Konstantin V. Getman

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.

Our analysis of Chandra X-ray data for over 27,000 pre-main sequence (PMS) stellar members of various young Galactic star-forming regions and open clusters suggests that, compared to the present-day Sun, the convective dynamo responsible for generating magnetic fields in PMS stars demonstrates much higher levels of surface activity: large spot areas, magnetic fluxes, baseline and flare X-ray luminosities, and magnetic loop sizes. The convective dynamo mechanism enables the production of X-ray super-flares (with X-ray energies E _x ranging from 10 ³⁴ to 10 ³⁶ erg) and mega-flares ( Ex> 10 ³⁶ erg) in fully convective (Hayashi track) PMS stars, far surpassing the capabilities of the contemporary Sun. Moreover, it leads to frequent weaker X-ray flares ( Ex< 10 ³⁴ erg) with an occurrence rate more than a million times higher than in older solar-type main sequence stars. Furthermore, we present two other PMS dynamo-related findings. First, we find no observational evidence of strong, long-term cyclical activity in PMS stars driven by convective dynamos. Second, our research offers tentative evidence for a transition from a convective dynamo to a solar-type tachoclinal dynamo as radiative cores develop in solar-mass stars at ages between 10 and 25 million years.

X-ray and ultraviolet (XUV) emission from young stars plays a critical role in shaping the evolution of planetary atmospheres and the conditions for habitability. To assess the long-term impact of high-energy stellar radiation, it is essential to empirically trace how X-ray luminosities and spectral hardness evolve during the first ≲1 Gyr, when atmospheric loss and chemical processing are most active. This study extends the X-ray activity–mass–age analysis of Chandra observations of five rich open clusters (∼45–100 Myr), and archival ROSAT and Chandra data for three older clusters (∼220–750 Myr). We find a mass-dependent decay in X-ray luminosity: solar-mass stars undergo a far more rapid and sustained decline, accompanied by coronal softening and the disappearance of hot plasma by ∼100 Myr, compared to their lower-mass siblings. These trends in solar-mass stars are likely linked to reduced magnetic dynamo efficiency and diminished ability to sustain large-scale, high-temperature coronal structures. The trends are significantly stronger than predicted by widely used XUV–rotation–age relations. The revised trends imply systematically lower rates of atmospheric mass loss and water photolysis, as well as altered ionization environments and chemical pathways relevant to the formation of prebiotic molecules, for planets in close orbits around solar analogs. These effects persist throughout at least the ≲750 Myr interval probed in this study.

X-ray and ultraviolet (XUV) emission from young stars plays a critical role in shaping the evolution of planetary atmospheres and the conditions for habitability. To assess the long-term impact of high-energy stellar radiation, it is essential to empirically trace how X-ray luminosities and spectral hardness evolve during the first ~<1 Gyr, when atmospheric loss and chemical processing are most active. This study extends the X-ray activity-mass-age analysis of <25 Myr stars by Getman et al. (2022) to ages up to 750 Myr, using Gaia-based cluster memberships, new Chandra observations of five rich open clusters (~45--100 Myr), and archival ROSAT and Chandra data for three older clusters (~220--750 Myr). We find a mass-dependent decay in X-ray luminosity: solar-mass stars undergo a far more rapid and sustained decline, accompanied by coronal softening and the disappearance of hot plasma by ~100 Myr, compared to their lower-mass siblings. These trends in solar-mass stars are likely linked to reduced magnetic dynamo efficiency and diminished ability to sustain large-scale, high-temperature coronal structures. The trends are significantly stronger than predicted by widely used XUV-rotation-age relations. The revised trends imply systematically lower rates of atmospheric mass loss and water photolysis, as well as altered ionization environments and chemical pathways relevant to the formation of prebiotic molecules, for planets in close orbits around solar analogs. These effects persist throughout at least the ~<750 Myr interval probed in this study.

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.

Aims. We investigate the James Webb Space Telescope (JWST) MIRI MRS gas molecular content of an externally irradiated Herbig disk, the F-type XUE 10 source, in the context of the eXtreme UV Environments (XUE) program. XUE 10 belongs to the massive star cluster NGC 6357 (1.69 kpc), where it is exposed to an external far-ultraviolet (FUV) radiation ≈10³ times stronger than in the solar neighborhood. Methods. We modeled the molecular features in the mid-infrared spectrum with local thermodynamic equilibrium (LTE) 0D slab models. We derived basic parameters of the stellar host from a VLT FORS2 optical spectrum using PHOENIX stellar templates. Results. We detected bright CO₂ gas with the first simultaneous detection (>5σ) of four isotopologues (¹²CO₂, ¹³CO₂, ¹⁶O¹²C¹⁸O, ¹⁶O¹²C¹⁷O) in a protoplanetary disk. We also detected faint CO emission (2σ) and the HI Pf α line (8σ). We placed strict upper limits on the water content, finding a total column density of ≲10¹⁸ cm⁻². The CO₂ species trace low gas temperatures (300–370 K) with a range of column densities of 7.4 × 10¹⁷ cm⁻² (¹⁶O¹²C¹⁷O)−1.3 × 10²⁰ cm⁻² (¹²CO₂) in an equivalent emitting radius of 1.15 au. The emission of ¹³CO₂ is likely affected by line optical depth effects. The ¹⁶O¹²C¹⁸O and ¹⁶O¹²C¹⁷O abundances may be isotopically anomalous compared to the ¹⁶O/¹⁸O and ¹⁶O/¹⁷O ratios measured in the interstellar medium and the Solar System. Conclusions. We propose that the mid-infrared spectrum of XUE 10 is explained by H₂O removal either via advection or strong photo-dissociation by stellar UV irradiation and enhanced local CO₂ gas phase production. Outer disk truncation supports the observed CO₂−H₂O dichotomy. A CO₂ vapor enrichment in ¹⁸O and ¹⁷O can be explained by means of external UV irradiation and early (10^4–5 yr) delivery of isotopically anomalous water ice to the inner disk.

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.

We explore the empirical power-law relationship between X-ray luminosity (L _X) and total surface magnetic flux (Φ), established across solar magnetic elements, time- and disk-averaged emission from the Sun, older active stars, and pre-main-sequence (PMS) stars. Previous models of large PMS X-ray flares, lacking direct magnetic field measurements, showed discrepancies from this baseline law, which MHD simulations attribute to unusually strong magnetic fields during flares. To test this, we used nearly simultaneous Chandra X-ray and HET-HPF near-infrared observations of four young Orion stars, measuring surface magnetic fields during or just after powerful PMS X-ray flares. We also modeled these PMS X-ray flares, incorporating their measured magnetic field strengths. Our findings reveal magnetic field strengths at the stellar surface typical of nonflaring PMS stars, ruling out the need for abnormally strong fields during flares. Both PMS and solar flares deviate from the L _X–Φ law, with PMS flares exhibiting a more pronounced deviation, primarily due to their much larger active regions on the surface and larger flaring loop volumes above the surface compared to their solar counterparts. These deviations likely stem from the fact that powerful flares are driven by magnetic reconnection, while baseline X-ray emission may involve less efficient mechanisms like Alfvén wave heating. Our results also indicate a preference for dipolar magnetic loops in PMS flares, consistent with Zeeman–Doppler imaging of fully convective stars. This requirement for giant dipolar loops aligns with MHD predictions of strong dipoles supported by polar magnetic surface active regions in fast-rotating, fully convective stars.

We explore the empirical power-law relationship between X-ray luminosity (Lx) and total surface magnetic flux (Phi), established across solar magnetic elements, time- and disk-averaged emission from the Sun, older active stars, and pre-main-sequence (PMS) stars. Previous models of large PMS X-ray flares, lacking direct magnetic field measurements, showed discrepancies from this baseline law, which MHD simulations attribute to unusually strong magnetic fields during flares. To test this, we used nearly simultaneous Chandra X-ray and HET-HPF near-infrared observations of four young Orion stars, measuring surface magnetic fields during or just after powerful PMS X-ray flares. We also modeled these PMS X-ray flares, incorporating their measured magnetic field strengths. Our findings reveal magnetic field strengths at the stellar surface typical of non-flaring PMS stars, ruling out the need for abnormally strong fields during flares. Both PMS and solar flares deviate from the Lx-Phi law, with PMS flares exhibiting a more pronounced deviation, primarily due to their much larger active regions on the surface and larger flaring loop volumes above the surface compared to their solar counterparts. These deviations likely stem from the fact that powerful flares are driven by magnetic reconnection, while baseline X-ray emission may involve less efficient mechanisms like Alfven wave heating. Our results also indicate a preference for dipolar magnetic loops in PMS flares, consistent with Zeeman-Doppler imaging of fully convective stars. This requirement for giant dipolar loops aligns with MHD predictions of strong dipoles supported by polar magnetic surface active regions in fast-rotating, fully convective stars.

The most powerful stellar flares driven by magnetic energy occur during the early pre-main-sequence (PMS) phase. The Orion Nebula represents the nearest region populated by young stars, showing the greatest number of flares accessible to a single pointing of Chandra. This study is part of a multi-observatory project to explore stellar surface magnetic fields (with the Hobby–Eberly Telescope Habitable-zone Planet Finder, HET-HPF), particle ejections (with the Very Long Baseline Array, VLBA), and disk ionization (with the Atacama Large Millimeter/submillimeter Array, ALMA) immediately following the detection of PMS superflares with Chandra. In 2023 December, we successfully conducted such a multi-telescope campaign. Additionally, by analyzing Chandra data from 2003, 2012, and 2016, we examine the multi-epoch behavior of PMS X-ray emission related to PMS magnetic cyclic activity and ubiquitous versus sample-confined megaflaring. Our findings are as follows. (1) We report detailed stellar quiescent and flare X-ray properties for numerous HET/ALMA/VLBA targets, facilitating ongoing multiwavelength analyses. (2) For numerous moderately energetic flares, we report correlations (or lack thereof) between flare energies and stellar mass/size (presence/absence of disks) for the first time. The former is attributed to the correlation between convection-driven dynamo and stellar volume, while the latter suggests the operation of solar-type flare mechanisms in PMS stars. (3) We find that most PMS stars exhibit minor long-term baseline variations, indicating the absence of intrinsic magnetic dynamo cycles or observational mitigation of cycles by saturated PMS X-rays. (4) We conclude that X-ray megaflares are ubiquitous phenomena in PMS stars, which suggests that all protoplanetary disks and nascent planets are subject to violent high-energy emission and particle irradiation events.