Veronika Witzke

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.

We model the solar radial velocity (RV) signal induced by faculae, the dominant contributors to RV variability in Sun-like stars. We use a representative case of a facular patch transiting the visible solar disk as the Sun rotates to disentangle various physical effects contributing to the RV signal. Our approach is based on 3D radiative magnetohydrodynamic (MHD) simulations of the solar photosphere and upper convection zone performed with the MURaM code, together with spectral synthesis using the MPS-ATLAS code. We show that the facula-induced RV signal strongly depends on the facular position on the solar disk. Near disk center, facular magnetic fields inhibit the convective blueshift and thus produce a relative redshift of the solar spectrum. Surprisingly, when located closer to the limb, namely at heliocentric angles greater than about 60°, faculae produce a relative blueshift. This transition from redshift to blueshift is caused by the effects of magnetic fields on horizontal flows, which dominate the signal near the limb, and on the visibility of these flows. In combination with solar rotation, this center-to-limb dependence of the facular effect leads to a complex RV profile during the facular transit and, in particular, to a phase lag between the maximum of the RV signal and the facular crossing of the central meridian. We further show that, in contrast to stellar reflex motion, the facular signal strongly depends on the spectral line in which it is measured.

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.

The intrinsic variability of stars, due to acoustic oscillations, surface granulation, and magnetic activity, introduces radial velocity (RV) jitter in spectral lines, obscuring true planetary signals and hindering the detection of Earth-like planets. Granulation is particularly challenging, as it affects even the most inactive stars, introducing substantial signals, with amplitudes up to 1 m s ⁻¹ . Disentangling granulation-induced RV jitter from the signal caused by planetary reflex motion requires reliable models of stellar granulation. In this study, we present a new approach for calculating the sensitivities of spectral lines to granulation. We simulate near-surface convection with the 3D radiative MHD code MURaM and calculate high-resolution emergent spectra with the radiative transfer code MPS-ATLAS. We then show that the spatial and temporal variability of spectral lines due to granulation are almost identical, and introduce a novel methodology that uses spatial variability at a single moment in time to compute their temporal variability. This approach significantly reduces computational costs. We apply our approach to analyze the response of lines from neutral and singly ionized elemental species to solar granulation. We find a clear distinction between the two groups of lines: those from neutral elements tend to show stronger variations in line strength, whereas those from singly ionized elements exhibit larger variations in central wavelength. These results enable the development of spectral line masks tailored to granulation sensitivity, offering a promising strategy to reduce granulation-induced RV noise and improve exoplanet detection.

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.

Available one-dimensional stellar models fail to reproduce the observed spectrum of the ultracool M dwarf TRAPPIST-1. In particular, current models predict strong iron hydride (FeH) absorption due to the Wing–Ford bands at 0.99 μ m, yet this spectral feature is only weakly present in TRAPPIST-1 and other mid-to-late M dwarf stars. Additionally, the shape of the continuum between the water bands in the near-infrared does not match between models and observations. Here, we show that assumptions about pressure broadening, specifically van der Waals broadening, have a dramatic effect on modeled broadband spectral features. We use Merged Parallelized Simplified-ATLAS to generate synthetic spectra over a range of van der Waals broadening strengths, adopting 1D PHOENIX temperature–pressure structures. We find that minimal broadening best matches the observed FeH profile at 0.99 μ m and in the pseudocontinuum between the large water bands. These results suggest that broadening prescriptions derived for Sun-like stars are not valid for lower-mass stars and that pressure broadening for molecular lines in cool stellar atmospheres must be reevaluated. Refining pressure broadening treatments will improve the accuracy of M dwarf spectral models, enabling more reliable determinations of stellar properties and atmospheric compositions of planets orbiting M dwarfs.

Available one-dimensional stellar models fail to reproduce the observed spectrum of the ultracool M dwarf TRAPPIST-1. In particular, current models predict strong iron hydride (FeH) absorption due to the Wing-Ford bands at 0.99$μ$m, yet this spectral feature is only weakly present in TRAPPIST-1 and other mid-to-late M dwarf stars. Additionally, the shape of the continuum between the water bands in the near-infrared does not match between models and observations. Here, we show that assumptions about pressure broadening, specifically van der Waals broadening, have a dramatic effect on modeled broadband spectral features. We use Merged Parallelized Simplified-ATLAS to generate synthetic spectra over a range of van der Waals broadening strengths, adopting 1D PHOENIX temperature-pressure structures. We find that minimal broadening best matches the observed FeH profile at 0.99$μ$m and in the pseudocontinuum between the large water bands. These results suggest that broadening prescriptions derived for Sun-like stars are not valid for lower-mass stars and that pressure broadening for molecular lines in cool stellar atmospheres must be reevaluated. Refining pressure broadening treatments will improve the accuracy of M dwarf spectral models, enabling more reliable determinations of stellar properties and atmospheric compositions of planets orbiting M dwarfs.

Differences in the ingress and egress shapes of transit light curves can indicate morning–evening temperature contrasts on transiting planets. Here, we pinpoint an alternative mechanism that can introduce asymmetries in transit light curves, potentially affecting the accurate determination of morning–evening differences. Small-scale magnetic field concentrations on the surfaces of the host star affect the visibility of stellar limb regions, making them brighter relative to the nonmagnetic case. A difference in magnetization between the star’s western and eastern limbs can thus create an asymmetry in limb brightness and, consequently, an asymmetry between transit ingress and egress. We model the limb darkening and stellar limb asymmetry in solar-like stars using the 3D radiative magnetohydrodynamics (MHD) code Max-Planck-Institute für Sonnensystemforshung (MPS)/University of Chicago Radiative MHD (MURaM) to simulate magnetized stellar atmospheres and the MPS-ATLAS code to synthesize spectra using a ray-by-ray approach. Our results show that ingress–egress depth differences can reach up to 600 ppm for a 10,000 ppm transit at 0.6 μm, depending on the magnetization of the stellar limbs, significantly interfering with planetary signals. Observations of the Sun show that such concentrations are often not accompanied by spots and do not manifest in photometric variability, indicating that even photometrically quiet stars can produce such asymmetries. However, planetary and stellar asymmetries exhibit distinct wavelength dependencies, which we propose to leverage for disentangling them.