No abstract

No abstract

Context. The major challenges for a fully polarized radiative transfer driven approach to Zeeman-Doppler imaging are still the enormous computational requirements. In every cycle of the iterative interplay between the forward process (spectral synthesis) and the inverse process (derivative based optimization) the Stokes profile synthesis requires several thousand evaluations of the polarized radiative transfer equation for a given stellar surface model. Aims. To cope with these computational demands and to allow for the incorporation of a full Stokes profile synthesis into Dopplerand Zeeman-Doppler imaging applications as well as into large scale solar Stokes profile inversions, we present a novel fast and accurate synthesis method for calculating local Stokes profiles. Methods. Our approach is based on artificial neural network models, which we use to approximate the complex non-linear mapping between the most important atmospheric parameters and the corresponding Stokes profiles. A number of specialized artificial neural networks, are used to model the functional relation between the model atmosphere, magnetic field strength, field inclination, and field azimuth, on one hand and the individual components (I, Q, U, V) of the Stokes profiles, on the other hand. Results. We performed an extensive statistical evaluation and show that our new approach yields accurate local as well as diskintegrated Stokes profiles over a wide range of atmospheric conditions. The mean rms errors for the Stokes I and V profiles are well below 0.2% compared to the exact numerical solution. Errors for Stokes Q and U are in the range of 1%. Our approach does not only offer an accurate approximation to the LTE polarized radiative transfer it, moreover, accelerates the synthesis by a factor of more than 1000.

Aims. We present the first Doppler imaging study of the two rapidly rotating, single K-giants DP CVn and DI Psc in order to study the surface spot configuration and to pinpoint their stellar evolutionary status. Methods. Optical spectroscopy and photometry were used to determine the fundamental astrophysical properties. Doppler imaging was applied to recover the surface temperature distribution for both stars, while photometric light-curve inversions were carried out for studying the long-term changes of the surface activity of DP CVn. Surface differential rotation of DP CVn was estimated from cross-correlating the available subsequent Doppler reconstructions separated by roughly one rotation period. Results. Both stars appear to have higher than normal lithium abundance, LTE log n of 2.28 (DP CVn) and 2.20 (DI Psc), and are supposed to be located at the end of the first Li dredge-up on the RGB. Photometric observations reveal rotational modulation with a period of 14.010 d (DP CVn) and 18.066 d (DI Psc). Doppler reconstructions from the available mapping lines well agree in the revealed spot patterns, recovering rather low latitude spots for both stars with temperature contrasts of ΔT ≈ 600−800 K below the unspotted photospheric background. Spots at higher latitudes are also found but either with less contrast (DP CVn) or with smaller extent (DI Psc). A preliminary antisolar-type differential rotation with α = −0.035 is found for DP CVn from cross-correlating the subsequent Doppler images. Long-term photometric analysis supports the existence of active longitudes, as well as the differential rotation.

We present the first ever study of the bright star HD 1. The star was chosen arbitrarily just because of its outstanding Henry Draper number. Surprisingly, almost nothing is known about this bright 7. m 4 star. Our observations were performed as part of the commissioning of the robotic telescope facility STELLA and its fiber-fed high-resolution optical echelle spectrograph SES in the years 2007-2010. We found long-term radial velocity variations with a full amplitude of 9 km s −1 with an average velocity of −29.8 km s −1 and suggest the star to be a hitherto unknown single-lined spectroscopic binary. A preliminary orbit with a period of 6.2 years (2279±69 days) and an eccentricity of 0.50±0.01 is given. Its rms uncertainty is just 73 m s −1 . HD 1 appears to be a G9-K0 giant of luminosity class IIIa with but no chromospheric activity is evident. We also present photometric monitoring BV (RI)C data taken in parallel with STELLA. The star is likely a small-amplitude (<10 mmag) photometric variable although no periodicity was found.