299 research outputs found
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
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
New Abundances for Old Stars - Atomic Diffusion at Work in NGC 6397
A homogeneous spectroscopic analysis of unevolved and evolved stars in the
metal-poor globular cluster NGC 6397 with FLAMES-UVES reveals systematic trends
of stellar surface abundances that are likely caused by atomic diffusion. This
finding helps to understand, among other issues, why the lithium abundances of
old halo stars are significantly lower than the abundance found to be produced
shortly after the Big Bang.Comment: 8 pages, 7 colour figures, 1 table; can also be downloaded via
http://www.eso.org/messenger
Weather in stellar atmosphere: the dynamics of mercury clouds in alpha Andromedae
The formation of long-lasting structures at the surfaces of stars is commonly
ascribed to the action of strong magnetic fields. This paradigm is supported by
observations of evolving cool spots in the Sun and active late-type stars, and
stationary chemical spots in the early-type magnetic stars. However, results of
our seven-year monitoring of mercury spots in non-magnetic early-type star
alpha Andromedae show that the picture of magnetically-driven structure
formation is fundamentally incomplete. Using an indirect stellar surface
mapping technique, we construct a series of 2-D images of starspots and
discover a secular evolution of the mercury cloud cover in this star. This
remarkable structure formation process, observed for the first time in any
star, is plausibly attributed to a non-equilibrium, dynamical evolution of the
heavy-element clouds created by atomic diffusion and may have the same
underlying physics as the weather patterns on terrestrial and giant planets.Comment: 10 pages, 2 figures; to be published in Nature Physic
Determination of the dominant factor affecting the change of the phase transition point in thin ferroelectric films
The study was supported by the Russian Foundation For Basic Research, Project № 19-32-50032
The search for magnetic fields in mercury-manganese stars
We performed a highly sensitive search for magnetic fields on a large set of
HgMn stars. With the aid of a new polarimeter attached to the HARPS
spectrometer at the ESO 3.6m-telescope, we obtained high-quality circular
polarization spectra of 41 single and double HgMn stars. Using a multi-line
analysis technique on each star, we co-added information from hundreds of
spectral lines resulting in significantly greater sensitivity to the presence
of magnetic fields, including very weak fields. For the 47 individual objects
studied, including 6 components of SB2 systems, we do not detect any magnetic
fields at greater than the 3 sigma level. The lack of detection in the circular
polarization profiles indicates that if strong fields are present on these
stars, they must have complex surface topologies. For simple global fields, our
detection limits imply upper limits to the fields present of 2-10 Gauss in the
best cases. We conclude that HgMn stars lack large-scale magnetic fields,
typical for spotted magnetic Ap stars, sufficient to form and sustain the
chemical spots observed on HgMn stars. Our study confirms that in addition to
magnetically altered atomic diffusion, there exists another differentiation
mechanism operating in the atmospheres of late-B main sequence stars which can
compositional inhomogeneities on their surfaces.Comment: 12 pages, 8 figures, 2 table
Chemical spots in the absence of magnetic field in the binary HgMn star 66 Eridani
According to our current understanding, a subclass of the upper main sequence
chemically peculiar stars, called mercury-manganese (HgMn), is non-magnetic.
Nevertheless, chemical inhomogeneities were recently discovered on their
surfaces. At the same time, no global magnetic fields stronger than 1-100 G are
detected by modern studies. The goals of our study are to search for magnetic
field in the HgMn binary system 66 Eri and to investigate chemical spots on the
stellar surfaces of both components. Our analysis is based on high quality
spectropolarimetric time-series observations obtained during 10 consecutive
nights with the HARPSpol instrument at the ESO 3.6-m telescope. To increase the
sensitivity of the magnetic field search we employed a least-squares
deconvolution (LSD). We used spectral disentangling to measure radial
velocities and study line profile variability. Chemical spot geometry was
reconstructed using multi-line Doppler imaging. We report a non-detection of
magnetic field in 66 Eri, with error bars 10-24 G for the longitudinal field.
Circular polarization profiles also do not indicate any signatures of complex
surface magnetic fields. For a simple dipolar field configuration we estimated
an upper limit of the polar field strength to be 60-70 G. For the HgMn
component we found variability in spectral lines of Ti, Ba, Y, and Sr with the
rotational period equal to the orbital one. The surface maps of these elements
reconstructed with the Doppler imaging technique, show relative underabundance
on the hemisphere facing the secondary component. The contrast of chemical
inhomogeneities ranges from 0.4 for Ti to 0.8 for Ba.Comment: 13 pages, 14 figure
Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 I. Spectropolarimetric observations in all four Stokes parameters
High-resolution spectropolarimetric observations provide simultaneous
information about stellar magnetic field topologies and three-dimensional
distributions of chemical elements. Here we present analysis of a unique full
Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap
star HD 24712. The goal of our work is to examine circular and linear
polarization signatures inside spectral lines and to study variation of the
stellar spectrum and magnetic observables as a function of rotational phase. HD
24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope over
a period of 2010-2011. The resulting spectra have S/N ratio of 300-600 and
resolving power exceeding 100000. The multiline technique of least-squares
deconvolution (LSD) was applied to combine information from the spectral lines
of Fe-peak and rare-earth elements. We used the HARPSPol spectra of HD 24712 to
study the morphology of the Stokes profile shapes in individual spectral lines
and in LSD Stokes profiles corresponding to different line masks. From the LSD
Stokes V profiles we measured the longitudinal component of the magnetic field,
, with an accuracy of 5-10 G. We also determined the net linear
polarization from the LSD Stokes Q and U profiles. We determined an improved
rotational period of the star, P_rot = 12.45812 +/- 0.00019d. We measured
from the cores of Halpha and Hbeta lines. The analysis of measurements
showed no evidence for a significant radial magnetic field gradient in the
atmosphere of HD 24712. We used our and net linear polarization
measurements to determine parameters of the dipolar magnetic field topology. We
found that magnetic observables can be reasonably well reproduced by the
dipolar model. We discovered rotational modulation of the Halpha core and
related it a non-uniform surface distribution of rare-earth elements.Comment: Accepted for publication in A&
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