arXiv subjects

N. Kostogryz

Publications and source records attributed to N. Kostogryz.

4 recordsLinked to original sources

arXiv2026

Effect of surface magnetic fields on limb darkening in main-sequence stars

Context.Stellar limb darkening encodes the thermal and radiative structure of stellar photospheres and is a key ingredient in modeling transit light curves and transmission spectra. It was recently shown that stellar surface magnetic fields modify limb darkening in stars with near-solar fundamental parameters, and that only magnetic models can reproduce high-precision transit observations for such stars. However, for stars with non-solar fundamental parameters, the magnitude of the magnetic effect on limb darkening remain unconstrained. Aims.We aim to investigate how surface magnetism affects stellar limb darkening across a range of fundamental parameters and to provide the community with center-to-limb spectra of stars at different magnetization levels. Methods. We use the MPS-ATLAS code to compute synthetic spectra from 3D radiative magnetohydrodynamic box-in-a-star simulations performed with the MURaM code. These simulations self-consistently capture photospheric magneto-convection without relying on ad hoc parameterizations. We perform calculations for main-sequence stars at solar metallicity with effective temperatures in the range Teff = 3200 - 6800 K. For stars with solar effective temperature we also consider metal-poor, M/H = -1.0, and metal-rich, M/H = 0.5, cases. Results. We show that the magnitude of the magnetic effect depends strongly on stellar fundamental parameters, increasing toward hotter and more metal-rich stars. Overall, limb darkening is significantly affected by magnetic fields in K, G, and F dwarfs, while the effect becomes negligible in M-dwarfs. We release a public database of synthetic spectra at 10 disk positions.

astro-ph.SR↗

arXiv2026

Sensitivity of spectral lines to granulation: from the Sun to K-type stars

Stellar granulation produces radial-velocity (RV) jitter at the 1 m/s level in Sun-like stars, limiting Earth-analog detection. A route beyond this limit is to weight spectral lines according to their granulation sensitivity. We apply a line-by-line diagnostic from 3D magneto-convection simulations that measures how each line's Doppler shift and strength respond to convective velocity and thermodynamic fluctuations. Extending our solar study, which used spatial line-profile variability across one granulation snapshot as an efficient proxy for temporal variability, we test whether this diagnostic transfers to cooler stars and examine how sensitivity changes with spectral type. We synthesize high-resolution spectra with MPS-ATLAS from 3D time-dependent MURaM simulations of the Sun and late-G and K dwarfs, focusing on FeI and FeII lines spanning broad ranges of excitation potential and strength. With decreasing $T_{\mathrm{eff}}$, weaker convective velocities and changing ionization balance produce a clearer separation between line families: FeI lines show lower velocity sensitivity and smaller fractional strength variability, while FeII lines are more sensitive. Cumulative contribution functions link spectroscopic velocity jitter to characteristic line-formation temperature. The diagnostic robustly separates stable and granulation-sensitive lines in late-G and K dwarfs, enabling spectral-type-aware cross-correlation masks and line-by-line RV weights. Solar-optimized line selections are therefore not generally portable to cooler stars, particularly when based on equivalent-width stability rather than velocity sensitivity.

astro-ph.SR↗

arXiv2025

The effect of stellar magnetic activity on measurements of morning and evening asymmetry of planetary terminator

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 non-magnetic 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 MHD code MURaM to simulate magnetized stellar atmospheres and the MPS-ATLAS code to synthesize spectra using ray-by-ray approach. Our results show that ingress-egress depth differences can reach up to 600 ppm for a 10000 ppm transit at 600 nm, 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.

astro-ph.SR↗

arXiv2023

MPS-ATLAS library of stellar model atmospheres and spectra

Stellar spectra contain a large amount of information about the conditions in stellar atmospheres. However, extracting this information is challenging and demands comprehensive numerical modelling. Here, we present stellar spectra synthesized using the recently developed state-of-the-art MPS-ATLAS code on a fine grid of stellar fundamental parameters. These calculations have been extensively validated against solar and stellar observations and can be used for various astrophysical applications. The spectra are available at the Max Planck Digital Library (MPDL, https://edmond.mpdl.mpg.de/dataset.xhtml?persistentId=doi:10.17617/3.NJ56TR) and have been also incorporated into the ExoTiC-LD python package (https://github.com/Exo-TiC/ExoTiC-LD/) which returns stellar limb darkening coefficients used for the software package Exoplanet Timeseries Characterisation (ExoTic).

astro-ph.SR↗