62 research outputs found
Abundances determined using Si II and Si III in B-type stars: evidence for stratification
It is becoming clear that determination of the abundance of Si using lines of
Si II and Si III can lead to quite discordant results in mid to late B-type
stars. The difference between the Si abundances derived from the two ion states
can exceed one dex in some cases.
We have carried out a study intended to clarify which kinds of B stars
exhibit this discrepancy, to try to identify regularities in the phenomenon,
and to explore possible explanations such as abundance stratification by
comparing models to observed spectra.
We used spectra from the ESPaDOnS spectropolarimeter and FEROS spectrograph,
supplemented with spectra from the ESO and ELODIE archives, of magnetic Bp,
HgMn, and normal B-type stars ranging in effective temperature from about 10500
to 15000 K. Using these spectra, we derived abundances using the spectrum
synthesis program ZEEMAN which can take into account the influence of magnetic
fields. For each star, accurate abundances of Si II, Si III, Ti, Cr, and Fe
were derived.
All magnetic Bp stars in our sample show a discordance between the derived
abundances of the first and second ions of silicon, with the latter being
between 0.6 - 1.7 dex higher. The same behaviour is observed in the
non-magnetic stars but to a much smaller extent: Si III is enhanced by between
0.3 - 0.8 dex compared to Si II.We do not detect the discrepancy in three
stars, HD 22136 (normal), HD 57608 (HgMn) and HD 27295 (HgMn); these are the
only stars in our sample for which the microturbulence parameter is
significantly different from zero, and which therefore probably have convection
occurring in their atmospheres.
We find that vertical stratification of silicon in the atmospheres of B-type
stars may provide an explanation of this phenomenon, but our detailed
stratification models do not completely explain the discrepancies, which may,
in part, be due to non-LTE effects.Comment: 9 pages, 2 figures, accepted for publication in A&
A new weak-field magnetic DA white dwarf in the local 20 pc volume. The frequency of magnetic fields in DA stars
We report the discovery of a new magnetic DA white dwarf (WD), WD0011-721,
which is located within the very important 20pc volume-limited sample of the
closest WDs to the Sun. This star has a mean field modulus of 343 kG, and
from the polarisation signal we deduce a line-of-sight field component of 75
kG. The magnetic field is sufficiently weak to have escaped detection in
classification spectra. We then present a preliminary exploration of the data
concerning the frequency of such fields among WDs with hydrogen-rich
atmospheres (DA stars). We find that 20 +- 5% of the DA WDs in this volume have
magnetic fields, mostly weaker than 1 MG. Unlike the slow field decay found
among the magnetic Bp stars of the upper main sequence, the WDs in this sample
show no evidence of magnetic field or flux changes over several Gyr
Searching for the weakest detectable magnetic fields in white dwarfs. Highly-sensitive measurements from first VLT and WHT surveys
Our knowledge of the magnetism in white dwarfs is based on an observational
dataset that is biased in favour of stars with very strong magnetic fields.
Most of the field measurements available in the literature have a relatively
low sensitivity, while current instruments allow us to detect magnetic fields
of white dwarfs with sub-kG precision. With the aim of obtaining a more
complete view of the incidence of magnetic fields in degenerate stars, we have
started a long-term campaign of high-precision spectropolarimetric observations
of white dwarfs. Here we report the results obtained so far with the
low-resolution FORS2 instrument of the ESO VLT and the medium-resolution ISIS
instrument of the WHT. We have considered a sample of 48 stars, of which five
are known magnetic or suspected magnetic stars, and obtained new longitudinal
magnetic field measurements with a mean uncertainty of about 0.6 kG. Overall,
in the course of our survey (the results of which have been partially published
in papers devoted to individual stars) we have discovered one new weak-field
magnetic white dwarf, confirmed the magnetic nature of another, found that a
suspected magnetic star is not magnetic, and suggested two new candidate
magnetic white dwarfs. Even combined with data previously obtained in the
literature, our sample is not sufficient yet to reach any final conclusions
about the actual incidence of very weak magnetic fields in white dwarfs, but we
have set the basis to achieve a homogeneous survey of an unbiased sample of
white dwarfs. As a by-product, our survey has also enabled us to carry out a
detailed characterisation of the ISIS and the FORS2 instruments for the
detection of extremely weak magnetic fields in white dwarfs, and in particular
to relate the S/N to measurement uncertainty for white dwarfs of different
spectral types. This study will help the optimisation of future observations.Comment: Accepted by A&
Discovery of kilogauss magnetic fields on the nearby white dwarfs WD1105-340 and WD2150+591
Magnetic fields are present in roughly 10% of white dwarfs. These fields
affect the structure and evolution of such stars, and may provide clues about
their earlier evolution history. Particularly important for statistical studies
is the collection of high-precision spectropolarimetric observations of (1)
complete magnitude-limited samples and (2) complete volume-limited samples of
white dwarfs. In the course of one of our surveys we have discovered previously
unknown kG-level magnetic fields on two nearby white dwarfs, WD1105-340 and
WD2150+591. Both stars are brighter than m_V = 15. WD2150+591 is within the
20-pc volume around the Sun, while WD1105-340 is just beyond 25 pc in distance.
These discoveries increase the small sample of such weak-field white dwarfs
from 21 to 23 stars. Our data appear consistent with roughly dipolar field
topology, but it also appears that the surface field structure may be more
complex on the older star than on the younger one, a result similar to one
found earlier in our study of the weak-field stars WD2034+372 and WD2359-434.
This encourages further efforts to uncover a clear link between magnetic
morphology and stellar evolution
Magnetic Fields of Slowly Pulsating B Stars and {\beta} Cep Variables: Comparing Results from FORS1/2 and ESPaDOnS
Recently announced magnetic models for four SPB and {\beta} Cep stars, along
with magnetic detections for two additional stars, have potentially doubled the
number of known magnetic SPB and \beta Cep stars (see Grunhut et al., these
proceedings). We have reanalyzed the published data and re-reduced archival low
resolution spectropolarimetry collected with the FORS1/2 instruments at VLT on
which the models were based, and compare them with high resolution data from
the ESPaDOnS spectropolarimeter at CFHT, investigating previously noted
inconsistencies between results from the two instruments.Comment: 5 pages, 4 figures, StarPol: Stellar Polarimetry from Birth to Deat
Rotation plays a role in the generation of magnetic fields in single white dwarfs
Recent surveys of close white dwarf binaries as well as single white dwarfs
have provided evidence for the late appearance of magnetic fields in white
dwarfs, and a possible generation mechanism a crystallization and
rotation-driven dynamo has been suggested. A key prediction of this dynamo is
that magnetic white dwarfs rotate, at least on average, faster than their
non-magnetic counterparts and/or that the magnetic field strength increases
with rotation. Here we present rotation periods of ten white dwarfs within 40
pc measured using photometric variations. Eight of the light curves come from
TESS observations and are thus not biased towards short periods, in contrast to
most period estimates that have been reported previously in the literature.
These TESS spin periods are indeed systematically shorter than those of
non-magnetic white dwarfs. This means that the crystallization and
rotation-driven dynamo could be responsible for a fraction of the magnetic
fields in white dwarfs. However, the full sample of magnetic white dwarfs also
contains slowly rotating strongly magnetic white dwarfs which indicates that
another mechanism that leads to the late appearance of magnetic white dwarfs
might be at work, either in addition to or instead of the dynamo. The
fast-spinning and massive magnetic white dwarfs that appear in the literature
form a small fraction of magnetic white dwarfs, and probably result from a
channel related to white dwarf mergers.Comment: 20 pages, 10 figures, 8 tables, accepted for publication in MNRA
Highly sensitive search for magnetic fields in white dwarfs using broad-band circular polarimetry
Circular polarisation measurements of white dwarfs of various ages and spectral types are useful to understand the origin and evolution of the magnetic field in degenerate stars. In the latest stages of white dwarf evolution, when stars are so cool that spectral lines are no longer formed in the normal H- or He-dominated atmospheres, magnetic fields can be probed only by means of circular polarimetry of the continuum. The study of the fields of featureless DC white dwarfs may reveal whether Ohmic decay acts on magnetic white dwarfs, or if magnetic fields continue to be generated even several billion years after white dwarf formation. Compared to spectropolarimetry, broad-band circular polarisation measurements have the advantage of reaching a higher accuracy in the continuum, with the potential of detecting magnetic fields as weak as a fraction of a MG in DC stars, if the telescope size is adequate for the star's magnitude. Here we present the results of a first (short) observing campaign with the DIPol-UF polarimeter, which we have used to measure broad-band circular polarisation of white dwarfs. Our observing run was in part aimed to fully characterise the instrument, and in part to study the relationship between magnetic field strength (when known from spectropolarimetry) and circular polarisation of the continuum. We also observed a small number of previously unexplored DC white dwarfs, and we present the discovery of two new magnetic white dwarfs of spectral class DC, probably the first discovery of this kind made with broad-band circular polarimetric techniques since the late 1970s. We also discuss the characteristics of our instrument, and predict the level of polarimetric accuracy that may be reached as a function of stellar magnitude, exposure time, and telescope size
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