119 research outputs found
Differential Rotation in F Stars
Differential rotation can be detected in single line profiles of stars
rotating more rapidly than about km s with the Fourier
transform technique. This allows to search for differential rotation in large
samples to look for correlations between differential rotation and other
stellar parameters. I analyze the fraction of differentially rotating stars as
a function of color, rotation, and activity in a large sample of F-type stars.
Color and rotation exhibit a correlation with differential rotation in the
sense that more stars are rotating differentially in the cooler, less rapidly
rotating stars. Effects of rotation and color, however, cannot be disentangled
in the underlying sample. No trend with activity is found.Comment: 4 pages, overview talk at the 5th Potsdam thinkshop, accepted by A
Turbulent Dynamo in Asymptotic Giant Branch Stars
Using recent results on the operation of turbulent dynamos, we show that a
turbulent dynamo can amplify a large scale magnetic field in the envelopes of
asymptotic giant branch (AGB) stars. We propose that a slow rotation of the AGB
envelope can fix the symmetry axis, leading to the formation of an axisymmetric
magnetic field structure. Unlike solar-type alpha-omega dynamos, the rotation
has only a small role in amplifying the toroidal component of the magnetic
field. The large-scale magnetic field is strong enough for the formation of
magnetic cool spots on the AGB stellar surface. The spots can regulate dust
formation, hence mass loss rate, leading to axisymmetric mass loss and the
formation of elliptical planetary nebulae (PNe). Despite its role in forming
cool spots, the large scale magnetic field is too weak to play a dynamic role
and directly influence the wind from the AGB star. We find other problems in
models where the magnetic field plays a dynamic role in shaping the AGB winds,
and argue that they cannot explain the formation of nonspherical PNe.Comment: 12 pages (1 ps file of a table); Submitted to MNRA
On the determination of oxygen abundances in chromospherically active stars
We discuss oxygen abundances derived from [O I] 6300 and the O I triplet in
stars spanning a wide range in chromospheric activity level, and show that
these two indicators yield increasingly discrepant results with higher
chromospheric/coronal activity measures. While the forbidden and permitted
lines give fairly consistent results for solar-type disk dwarfs, spuriously
high O I triplet abundances are observed in young Hyades and Pleiades stars, as
well as in individual components of RS CVn binaries (up to 1.8 dex). The
distinct behaviour of the [O I]-based abundances which consistently remain
near-solar suggests that this phenomenon mostly results from large departures
from LTE affecting the O I triplet at high activity level that are currently
unaccounted for, but also possibly from a failure to adequately model the
atmospheres of K-type stars. These results suggest that some caution should be
exercised when interpreting oxygen abundances in active binaries or young open
cluster stars.Comment: 8 pages, accepted for publication in A&
Observational evidence for the origin of X-ray sources in globular clusters
Low-mass X-ray binaries, recycled pulsars, cataclysmic variables and
magnetically active binaries are observed as X-ray sources in globular
clusters. We discuss the classification of these systems, and find that some
presumed active binaries are brighter than expected. We discuss a new
statistical method to determine from observations how the formation of X-ray
sources depends on the number of stellar encounters and/or on the cluster mass.
We show that cluster mass is not a proxy for the encounter number, and that
optical identifications are essential in proving the presence of primordial
binaries among the low-luminosity X-ray sources.Comment: 10 pages, 7 figures, to appear in IAUS 246, Dynamical evolution of
dense stellar systems, ed. E. Vesperin
X-ray emission from the remarkable A-type star HR 8799
We present a Chandra observation of the exceptional planet bearing A5V star
HR 8799, more precisely classified as a kA5hF0mA5 star and search for intrinsic
X-ray emission. We clearly detect HR 8799 at soft X-ray energies with the
ACIS-S detector in a 10 ks exposure; minor X-ray brightness variability is
present during the observation. The coronal plasma is described well by a model
with a temperature of around 3 MK and an X-ray luminosity of about Lx = 1.3 x
10^28 erg/s in the 0.2-2.0 keV band, corresponding to an activity level of log
Lx/Lbol ~ -6.2. Altogether, these findings point to a rather weakly active and
given a RASS detection, long-term stable X-ray emitting star. The X-ray
emission from HR 8799 resembles those of a late A/early F-type stars, in
agreement with its classification from hydrogen lines and effective temperature
determination and thus resolving the apparent discrepancy with the standard
picture of magnetic activity that predicts mid A-type stars to be virtually
X-ray dark.Comment: 4 pages, 3 figures, accepted by A&
Strong latitudinal shear in the shallow convection zone of a rapidly rotating A-star
We have derived the mean broadening profile of the star V102 in the region of
the open cluster IC4665 from high resolution spectroscopy. At a projected
equatorial rotation velocity of vsini = (105 +- 12)km/s we find strong
deviation from classical rotation. We discuss several scenarios, the most
plausible being strong differential rotation in latitudinal direction. For this
scenario we find a difference in angular velocity of DeltaOmega = 3.6 +- 0.8
rad/d (DeltaOmega/Omega = 0.42 +- 0.09). From the Halpha line we derive a
spectral type of A9 and support photometric measurements classifying IC4665
V102 as a non-member of IC4665. At such early spectral type this is the
strongest case of differential rotation observed so far. Together with three
similar stars, IC4665 V102 seems to form a new class of objects that exhibit
extreme latitudinal shear in a very shallow convective envelope.Comment: accepted for A&A Letter
Castor A and Castor B resolved in a simultaneous Chandra and XMM-Newton observation
We present a simultaneous Chandra and XMM-Newton observation of the Castor
sextett, focusing on Castor A and Castor B, two spectroscopic binaries with
early-type primaries. Of the present day X-ray instruments only Chandra can
isolate the X-ray lightcurves and spectra of A and B. We compare the Chandra
observation with XMM-Newton data obtained simultaneously. Albeit not able to
resolve Castor A and Castor B from each other, the higher sensitivity of
XMM-Newton allows for a quantitative analysis of their combined high-resolution
spectrum. He-like line triplets are used to examine the temperature and the
density in the corona of Castor AB. The temporal variability of Castor AB is
studied using data collected with the European Photon Imaging Camera onboard
XMM-Newton. Strong flare activity is observed, and combining the data acquired
simultaneously with Chandra and XMM-Newton each flare can be assigned to its
host. Our comparison with the conditions of the coronal plasma of other stars
shows that Castor AB behave like typical late-type coronal X-ray emitters
supporting the common notion that the late-type secondaries within each
spectroscopic binary are the sites of the X-ray production.Comment: accepted for publication in A&
Fe XIII coronal line emission in cool M dwarfs
We report on a search for the Fe xiii forbidden coronal line at 3388.1 \AA in
a sample of 15 M-type dwarf stars covering the whole spectral class as well as
different levels of activity. A clear detection was achieved for LHS 2076
during a major flare and for CN Leo, where the line had been discovered before.
For some other stars the situation is not quite clear. For CN Leo we
investigated the timing behaviour of the Fe xiii line and report a high level
of variability on a timescale of hours which we ascribe to microflare heating.Comment: 13 pages, 10 figure
Direct detection of a magnetic field in the photosphere of the single M giant EK Boo: How common is magnetic activity among M giants?
We study the fast rotating M5 giant EK Boo by means of spectropolarimetry to
obtain direct and simultaneous measurements of both the magnetic field and
activity indicators, in order to infer the origin of the activity in this
fairly evolved giant. We used the new spectropolarimeter NARVAL at the Bernard
Lyot Telescope (Observatoire du Pic du Midi, France) to obtain a series of
Stokes I and Stokes V profiles for EK Boo. Using the Least Square Deconvolution
technique we were able to detect the Zeeman signature of the magnetic field. We
measured its longitudinal component by means of the averaged Stokes V and
Stokes I profiles. The spectra also permitted us to monitor the CaII K&H
chromospheric emission lines, which are well known as indicators of stellar
magnetic activity. From ten observations obtained between April 2008 and March
2009, we deduce that EK Boo has a magnetic field, which varied in the range of
-0.1 to -8 G. We also determined the initial mass and evolutionary stage of EK
Boo, based on up-to-date stellar evolution tracks. The initial mass is in the
range of 2.0-3.6 M_sun, and EK Boo is either on the asymptotic giant branch
(AGB), at the onset of the thermal pulse phase, or at the tip of the first (or
red) giant branch (RGB). The fast rotation and activity of EK Boo might be
explained by angular momentum dredge-up from the interior, or by the merging of
a binary. In addition, we observed eight other M giants, which are known as
X-ray emitters, or to be rotating fast for their class. For one of these, beta
And, presumably also an AGB star, we have a marginal detection of magnetic
field, and a longitudinal component Bl of about 1G was measured. More
observations like this will answer the question whether EK Boo is a special
case, or whether magnetic activity is, rather, more common among M giants than
expected.Comment: Accepted for publication in Astronomy & Astrophysics, 10 pages, 8
figure
Origin and evolution of magnetars
We present a population synthesis study of the observed properties of the
magnetars, which allows for X-ray selection effects, investigating the
hypothesis that they are drawn from a population of progenitors that are more
massive than those of the normal radio pulsars. We assume that the anomalous
X-ray emission is caused by the decay of a toroidal or tangled up field that
does not partake in the spin down of the star.
We find that we can explain the observed properties, such as the period and
field distributions and the Period - Period derivative diagram, if we suitably
parametrise the time evolution of the anomalous X-ray luminosity as an
exponentially decaying function of time.
The magnetic flux of the neutron stars is required to be a strong function of
the progenitor mass with the magnetars arising from the mass range 20-45 solar
masses.
Unlike with the radio pulsars, the magnetars only weakly constrain the birth
spin period, due to their rapid spin-down. Our model predicts a birthrate of
about 0.15-0.3 per century.Comment: 5 pages, 2 figures, accepted for publication in the MNRAS (letters
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