773 research outputs found
Inhomogeneous distribution of mercury on the surfaces of rapidly rotating HgMn stars
Starspots are usually associated with the action of magnetic fields at the
stellar surfaces. However, recently an inhomogeneous chemical distribution of
mercury was found for the mercury-manganese (HgMn) star alpha And -- a
well-established member of a non-magnetic subclass of the chemically peculiar
stars of the upper main sequence. In this study we present first results of the
high-resolution survey of the HgII 3984 resonance line in the spectra of
rapidly rotating HgMn stars with atmospheric parameters similar to those of
alpha And. We use spectrum synthesis modelling and take advantage of the
Doppler resolution of the stellar surfaces to probe horizontal structure of
mercury distribution. Clear signatures of spots are found in the HgII 3984 line
profiles of HR 1185 and HR 8723. Two observations of the latter star separated
by two days give evidence for the line profile variability. We conclude that
inhomogeneous distribution of Hg is a common phenomenon for the rapidly
rotating HgMn stars in the 13000--13800 K effective temperature range
independently of the stellar evolutionary stage. These results establish
existence of a new class of spectrum variable spotted B-type stars. It is
suggested that the observed Hg inhomogeneities arise from dynamical
instabilities in the chemical diffusion processes and are unrelated to magnetic
phenomena.Comment: 6 pages, 3 figures, accepted by Astronomy & Astrophysic
The evolution of luminosity, colour and the mass-to-luminosity ratio of Galactic open clusters: comparison of discrete vs. continuous IMF models
(abridged) We found in previous studies that standard Simple Stellar
Population (SSP) models are unable to describe or explain the colours of
Galactic open clusters both in the visible and in the NIR spectral range. (...)
We construct a numerical SSP-model, with an underlying Salpeter IMF, valid
within an upper and lower stellar mass range, and with total masses
typical of open clusters. We assume that the mass
loss from a cluster is provided by mass loss from evolved stars and by the
dynamical evaporation of low-mass members due to two-body relaxation. The data
for the latter process were scaled to the models from high-resolution N-body
calculations. We also investigate how a change of the -limit influences
magnitudes and colours of clusters of a given mass and derive a necessary
condition for a luminosity and colour flash. The discreteness of the IMF leads
to bursts in magnitude and colour of model clusters at moments when red
supergiants or giants appear and then die. The amplitude of the burst depends
on the cluster mass and on the spectral range; it is strongly increased in the
NIR compared to optical passbands. In the discrete case, variations of the
parameter are able to substantially change the magnitude-age and
-age relations. For the colours, the lowering of considerably
amplifies the discreteness effect. The influence of dynamical mass loss on
colour and magnitude is weak, although it provides a change of the slopes of
the considered relations, improving their agreement with observations. For the
Galactic open clusters we determined luminosity and tidal mass independent of
each other. The derived mass-to-luminosity ratio shows, on average, an increase
with cluster age in the optical, but gradually declines with age in the NIR.
The observed flash statistics can be used to constrain in open clusters.Comment: 15 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Exploring the magnetic field complexity in M dwarfs at the boundary to full convection
Based on detailed spectral synthesis we carry out quantitative measurements
of the strength and complexity of surface magnetic fields in the four
well-known M-dwarfs GJ 388, GJ 729, GJ 285, and GJ 406 populating the mass
regime around the boundary between partially and fully convective stars. Very
high resolution R=100000, high signal-to-noise (up to 400) near-infrared Stokes
I spectra were obtained with CRIRES at ESO's Very Large Telescope covering
regions of the FeH Wing-Ford transitions at 1mum. The field distributions in
all four stars are characterized by three distinct groups of field components,
the data are neither consistent with a smooth distribution of different field
strengths, nor with one average field strength covering the full star. We find
evidence of a subtle difference in the field distribution of GJ 285 compared to
the other three targets. GJ 285 also has the highest average field of 3.5kG and
the strongest maximum field component of 7-7.5kG. The maximum local field
strengths in our sample seem to be correlated with rotation rate. While the
average field strength is saturated, the maximum local field strengths in our
sample show no evidence for saturation. We find no difference between the field
distributions of partially and fully convective stars. The one star with
evidence for a field distribution different to the other three is the most
active star (i.e. with largest x-ray luminosity and mean surface magnetic
field) rotating relatively fast. A possible explanation is that rotation
determines the distribution of surface magnetic fields, and that local field
strengths grow with rotation even in stars in which the average field is
already saturated.Comment: 15 pages, 8 figure
HD 178892 - a cool Ap star with extremely strong magnetic field
We report a discovery of the Zeeman resolved spectral lines, corresponding to
the extremely large magnetic field modulus =17.5 kG, in the cool Ap star HD
178892. The mean longitudinal field of this star reaches 7.5 kG, and its
rotational modulation implies the strength of the dipolar magnetic component
Bp>=23 kG. We have revised rotation period of the star using the All Sky
Automated Survey photometry and determined P=8.2478 d. Rotation phases of the
magnetic and photometric maxima of the star coincide with each other. We
obtained Geneva photometric observation of HD 178892 and estimated
Teff=7700+/-250 K using photometry and the hydrogen Balmer lines. Preliminary
abundance analysis reveals abundance pattern typical of rapidly oscillating Ap
stars.Comment: Accepted by Astronomy & Astrophysics; 4 pages, 4 figure
Global survey of star clusters in the Milky Way II. The catalogue of basic parameters
Although they are the main constituents of the Galactic disk population, for
half of the open clusters in the Milky Way reported in the literature nothing
is known except the raw position and an approximate size. The main goal of this
study is to determine a full set of uniform spatial, structural, kinematic, and
astrophysical parameters for as many known open clusters as possible. On the
basis of stellar data from PPMXL and 2MASS, we used a dedicated data-processing
pipeline to determine kinematic and photometric membership probabilities for
stars in a cluster region. For an input list of 3784 targets from the
literature, we confirm that 3006 are real objects, the vast majority of them
are open clusters, but associations and globular clusters are also present. For
each confirmed object we determined the exact position of the cluster centre,
the apparent size, proper motion, distance, colour excess, and age. For about
1500 clusters, these basic astrophysical parameters have been determined for
the first time. For the bulk of the clusters we also derived the tidal radius.
We estimated additionally average radial velocities for more than 30% of the
confirmed clusters. The present sample (called MWSC) reaches both the central
parts of the Milky Way and its outer regions. It is almost complete up to 1.8
kpc from the Sun and also covers neighbouring spiral arms. However, for a small
subset of the oldest open clusters () we found some evidence
of incompleteness within about 1 kpc from the Sun.Comment: 8 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Global survey of star clusters in the Milky Way IV. 63 new open clusters detected by proper motions
AIMS: In their 1st extension to the Milky Way Star Clusters (MWSC) survey,
Schmeja et al. applied photometric filters to the 2MASS to find new cluster
candidates that were subsequently confirmed or rejected by the MWSC pipeline.
To further extend the MWSC census, we aimed at discovering new clusters by
conducting an almost global search in proper motion catalogues as a starting
point. METHODS: We first selected high-quality samples from the PPMXL and UCAC4
for comparison and verification of the proper motions. For 441 circular proper
motion bins (radius 15 mas/yr) within 50 mas/yr, the sky outside a thin
Galactic plane zone (5) was binned in small areas ('sky
pixels') of 0.250.25 deg. Sky pixels with enhanced numbers of stars
with a certain common proper motion in both catalogues were considered as
cluster candidates. After visual inspection of the sky images, we built an
automated procedure that combined these representations of the sky for
neighbouring proper motion subsamples after a background correction. RESULTS:
About half of our 692 candidates overlapped with known clusters (46 globular
and 68 open clusters in the Galaxy, about 150 known clusters of galaxies) or
the Magellanic Clouds. About 10% of our candidates turned out to be 63 new open
clusters confirmed by the MWSC pipeline. They occupy predominantly the two
inner Galactic quadrants and have apparent sizes and numbers of high-probable
members slightly larger than those of the typically small MWSC clusters,
whereas their other parameters (ages, distances, tidal radii) fall in the
typical ranges. As our search aimed at finding compact clusters, we did not
find new very nearby (extended) clusters. (abridged)Comment: 14 pages, 14 figures, accepted for publication in Astronomy and
Astrophysic
Shape parameters of Galactic open clusters
(abridged) In this paper we derive observed and modelled shape parameters
(apparent ellipticity and orientation of the ellipse) of 650 Galactic open
clusters identified in the ASCC-2.5 catalogue. We provide the observed shape
parameters of Galactic open clusters, computed with the help of a
multi-component analysis. For the vast majority of clusters these parameters
are determined for the first time. High resolution ("star by star") N-body
simulations are carried out with the specially developed GRAPE code
providing models of clusters of different initial masses, Galactocentric
distances and rotation velocities. The comparison of models and observations of
about 150 clusters reveals ellipticities of observed clusters which are too low
(0.2 vs. 0.3), and offers the basis to find the main reason for this
discrepancy. The models predict that after Myr clusters reach an
oblate shape with an axes ratio of , and with the major axis
tilted by an angle of with respect to the
Galactocentric radius due to differential rotation of the Galaxy. Unbiased
estimates of cluster shape parameters require reliable membership determination
in large cluster areas up to 2-3 tidal radii where the density of cluster stars
is considerably lower than the background. Although dynamically bound stars
outside the tidal radius contribute insignificantly to the cluster mass, their
distribution is essential for a correct determination of cluster shape
parameters. In contrast, a restricted mass range of cluster stars does not play
such a dramatic role, though deep surveys allow to identify more cluster
members and, therefore, to increase the accuracy of the observed shape
parameters.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Modelling the molecular Zeeman effect in M-dwarfs: methods and first results
We present first quantitative results of the surface magnetic field
measurements in selected M-dwarfs based on detailed spectra synthesis conducted
simultaneously in atomic and molecular lines of the FeH Wing-Ford
transitions. A modified version of the Molecular
Zeeman Library (MZL) was used to compute Land\'e g-factors for FeH lines in
different Hund's cases. Magnetic spectra synthesis was performed with the
Synmast code. We show that the implementation of different Hund's case for FeH
states depending on their quantum numbers allows us to achieve a good fit to
the majority of lines in a sunspot spectrum in an automatic regime. Strong
magnetic fields are confirmed via the modelling of atomic and FeH lines for
three M-dwarfs YZ~CMi, EV~Lac, and AD~Leo, but their mean intensities are found
to be systematically lower than previously reported. A much weaker field
(~kG against ~kG) is required to fit FeH lines in the spectra of
GJ~1224. Our method allows us to measure average magnetic fields in very
low-mass stars from polarized radiative transfer. The obtained results indicate
that the fields reported in earlier works were probably overestimated by about
\%. Higher quality observations are needed for more definite results.Comment: Accepted by A&A, 13 pages, 7 figures, 1 tabl
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