Abstract. Modelling stellar atmospheres becomes increasingly demanding as more accurate observations draw a more complex picture of how real stars look like. What could be called a normal star becomes increasingly rare because of, e.g., significant deviations from the classical solar abundance pattern and clear evidence for stratification of elements in the atmospheres as well as surface inhomogeneities (spots) causing further severe deviations from "standard" atmospheres. We describe here a new code for calculating LTE plane-parallel stellar model atmospheres for early and intermediate type of stars which has been written in Compaq Fortran 95 and can be compiled for Windows and Linux/UNIX computer platforms. The code is based on modified 9 subroutines (Kurucz) and on spectrum synthesis codes written by V. Tsymbal with the main modifications of input physics concerning the block for opacity calculation. Each line contributing to opacity is taken into account for modelling the atmosphere, similar to synthetic spectrum calculations. This approach, which we call the line-by-line (LL) technique, avoids problems resulting from statistical methods (ODF, OS) and allows to calculate complex models with abundances which are not simply scaled from a standard pattern (usually the solar abundances) and which can be even depth dependent. Stratification is considered in this context as an empirical input parameter which has to be derived from observations. Due to the implemented numerical methods, mainly in the opacity calculation module, our code produces model atmospheres with modern PCs in a time comparable to that required by classical routines.

M dwarfs are the most numerous stars in our Galaxy with masses between approximately 0.5 and 0.1 solar mass. Many of them show surface activity qualitatively similar to our Sun and generate flares, high X-ray fluxes, and largescale magnetic fields 1-4 . Such activity is driven by a dynamo powered by the convective motions in their interiors 2,5-8 . Understanding properties of stellar magnetic fields in these stars finds a broad application in astrophysics, including, e.g., theory of stellar dynamos and environment conditions around planets that may be orbiting these stars. Most stars with convective envelopes follow a rotation-activity relationship where various activity indicators saturate in stars with rotation periods shorter than a few days 2,6,8 . The activity gradually declines with rotation rate in stars rotating more slowly. It is thought that due to a tight empirical correlation between X-ray and magnetic flux 9 , the stellar magnetic fields will also saturate, to values around ∼ 4 kG 10 . Here we report the detection of magnetic fields above the presumed saturation limit in four fully convective M-dwarfs. By combining results from spectroscopic and polarimetric studies we explain our findings in terms of bistable dynamo models 11,12 : stars with the strongest magnetic fields are those in a dipole dynamo state, while stars in a multipole state cannot generate fields stronger than about four kilogauss. Our study provides observational evidence that dynamo in fully convective M dwarfs generates magnetic fields that can differ not only in the geometry of their large scale component, but also in the total magnetic energy.Our understanding of origin and evolution of the magnetic fields in M dwarfs is based on the models of the rotationally driven convective dynamos. Modern observations provide two important constraints for these models.First, from the analysis of circular polarization in spectral lines we infer that large-scale magnetic fields tend to have simple axisymmetric geometry with dominant poloidal component in stars that are fully convective. In contrast, M dwarfs that are hotter and therefore only 1 arXiv:1801.08571v1 [astro-ph.SR] 25 Jan 2018 partly convective tend to have more complex fields with strong toroidal components 13 . However, there is a number of exceptions when a rapidly-rotating fully convective star generates a large-scale magnetic field with a complex multipole geometry. This dichotomy of magnetic properties in stars that have similar stellar parameters may be explained in terms of dynamo bistability: stars can relax to either dipole or multipole states depending on the geometry and the amplitude of an initial seed magnetic field 11,12 . Note, however, that dynamo bistability was observed only in models of stars with masses M 0.2M .The second observational constraint is the rotation-activity relation 2,8,14,15 . A remarkable feature of this relation is the existence of two branches, a saturated and a non-saturated branch. In the non-saturated branch, the amount of non-thermal (e.g...

Context. The Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs (CARMENES) survey is searching for Earth-like planets orbiting M dwarfs using the radial velocity method. Studying the stellar activity of the target stars is important to avoid false planet detections and to improve our understanding of the atmospheres of late-type stars. Aims. In this work we present measurements of activity indicators at visible and near-infrared wavelengths for 331 M dwarfs observed with CARMENES. Our aim is to identify the activity indicators that are most sensitive and easiest to measure, and the correlations among these indicators. We also wish to characterise their variability. Methods. Using a spectral subtraction technique, we measured pseudo-equivalent widths of the He I D3, Hα, He I λ10833 Å, and Pa β lines, the Na I D doublet, and the Ca II infrared triplet, which have a chromospheric component in active M dwarfs. In addition, we measured an index of the strength of two TiO and two VO bands, which are formed in the photosphere. We also searched for periodicities in these activity indicators for all sample stars using generalised Lomb-Scargle periodograms. Results. We find that the most slowly rotating stars of each spectral subtype have the strongest Hα absorption. Hα is correlated most strongly with He I D3, whereas Na I D and the Ca II infrared triplet are also correlated with Hα. He I λ10833 Å and Paβ show no clear correlations with the other indicators. The TiO bands show an activity effect that does not appear in the VO bands. We find that the relative variations of Hα and He I D3 are smaller for stars with higher activity levels, while this anti-correlation is weaker for Na I D and the Ca II infrared triplet, and is absent for He I λ10833 Å and Paβ. Periodic variation with the rotation period most commonly appears in the TiO bands, Hα, and in the Ca II infrared triplet.

We present precise photospheric parameters of 282 M dwarfs determined from fitting the most recent version of PHOENIX models to high-resolution CARMENES spectra in the visible (0.52–0.96 μm) and NIR wavelength range (0.96–1.71 μm). With its aim to search for habitable planets around M dwarfs, several planets of different masses have been detected. The characterization of the target sample is important for the ability to derive and constrain the physical properties of any planetary systems that are detected. As a continuation of previous work in this context, we derived the fundamental stellar parameters effective temperature, surface gravity, and metallicity of the CARMENES M-dwarf targets from PHOENIX model fits using a χ2 method. We calculated updated PHOENIX stellar atmosphere models that include a new equation of state to especially account for spectral features of low-temperature stellar atmospheres as well as new atomic and molecular line lists. We show the importance of selecting magnetically insensitive lines for fitting to avoid effects of stellar activity in the line profiles. For the first time, we directly compare stellar parameters derived from multiwavelength range spectra, simultaneously observed for the same star. In comparison with literature values we show that fundamental parameters derived from visible spectra and visible and NIR spectra combined are in better agreement than those derived from the same spectra in the NIR alone.

Context. For an asteroseismic modeling, the analysis of the high-accuracy light curves delivered by the Kepler satellite mission needs support by ground-based multi-colour and spectroscopic observations. Aims. We determine the fundamental parameters of SPB and β Cep candidate stars observed by the Kepler satellite mission and estimate the expected types of non-radial pulsators. Methods. We compare newly obtained high-resolution spectra with synthetic spectra computed on a grid of stellar parameters assuming LTE and check for NLTE effects for the hottest stars. For comparison, we determine T eff independently from fitting the spectral energy distribution of the stars obtained from the available photometry. Results. We determine T eff , log g, micro-turbulent velocity, v sin i, metallicity, and elemental abundance for 14 of the 16 candidate stars, two of the stars are spectroscopic binaries. No significant influence of NLTE effects on the results could be found. For hot stars, we find systematic deviations of the determined effective temperatures from those given in the Kepler Input Catalogue. The deviations are confirmed by the results obtained from ground-based photometry. Five stars show reduced metallicity, two stars are He-strong, one is He-weak, and one is Si-strong. Two of the stars could be β Cep/SPB hybrid pulsators, four SPB pulsators, and five more stars are located close to the borders of the SPB instability region.