1,149 research outputs found
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
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
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
Observations of magnetic fields in hot stars
The presence of magnetic fields at the surfaces of many massive stars has
been suspected for decades, to explain the observed properties and activity of
OB stars. However, very few genuine high-mass stars had been identified as
magnetic before the advent of a new generation of powerful spectropolarimeters
that has resulted in a rapid burst of precise information about the magnetic
properties of massive stars. During this talk, I will briefly review modern
methods used to diagnose magnetic fields of higher-mass stars, and summarize
our current understanding of the magnetic properties of OB stars.Comment: Proceedings of IAUS272: Active OB stars, review talk (11 pages
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
Evolution of magnetic fields in stars across the upper main sequence: I. Catalogue of magnetic field measurements with FORS1 at the VLT
To properly understand the physics of Ap and Bp stars it is particularly
important to identify the origin of their magnetic fields. For that, an
accurate knowledge of the evolutionary state of stars that have a measured
magnetic field is an important diagnostic. Previous results based on a small
and possibly biased sample suggest that the distribution of magnetic stars with
mass below 3 M_sun in the H-R diagram differs from that of normal stars in the
same mass range (Hubrig et al. 2000). In contrast, higher mass magnetic Bp
stars may well occupy the whole main-sequence width (Hubrig, Schoeller & North
2005). In order to rediscuss the evolutionary state of upper main sequence
magnetic stars, we define a larger and bias-free sample of Ap and Bp stars with
accurate Hipparcos parallaxes and reliably determined longitudinal magnetic
fields. We used FORS1 at the VLT in its spectropolarimetric mode to measure the
magnetic field in chemically peculiar stars where it was unknown or poorly
known as yet. In this first paper we present our results of the mean
longitudinal magnetic field measurements in 136 stars. Our sample consists of
105 Ap and Bp stars, two PGa stars, 17 HgMn stars, three normal stars, and nine
SPB stars. A magnetic field was for the first time detected in 57 Ap and Bp
stars, in four HgMn stars, one PGa star, one normal B-type star and four SPB
stars.Comment: 9 pages, 2 figures, 2 tables, to appear in AN 327, 28
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