241 research outputs found
The variation of the magnetic field of the Ap star HD~50169 over its 29 year rotation period
Context. The Ap stars that rotate extremely slowly, with periods of decades
to centuries, represent one of the keys to the understanding of the processes
leading to the differentiation of stellar rotation.
Aims. We characterise the variations of the magnetic field of the Ap star HD
50169 and derive constraints about its structure.
Methods. We combine published measurements of the mean longitudinal field
of HD 50169 with new determinations of this field moment from circular
spectropolarimetry obtained at the 6-m telescope BTA of the Special
Astrophysical Observatory of the Russian Academy of Sciences. For the mean
magnetic field modulus , literature data are complemented by the analysis of
ESO spectra, both newly acquired and from the archive. Radial velocities are
also obtained from these spectra.
Results. We present the first determination of the rotation period of HD
50169, Prot = (29.04+/-0.82) y. HD 50169 is currently the longest-period Ap
star for which magnetic field measurements have been obtained over more than a
full cycle. The variation curves of both and have a significant degree
of anharmonicity, and there is a definite phase shift between their respective
extrema. We confirm that HD 50169 is a wide spectroscopic binary, refine its
orbital elements, and suggest that the secondary is probably a dwarf star of
spectral type M.
Conclusions. The shapes and mutual phase shifts of the derived magnetic
variation curves unquestionably indicate that the magnetic field of HD 50169 is
not symmetric about an axis passing through its centre. Overall, HD 50169
appears similar to the bulk of the long-period Ap stars.Comment: 10 pages, 3 figures, accepted for publication in A&
HD 965: An extremely peculiar A star with an extremely long rotation period
Context. One of the keys to understanding the origin of the Ap stars and
their significance in the general context of stellar astrophysics is the
consideration of the most extreme properties displayed by some of them. In that
context, HD 965 is particularly interesting, as it combines some of the most
pronounced chemical peculiarities with one of the longest rotation periods
known.
Aims. We characterise the variations of the magnetic field of the Ap star HD
965 and derive constraints about its structure.
Methods. We combine published measurements of the mean longitudinal field
of HD 965 with new determinations of this field moment from circular
spectropolarimetry obtained at the 6-m telescope BTA of the Special
Astrophysical Observatory of the Russian Academy of Sciences. For the mean
magnetic field modulus , literature data are complemented by the analysis of
ESO archive spectra.
Results. We present the first determination of the rotation period of HD 965,
P = (16.5+/-0.5) y. HD 965 is only the third Ap star with a period longer than
10 years for which magnetic field measurements have been obtained over more
than a full cycle. The variation curve of is well approximated by a cosine
wave. does not show any significant variation. The observed behaviour of
these field moments is well represented by a simple model consisting of the
superposition of collinear dipole, quadrupole and octupole. The distribution of
neodymium over the surface of HD 965 is highly non-uniform. The element appears
concentrated around the magnetic poles, especially the negative one.
Conclusions. The shape of the longitudinal magnetic variation curve of HD 965
indicates that its magnetic field is essentially symmetric about an axis
passing through the centre of the star. Overall, as far as its magnetic field
is concerned, HD 965 appears similar to the bulk of the long-period Ap stars.Comment: 7 pages, 4 figures, accepted for publication in Astronomy &
Astrophysics. arXiv admin note: text overlap with arXiv:1902.0586
Search for stellar spots in field blue horizontal-branch stars
Blue horizontal-branch stars are Population II objects which are burning
helium in their core and possess a hydrogen-burning shell and radiative
envelope. Because of their low rotational velocities, diffusion has been
predicted to work in their atmospheres. In many respects, blue
horizontal-branch stars closely resemble the magnetic chemically peculiar stars
of the upper main sequence, which show photometric variability caused by
abundance spots on their surfaces. These spots are thought to be caused by
diffusion and the presence of a stable magnetic field. However, the latter does
not seem to be axiomatic. We searched for rotationally induced variability in
30 well-established bright field blue horizontal-branch stars in the solar
neighbourhood and searched the literature for magnetic fields measurements of
our targets. We employed archival photometric time series data from the ASAS,
ASAS-SN, and SuperWASP surveys. The data were carefully reduced and processed,
and a time series analysis was applied using several different techniques. We
also synthesized existing photometric and spectroscopic data of magnetic
chemically peculiar stars in order to study possible different surface
characteristics producing lower amplitudes. In the accuracy limit of the
employed data, no significant variability signals were found in our sample
stars. The resulting upper limits for variability are given. We conclude that
either no stellar surface spots are present in field blue horizontal-branch
stars, or their characteristics (contrast, total area, and involved elements)
are not sufficient to produce amplitudes larger than a few millimagnitudes in
the optical wavelength region. New detailed models taking into account the
elemental abundance pattern of blue horizontal-branch stars are needed to
synthesize light curves for a comparison with our results.Comment: 6 pages, 2 figures, 2 tables, accepted for publication in Astronomy &
Astrophysic
Spatial structure of an individual Mn acceptor in GaAs
The wave function of a hole bound to an individual Mn acceptor in GaAs is
spatially mapped by scanning tunneling microscopy at room temperature and an
anisotropic, cross-like shape is observed. The spatial structure is compared
with that from an envelope-function, effective mass model, and from a
tight-binding model. This demonstrates that anisotropy arising from the cubic
symmetry of the GaAs crystal produces the cross-like shape for the hole
wave-function. Thus the coupling between Mn dopants in GaMnAs mediated by such
holes will be highly anisotropic.Comment: 3 figures, submitted to PR
Specular reflection intensity modulated by grazing-incidence diffraction in a wide angular range
Grazing-incidence X-ray diffraction (GID) is a well known technique for the characterization of crystal surfaces. A theoretical study has been performed of the sensitivity of GID to the structure of a crystal surface and distorted nanometre-thin surface layers. To simulate GID from crystals that have a complex subsurface structure, a matrix formalism of the dynamical diffraction theory has been applied. It has been found that the azimuthal rocking curves of a crystal that has a distorted subsurface, measured over a wide angular range, show asymmetric thickness oscillations with two distinguishable sets of frequencies: one corresponding to the diffraction in the single-crystal subsurface layer and the second corresponding to the diffraction in the single-crystal substrate. Therefore, azimuthal rocking curves allow characterization of the subsurface structure of a single crystal. Furthermore, thickness oscillations induced by evanescent diffraction modulate the specular reflection intensity, showing high-intensity modulations. This will potentially allow implementation of subsurface crystal characterization using, for instance, a laboratory-scale X-ray diffractometer.</p
Assessing the impact of high environmental temperatures on the frequency and structural characteristics of violations of traffic rules
In the hot season, there is a positive relationship between the number of accidents per thousand registered vehicles and air temperature, due to the fact that high temperatures and solar activity can have a significant negative impact on the condition of drivers. As a rule, the creation of an emergency is preceded by a violation of traffic rules. Therefore, the identification of the type and number of violations committed by drivers at high ambient temperatures is relevant.
The purpose of the work is to assess the impact of high ambient temperatures on the structure and frequency of traffic violations.
Based on a literature review, the article states that at present there is no methodology for assessing the impact of high ambient temperatures on the characteristics of traffic violations.
As an information platform, data from the Wialon — GPS/Glonass system are used, which allow you to objectively assess the characteristics of driving activity and the number of traffic violations. The work uses methods of mathematical statistics.
The main result of the work, which constitutes the scientific novelty of the study, is the revealed increasing dependence of the total number of traffic violations on the average monthly ambient temperature. The structure of such violations is determined. The most significant, based on the impact on the accident rate, violations are: exceeding the speed limit and abrupt and dangerous maneuvers. It has been found that the total number of violations of the rules increases during periods with high ambient temperatures by 94% compared to months with moderate temperatures. The largest number of violations consists in exceeding the speed limit — 97.8% of their total number.
The practical significance of the work consists in obtaining objective information about accidents at high ambient temperatures, on the basis of which it becomes possible to develop organizational and technological measures aimed at improving road safety.
The areas of further research are related to the creation of an information platform for theoretical and methodological tools aimed at preventing high accidents of road transport in the hot season
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