We present and interpret simultaneous new photometric and spectroscopic observations of the peculiar magnetic white dwarf WD1953-011. The flux in the V-band filter and intensity of the Balmer spectral lines demonstrate variability with the rotation p
eriod of about 1.45 days. According to previous studies, this variability can be explained by the presence of a dark spot having a magnetic nature, analogous to a sunspot. Motivated by this idea, we examine possible physical relationships between the suggested dark spot and the strong-field magnetic structure (magnetic spot, or tube) recently identified on the surface of this star. Comparing the rotationally-modulated flux with the variable spectral observables related to the magnetic spot we establish their correlation, and therefore their physical relationship. Modeling the variable photometric flux assuming that it is associated with temperature variations in the stellar photosphere, we argue that the strong-field area and dark, low-temperature spot are comparable in size and located at the same latitudes, essentially overlapping each other with a possible slight longitudinal shift. In this paper we also present a new, improved value of the stars rotational period and constrain the characteristics of the thermal inhomogeneity over the degenerates surface.
Aims: Our primary goal is to search for planets around intermediate mass stars. We are also interested in studying the nature of radial velocity (RV) variations of K giant stars. Methods: We selected about 55 early K giant (K0 - K4) stars brighter
than fifth magnitude that were observed using BOES, a high resolution spectrograph attached to the 1.8 m telescope at BOAO (Bohyunsan Optical Astronomy Observatory). BOES is equipped with $I_2$ absorption cell for high precision RV measurements. Results: We detected a periodic radial velocity variations in the K0 III star gam1leo with a period of P = 429 days. An orbital fit of the observed RVs yields a period of P = 429 days, a semi-amplitude of K = 208 mps, and an eccentricity of e = 0.14. To investigate the nature of the RV variations, we analyzed the photometric, CaII $lambda$ 8662 equivalent width, and line-bisector variations of gam1leo. We conclude that the detected RV variations can be best explained by a planetary companion with an estimated mass of m $sin i = 8.78 M_{Jupiter}$ and a semi-major axis of $a = 1.19$ AU, assuming a stellar mass of 1.23 Msun.
We present a high resolving power ($lambda$ / $Deltalambda$ = 90,000) and high signal-to-noise ratio ($sim$700) spectral atlas of Vega covering the 3850 -- 6860 AA wavelength range. The atlas is a result of averaging of spectra recorded with the aid
of the echelle spectrograph BOES fed by the 1.8-m telescope at Bohyunsan observatory (Korea). The atlas is provided only in machine-readable form (electronic data file) and will be available in the SIMBAD database upon publication.
We present and interpret new spectropolarimetric observations of the magnetic white dwarf WD 1953-011. Circular polarization and intensity spectra of the H$alpha$ spectral line demonstrate the presence of two-component magnetic field in the photosphe
re of this star. The geometry consists of a weak, large scale component, and a strong, localized component. Analyzing the rotationally modulated low-field component, we establish a rotation period $P_{rot} = 1.4480 pm 0.0001$ days. Modeling the measured magnetic observables, we find that the low-field component can be described by the superposition of a dipole and quadrupole. According to the best-fit model, the inclination of the stellar rotation axis with respect to the line of sight is $i approx 20^circ$, and the angle between the rotation axis and the dipolar axis is $beta approx 10^circ$. The dipole strength at the pole is about 180 kG, and the quadrupolar strength is about 230 kG. These data suggest a fossil origin of the low-field component. In contrast, the strong-field component exhibits a peculiar, localized structure (``magnetic spot) that confirms the conclusions of Maxted and co-workers. The mean field modulus of the spot ($|B_{spot}| = 520 pm 7$ kG) together with its variable longitudinal magnetic field having a maximum of about +400 kG make it difficult to describe it naturally as a high-order component of the stars global poloidal field. Instead, we suggest that the observed strong-field region has a geometry similar to a magnetic flux tube.
We introduce a new polarimeter installed on the high-resolution fiber-fed echelle spectrograph (called BOES) of the 1.8-m telescope at the Bohyunsan Optical Astronomy Observatory, Korea. The instrument is intended to measure stellar magnetic fields w
ith high-resolution (R $sim$ 60000) spectropolarimetric observations of intrinsic polarization in spectral lines. In this paper we describe the spectropolarimeter and present test observations of the longitudinal magnetic fields in some well-studied F-B main sequence magnetic stars (m_v < 8.8^m). The results demonstrate that the instrument has a high precision ability of detecting the fields of these stars with typical accuracies ranged from about 2G to a few tens of gauss.
