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We present and interpret new spectropolarimetric observations of the magnetic white dwarf WD 1953À011. Circular polarization and intensity spectra of the H spectral line demonstrate the presence of two-component magnetic field in the photosphere 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 ¼ 1:4480 AE 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 % 20 , and the angle between the rotation axis and the dipolar axis is % 10 . 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 coworkers. The mean field modulus of the spot (jB spot j ¼ 520 AE 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 star's global poloidal field. Instead, we suggest that the observed strong-field region has a geometry similar to a magnetic flux tube.

Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young.

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 with high-resolution (R ∼ 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 2 to a few tens of gauss.Subject headings: Astronomical instrumentation: polarimetry -magnetic fields -stars: magnetic stars IntroductionThe presence of intrinsic linear and circular polarizations in spectra of stellar objects provides an important information for diagnostics of their magnetism, wind surroundings, atmospheric inhomogeneities and other properties. For example, non-zero continuum linear polarization due to Thomson and Rayleigh scattering demonstrates the presence of non-symmetric patterns in the distribution of atmospheric or wind medium. The broad-band circular polarization as well as circular and linear polarizations in spectral lines exhibit information on the magnetic fields. The spectropolarimetric observation is therefore one of the most important tools for the experimental studies of

Aims. We investigate the nature of the long-period radial velocity variations in α Tau first reported over 20 yr ago. Methods. We analyzed precise stellar radial velocity measurements for α Tau spanning over 30 yr. An examination of the Hα and Ca II λ8662 spectral lines, and H photometry was also done to help discern the nature of the long-period radial velocity variations. Results. Our radial velocity data show that the long-period, low amplitude radial velocity variations are long-lived and coherent. Furthermore, Hα equivalent width measurements and H photometry show no significant variations with this period. Another investigation of this star established that there was no variability in the spectral line shapes with the radial velocity period. An orbital solution results in a period of P = 628.96 ± 0.90 d, eccentricity, e = 0.10±0.05, and a radial velocity amplitude, K = 142.1±7.2 m s −1 . Evolutionary tracks yield a stellar mass of 1.13 ± 0.11 M , which corresponds to a minimum companion mass of 6.47 ± 0.53 M Jup with an orbital semi-major axis of a = 1.46 ± 0.27 AU. After removing the orbital motion of the companion, an additional period of ≈520 d is found in the radial velocity data, but only in some time spans. A similar period is found in the variations in the equivalent width of Hα and Ca II. Variations at one-third of this period are also found in the spectral line bisector measurements. The ∼520 d period is interpreted as the rotation modulation by stellar surface structure. Its presence, however, may not be long-lived, and it only appears in epochs of the radial velocity data separated by ∼10 yr. This might be due to an activity cycle. Conclusions. The data presented here provide further evidence of a planetary companion to α Tau, as well as activity-related radial velocity variations.

We show that the equivalent widths of the well-known interstellar Ca ii H and K lines can be used to determine the distances to OB stars in our Galaxy. The equivalent widths, measured in the spectra of 147 early-type stars, are strongly related to the Hipparcos parallaxes of those objects. The lines fitted to the parallax-equivalent width data are given by the formulaewhere is in arcseconds and EW is in milliangstroms. The form of the formulae, yielding a finite parallax even for zero absorption, shows that space within %100 pc of the Sun contains very little Ca ii, which is in agreement with the known dimensions of the Local Bubble. Using Ca ii lines for distance determination does not require the knowledge of the absolute magnitude of the object; it is thus well suited for targets for which the absolute calibration is either not precise (OB supergiants) or not available at all (peculiar objects). We also demonstrate that neither the reddening E (B À V ) nor the equivalent widths of interstellar K i and CH lines are suitable candidates for distance estimation, their relation with parallaxes being far less tight than for Ca ii.

We present the first results of a long-term program of a radial velocity (RV) study of Cepheid Polaris (F7 Ib), with the aim of finding the amplitude and period of its pulsations and the nature of its secondary periodicities. A total of 264 new precise RV measurements were obtained during 2004-2007 with the fiber-fed echelle spectrograph Bohyunsan Observatory Echelle Spectrograph (BOES) of the 1.8 m telescope at Bohyunsan Optical Astronomy Observatory (BOAO) in Korea. We find a pulsational RV amplitude and period of Polaris for the three seasons 2005.183, 2006.360, and 2007.349 as 2K = 2.210 ± 0.048 km s −1 , 2K = 2.080 ± 0.042 km s −1 , and 2K = 2.406 ± 0.018 km s −1 respectively, indicating that the pulsational amplitudes of Polaris that had decayed during the last century are now increasing rapidly. The pulsational period was also found to be increasing. This is the first detection of a historical turnaround of a pulsational amplitude change in the Cepheids. We clearly find the presence of additional RV variations on a timescale of about 119 days and an amplitude of about ±138 m s −1 , which is quasi-periodic rather than strictly periodic. From our data, we do not confirm the presence of the variation on a timescale of 34-45 days found in the RV data obtained in the 1980s and 1990s. We assume that both the 119 day quasi-periodic, noncoherent variations found in our data and the 34-45 day variations found previously can be caused by the 119 day rotation periods of Polaris and by surface inhomogeneities such as single-or multiple-spot configuration varying with time.