Nadiia M. Kostogryz

We present a systematic investigation of the connection between magnetic activity and the low-surface-gravity-like near-infrared spectral morphology of late M dwarfs. Our sample includes TRAPPIST-1 observed with JWST NIRISS/Single Object Slitless Spectroscopy (SOSS) and 22 late M dwarfs observed with Infrared Telescope Facility/SpeX, together with flare observations from K2, TESS, JWST NIRISS/SOSS, and literature-reported X-ray luminosities. Using one-component PHOENIX/NewEra spectral modeling, we find that all targets are better reproduced by lower surface gravity model spectra than by models with the expected high gravities of field late M dwarfs. The retrieved surface gravities moderately correlate with independently measured gravity-sensitive spectral indices, with the strongest trends seen for FeH _z at 0.998 μ m and FeH _J at 1.2 μ m, both showing statistically significant Pearson and Spearman correlations with p < 0.005. They also show strong anticorrelations with flare luminosity and X-ray luminosity: for both relations, Pearson and Spearman coefficients are <−0.9, with p ≤ 1.0 × 10 ⁻⁴ , indicating that more active stars tend to have spectra favoring lower-gravity morphologies. Time-series spectra of TRAPPIST-1 during two large flares show no clear short-timescale variations in selected photospheric indices, suggesting that persistent small-scale magnetic activity may play a more important role than large flares in modifying the photospheric temperature–pressure structure. A simplified radiative transfer experiment further shows that activity-associated heating can produce lower-gravity-like spectral features even when the input surface gravity is fixed. Our results suggest that underestimated surface gravities in late M dwarfs may reflect atmospheres modified by frequent, small-scale magnetic activity, such as microflares, producing emergent spectra that mimic lower-gravity morphology.

Stellar activity complicates exoplanet transmission spectra, particularly for smaller planets around M dwarfs with JWST. The transit-light-source (TLS) effect, the imprinting of spectral differences between the average stellar disk and the occulted transit chord onto the transmission spectrum, makes it challenging to directly use the out-of-transit spectrum to correct for stellar contamination. Theory and observations suggest that spots may concentrate toward higher latitudes when the Coriolis force is substantial relative to buoyancy, leaving the equatorial region relatively quiet. Here, we evaluate how the latitudinal distribution of active regions shapes the strength of the TLS effect for planets spanning a range of impact parameters ( b ), using TRAPPIST-1 as a testbed. We first construct a fiducial model to illustrate two distribution regimes. With our model, the moderate- b outer TRAPPIST-1 planets (f, g, h) occult a more typical region of the stellar disk than the inner planets and are thereby less affected by the TLS effect, though their bias may vary more from visit to visit as these active regions evolve with time. More generally, our results imply an impact parameter “sweet spot” for atmospheric characterization, independent of the sign of the active-region temperature contrast, whose location depends on the distribution of active regions. The distribution may be revealed by transit residuals as multiple planets probe different latitudes, while longitudes are sampled in time, such that the variance and frequency of the correlated scatter could constrain active-region filling factors, sizes, and separations.

The study of exoplanet atmospheres has brought a renewed interest in stellar astrophysics. Specifically, stellar magnetic activity from the host star contaminates exoplanet atmosphere transmission spectra, as evidenced by observations from the James Webb Space Telescope. Stellar surface magnetic features in the form of dark spots caused by large concentrations of surface magnetic fields and the more diffuse, extended faculae caused by small-scale magnetic field concentrations change the apparent size of the star in a wavelength-dependent way. These spot and faculae contributions to the change in transit depth contaminate the planetary signal. Here, we study the transition from bright faculae on G and K dwarfs to dark faculae on M dwarfs. This dark appearance of faculae is in significant contrast to the conventional picture that faculae are brighter than the quiet star region as they are on the Sun. We use the 3D radiative magnetohydrodynamics code MURaM to simulate faculae and calculate their spectra with the MPS-ATLAS radiative transfer code. We present a qualitative explanation for the transition from bright to dark faculae, attributing it to shallower flux tubes and reduced vertical temperature gradients at the surfaces of M dwarfs relative to the Sun.

Stellar granulation is a source of radial-velocity (RV) jitter of 1 m s ⁻¹ on Sun-like stars, limiting the detection of Earth analogs. One way to move beyond this limit is to use line-weighting schemes to mitigate granulation-driven RV variability in a spectral-type-dependent manner. A physics-based granulation-sensitivity diagnostic is essential. Here, we use a line-by-line sensitivity measure from 3D magnetoconvection simulations that quantifies how strongly each spectral line’s Doppler shift and strength respond to convective velocity and thermodynamic fluctuations. Extending our previous solar analysis, which used the spatial variability of line profiles across a single granulation snapshot as a computationally efficient proxy for temporal variability, we test the transferability of this diagnostic to cooler stars and characterize how line sensitivity varies with spectral type. We synthesize high-resolution spectra with the Merged Parallelised Simplified ATLAS code from 3D time-dependent MURaM simulations representative of the Sun and late-G and K dwarfs, focusing on neutral and singly ionized iron lines spanning a wide range of excitation potentials and strengths. As T _eff decreases, convective velocities weaken, and the ionization balance shifts, producing a clearer separation between line families: Fe i lines show lower velocity sensitivity and smaller fractional strength variability, whereas Fe ii lines show the opposite. Using cumulative contribution functions, we further relate spectroscopic velocity jitter to characteristic line-formation temperature. The diagnostic continues to separate robust and sensitive lines in late-G and K dwarfs, enabling spectral-type-aware cross-correlation masks and line-by-line weights. Solar-optimized line selections are not generally portable to cooler stars, especially when based on equivalent-width stability rather than velocity sensitivity.

TRAPPIST-1 is an ultracool dwarf that flares frequently. These flares shape the surrounding planets’ high-energy irradiation environment, with consequences for atmospheric chemistry and escape, and they can contaminate transmission spectroscopy of those planets. A quantitative flare-frequency distribution (FFD) spanning the full energy range is therefore essential for both interpreting JWST spectra and modeling the planets’ irradiation histories. Here we present a unified FFD over 4 orders of magnitude in energy by jointly analyzing ≈87 hr of JWST/NIRISS and JWST/NIRSpec time-series spectroscopy together with ≈74 days of Kepler Space Telescope (Kepler) K2 photometry. To enable a consistent comparison across these heterogeneous datasets, we convert all events to energies in the Transiting Exoplanet Survey Satellite (TESS) bandpass. For the Kepler-to-TESS conversion we adopt a cooler flare continuum appropriate for ultracool dwarfs ( T flare = 3500 K). After correcting for flare-detection sensitivities, the combined JWST + K2 cumulative FFD is consistent with a single power law, N ( ≥ E TESS ) ∝ E TESS − β , with β = 0.753 over E _TESS ≃ 10 ²⁹ –10 ³³ erg. The slope of the distribution indicates that the time-averaged flare energy budget is dominated by rare, high-energy events rather than by the more numerous low-energy flares. This bandpass-consistent FFD provides a practical basis for JWST transit-spectroscopy planning and for modeling the flare-driven irradiation environment of the TRAPPIST-1 planets.

Context . Stellar convection in the presence of magnetic field affects the emergent intensity, as well as the structure and evolution of cool main-sequence dwarfs. Aims . We aim to understand the effect of faculae-like field strengths on near-surface stellar convection using 3D radiative MHD simulations of near-surface magneto-convection. Methods . We compare simulations of F, G, K, and M main-sequence stars with a small-scale dynamo (SSD) to faculae-like spatially averaged field strengths (from 100 to 500 G). We focus on the effect of imposed magnetic field on the thermodynamic stratification and velocities, along with the bolometric intensity and surface field strength. Results . Imposed magnetic fields result in reduced average density and gas pressure near the surface compared to the SSD simulations. The temperature stratification also shows a dip at and just below the stellar surface. The changes in average bolometric intensity are within a percent, with different trends with field strength for different stellar types. In addition, the convective velocities are reduced. The magnitude of changes in thermodynamic quantities are related to field strength as well as the stellar T _eff . Conclusions . Faculae-strength magnetic fields modify the near surface convection by reducing gas pressure and density as well as suppressing convection in regions with strong field concentrations. The strength of these effects depends on the stellar type.

Early JWST observations of TRAPPIST-1 have revealed an unexpected puzzle: energetic white-light flares ( E > 10 ³⁰ erg) reach temperatures of only ∼3500–4000 K, nearly 3 times cooler than typical solar flares, which peak around 9000–10,000 K. Here we explain this difference by identifying the physical mechanism that regulates flare temperatures on late M dwarfs. The key factor is that in the cool, dense atmosphere of TRAPPIST-1, magnetic heating is strongly moderated by the dissociation of molecular hydrogen (H ₂ ) into atomic hydrogen. This “H ₂ dissociation thermostat” acts as an efficient energy sink, preventing flare regions from heating above ∼4000 K. Our chemical equilibrium and heat capacity calculations show that this effect depends sensitively on stellar atmospheric pressure and the local abundance of H ₂ . In hotter stars, from early M dwarfs to solar-type stars, the scarcity of molecular hydrogen renders this mechanism ineffective; instead, atomic hydrogen ionization limits flare temperatures near ∼9000 K.

Context. Helioseismology aims to infer the properties of the solar interior by analyzing observations of acoustic oscillations. Interpreting helioseismic data, however, is complicated by the non-trivial relationship between helioseismic observables and the physical perturbations associated with acoustic modes as well as by various instrumental effects. Aims. We aim to improve our understanding of the signature of acoustic modes measured in the Helioseismic and Magnetic Imager (HMI) continuum intensity and Doppler velocity observables by accounting for radiative transfer, solar background rotation, and spacecraft velocity. Methods. We started with a background model atmosphere that accurately reproduces solar limb darkening and the Fe I 6173Å spectral line profile. We employed first-order perturbation theory to model the effect of acoustic oscillations on inferred intensity and velocity. By solving the radiative transfer equation in the atmosphere, we synthesized the spectral line, convolved it with the six HMI spectral windows, and deduced the continuum intensity (hmi.Ic_45s) and Doppler velocity (hmi.V_45s) according to the HMI algorithm. Results. We analytically derived the relationship between mode displacement in the atmosphere and the HMI observables and show that both the intensity and velocity deviate significantly from simple approximations. Specifically, the continuum intensity does not simply reflect the true continuum value, while the line-of-sight velocity does not correspond to a straightforward projection of the velocity at a fixed height in the atmosphere. Our results indicate that these deviations are substantial, with amplitudes of approximately 10% and phase shifts of around 10° across the detector for both observables. Moreover, these effects are highly dependent on the acoustic mode under consideration and the position on the solar disk. To achieve accurate modeling of the observables, it is important to account for the impact of radiative transfer on oscillation velocities and perturbations in atmospheric thermodynamic quantities, which influence the line profile. Conclusions. The combination of these effects leads to non-trivial systematic errors (in amplitude and phase) across the detector that must be taken into account to understand the observables. This framework can be used to study mode visibility across the solar disk and for asteroseismology applications.

TRAPPIST-1 is an M8 dwarf hosting seven known exoplanets and is currently one of the most frequently observed targets of the James Webb Space Telescope (JWST). However, it is notoriously active, and its surface is believed to be covered by magnetic features that contaminate the planetary transmission spectra. The radiative spectra of these magnetic features are needed to clean transmission spectra, but they currently remain unknown. Here, we develop a new approach for measuring these spectra using time-resolved JWST/NIRISS observations. We detect a persistent postflare enhancement in the spectral flux of TRAPPIST-1. Our analysis rules out lingering flare decay as the cause of the flux enhancement and thus, points to structural changes on the stellar surface induced by flares. We suggest that the flaring event triggers the disappearance of (part of) a dark magnetic feature, producing a net brightening. This suggestion is motivated by solar data: flare-induced disappearance of magnetic features on the solar surface has been directly detected in high spatial resolution images, and our analysis shows that this process produces changes in solar brightness very similar to those we observe on TRAPPIST-1. The proposed explanation for the flux enhancement enables, to our knowledge, the first measurement of the spectrum of a magnetic feature on an M8 dwarf. Our analysis indicates that the disappearing magnetic feature is cooler than the TRAPPIST-1 photosphere, but by at most a few hundred kelvins.

TRAPPIST-1 is an M8 dwarf hosting seven known exoplanets and is currently one of the most frequently observed targets of the James Webb Space Telescope (JWST). However, it is notoriously active, and its surface is believed to be covered by magnetic features that contaminate the planetary transmission spectra. The radiative spectra of these magnetic features are needed to clean transmission spectra, but they currently remain unknown. Here, we develop a new approach for measuring these spectra using time-resolved JWST/NIRISS observations. We detect a persistent post-flare enhancement in the spectral flux of TRAPPIST-1. Our analysis rules out lingering flare decay as the cause of the flux enhancement and, thus, points to structural changes on the stellar surface induced by flares. We suggest that the flaring event triggers the disappearance of (part of) a dark magnetic feature, producing a net brightening. This suggestion is motivated by solar data: flare-induced disappearance of magnetic features on the solar surface has been directly detected in high spatial resolution images, and our analysis shows that this process produces changes in solar brightness very similar to those we observe on TRAPPIST-1. The proposed explanation for the flux enhancement enables, to our knowledge, the first measurement of the spectrum of a magnetic feature on an M8 dwarf. Our analysis indicates that the disappearing magnetic feature is cooler than the TRAPPIST-1 photosphere, but by at most a few hundred kelvins.