Gregory J. Herczeg

Context . Very low-mass stars ( M _⋆ < 0.2 M _⊙ ) are known to host a larger fraction of terrestrial planets compared to higher mass stars. Studying the structure of their protoplanetary disk is therefore crucial to understanding the formation of Earth-like planets. Aims . In this work, we aim to characterize the innermost regions of the disk around a known very low-mass star, 2MASS J16053215-1933159 (J1605 hereafter), by analyzing new JWST/NIRSpec observations taken in 2024, two years after the first observation with JWST/MIRI in 2022. We intend to investigate the atomic and molecular emission in both spectra to establish a global view of the system and study its variability. Methods . After identifying both the fundamental band of CO and HI recombination emission lines from both spectra corrected from the contribution of the stellar photosphere, we used the HI lines to derive the stellar accretion rate at each epoch. The CO emission was fitted with a slab model to derive the gas characteristics. Results . We find that between the two epochs (2022 and 2024), the CO flux is reduced by a factor of ∼3, the flux of the only HI line seen by both NIRSpec and MIRI (the HI 10-6 line) is reduced by a factor of ∼13, and the total continuum-density flux is reduced by ∼15%. The accretion rate derived from individual HI lines is consistent with a decrease of a factor of ∼13 in two years. After correcting the spectrum from the CO absorption present in the J1605 stellar photosphere, we find that the decrease in the disk’s CO emission is consistent with a CO gas of the same column density and temperature at each epoch, but originating from a smaller emitting area when the accretion rate is low (NIRSpec’s epoch). While warm hydrocarbons (≃500 K) are detected with MIRI-MRS (2022) with extremely high column densities of C ₂ H ₂ , no hydrocarbon features are seen with NIRSpec (2024). We propose that non-Local Thermal Equilibrium (LTE) effects quench the near-IR emission of hydrocarbons in the warm reservoir. Taking the newly detected hot CO (∼800 K) as a reference and assuming LTE, we constrained the column density of C ₂ H ₂ , HCN, and CH ₄ to be less than 30% of CO in the hot reservoir. This value suggests that the C/O in the inner disk is unlikely to be much larger than unity. Conclusions . The variation of the accretion luminosity of the very low-mass star J1605 correlates with the variation of its fundamental CO luminosity similarly to T-Tauri stars, supporting the idea that very low-mass stars could be seen as scaled-down versions of T-Tauri stars. The detection of the emission of the fundamental band of CO in disks around cold objects (very low-mass stars, brown dwarfs, and giant planets) is challenging because of the presence of CO absorption in their photosphere. Future observations (e.g., with ELT/METIS) looking for such emission will require robust ways to constrain the photosphere emission and follow the time variability of the sources.

Hydrogen lines are widely used as tracers of stellar and planetary accretion. In classical T Tauri stars, hydrogen lines are usually interpreted as arising from magnetospheric accretion columns, whereas in lower-mass counterparts (subsolar-mass objects including brown dwarfs and gas giant planets), the post-accretion-shock region can directly emit a substantial fraction of the hydrogen-line luminosity. However, the boundary between nonshock-dominated and shock-dominated cases has remained unclear. In this study, we compare hydrogen-line profiles predicted by the shock emission model with 254 observations of 164 low-mass accreting objects (≲0.5 M _⊙ ) in the Very Large Telescope/X-shooter archive. We simultaneously fit seven hydrogen lines (H β , H γ , H6, H8, H9, Pa β , and Br γ ), testing both line profiles and flux ratios within a single framework, and introduce a phenomenological broad-component-subtracted fit for cases with mixed nonshock and shock contributions. We find that shock emission dominates the hydrogen-line emission at object masses M ≲ 0.05 M _⊙ or freefall velocities v _ff < 175 km s ⁻¹ , but becomes minor at M ≳ 0.2 M _⊙ . The inferred flow velocities at the shock front are often significantly smaller than the freefall velocity from infinity, implying smaller truncation radii and surface dipole magnetic field strengths of sub-kG. The accretion luminosities inferred from the shock-emission fitting are systematically larger than literature values, often by orders of magnitude, likely because conventional estimates neglect line emission that is nonnegligible in low-mass objects. We also confirm that H α is more susceptible than the other hydrogen lines to additional nonshock components.

The physical properties of circumplanetary disks offer insights into the formation of giant planets and their satellites. The circumbinary planetary-mass companion SR 12 c (1200 au from SR 12 AB) hosts an isolated dust disk directly detected with ALMA. We present JWST/MIRI imaging of SR 12 c from 5.6 to 21 μ m and analyze its spectral energy distribution from infrared to submillimeter wavelengths. Phenomenologically, the dust disk can be described by a two-component blackbody with large contrasts in characteristic temperatures and radii. Physically motivated models with varied geometries and heating mechanisms suggest a predominantly flat, irradiation-heated disk with a smaller contribution from viscous dissipation. The disk is inferred to be compact, with an inner radius of ∼3 R _Jup . The outer radius is only loosely constrained, with a characteristic value of ∼3 au. The lack of strong 10 μ m excess emission suggests significant grain growth and dust settling. We also find that SR 12 c is broadly consistent with the empirical star–disk scaling relations, providing context for its possible formation pathway. These results demonstrate the power of combining mid-infrared and submillimeter observations with disk modeling to better understand the physical properties of circumplanetary disks.

We present the Atacama Large Millimeter/submillimeter Array (ALMA) and the NOrthern Extended Millimeter Array (NOEMA) observations of the molecular outflow from the high-mass young stellar object (HMYSO) G24.33+0.14, which has recently undergone an accretion burst. CO (2–1) and SiO (5–4) reveal a bipolar outflow with pronounced asymmetry and multiple shells, indicating episodic mass ejection. CO reaches projected velocities up to ∼150 km s ⁻¹ , placing the outflow in the extremely high-velocity (EHV) regime. Position–velocity (PV) diagrams show distinct Hubble-wedge structures, and high-velocity CO knots cluster into two separate wedges, supporting nonsteady, multiepoch ejection. Crucially, CH ₃ CN ( J = 12–11, K = 0–4) emission is detected along the outflow axis and spatially coincides with SiO (5–4). Its velocity gradient aligns with the outflow, rather than the disk, and the derived rotational temperatures have a median value of 135 K, which, together with its spatial and kinematic association with SiO, supports a shock-related origin of the CH ₃ CN emission. Comparison of burst-phase (ALMA 2019) and postburst (NOEMA 2020) data shows significant intensity variations in CO and CH ₃ OH masers, confirming that accretion bursts modify outflow physical conditions on observable timescales. These results support a possible connection between episodic accretion, shock chemistry, and extreme-velocity mass ejection in a massive protostar.

Multiwavelength observations of stellar flares trace the activity of different components of the stars’ outer atmosphere, providing insight into their interactions. In the present paper, we report a superflare from BP Tau, simultaneously observed with the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) satellite and TESS. While we attribute the X-ray flux increase to a magnetically powered flare, the optical light curve likely results from the superposition of the flare and an accretion burst. The X-ray flare has a mean flux of (1.5 _−0.04 ^+0.3 ) × 10 ⁻¹¹ erg cm ⁻² s ⁻¹ in the WXT energy band (0.5−4.0 keV), with e-folding times of 1.7 ± 1.0 ks and 14 ± 5 ks for the rise and decay phase, respectively. The corresponding time-integrated flare energy is (1.0 ± 0.2)×10 ³⁶ erg. The optical flare has an e-folding time of 0.33 ± 0.04 ks for the rise phase, but the data do not constrain the decay timescale. Assuming a decay phase equal to the rise phase, the resulting optical flare energy is (2.8 ± 0.4)×10 ³⁴ erg in the TESS band (∼6000 − ∼10 000 Å), corresponding to a bolometric energy of (1.9 ± 0.3)×10 ³⁵ erg (assuming a blackbody at 11 000 K). The Follow-up X-ray Telescope (FXT) on EP triggered an observation ∼1.5 day after the flare, with a flux of (4.6 _−0.5 ^+0.2 ) × 10 ⁻¹³ erg cm ⁻² s ⁻¹ (0.5−10.0 keV), indicating that BP Tau had returned to quiescence. This work demonstrates the potential of jointly analyzing EP and TESS data for superflares. WXT is expected to detect ∼800 superflares per year, with FXT capable of slewing to the flaring star within ∼3 − 5 minutes. The large field of view of both missions offers us the opportunity to study multiwavelength variability during energetic flares.

Context. The unstable mass accretion process in young stellar objects (YSOs) often triggers observable outbursts. These episodic accretion events play a critical role in stellar mass assembly during the pre-main-sequence phase. In this paper, we present observations of an eruptive young star in the Rosette Nebula, identified by the Gaia Science Alerts system using Gaia time-series data. Aims. We aim to investigate the evolution of the brightness and mass accretion rate of V557 Mon throughout its outburst and subsequent decline. In addition, we trace the evolution of the inner accretion disk during the outburst by monitoring molecular emission features. Methods. We compiled multi-band photometric time-series from Gaia, ZTF, and several 1 m-class ground-based telescopes and obtained optical and near-infrared spectra at multiple epochs covering the outburst and fading phases. Stellar parameters were derived from quiescent colour, spectra and spectral energy distribution (SED) fitting. We also measured the mass accretion rate and fit models to molecular emission bands. Results. Since late 2024, V557 Mon has undergone a year-long outburst consistent with EXor variability. Based on quiescent photometry, V557 Mon has a spectral type of M1 with an extinction of A _V = 1.8 ± 0.3 mag, consistent with a 0.4–0.5 M _⊙ star at an age of 2 Myr. Our multi-epoch spectra and u -band photometry indicate a peak accretion rate of 6.3 × 10 ⁻⁷ M _⊙ yr ⁻¹ during the outburst, roughly 70 times higher than in quiescence. We report the detection of hot water vapour emission bands, together with TiO, VO, and CO emission features. Using ExoMol models, we measured that the inner-disk temperature changed from 3000 K to 2000 K during the fading phase of the outburst. Conclusions. We report a recent EXor outburst in a low-mass Class II YSO. Our observations reveal the transient formation of a hot molecular inner disk, traced by variable water vapour emission during the EXor event. A positive correlation is found between the molecular excitation temperature and the overall stellar brightness.

The evolution and dispersal of protoplanetary disks—governed by accretion, magnetically launched jets and winds, and photoevaporative winds—fundamentally shape planetary systems. Determining how these mass-loss processes coevolve is crucial for constraining planet formation pathways. We analyze archival James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) integral field unit data of 72 inclined ( i > 40°), mostly Class II disks to identify and characterize spatially resolved jets and winds, focusing on [Ne II ] and H ₂ lines. Extended emission in H ₂ S(1), S(3), S(5), S(7), and/or [Ne II ] is detected toward 66 disks, revealing diverse morphologies. We develop a framework to identify conical H ₂ winds and high-velocity [Ne II ] jets perpendicular to the disk, detecting them toward 46 and 40 disks, respectively. All sources with [Ne II ] jets exhibit a corresponding wind traced in either H ₂ (85%) or [O I ], establishing a connection between jets and winds. The detection fractions of [Ne II ] jets and H ₂ winds correlate positively with mass accretion rate, with no dependence on disk inclination or stellar mass. Conversely, marginally resolved low-velocity [Ne II ] winds are found preferentially toward lower accretors. Among sources with H ₂ winds, detection of hotter winds traced by S(7) and S(5) declines more rapidly with decreasing accretion rate than that of the colder S(1) component. Comparison with high-resolution [O I ] 6300Å spectroscopy reveals [O I ] low-velocity components and extended H ₂ wind detections preferentially toward moderate-to-high accretors ( ≳ 1 0 − 8.5 M ̇ ⊙ yr − 1 ), whereas lower accretors exhibit only [O I ] and [Ne II ] winds. Together, these results indicate that atomic jets and atomic+molecular winds, consistent with an MHD disk-wind origin, dominate during early, actively accreting disk phases, while at lower accretion rates, jets weaken and winds become predominantly atomic.

Many details of the gas accretion phase during giant planet formation remain untested. We present new 0.2–0.7 μ m UV-through-red optical imaging of the young, wide-orbit planetary-mass companion SR 12 c from the Wide Field Camera 3 (WFC3) instrument on board the Hubble Space Telescope. SR 12 c exhibits strong accretion-related continuum excess blueward of ∼5000 Å and clear signs of the Balmer jump at 3646 Å. We derive a total accretion luminosity of 1.65 ± 0.19 × 10 ⁻⁵ L _⊙ and a mass accretion rate of 8 ± 2 × 10 ⁻¹² M _⊙ yr ⁻¹ . Based on its mass and age, SR 12 c will not grow by an appreciable amount at its current accretion rate; it is at the end stages of assembly. No accretion variability is evident between the two epochs of the WFC3 observations spanning a month-long baseline, but the H α emission line strength decreases by 90% compared to the reported flux from five years earlier. Combined with previous observations of SR 12 c, we assemble one of the most complete spectral energy distributions of a young giant planet to date, spanning the UV through sub-mm wavelengths (0.2–880 μ m). This adds SR 12 c to the small yet growing sample of planets with detailed accretion and disk constraints, which together are beginning to establish the diversity of timescales and physical processes governing the formation of giant planets.

Erratum: “Tracking Protostellar Variability in Massive Protoclusters with ALMA. I. Insights from QUARKS and MaMMOtH” (2026, ApJ, 1003, 60), Yang, Yuhan, Liu, Tie, Liu, Sheng-Yuan, Johnstone, Doug, Herczeg, Gregory, Jiao, Wenyu, Su, Yu-Nung, Mai, Xiaofeng, Xu, Fengwei, Meyer, D. M.-A., Zhang, Siju, Vorobyov, Eduard, Zhang, Suinan, Luo, Qiuyi, Garay, Guido, Chen, Xi, Jiao, Yunfan, Gu, Qilao, Zhang, Yankun, Tatematsu, Ken’ichi, Kim, Kee-Tae, Sobolev, Andrey M., Parfenov, Sergey, Ladeyschikov, Dmitry A., Bronfman, Leonardo Erratum: “Tracking Protostellar Variability in Massive Protoclusters with ALMA. I. Insights from QUARKS and MaMMOtH” (2026, ApJ, 1003, 60), Yang, Yuhan, Liu, Tie, Liu, Sheng-Yuan, Johnstone, Doug, Herczeg, Gregory, Jiao, Wenyu, Su, Yu-Nung, Mai, Xiaofeng, Xu, Fengwei, Meyer, D. M.-A., Zhang, Siju, Vorobyov, Eduard, Zhang, Suinan, Luo, Qiuyi, Garay, Guido, Chen, Xi, Jiao, Yunfan, Gu, Qilao, Zhang, Yankun, Tatematsu, Ken’ichi, Kim, Kee-Tae, Sobolev, Andrey M., Parfenov, Sergey, Ladeyschikov, Dmitry A., Bronfman, Leonardo

We present two-epoch JWST NIRSpec and MIRI observations of the young protostar EC 53 (V371 Ser), a periodically variable source with well-characterized quiescent and burst phases. The spectra in both epochs show absorption in the CO overtone (∼2.3 μ m) and fundamental (∼4.6 μ m) bands and the H ₂ O stretching (∼2.7 μ m) and bending (∼6.0 μ m) modes. We also obtained high-resolution ( R ≈ 45,000) IGRINS spectra during the burst to constrain the CO overtone line profiles. Local thermodynamic equilibrium (LTE) slab modeling yields gas temperatures of ∼1800 K (CO overtone) and ∼1200 K (CO fundamental), consistent with the overtone tracing hotter gas at smaller radii. The H ₂ O stretching-mode absorption shows no compelling evidence for variability, and the current JWST CO overtone data do not provide a robust constraint on overtone variability. In contrast, the CO fundamental and H ₂ O bending-mode features weaken by a factor of ∼2 during the burst, which is most naturally explained by continuum changes rather than large variations in absorbing gas. To quantify continuum dilution, we introduce a “relative veiling” that treats the quiescent spectrum as an internal reference and measures the change in the continuum excess between the two epochs. This formalism yields burst-to-quiescent hot-continuum ratios of 2.9 ± 0.2 for the CO overtone and 1.71 ± 0.11 for the CO fundamental. Using a viscous-disk prescription, these imply representative accretion-rate ratios of ∼3.6 and ∼2.0, respectively. The differing ratios suggest that inner-disk regions traced at different temperatures, and thus radii, respond differently across the burst cycle, consistent with episodic mass buildup in the inner disk during quiescence followed by more efficient transport through the innermost disk onto the protostar during the burst.