Context. About 10% of white dwarfs have magnetic fields with strength in the range between about 10 5 and 5 × 10 8 G. It is not known whether the remaining white dwarfs are not magnetic, or if they have magnetic fields too weak to be detected with the techniques adopted in the large surveys. Information is particularly lacking for the cooler (and generally fainter) white dwarfs. Aims. We describe the results of the first survey specifically devised to clarify the detection frequency of kG-level magnetic fields in cool DA white dwarfs. Methods. Using the FORS1 instrument of the ESO VLT, we have obtained Balmer line circular spectropolarimetric measurements of a small sample of cool (DA6 -DA8) white dwarfs. Using FORS and UVES archive data, we have also revised numerous white dwarf field measurements previously published in the literature. Results. We have discovered an apparently constant longitudinal magnetic field of ∼9.5 kG in the DA6 white dwarf WD 2105−820. This star is the first weak-field white dwarf that has been observed sufficiently to roughly determine the characteristics of its field. The available data are consistent with a simple dipolar morphology with magnetic axis nearly parallel to the rotation axis, and a polar strength of 56 kG. Our re-evaluation of the FORS archive data for white dwarfs indicates that longitudinal magnetic fields weaker than 10 kG have previously been correctly identified in at least three white dwarfs. However, for one of these three weak-field stars (WD 2359−434), UVES archive data show a ∼100 kG mean field modulus. Either at the time of the FORS observations the star's magnetic field axis was nearly perpendicular to the line of sight, or the star's magnetic field has rather complex structure. Conclusions. We find that the probability of detecting a field of kG strength in a DA white dwarf is of the order of 10% for each of the cool and hot DA stars. If there is a lower cutoff to field strength in white dwarfs, or a field below which all white dwarfs are magnetic, the current precision of measurements is not yet sufficient to reveal it.

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.

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

The laboratory gas phase spectrum recently published by Campbell et al. has reinvigorated attempts to confirm the presence of the C + 60 cation in the interstellar medium, thorough an analysis of the spectra of hot, reddened stars. This search is hindered by at least two issues that need to be addressed: (i) the wavelength range of interest is severely polluted by strong water-vapour lines coming from the Earth's atmosphere; (ii) one of the major bands attributed to C + 60 , at 9633Å, is blended with the stellar Mgii line, which is susceptible to non-local-thermodynamic equilibrium effects in hot stellar atmospheres. Both these issues are here carefully considered here for the first time, based on high-resolution and high signal-to-noise ratio echellé spectra for 19 lines of sight. The result is that the presence of C + 60 in interstellar clouds is brought into question.

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.