phenomenon in the Orion Nebula, which harbours a subset of stars that show infrared excess detected by Spitzer (which indicates the presence of protoplanetary discs) and H α excess emission (which traces ongoing mass accretion), yet seem to have significantly older isochronal ages (≳10 Myr) than the bulk population (∼1−3 Myr) in the r , ( r − i ) CMD. This raises the question of whether these stars are truly older or whether their photometric properties are affected by observational biases or other physical processes. Methods. We performed a detailed spectroscopic analysis of 40 Orion Nebula stars using VLT/X-Shooter, covering CMD-based isochronal ages from 1 to over 30 Myr. We derived extinction values, stellar properties, and accretion parameters by modelling the ultraviolet excess emission via a multi-component fitting procedure. The sample spans spectral types from M4.5 up to K6, and masses in the range ∼0.1−0.8 M _⊙ . Results. We demonstrate that when extinction and, more importantly, accretion effects are accurately constrained, the stellar luminosity and effective temperature of the majority of the seemingly old stars become consistent with a younger population (∼1−5 Myr). This is supported by strong lithium absorption (EW _Li ≳ 400 mÅ), which corroborates their youth, and by the accretion-to-stellar luminosity ratios ( L _acc / L _⋆ ) typical for young, accreting stars. Three of these sources, however, remain old even after our analysis, despite showing signatures consistent with ongoing accretion from a protoplanetary disc. More generally, our analysis indicates that excess continuum emission from accretion shocks affects the placement of PMS stars in the CMD, displacing sources towards bluer optical colours. Conclusions. This study highlights the critical role of accretion in shaping stellar property estimates (including age) derived from optical CMDs and emphasises the need to carefully account for accretion effects when interpreting age distributions in star-forming regions. Understanding these biases is essential for accurately constraining the early evolution of PMS stars.

Aims . Accretion variability is ubiquitous in young stellar objects. While large outbursts (2.5–6 mag) may strongly affect the disk structure, the effects of moderate bursts (1–2.5 mag) are less understood. Our aim is to characterize the physical response of the disk around the eccentric binary system DQ Tau to its periodic accretion changes. Methods . We organized a multi-wavelength observing campaign centered on four JWST/MIRI spectra. We targeted three consecutive periastrons (high accretion state) and one apastron (quiescence). We used optical and near-infrared spectroscopy and photometry to measure how the accretion luminosity varies. We decomposed the multi-epoch spectral energy distributions into stellar, accretion, and rim components. In the MIRI spectra, we fitted the solid-state features using various opacity curves and the molecular features using slab models. Results . We find the inner disk of DQ Tau to be highly dynamic. The temperature, luminosity, and location of the inner dust rim vary in response to the movement of stars and the L _acc variations (0.10–0.40 L _⊙ ). This causes variable shadowing of the outer disk, leading to an anti-correlation between the rim temperature and the strength of the 10 µm silicate feature. The dust mineralogy remains constant across all epochs, dominated by large (>2 µm) amorphous olivine and pyroxene grains, with smaller fractions of crystalline forsterite. The excitation of CO (1550–2260 K), HCN (880–980 K), and hot H ₂ O (740–860 K) molecules, as well as the luminosity of the [NeII] line correlate with the accretion rate, while the warm (~650 K) and cold (~170–200 K) H ₂ O components are mostly constant. CO emission, originating from a hot region (>1500 K) likely within the dust sublimation radius, is the most sensitive to L _acc changes. In comparison with other T Tauri disks, DQ Tau is highly C-poor and displays moderately inefficient pebble drift. Conclusions . We conclude that even moderate accretion rate changes affect the thermal structure in the planet-forming disk regions on short timescales, providing a crucial benchmark for understanding disk evolution.

Unveiling the physical structure of protoplanetary disks is crucial for interpreting the diversity of the exoplanet population. Until recently, the census of the physical properties of protoplanetary disks probed by mid-infrared observations was limited to the solar neighborhood (d ≲ 250 pc). However, nearby star-forming regions (SFRs) such as Taurus—where no O-type stars reside—are not representative of the environments where the majority of the planet formation occurs in the Galaxy. The James Webb Space Telescope (JWST) now enables observations of disks in distant high-mass SFRs, where strong external far-ultraviolet radiation is expected to impact those disks. Nevertheless, a detailed characterization of the population of externally irradiated disks is still lacking. We use the thermochemical code ProDiMo to model JWST/MIRI spectroscopy and archival visual/near-infrared photometry aiming to constrain the physical structure of the irradiated disk around the solar-mass star XUE 1 in NGC 6357 (d ≈ 1690 pc). Our findings are as follows. (1) Mid-infrared dust emission features are explained by amorphous and crystalline silicates with compositions similar to nearby disks. (2) The molecular features detected with MIRI originate within the first ∼1 au, consistent with results from slab models. (3) Our model favors a disk truncated at 10 au with a gas-to-dust ratio of unity in the outskirts. (4) Comparing models of the same disk structure under different irradiation levels, we find that strong external irradiation raises gas temperature tenfold and boosts water abundance beyond 10 au by a factor of 100.

Recent years have seen a surge of interest in the community studying the effect of ultraviolet radiation environment, predominantly set by OB stars, on protoplanetary disc evolution and planet formation. This is important because a significant fraction of planetary systems, potentially including our own, formed in close proximity to OB stars. This is a rapidly developing field, with a broad range of observations across many regions recently obtained or recently scheduled. In this paper, stimulated by a series of workshops on the topic, we take stock of the current and upcoming observations. We discuss how the community can build on this recent success with future observations to make progress in answering the big questions of the field, with the broad goal of disentangling how external photoevaporation contributes to shaping the observed (exo)planet population. Both existing and future instruments offer numerous opportunities to make progress towards this goal.

The gas masses of protoplanetary disks are important but elusive quantities. In this work we present new Atacama Large Millimeter/submillimeter Array (ALMA) observations of N₂H⁺ (3–2) for 11 exoALMA disks. N₂H⁺ is a molecule sensitive to CO freeze-out and has recently been shown to significantly improve the accuracy of gas masses estimated from CO line emission. We combine these new observations with archival N₂H⁺ and CO isotopologue observations to measure gas masses for 19 disks, predominantly from the exoALMA large program. For 15 of these disks the gas mass has also been measured using gas rotation curves. We show that the CO + N₂H⁺ line emission-based gas masses typically agree with the kinematically measured ones within a factor of 3 (∼1σ–2σ). Gas disk masses from CO + N₂H⁺ are on average a factor of 2.3 − 1.0 + 0.7 × lower than the kinematic disk masses, which could suggest slightly lower N₂ abundances and/or lower midplane ionization rates than typically assumed. Herbig disks are found to have CO gas abundances at the level of the interstellar medium based on their CO and N₂H⁺ fluxes, which sets them apart from T Tauri disks, where abundances are typically ∼3−30× lower. The agreement between CO + N₂H⁺-based and kinematically measured gas masses is promising and shows that multimolecule line fluxes are a robust tool to accurately measure disk masses at least for extended disks.

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