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The results of photometric, spectral and radio studies of the flare star EV Lac we obtained during the course of cooperative observations in 1994 and 1995 are presented. A quantitative analysis of the radiation emitted by two powerful flares using the colour-colour diagram confirms the previous conclusion on the essential heterogeneity of matter radiating in optical flares. From simultaneous observations of the star in UBVRI, K and H bands no significant brightness variations in IR were found in coincidence with observed small-amplitude optical flares, except a gradual decrease in the K-band following a 1.0 mag flare in the U-band. A differential spottedness of bright and dark hemispheres of EV Lac is estimated using observations in separate seasons and the total stellar spottedness is determined within the framework of the zonal spottedness model. Spectra of quiet and active states of the star in the blue-green region in 1994 and in the red region in 1995 are described: the characteristics of th...

This paper investigates the Paschen-Back effect in the 0 - 0 band of the A6Σ+ - X6Σ+ system of the CrH molecule and demonstrates a possibility to measure magnetic fields on cool dwarfs. The quantum mechanical calculations were done in order to obtain the energy level structure of the CrH molecule both in the absence and in the presence of the magnetic field. In addition, we calculated the frequencies of the transitions between the magnetic levels according to the selection rules and their strengths (Honl-London factors). Employing these results and the analytical solution of the transport equation for polarized radiative transfer in a stellar atmosphere, based on the Milne-Eddington approximation, we calculated the four Stokes parameters and estimated the polarization signal in dependence of the strength and orientation of the magnetic field. We show that net polarisation is produced in CrH bands due to the Paschen-Back effect.

Iron hydride (FeH) bands are the most sensitive indicators of magnetic fields in cool atmospheres, where atomic lines lose their utility. In the first part of our work, we established the theoretical foundations for the analysis of the astrophysically important Wing‐Ford FeH F4Δ−X4Δ system around 1 μm. We calculated the molecular Zeeman effect, computed the Lande factors of the energy levels and transitions, and calibrated molecular constants to account for unknown perturbations by comparing observed and modeled sunspot spectra. In the second part we illustrate the applicability of the FeH lines to diagnose stellar magnetic fields. We modeled highly magnetically sensitive FeH line profiles employing our novel quantum‐mechanical treatment of the Zeeman effect in the FeH molecule and compared them with observed M dwarf spectra to infer magnetic field strengths. Measurements of magnetic fields on cool M dwarfs provide important information on stellar dynamo processes, since the transformation between the solar type to a turbulent dynamo takes place roughly at a spectral type M3 to M4 and the dynamo mechanism that generates and amplifies magnetic fields in fully convective stars is still unknown.

Stellar spectra usually are very limited in the signal‐to‐noise ratio (SNR) that can be obtained. In order to increase their informative value, different techniques have been developed in the past ten years which combine multiple spectral lines by cutting out individual line profiles and analysing them by means of least‐squares errors or PCA. They usually neglect that the bulk of lines are blended, resulting in artificial broadening of the retrieved common line pattern, while those that care to disentangle blended profiles assume linear line adding.Based on the well‐known least‐squares deconvolution (LSD) method, we developed a new technique, which truly accounts for blended profiles and deconvolves them in a physically meaningful way by taking into account the nonlinearity when abandoning the regime of optically thin lines. The so‐called interpolation formula by M. Minnaert is a unique tool to describe a line profile both in the optically thin and the optically thick regime. It enables us to write a tota...

V889 Herculis and EK Draconis are both young active dwarfs that can be considered as young solar analogues. Earlier Doppler imaging studies of these stars have shown that they have quite different spot distributions. Despite of that, latitudinal spot distribution seems to have a connection to the photometrically determined activity cycle. However, especially for EK Dra there have not been many Doppler images available for a detailed study. Here we present new surface temperature maps for both stars, and discuss further possible connections to the activity cycles and compare the results with the solar case.

We apply stellar surface imaging techniques for studying non-radial pulsations. Stellar surface imaging is based on inversions of time series of variable spectral line proflles without making assumptions on the speciflc shape of the pulsations. The inversion results in an image of the stellar surface in which the sectoral and tesseral modes can in many cases be distinguished, and the pulsational degree l and the azimuthal order jmj can be determined. We apply this method to simulated data for testing the method (Berdyugina et al. 2003a). The tests show that the tesseral modes with l ijmj = 2 (or any even number) can be successfully recovered. Pulsation modes that are antisymmetric with respect to the equator, i.e. when lijmj = 1 (or any odd number), are di‐cult to recover accurately.