4,356 research outputs found
Magnetic activity on AB Doradus: Temporal evolution of starspots and differential rotation from 1988 to 1994
Surface brightness maps for the young K0 dwarf AB Doradus are reconstructed
from archival data sets for epochs spanning 1988 to 1994. By using the
signal-to-noise enhancement technique of Least-Squares Deconvolution, our
results show a greatly increased resolution of spot features than obtained in
previously published surface brightness reconstructions. These images show that
for the exception of epoch 1988.96, the starspot distributions are dominated by
a long-lived polar cap, and short-lived low to high latitude features. The
fragmented polar cap at epoch 1988.96 could indicate a change in the nature of
the dynamo in the star. For the first time we measure differential rotation for
epochs with sufficient phase coverage (1992.05, 1993.89, 1994.87). These
measurements show variations on a timescale of at least one year, with the
strongest surface differential rotation ever measured for AB Dor occurring in
1994.86. In conjunction with previous investigations, our results represent the
first long-term analysis of the temporal evolution of differential rotation on
active stars.Comment: accepted by MNRAS 18 pages 18 figure
A polarity reversal in the large-scale magnetic field of the rapidly rotating Sun HD 190771
Aims. We investigate the long-term evolution of the large-scale photospheric
magnetic field geometry of the solar-type star HD 190771. With fundamental
parameters very close to those of the Sun except for a shorter rotation period
of 8.8 d, HD 190771 provides us with a first insight into the specific impact
of the rotation rate in the dynamo generation of magnetic fields in 1
stars.
Methods. We use circularly polarized, high-resolution spectra obtained with
the NARVAL spectropolarimeter (Observatoire du Pic du Midi, France) and compute
cross-correlation line profiles with high signal-to-noise ratio to detect
polarized Zeeman signatures. From three phase-resolved data sets collected
during the summers of 2007, 2008, and 2009, we model the large-scale
photospheric magnetic field of the star by means of Zeeman-Doppler imaging and
follow its temporal evolution.
Results. The comparison of the magnetic maps shows that a polarity reversal
of the axisymmetric component of the large-scale magnetic field occurred
between 2007 and 2008, this evolution being observed in both the poloidal and
toroidal magnetic components. Between 2008 and 2009, another type of global
evolution occured, characterized by a sharp decrease of the fraction of
magnetic energy stored in the toroidal component. These changes were not
accompanied by significant evolution in the total photospheric magnetic energy.
Using our spectra to perform radial velocity measurements, we also detect a
very low-mass stellar companion to HD 190771.Comment: Accepted by Astronomy and Astrophysics (Letter to the Editor
NGC 6535: the lowest mass Milky Way globular cluster with a Na-O anti-correlation? Cluster mass and age in the multiple population context
To understand globular clusters (GCs) we need to comprehend how their
formation process was able to produce their abundance distribution of light
elements. In particular, we seek to figure out which stars imprinted the
peculiar chemical signature of GCs. One of the best ways is to study the
light-element anti-correlations in a large sample of GCs that are analysed
homogeneously. As part of our spectroscopic survey of GCs with FLAMES, we
present here the results of our study of about 30 red giant member stars in the
low-mass, low-metallicity Milky Way cluster NGC 6535. We measured the
metallicity (finding [Fe/H]=-1.95, rms=0.04 dex in our homogeneous scale) and
other elements of the cluster and, in particular, we concentrate here on O and
Na abundances. These elements define the normal Na-O anti-correlation of
classical GCs, making NGC 6535 perhaps the lowest mass cluster with a confirmed
presence of multiple populations. We updated the census of Galactic and
extragalactic GCs for which a statement on the presence or absence of multiple
populations can be made on the basis of high-resolution spectroscopy
preferentially, or photometry and low-resolution spectroscopy otherwise; we
also discuss the importance of mass and age of the clusters as factors for
multiple populations.Comment: In press on A&A. Table 2 available at CD
The relation between stellar magnetic field geometry and chromospheric activity cycles – II The rapid 120-day magnetic cycle of <i>τ</i> Bootis
One of the aims of the BCool programme is to search for cycles in other stars and to understand how similar they are to the Sun. In this paper, we aim to monitor the evolution of τ Boo’s large-scale magnetic field using high-cadence observations covering its chromospheric activity maximum. For the first time, we detect a polarity switch that is in phase with τ Boo’s 120-day chromospheric activity maximum and its inferred X-ray activity cycle maximum. This means that τ Boo has a very fast magnetic cycle of only 240 days. At activity maximum τ Boo’s large-scale field geometry is very similar to the Sun at activity maximum: it is complex and there is a weak dipolar component. In contrast, we also see the emergence of a strong toroidal component which has not been observed on the Sun, and a potentially overlapping butterfly pattern where the next cycle begins before the previous one has finished
Photometric and spectroscopic study of the intermediate-age open cluster NGC 2355
In this paper we analyse the evolutionary status and properties of the old
open cluster NGC 2355, located in the Galactic anticentre direction, as a part
of the long term programme BOCCE. NGC 2355 was observed with LBC@LBT using the
Bessel , , and filters. The cluster parameters have been obtained
using the synthetic colour-magnitude diagram (CMD) method, as done in other
papers of this series. Additional spectroscopic observations with FIES@NOT of
three giant stars were used to determine the chemical properties of the
cluster. Our analysis shows that NGC 2355 has metallicity slightly less than
solar, with [Fe/H] dex, age between 0.8 and 1 Gyr, reddening
in the range 0.14 and 0.19 mag, and distance modulus of about 11 mag.
We also investigated the abundances of O, Na, Al, , iron-peak, and
neutron capture elements, showing that NGC 2355 falls within the abundance
distribution of similar clusters (same age and metallicity). The Galactocentric
distance of NGC~2355 places it at the border between two regimes of metallicity
distribution; this makes it an important cluster for the study of the chemical
properties and evolution of the disc.Comment: 20 pages, 11 figures, Accepted on MNRA
NGC 6139: a normal massive globular cluster or a first-generation dominated cluster? Clues from the light elements
Information on globular clusters (GC) formation mechanisms can be gathered by
studying the chemical signature of the multiple populations that compose these
stellar systems. In particular, we are investigating the anticorrelations among
O, Na, Al, and Mg to explore the influence of cluster mass and environment on
GCs in the Milky Way and in extragalactic systems. We present here the results
obtained on NGC 6139 which, on the basis of its horizontal branch morphology,
had been proposed to be dominated by first-generation stars. In our extensive
study based on high resolution spectroscopy, the first for this cluster, we
found a metallicity of [Fe/H]= -1.579 +/- 0.015 +/- 0.058 (rms=0.040 dex, 45
bona fide member stars) on the UVES scale defined by our group. The stars in
NGC 6139 show a chemical pattern normal for GCs, with a rather extended Na-O
(and Mg-Al) anticorrelation. NGC 6139 behaves like expected from its mass and
contains a large fraction (about two thirds) of second-generation stars.Comment: Accepted for publication on A&
Three-dimensional Simulations of Accretion to Stars with Complex Magnetic Fields
Disk accretion to rotating stars with complex magnetic fields is investigated
using full three-dimensional magnetohydrodynamic (MHD) simulations. The studied
magnetic configurations include superpositions of misaligned dipole and
quadrupole fields and off-centre dipoles. The simulations show that when the
quadrupole component is comparable to the dipole component, the magnetic field
has a complex structure with three major magnetic poles on the surface of the
star and three sets of loops of field lines connecting them. A significant
amount of matter flows to the quadrupole "belt", forming a ring-like hot spot
on the star. If the maximum strength of the magnetic field on the star is
fixed, then we observe that the mass accretion rate, the torque on the star,
and the area covered by hot spots are several times smaller in the
quadrupole-dominant cases than in the pure dipole cases. The influence of the
quadrupole component on the shape of the hot spots becomes noticeable when the
ratio of the quadrupole and dipole field strengths , and
becomes dominant when . In the case of an off-centre dipole
field, most of the matter flows through a one-armed accretion stream, forming a
large hot spot on the surface, with a second much smaller secondary spot. The
light curves may have simple, sinusoidal shapes, thus mimicking stars with pure
dipole fields. Or, they may be complex and unusual. In some cases the light
curves may be indicators of a complex field, in particular if the inclination
angle is known independently. We also note that in the case of complex fields,
magnetospheric gaps are often not empty, and this may be important for the
survival of close-in exosolar planets.Comment: 13 pages, 21 figures, accepted for publication in MNRA
Letters
www.elsevier.com/locate/dsw A branch and bound algorithm for the robust shortest path problem with interval data
Spots structure and stratification of helium and silicon in the atmosphere of He-weak star HD 21699
The magnetic star HD 21699 possesses a unique magnetic field structure where
the magnetic dipole is displaced from the centre by 0.4 +/- 0.1 of the stellar
radius (perpendicularly to the magnetic axis), as a result, the magnetic poles
are situated close to one another on the stellar surface with an angular
separation of 55 and not 180 as seen in the case of a centred dipole.
Respectively, the two magnetic poles form a large "magnetic spot".
High-resolution spectra were obtained allowing He I and Si II abundance
variations to be studied as a function of rotational phase. The results show
that the helium abundance is concentrated in one hemisphere of the star, near
the magnetic poles and it is comparatively weaker in another hemisphere, where
magnetic field lines are horizontal with respect to the stellar surface. At the
same time, the silicon abundance is greatest between longitudes of 180 -
320, the same place where the helium abundance is the weakest. These
abundance variations (with rotational phase) support predictions made by the
theory of atomic diffusion in the presence of a magnetic field. Simultaneously,
these result support the possibility of the formation of unusual structures in
stellar magnetic fields. Analysis of vertical stratification of the silicon and
helium abundances shows that the boundaries of an abundance jump (in the two
step model) are similar for each element; = 0.8-1.2 for helium
and 0.5-1.3 for silicon. The elemental abundances in the layers of effective
formation of selected absorption lines for various phases are also correlated
with the excitation energies of low transition levels: abundances are enhanced
for higher excitation energy and higher optical depth within the applied model
atmosphere.Comment: accepted by MN, 7 pagers, 10 figs, 3 table
- …