scholarly journals Mapping the Surface Distribution of Elements on Ap Stars Using the Maximum Entropy Method

1993 ◽  
Vol 138 ◽  
pp. 258-271
Author(s):  
Artie P. Hatzes
Keyword(s):  
Maximum Entropy ◽  
Rotation Period ◽  
Entropy Method ◽  
Elemental Distribution ◽  
Surface Distribution ◽  
Horizontal Diffusion ◽  
Distribution Of Elements ◽  
Characteristic Features ◽  
Magnetic Poles ◽  
Ap Stars

AbstractA technique for deriving the distribution of elements on the surface of Ap stars using maximum entropy reconstruction principles is described. The technique is applied to deriving the silicon distribution on 56 Ari, CU Vir, 11 Ori and the chromium distribution on ϒ2 Ari. Silicon on these stars is depleted at the magnetic poles and is enhanced in regions between the magnetic equator and poles. The chromium distribution on ϒ2 Ari is markedly different than the chromium distribution seen on other Ap stars. It shows depletions at one of the magnetic poles (as do other Ap stars) but it does not show the depleted band at the equator as has been seen on θ Aur, 45 Her, and ω Her. The silicon distribution on 11 Ori also differs from that found on other stars in that it shows evidence for a depleted band, similar to what has been seen in the chromium distribution is some stars. Characteristic features in the abundance maps such as spots or bands appear to mark the location of the magnetic poles or equator so that these maps can be used to infer the magnetic field geometries on these stars. Dipole decentering parameters derived from the abundance maps yield decentering parameters of about 0.2 stellar radii. The amount of decentering seems to be correlated with rotation period (longer period Ap stars have less decentering). Horizontal diffusion can complicate the use of abundance maps to determine the field geometry. The effects of horizontal diffusion can only be understood by a proper theoretical study of its effects or by mapping the elemental distribution on Ap stars of known age.

scholarly journals Doppler Images of Rapidly Rotating Ap Stars

1988 ◽  
Vol 132 ◽  
pp. 199-204
Author(s):  
Artie P. Hatzes
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Stellar Surface ◽  
Surface Distribution ◽  
Distribution Of Elements ◽  
Local Equivalent ◽  
Field Geometry ◽  
A New Technique ◽  
Ap Stars

The magnetic Ap stars are characterized by the presence of large magnetic fields which undergo periodic variations. These magnetic field variations are accompanied by spectral variations caused by the inhomogeneous distribution of elements on the stellar surface. It is believed that the magnetic field plays an important role in determining this distribution. Accurate maps of the surface distribution of elements would provide valuable probes as to the field geometry as well as provide clues to the role of the magnetic fields in the atmospheres of these stars. We have developed a new technique for mapping the local equivalent width on a stellar surface from the observed spectral line variations.

scholarly journals HD 965: An extremely peculiar A star with an extremely long rotation period

2019 ◽  
Vol 629 ◽  
pp. A39 ◽  
Author(s):  
G. Mathys ◽  
I. I. Romanyuk ◽  
S. Hubrig ◽  
D. O. Kudryavtsev ◽  
M. Schöller ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Rotation Period ◽  
Field Measurements ◽  
Special Astrophysical Observatory ◽  
General Context ◽  
Rotation Periods ◽  
Variation Curve ◽  
The Mean ◽  
Magnetic Poles ◽  
Ap Stars

Context. One of the keys to understanding the origin of Ap stars and their significance in the general context of stellar astrophysics is by considering the most extreme properties displayed by some of them. For this reason, 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 combined published measurements of the mean longitudinal field ⟨Bz⟩ 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 ⟨B⟩, literature data were complemented by the analysis of ESO archive spectra. Results. We present the first determination of the rotation period of HD 965, Prot = (16.5 ± 0.5) yr. The star HD 965 is only the third Ap star with a period longer than ten years for which magnetic field measurements have been obtained over more than a full cycle. The variation curve of ⟨Bz⟩ is well approximated by a cosine wave. Furthermore, ⟨B⟩ 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, HD 965 appears similar to the bulk of the long-period Ap stars, as far as its magnetic field is concerned.

Towards compositional imaging using auger electron signals

1991 ◽  
Vol 49 ◽  
pp. 6-7
Author(s):  
M M El Gomati ◽  
M Prutton ◽  
R H Roberts ◽  
I R Barkshire ◽  
P G Kenny ◽  
...  
Keyword(s):  
Electron Beam ◽  
Auger Electron ◽  
Elemental Distribution ◽  
Beam Diameter ◽  
Surface Distribution ◽  
Analytical Technique ◽  
Electron Production ◽  
Distribution Of Elements ◽  
Low Efficiency ◽  
Surface Analytical Technique

Auger electron spectroscopy (AES) is a well established, quantitative surface analytical technique with reasonable accuracy of the order of 1% of an atomic monolayer. When it is combined with a small electron beam diameter, high resolution concentration maps of the surface distribution of elements can be obtained. This has been demonstrated to be a useful and powerful method in surface analysis. However, because of the rather low efficiency of Auger electron production (∼ 10-5 - 10-4 per incident electron) long frame scan times (of the order of hours) have to be employed in the case of multi-element composite samples. The raw images often reflect not only surface elemental distribution but also electron beam fluctuations. In addition, subsurface atomic number variations as well as local surface topography are known to alter the contrast of these images. In order to quantify Auger maps to give concentration distribution of the surface elements, sample and instrumental effects have to be separated.

scholarly journals Doppler imaging results for Ap stars

1996 ◽  
Vol 176 ◽  
pp. 305-320 ◽  
Author(s):  
Artie P. Hatzes
Keyword(s):  
Diffusion Theory ◽  
Dipole Field ◽  
Doppler Imaging ◽  
Magnetic Pole ◽  
Quadrupole Field ◽  
Imaging Results ◽  
Distribution Of Elements ◽  
Magnetic Poles ◽  
Ap Stars ◽  
The Magnetic Field

Doppler imaging results for the surface abundance distribution on Ap stars are reviewed. The silicon distribution on these stars is often characterized by depleted spots near the magnetic poles and enhancements in ring-like structures at intermediate magnetic latitudes. These results are consistent with the predictions of diffusion theory. On two stars, γ Ari and CU Vir, the silicon enhancements appear very close to a magnetic pole. This can be explained if silicon is enhanced where the magnetic field has its maximum horizontal field strength and these stars possess a decentered dipole field. Doppler images of abundance distributions on Ap stars may be used to accurately measure the obliquity angle and to estimate the decentering parameter for a dipole field. The chromium distribution on at least four Ap stars show a depleted band coincident with the magnetic equator and depleted spots at the magnetic poles. Such a distribution can only arise if the star has a predominantly quadrupole field, or if horizontal diffusion is playing a role in the distribution of elements.

X-ray mapping without STEM

1994 ◽  
Vol 52 ◽  
pp. 508-509
Author(s):  
Judith M. Brock ◽  
Max T. Otten
Keyword(s):  
Life Science ◽  
Materials Science ◽  
Chemical Elements ◽  
Full Description ◽  
Scanning System ◽  
Elemental Distribution ◽  
X Ray ◽  
Transmission Electron ◽  
Distribution Of Elements ◽  
Made In

A knowledge of the distribution of chemical elements in a specimen is often highly useful. In materials science specimens features such as grain boundaries and precipitates generally force a certain order on mental distribution, so that a single profile away from the boundary or precipitate gives a full description of all relevant data. No such simplicity can be assumed in life science specimens, where elements can occur various combinations and in different concentrations in tissue. In the latter case a two-dimensional elemental-distribution image is required to describe the material adequately. X-ray mapping provides such of the distribution of elements.The big disadvantage of x-ray mapping hitherto has been one requirement: the transmission electron microscope must have the scanning function. In cases where the STEM functionality – to record scanning images using a variety of STEM detectors – is not used, but only x-ray mapping is intended, a significant investment must still be made in the scanning system: electronics that drive the beam, detectors for generating the scanning images, and monitors for displaying and recording the images.

scholarly journals Compositional Variation of Eudialyte-Group Minerals from the Lovozero and Ilímaussaq Complexes and on the Origin of Peralkaline Systems

Minerals ◽  
2021 ◽  
Vol 11 (6) ◽  
pp. 548
Author(s):  
Lia N. Kogarko ◽  
Troels F. D. Nielsen
Keyword(s):  
Linear Trend ◽  
Major And Trace Elements ◽  
High Tech ◽  
Compositional Variation ◽  
Elemental Distribution ◽  
Magma Chambers ◽  
Eudialyte Group ◽  
System A ◽  
Distribution Of Elements ◽  
World Class

The Lovozero complex, Kola peninsula, Russia and the Ilímaussaq complex in Southwest Greenland are the largest known layered peralkaline intrusive complexes. Both host world-class deposits rich in REE and other high-tech elements. Both complexes expose spectacular layering with horizons rich in eudialyte group minerals (EGM). We present a detailed study of the composition and cryptic variations in cumulus EGM from Lovozero and a comparison with EGM from Ilímaussaq to further our understanding of peralkaline magma chambers processes. The geochemical signatures of Lovozero and Ilímaussaq EGM are distinct. In Lovozero EGMs are clearly enriched in Na + K, Mn, Ti, Sr and poorer Fe compared to EGM from Ilímaussaq, whereas the contents of ΣREE + Y and Cl are comparable. Ilímaussaq EGMs are depleted in Sr and Eu, which points to plagioclase fractionation and an olivine basaltic parent. The absence of negative Sr and Eu anomalies suggest a melanephelinitic parent for Lovozero. In Lovozero the cumulus EGMs shows decrease in Fe/Mn, Ti, Nb, Sr, Ba and all HREE up the magmatic layering, while REE + Y and Cl contents increase. In Lovozero EGM spectra show only a weak enrichment in LREE relative to HREE. The data demonstrates a systematic stratigraphic variation in major and trace elements compositions of liquidus EGM in the Eudialyte Complex, the latest and uppermost part of Lovozero. The distribution of elements follows a broadly linear trend. Despite intersample variations, the absence of abrupt changes in the trends suggests continuous crystallization and accumulation in the magma chamber. The crystallization was controlled by elemental distribution between EGM and coexisting melt during gravitational accumulation of crystals and/or mushes in a closed system. A different pattern is noted in the Ilimaussaq Complex. The elemental trends have variable steepness up the magmatic succession especially in the uppermost zones of the Complex. The differences between the two complexes are suggested to be related dynamics of the crystallization and accumulation processes in the magma chambers, such as arrival of new liquidus phases and redistributions by mush melts.

Enhanced Information Recovery in 2D On-and Off-Resonance Nutation NQR using the Maximum Entropy Method

1996 ◽  
Vol 51 (5-6) ◽  
pp. 337-347 ◽  
Author(s):  
Mariusz Maćkowiak ◽  
Piotr Kątowski
Keyword(s):  
Maximum Entropy ◽  
Maximum Entropy Method ◽  
Theoretical Description ◽  
Entropy Method ◽  
Zero Field ◽  
Resonance Effects ◽  
Information Recovery ◽  
Truncation Errors ◽  
Enhanced Resolution ◽  
2D Data

Abstract Two-dimensional zero-field nutation NQR spectroscopy has been used to determine the full quadrupolar tensor of spin - 3/2 nuclei in serveral molecular crystals containing the 3 5 Cl and 7 5 As nuclei. The problems of reconstructing 2D-nutation NQR spectra using conventional methods and the advantages of using implementation of the maximum entropy method (MEM) are analyzed. It is shown that the replacement of conventional Fourier transform by an alternative data processing by MEM in 2D NQR spectroscopy leads to sensitivity improvement, reduction of instrumental artefacts and truncation errors, shortened data acquisition times and suppression of noise, while at the same time increasing the resolution. The effects of off-resonance irradiation in nutation experiments are demonstrated both experimentally and theoretically. It is shown that off-resonance nutation spectroscopy is a useful extension of the conventional on-resonance experiments, thus facilitating the determination of asymmetry parameters in multiple spectrum. The theoretical description of the off-resonance effects in 2D nutation NQR spectroscopy is given, and general exact formulas for the asymmetry parameter are obtained. In off-resonance conditions, the resolution of the nutation NQR spectrum decreases with the spectrometer offset. However, an enhanced resolution can be achieved by using the maximum entropy method in 2D-data reconstruction.

Estimating the distribution of primary reflection coefficients

Geophysics ◽  
2003 ◽  
Vol 68 (4) ◽  
pp. 1417-1422 ◽  
Author(s):  
Danilo R. Velis
Keyword(s):  
Maximum Entropy ◽  
Maximum Entropy Method ◽  
Entropy Method ◽  
Reflection Coefficients ◽  
Log Data ◽  
Neuquen Basin ◽  
Neuquén Basin ◽  
Skewness And Kurtosis

The distribution of primary reflection coefficients can be estimated by means of the maximum entropy method, giving rise to smooth nonparametric functions which are consistent with the data. Instead of using classical moments (e.g. skewness and kurtosis) to constraint the maximization, nonconventional sample statistics help to improve the quality of the estimates. Results using real log data from various wells located in the Neuquen Basin (Argentina) show the effectiveness of the method to estimate both robust and consistent distributions that may be used to simulate realistic sequences.

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