585 research outputs found
Accurate age determinations of several nearby open clusters containing magnetic Ap stars
Aims: Our aim is to obtain ages that are as accurate as possible for the
seven nearby open clusters alpha Per, Coma Ber, IC 2602, NGC 2232, NGC 2451A,
NGC 2516, and NGC 6475, each of which contains at least one magnetic Ap or Bp
star. Simultaneously, we test the current calibrations of Te and luminosity for
the Ap/Bp star members, and identify clearly blue stragglers in the clusters
studied. Methods: We explore the possibility that isochrone fitting in the
theoretical Hertzsprung-Russell diagram (i.e. log (L/L&sun;) vs. log Te),
rather than in the conventional colour-magnitude diagram, can provide more
precise and accurate cluster ages, with well-defined uncertainties. Results:
Well-defined ages are found for all the clusters studied. For the nearby
clusters studied, the derived ages are not very sensitive to the small
uncertainties in distance, reddening, membership, metallicity, or choice of
isochrones. Our age determinations are all within the range of previously
determined values, but the associated uncertainties are considerably smaller
than the spread in recent age determinations from the literature. Furthermore,
examination of proper motions and HR diagrams confirms that the Ap stars
identified in these clusters are members, and that the presently accepted
temperature scale and bolometric corrections for Ap stars are approximately
correct. We show that in these theoretical HR diagrams blue stragglers are
particularly easy to identify. Conclusions: Constructing the theoretical HR
diagram of a nearby open cluster makes possible an accurate age determination,
with well defined uncertainty. This diagnostic of a cluster also provides a
useful tool for studying unusual stars such as Ap stars and blue stragglers
On the incidence of weak magnetic fields in DA white dwarfs
Context: About 10% of white dwarfs have magnetic fields with strength in the
range between about 10^5 and 3x10^8 G. It is not known whether the remaining
white dwarfs are not magnetic, or if they have a magnetic field too weak to be
detected with the techniques adopted in the large surveys. 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 \sim9.5
kG in the DA6 white dwarf WD2105-820. 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 \simeq 56 kG. Our re-evaluation of the FORS archive data
for white dwarfs indicates that longitudinal magnetic fields weaker than 10 kG
had previously been correctly identified in at least three white dwarfs.
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.Comment: Accepted for publication in Astronomy & Astrophysic
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
An Analysis of the Rapidly Rotating Bp Star HD 133880
HD 133880 is a rapidly rotating Bp star and host to one of the strongest
magnetic fields of any Bp star. A member of the Upper Centaurus Lupus
association, it is a star with a well-determined age of 16 Myr. Twelve new
spectra obtained from the FEROS, ESPaDOnS, and HARPS instruments, provide
sufficient material from which to re-evaluate the magnetic field and obtain a
first approximation to the atmospheric abundance distributions of various
elements. Using the new magnetic field measurements and optical photometry
together with previously published data, we refine the period of HD 133880 to P
= 0.877476 \pm 0.000009 days. The magnetic field structure was characterised by
a colinear multipole expansion from the observed variations of the longitudinal
and surface fields with rotational phase. This simple axisymmetric magnetic
field model is based on a predominantly quadrupolar component that roughly
describes the field variations. Using spectrum synthesis, we derived mean
abundances for O, Mg, Si, Ti, Cr, Fe, Nd and Pr. All elements, except Mg (which
has a uniform distribution), are overabundant compared to the Sun and are more
abundant in the negative than in the positive magnetic hemisphere. In contrast
to most Bp stars the abundance of O in HD 133880 is overabundant compared to
the solar abundance ratio. In studying the Halpha and Paschen lines in the
optical spectra we could not unambiguously detect information about the
magnetosphere of HD 133880. However, radio emission data at both 3 and 6 cm
suggests that the magnetospheric plasma is held in rigid rotation with the star
by the magnetic field and further supported against collapse by the rapid
rotation. Subtle differences in the shapes of the optically thick radio light
curves at 3 and 6 cm suggest that the large-scale magnetic field is not fully
axisymmetric at large distances from the star.Comment: Accepted for publication in MNRAS. 17 pages, 10 figure
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&
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
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