The magnetic fields of hot subdwarf stars

Detection of magnetic fields has been reported in several sdO and sdB stars. Recent literature has cast doubts on the reliability of most of these detections. We revisit data previously published in the literature, and we present new observations to clarify the question of how common magnetic fields are in subdwarf stars. We consider a sample of about 40 hot subdwarf stars. About 30 of them have been observed with the FORS1 and FORS2 instruments of the ESO VLT. Here we present new FORS1 field measurements for 17 stars, 14 of which have never been observed for magnetic fields before. We also critically review the measurements already published in the literature, and in particular we try to explain why previous papers based on the same FORS1 data have reported contradictory results. All new and re-reduced measurements obtained with FORS1 are shown to be consistent with non-detection of magnetic fields. We explain previous spurious field detections from data obtained with FORS1 as due to a non-optimal method of wavelength calibration. Field detections in other surveys are found to be uncertain or doubtful, and certainly in need of confirmation. There is presently no strong evidence for the occurrence of a magnetic field in any sdB or sdO star, with typical longitudinal field uncertainties of the order of 2-400 G. It appears that globally simple fields of more than about 1 or 2 kG in strength occur in at most a few percent of hot subdwarfs, and may be completely absent at this strength. Further high-precision surveys, both with high-resolution spectropolarimeters and with instruments similar to FORS1 on large telescopes, would be very valuable

Linear spectro-polarimetry: a new diagnostic tool for the classification and characterisation of asteroids

We explore the use of spectro-polarimetry as a remote sensing tool for asteroids in addition to traditional reflectance measurements. In particular we are interested in possible relationships between the wavelength-dependent variation of linear polarization and the properties of the surfaces, including albedo and composition. We have obtained optical spectro-polarimetric measurements of a dozen asteroids of different albedo and taxonomic classes and of two small regions at the limb of the Moon. We found that objects with marginally different relative reflectance spectra (in the optical) may have totally different polarization spectra. This suggests that spectro-polarimetry may be used to refine the classification of asteroids. We also found that in some cases the Umov law may be violated, that is, in contrast to what is expected from basic physical considerations, the fraction of linear polarization and the reflectance may be positively correlated. In agreement with a few previous studies based on multi-colour broadband polarimetry, we found that the variation of linear polarization with wavelength and with phase-angle is correlated with the albedo and taxonomic class of the objects. Finally, we have serendipitously discovered that spinel-rich asteroid (599) Luisa, located very close to the Watsonia family, is a member of the rare class of Barbarian asteroids. We suggest that future modelling attempts of the surface structure of asteroids should be aimed at explaining both reflectance and polarization spectra.Comment: Accepted for publication in MNRAS Letter

On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive

CONTEXT. The ESO archive of FORS1 spectropolarimetric observations may be used to create a homogeneous database of magnetic field measurements. However, no systematic comparison of FORS field measurements to those obtained with other instruments has been undertaken so far. AIMS. We exploit the FORS archive of circular spectropolarimetric data to examine in a general way how reliable and accurate field detections obtained with FORS are. METHODS. We examine the observations of Ap and Bp stars, on the grounds that almost all of the unambiguous detections of magnetic fields in the FORS1 archive are in these kinds of stars. We assess the overall quality of the FORS1 magnetic data by examining the consistency of field detections with what is known from previous measurements obtained with other instruments, and we look at patterns of internal consistency. RESULTS. FORS1 magnetic measurements are fully consistent with those made with other instruments, and the internal consistency of the data is excellent. However, it is important to recognise that each choice of grism and wavelength window constitutes a distinct instrumental measuring system, and that simultaneous field measurements in different instrumental systems may produce field strength values that differ up to 20 %, or more. Furthermore, we found that field measurements using hydrogen lines only yield results that meaningfully reflect the field strength as sampled specifically by lines of hydrogen for stars with effective temperatures above about 9000 K. CONCLUSIONS. In general the magnetic field measurements of Ap and Bp stars obtained with FORS1 are of excellent quality, accuracy and precision, and FORS1 provides an extremely useful example that offers valuable lessons for field measurements with other low- resolution Cassegrain spectropolarimeters.Comment: 14 pages, 8 figures. Accepted for publication in section 13 of Astronomy & Astrophysics on 13 October 201

Magnetic fields in central stars of planetary nebulae?

Most of the planetary nebulae (PN) have bipolar or other non-spherically symmetric shapes. The presence of a magnetic field in the central star may be the reason for this lack of symmetry, but observational works published in the literature have so far reported contradictory results. We try to correlate the presence of a magnetic field with the departures from the spherical geometry of the envelopes of planetary nebulae. We determine the magnetic field from spectropolarimetric observations of ten central stars of planetary nebulae. The results of the analysis of the observations of four stars was previously presented and discussed in the literature, while the observations of six stars, plus additional measurements for a star previously observed, are presented here for the first time. All our determinations of magnetic field in the central planetary nebulae are consistent with null results. Our field measurements have a typical error bar of 150-300 G. Previous spurious field detections obtained with FORS were probably due to the use of different wavelength calibration solutions for frames obtained at different position angles of the retarder waveplate. Currently, there is no observational evidence for the presence of magnetic fields with a strength of the order of hundreds Gauss or higher in the central stars of planetary nebulae.Comment: 5 pages, 2 figures, accepted for publication by Astronomy and Astrophysic

Dust in the diffuse interstellar medium: Extinction, emission, linear and circular polarisation

We present a model for the diffuse interstellar dust that explains the observed wavelength-dependence of extinction, emission, linear and circular polarisation of light. The model is set-up with a small number of parameters. It consists of a mixture of amorphous carbon and silicate grains with sizes from the molecular domain of 0.5 up to about 500nm. Dust grains with radii larger than 6nm are spheroids. Spheroidal dust particles have a factor 1.5 - 3 larger absorption cross section in the far IR than spherical grains of the same volume. Mass estimates derived from submillimeter observations that ignore this effect are overestimated by the same amount. In the presence of a magnetic field, spheroids may be partly aligned and polarise light. We find that polarisation spectra help to determine the upper particle radius of the otherwise rather unconstrained dust size distribution. Stochastically heated small grains of graphite, silicates and polycyclic aromatic hydrocarbons (PAHs) are included. We tabulate parameters for PAH emission bands in various environments. They show a trend with the hardness of the radiation field that can be explained by the ionisation state or hydrogenation coverage of the molecules. For each dust component its relative weight is specified, so that absolute element abundances are not direct input parameters. The model is confronted with the average properties of the Milky Way, which seems to represent dust in the solar neighbourhood. It is then applied to four specific sight lines including the reflection nebula NGC2023. For these sight lines, we present linear and circular spectro-polarimetric observations obtained with FORS/VLT. Using prolate rather than oblate grains gives a better fit to observed spectra; the axial ratio of the spheroids is typically two and aligned silicates are the dominant contributor to the polarisation.Comment: accepted by A&A Edito

Discovery of secular variations in the atmospheric abundances of magnetic Ap stars

The stars of the middle main sequence have relatively quiescent outer layers, and unusual chemical abundance patterns may develop in their atmospheres. The presence of chemical peculiarities reveal the action of such subsurface phenomena as gravitational settling and radiatively driven levitation of trace elements, and their competition with mixing processes such as turbulent diffusion. We want to establish whether abundance peculiarities change as stars evolve on the main sequence, and provide observational constraints to diffusion theory. We have performed spectral analysis of 15 magnetic Bp stars that are members of open clusters (and thus have well-known ages), with masses between about 3 and 4 M_sun. For each star, we measured the abundances of He, O, Mg, Si, Ti, Cr, Fe, Pr and Nd. We have discovered the systematic time evolution of trace elements through the main-sequence lifetime of magnetic chemically peculiar stars as their atmospheres cool and evolve toward lower gravity. During the main sequence lifetime, we observe clear and systematic variations in the atmospheric abundances of He, Ti, Cr, Fe, Pr and Nd. For all these elements, except He, the atmospheric abundances decrease with age. The abundances of Fe-peak elements converge toward solar values, while the rare-earth elements converge toward values at least 100 times more abundant than in the Sun. Helium is always underabundant compared to the Sun, evolving from about 1% up to 10% of the solar He abundance. We have attempted to interpret the observed abundance variations in the context of radiatively driven diffusion theory, which appears to provide a framework to understand some, but not all, of the observed anomalous abundance levels and variations.Comment: 13 pages, 5 figures, 5 tables, accepted for publication in A&

The enigmatic He-sdB pulsator LS IV−-14∘^\circ116: new insights from the VLT

The intermediate Helium subdwarf B star LS IV$-$14$^\circ$116 is a unique object showing extremely peculiar atmospheric abundances as well as long-period pulsations that cannot be explained in terms of the usual opacity mechanism. One hypothesis invoked was that a strong magnetic field may be responsible. We discredit this possibility on the basis of FORS2 spectro-polarimetry, which allows us to rule out a mean longitudinal magnetic field down to 300 G. Using the same data, we derive the atmospheric parameters for LS IV$-$14$^\circ$116 to be $T_{\rm eff}$ = 35,150$\pm$111 K, $\log{g}$ = 5.88$\pm$0.02 and $\log{N(\rm He)/N(\rm H)}$ = $-$0.62$\pm$0.01. The high surface gravity in particular is at odds with the theory that LS IV$-$14$^\circ$116 has not yet settled onto the Helium Main Sequence, and that the pulsations are excited by an $\epsilon$ mechanism acting on the Helium-burning shells present after the main Helium flash. Archival UVES spectroscopy reveals LS IV$-$14$^\circ$116 to have a radial velocity of 149.1$\pm$2.1 km/s. Running a full kinematic analysis, we find that it is on a retrograde orbit around the Galactic centre, with a Galactic radial velocity component $U$=13.23$\pm$8.28 km/s and a Galactic rotational velocity component $V$=$-$55.56$\pm$22.13 km/s. This implies that LS IV$-$14$^\circ$116 belongs to the halo population, an intriguing discovery.Comment: accepted for publication in A&

The FORS1 catalogue of stellar magnetic field measurements

The FORS1 instrument on the ESO Very Large Telescope was used to obtain low-resolution circular polarised spectra of nearly a thousand different stars, with the aim of measuring their mean longitudinal magnetic fields. A catalogue of FORS1 magnetic measurements would provide a valuable resource with which to better understand the strengths and limitations of this instrument and of similar low-dispersion, Cassegrain spectropolarimeters. However, FORS1 data reduction has been carried out by a number of different groups using a variety of reduction and analysis techniques. Our understanding of the instrument and our data reduction techniques have both improved over time. A full re-analysis of FORS1 archive data using a consistent and fully documented algorithm would optimise the accuracy and usefulness of a catalogue of field measurements. Based on the ESO FORS pipeline, we have developed a semi-automatic procedure for magnetic field determinations, which includes self-consistent checks for field detection reliability. We have applied our procedure to the full content of circular spectropolarimetric measurements of the FORS1 archive. We have produced a catalogue of spectro-polarimetric observations and magnetic field measurements for about 1400 observations of about 850 different objects. The spectral type of each object has been accurately classified. We have also been able to test different methods for data reduction is a systematic way. The resulting catalogue has been used to produce an estimator for an upper limit to the uncertainty in a field strength measurement of an early type star as a function of the signal-to-noise ratio of the observation. While FORS1 is not necessarily an optimal instrument for the discovery of weak magnetic fields, it is very useful for the systematic study of larger fields, such as those found in Ap/Bp stars and in white dwarfs.Comment: Accepted for publication by A&