Surface Distribution Of Elements Research Articles

The nature of medium-period variables on the extreme horizontal branch

A fraction of the extreme horizontal branch stars of globular clusters exhibit a periodic light variability that has been attributed to rotational modulation caused by surface spots. These spots are believed to be connected to inhomogeneous surface distribution of elements. However, the presence of such spots has not been tested against spectroscopic data. We analyzed the phase-resolved ESO X-shooter spectroscopy of three extreme horizontal branch stars that are members of the globular cluster ω Cen and also display periodic light variations. The aim of our study is to understand the nature of the light variability of these stars and to test whether the spots can reproduce the observed variability. Our spectroscopic analysis of these stars did not detect any phase-locked abundance variations that are able to reproduce the light variability. Instead, we revealed the phase variability of effective temperature and surface gravity. In particular, the stars show the highest temperature around the light maximum. This points to pulsations as a possible cause of the observed spectroscopic and photometric variations. However, such an interpretation is in a strong conflict with Ritter’s law, which relates the pulsational period to the mean stellar density. The location of the ω Cen variable extreme horizontal branch stars in HR diagram corresponds to an extension of PG 1716 stars toward lower temperatures or blue, low-gravity, large-amplitude pulsators toward lower luminosities, albeit with much longer periods. Other models of light variability, namely, related to temperature spots, should also be tested further. The estimated masses of these stars in the range of 0.2–0.3 M⊙ are too low for helium-burning objects.

Selected properties of single-sided resistance spot welded joints on 18650 battery tab

The aim of this study was to characterize selected properties of joints made by single-sided serial resistance spot welding of 0.15 mm thickness Hilumin connectors to the negative terminal of LGDBHG21865 lithium-ion cylindrical battery cell. Welding process was performed using production parameters to properly represent actual manufacturing conditions. Mechanical properties of the joints were examined by means of tensile stress test and microhardness measurements while additional studies include metallographic analysis of the joint cross sections by means of optical and scanning microscopy. Energy dispersive spectroscopy was performed in order to determine surface distribution of elements within the weld nugget. Studies confirmed suitable microstructure of welded joints, repeatability of the welding process and proper selection of the welding parameters.

Doppler imaging of a southern ApSi star HD 152564

ABSTRACT We present the results of the spectroscopic study of a chemically peculiar star HD 152564. Using medium-resolution (R = 37 000) observations obtained with the high-resolution fibre échelle spectrograph mounted on the South African Large Telescope, we determined atmospheric parameters Teff = 11 950 ± 200 K and log g = 3.6 ± 0.2 dex. Abundance analysis revealed mild deficiency of the light elements and an overabundance of up to ∼2 dex of metals, with the greatest excess for silicon. With these characteristics, HD 152564 is a typical member of the silicon subgroup of Ap stars. The rotational modulation of the light curve and line profiles of HD 152564 are typical for the inhomogeneous surface distribution of elements in its atmosphere. We performed multi-element Doppler imaging of the surface of HD 152564. Abundance maps constructed for He, O, Mg, Si, and Fe revealed the concentration of these elements in a sequence of equatorial spots as well as in the circumpolar rings. The photometric maximum of the light curve coincided with the visibility of two most overabundant silicon spots. Abundances determined from the different ionization stages of Fe and Si show clear evidence for vertical stratification of these elements in the atmosphere of HD 152564. Meanwhile, the horizontal distribution of silicon reconstructed from the lines of different ionization stages and excitation energies appears to be identical with increasing average abundance deeper in atmosphere.

Stability Studies of Highly Active Cobalt Catalyst for the Ammonia Synthesis Process

Ammonia is currently considered a promising compound for the chemical storage of hydrogen and as an energy carrier. However, large-scale ammonia production is not possible without an active and stable catalyst enabling efficient, long-term work without the need for its replacement. In this paper, the extended stability studies of the highly active promoted cobalt catalyst for ammonia synthesis were carried out. The long-term activity measurements in NH3 synthesis reaction under conditions close to the industrial ones (400–470 °C, 6.3 MPa, H2/N2 = 3) were compiled with the characterization of catalyst properties on different stages of its work using N2 physisorption, XRPD, STEM-EDX, and H2-TPD. The accelerated aging method was used to simulate the deterioration of catalyst performance during industrial operation. Textural and structural characteristics revealed that the tested catalyst is highly resistant to high temperatures. The lack of significant changes in the specific surface area, morphology of the catalyst particles, surface distribution of elements, and chemisorption properties of cobalt surface during long-term heating (436 h) at 600 °C suggests that stable operation of the catalyst is possible in an ammonia synthesis reactor in the temperature range of 400–470 °C without the risk of losing its beneficial catalytic properties over time. The decline in catalyst activity during the long-term stability test was less than 10%.

A case study of ACV variables discovered in the Zwicky Transient Facility survey

Context. Magnetic chemically peculiar (mCP) stars exhibit complex atmospheres that allow the investigation of the interplay of atomic diffusion, magnetic fields, and stellar rotation. A non-uniform surface distribution of chemical elements and the non-alignment of the rotational and magnetic axes result in the variability of several observables. Photometrically variable mCP stars are referred to as α2 Canum Venaticorum (ACV) variables. Aims. The present work presents a case study of known variables from the Zwicky Transient Facility (ZTF) survey, with the aim of investigating the survey’s suitability for the detection and study of new ACV variables. Methods. Using suitable selection criteria based on the known characteristics of ACV variables, candidate ACV stars were selected from the ZTF Catalog of Periodic Variable Stars. All light curves were inspected in detail to select the most promising candidates. Where available, low-resolution spectra from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) were employed to classify the stars on the MK system and confirm their status as mCP stars. Results. We have identified 86 new promising ACV star candidates. Fifteen of these stars have LAMOST spectra available that, in all cases, confirm them as classical mCP stars, which highlights the viability of our approach. We present astrophysical parameters for all sample stars that can be sorted into four subgroups characterized by distinct light curve shapes. Anti-phase variations in different photometric passbands, in particular, are a unique characteristic of a subset of ACV stars readily usable for their identification. The availability of data in three different passbands (g, r, and i) is a major advantage of the ZTF survey. Conclusions. On the basis of our experience with other photometric surveys and the analysis of light curves, we conclude that the ZTF is well suited to the search for, and the analysis of, ACV variables that are not considered in the available ZTF variable star catalogs. Further work will focus on the development and refinement of a search algorithm to correctly identify these stars in ZTF data and, subsequently, in massive photometric time series databases in general.

Physicochemical characteristics of biomass‐coal blend char: The role of co‐pyrolysis synergy

AbstractCo‐pyrolysis synergy between cornstalk and bituminous coal was investigated based on the physicochemical properties of the char. The char was prepared by directly putting the individual or blended materials into a heated resistance furnace at 600°C or 800°C. The performance of surface morphology and mineral matters was not notably changed by co‐pyrolysis, indicated by scanning electron microscope and X‐ray diffraction. However, N2 and CO2 adsorption‐desorption isotherms confirmed that surface area and pore structure were strongly influenced, particularly in large micropores and mesopores. Ultimate analysis and inductively coupled plasma optical emission spectrometer measurement revealed excessive depositions of carbon, alkali, and alkaline‐earth metal species in co‐pyrolysis char. Energy‐dispersive X‐ray spectrometer analysis examined the elemental surface distribution, suggesting that co‐pyrolysis under moderate temperature strongly favored inhibiting potassium release. The non‐isothermal thermogravimetry‐differential scanning calorimeter experiments under air atmosphere were conducted for char combustion reactivity, which was expected to be instructive for deeper understanding about co‐pyrolysis synergy and reutilization of the char. The thermal behaviors during combustion of co‐pyrolysis char clearly resembled a single fuel, which completely varied from those of the char blends. Besides, co‐pyrolysis chars and char blends were superior in total heat release compared with the simple addition of calculated values from individual char. Based on these observations, key factors of the synergy were supposed to have consisted of free radical reaction, secondary char formation, and the migration of alkali and alkaline‐earth metal species.

Thermal History Induced Surface Restructuring of La0.6Sr0.4CoO3-δ thin-Film Solid Oxide Electrodes and Its Significance for the Catalytic Activity and Stability

Thin-films deposited by Pulsed Laser Deposition (PLD) have enabled the fabrication of cathodes for solid oxide cells (SOC) to possess various microstructures, crystalline states and surface chemistry. These films are often subjected to various processing and thermal treatments which influence the chemical activity and stability of the films. Here, we investigate how the catalytic activity and stability is affected by the thermally induced surface restructuring in complex transition metal oxides, in particular, La0.6Sr0.4CoO3-δ (LSC) thin films grown by PLD.(1) To this end, the influence of substrate temperature during PLD as well as post-annealing treatments on the film microstructure and surface chemistry were studied. In comparison to high-temperature grown films, the post-annealed low-temperature grown films showed 2-fold lower degradation rate in the long-term electrochemical tests at 500 °C. The differences in the electrochemical stability were found to be related to the initial microstructure and morphology of the films (Fig 1). The post-annealed low-temperature grown film showed localised Sr-segregation into protruding particles, leaving the remaining surface with catalytically active stoichiometry. On the other hand, the high-temperature grown films showed a fully covered surface with Sr-rich particles, which continued to thicken with time. Here we emphasise the importance of processing and thermal history in the elemental surface distribution, especially for the stability of LSC64 electrodes and propose that it should be considered as among the main pillars in the design of the active surface.Fig. 1 Top-view (a,d,g) and cross-section view (b,e,h) SEM micrographs and corresponding AFM images (c,f,i) showing the topography of LSC64 films deposited by the PLD: (a−c) LT-grown film, (d−f) postannealed LT-grown film, and (g−i) HT-grown film. (https://pubs.acs.org/doi/10.1021/acsami.0c08308) Reprinted with the permission of the American Chemical Society. Further permissions related to the material excerpted should be directed to the ACS.(1) Celikbilek, O.; Cavallaro, A.; Kerherve, G.; Fearn, S.; Chaix-Pluchery, O.; Aguadero, A.; Kilner, J. A.; Skinner, S. J.. ACS Appl. Mater. Interfaces 2020, 12, 34388−34401. Figure 1

Surface Restructuring of Thin-Film Electrodes Based on Thermal History and Its Significance for the Catalytic Activity and Stability at the Gas/Solid and Solid/Solid Interfaces.

Electrodes in solid-state energy devices are subjected to a variety of thermal treatments, from film processing to device operation at high temperatures. All these treatments influence the chemical activity and stability of the films, as the thermally induced chemical restructuring shapes the microstructure and the morphology. Here, we investigate the correlation between the oxygen reduction reaction (ORR) activity and thermal history in complex transition metal oxides, in particular, La0.6Sr0.4CoO3−δ (LSC64) thin films deposited by pulsed laser deposition. To this end, three ∼200 nm thick LSC64 films with different processing and thermal histories were studied. A variety of surface-sensitive elemental characterization techniques (i.e., low-energy ion scattering, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry) were employed to thoroughly investigate the cationic distribution from the outermost surface to the film/substrate interface. Moreover, electrochemical impedance spectroscopy was used to study the activity and the stability of the films. Our investigations revealed that, despite the initial comparable ORR activity at 600 °C, the degradation rates of the films differed by twofold in the long-term stability tests at 500 °C. Here, we emphasize the importance of processing and thermal history in the elemental surface distribution, especially for the stability of LSC64 electrodes and propose that they should be considered as among the main pillars in the design of active surfaces.

The environmental risk of storing vehicles over the soil

Automotive vehicles are a potential source of heavy metal contamination of the soil due to their lead-acid batteries and the use of metals in their structure and paint pigments. Understanding the characteristics of heavy metal soil contamination and identifying their environmental exposure provides important information for making decisions regarding remediation of contaminated soils. The objective of the present work was to evaluate metal contamination (Cd, Pb, Ni, Zn, Cu e Cr) in 35 soil samples (fraction<2 mm). The samples were collected from two unpaved/unsealed storage areas used by the Highway Patrol (Polícia Rodoviária Federal - PRF) for storing apprehended vehicles, one located in Araucária (AR) and one Colombo (CL), in the metropolitan region of Curitiba/PR, Brazil. Assays were performed to determine organic carbon (OC) and pH. For analysis of metals, the samples were prepared by acid digestion (method EPA 3050-B) and measured by ICP-OES. The results were evaluated according to Resolution CONAMA 420, geoaccumulation index (Igeo), contamination factor (CF) and Pearson correlation analysis. The textural composition of the soil at AR and CL reflect a high degree of heterogeneity and distinct chemical and physical characteristics. The AR site had a higher level of sand and a more acidic pH than CL, and concentrations of the analyzed elements between prevention and investigation values according to Resolution CONAMA 420/2009. The CL site had a higher concentration of clay and OC, and a less acidic pH than AR. Only one collected point at CL had a concentration of Cu above the prevention value. In both AR and CL, the element Cd was below detectability with methodology employed. The spatial distribution of the cars associated with the soil texture and the analyzed elements exhibited a random surface distribution of elements. The OC content, soil pH and texture associated with Igeo, CF and Pearson correlation suggests that both environments suffer an input of metals at different points. The input of heavy metals from stored vehicles and the possible anthropogenic impact on the soil is evident.

Properties of hens’ eggs after surface irradiation by nanosecond electron beam

There are presented some details of the technology of surface disinfection of the fresh eggs. Irradiation experiments were carried out by means of the pulsed repetitive nanosecond accelerator URT-0.5 (electron energy of up to 500 keV, a pulse duration of 50 ns, pulse repetition rate up to 200 pps, which significantly reduce the costs of the irradiation in comparison with high-energy electrons (several MeV) or γ-radiation.The determination of the distribution of the absorbed dose in the depth of the polyethylene was conducted by the gray wedge method. Measurement of the electron beam dose on the surface of the shell and under the shell, as well as beneath the absorber layer (polyethylene 80 μm thick) was also performed by a film dosimeter. The electron average dose rate on the egg surface was 200 Gy/pulse and the instantaneous dose rate was 4 GGy/s. The non-uniform distribution of the electron beam current density on the output foil was ±15%.Thermoluminescent dosimeters, TLD-500, were used to determine the distribution of the bremsstrahlung dose inside hens’ eggs. It has been established that for nanosecond electron beam irradiation with dose higher than 5 kGy sufficient for complete disinfection on the hens’ egg surface, due to bremsstrahlung, the dose inside it will not exceed 8 cGy. This dose value should not lead to biological changes in the white or yolk.Dose 5 kGy does not influence the observed shell surface; there are no changes on the egg surface according to scanning electron microscope data. For heavy doses, rounded areas of the surface relief are observed.Data from energy-dispersive X-ray spectroscopy analysis for high dose show a significant increase in the amount of phosphorus and especially calcium in the shell and some reduction in the amount of oxygen, most likely due to change in the surface distribution of elements in the shell.

Magnetic characterization and variability study of the magnetic SPB staroLupi

Thanks to large dedicated surveys, large-scale magnetic fields have been detected for about 10% of early-type stars. We aim to precisely characterize the large-scale magnetic field of the magnetic component of the wide binaryoLupi, by using high-resolution ESPaDOnS and HARPSpol spectropolarimetry to analyze the variability of the measured longitudinal magnetic field. In addition, we have investigated the periodic variability using space-based photometry collected with the BRITE-Constellation by means of iterative prewhitening. The rotational variability of the longitudinal magnetic field indicates a rotation periodProt = 2.95333(2) d and that the large-scale magnetic field is dipolar, but with a significant quadrupolar contribution. Strong differences in the strength of the measured magnetic field occur for various chemical elements as well as rotational modulation for Fe and Si absorption lines, suggesting a inhomogeneous surface distribution of chemical elements. Estimates of the geometry of the large-scale magnetic field indicatei = 27 ± 10°,β= 74−9+7°, and a polar field strength of at least 5.25 kG. The BRITE photometry reveals the rotation frequency and several of its harmonics, as well as two gravity mode pulsation frequencies. The high-amplitudeg-mode pulsation atf = 1.1057 d−1dominates the line-profile variability of the majority of the spectroscopic absorption lines. We do not find direct observational evidence of the secondary in the spectroscopy. Therefore, we attribute the pulsations and the large-scale magnetic field to the B5IV primary of theoLupi system, but we discuss the implications should the secondary contribute to or cause the observed variability.

Selected Tribological Properties of A390.0 Alloy

AbstractEmergence of new designs for internal combustion engines resulted in a necessity to search for new materials which will rise to excessive technological requirements under operating conditions of modern internal combustion engines of up to 150 kW. Focusing only on material properties, theoretically existing alloys should meet presents requirements. More importantly, existing materials are well fitted to the entire crank-piston system. Thus, there is a need for a more thorough examination of these materials. The paper presents studies on determination of coefficient of friction μ and wear for the A390.0 alloy modified with AlTi5B master alloy combined with EN GJL-350 cast iron. The characteristics of a T-11 tribological tester (pin on disc) used for the tests, as well as the methodology of the tribological tests, were described. Also, the analysis of the surface distribution of elements for the pin and the disc was presented. The studies were realized in order to find whether the analyzed alloy meets the excessive requirements for the materials intended for pistons of modern internal combustion engines. The results show that the A390.0 alloy can only be applied to a load of 1.4 MPa. Above this value was observed destructive wear, which results in the inability to use it in modern internal combustion engines.

Separation Process of Fine Coals by Ultrasonic Vibration Gas-Solid Fluidized Bed.

Ultrasonic vibration gas-solid fluidized bed was proposed and introduced to separate fine coals (0.5–0.125 mm fraction). Several technological methods such as XRF, XRD, XPS, and EPMA were used to study the composition of heavy products to evaluate the separation effect. Results show that the ultrasonic vibration force field strengthens the particle separation process based on density when the vibration frequency is 35 kHz and the fluidization number is 1.8. The ash difference between the light and heavy products and the recovery of combustible material obtain the maximum values of 47.30% and 89.59%, respectively. The sulfur content of the heavy product reaches the maximum value of 6.78%. Chemical state analysis of sulfur shows that organic sulfur (-C-S-), sulfate-sulfur (-SO4), and pyrite-sulfur (-S2) are confirmed in the original coal and heavy product. Organic sulfur (-C-S-) is mainly concentrated in the light product, and pyrite-sulfur (-S2) is significantly enriched in the heavy product. The element composition, phase composition, backscatter imagery, and surface distribution of elements for heavy product show concentration of high-density minerals including pyrite, quartz, and kaolinite. Some harmful elements such as F, Pb, and As are also concentrated in the heavy product.

Modelling of variability of the chemically peculiar starϕDraconis

Context: The presence of heavier chemical elements in stellar atmospheres influences the spectral energy distribution (SED) of stars. An uneven surface distribution of these elements, together with flux redistribution and stellar rotation, are commonly believed to be the primary causes of the variability of chemically peculiar (CP) stars. Aims: We aim to model the photometric variability of the CP star PHI Dra based on the assumption of inhomogeneous surface distribution of heavier elements and compare it to the observed variability of the star. We also intend to identify the processes that contribute most significantly to its photometric variability. Methods: We use a grid of TLUSTY model atmospheres and the SYNSPEC code to model the radiative flux emerging from the individual surface elements of PHI Dra with different chemical compositions. We integrate the emerging flux over the visible surface of the star at different phases throughout the entire rotational period to synthesise theoretical light curves of the star in several spectral bands. Results: The synthetic light curves in the visible and in the near-UV regions are in very good agreement with the observed variability of the star. The lack of usable far-UV measurements of the star precludes making any conclusions about the correctness of our model in this spectral region. We also obtained 194 new BVRI observations of PHI Dra and improved its rotational period to P=1.716500(2). Conclusions: We show that the inhomogeneous distribution of elements, flux redistribution, and rotation of the star are fully capable of explaining the stellar variability in the visible and the near-UV regions. The flux redistribution is mainly caused by bound--free transitions of silicon and bound--bound transitions of iron.

Behavior of Fe/nano-Si particles composite electrodeposition with a vertical electrode system in a static parallel magnetic field

Composite iron coatings containing nano-silicon particles were produced using a vertical electrode system in the presence of and the absence of magnetic field parallel to the current at 25°C. The surface morphology, the surface distribution of elements, the cathode current efficiency and the silicon particles content of coatings have been investigated respectively under the influence of magnetic field. Compared to the rather smooth surface morphology of Fe-Si composite coating without magnetic field, it became wavy and striped obviously with imposing magnetic field, meanwhile, the surface roughness Ra in the right region of composite coatings was significantly larger than that in the left region due to the existence of more needlelike iron deposits. Furthermore, silicon particles dispersed uniformly on the surface of coating when no magnetic field was applied, whereas silicon particles connected gradually to form the distribution of some stripes under the influence of magnetic field. Meanwhile, the silicon particles content of coatings presented a trend of increasing with increasing the magnetic flux density. Finally, a possible mechanism has been put forward to illustrate those phenomena in this paper.

Ultraviolet and visual flux and line variations of one of the least variable Bp stars HD 64740

The light variability of hot magnetic chemically peculiar stars is typically caused by the flux redistribution in spots with peculiar abundance. This raises the question why some stars with surface abundance spots show significant rotational light variability, while others do not. We study the Bp star HD 64740 to investigate how its remarkable inhomogeneities in the surface distribution of helium and silicon, and the corresponding strong variability of many spectral lines, can result in one of the faintest photometric variabilities among the Bp stars. We used model atmospheres and synthetic spectra calculated for the silicon and helium abundances from surface abundance maps to predict the ultraviolet and visual light and line variability of HD 64740. The predicted fluxes and line profiles were compared with the observed ones derived with the IUE, HST, and Hipparcos satellites and with spectra acquired using the FEROS spectrograph at the 2.2m MPG/ESO telescope. We are able to reproduce the observed visual light curve of HD 64740 assuming an inhomogeneous distribution of iron correlated with silicon distribution. The light variations in the ultraviolet are hardly detectable. We detect the variability of many ultraviolet lines of carbon, silicon, and aluminium and discuss the origin of these lines and the nature of their variations. The maximum abundances of helium and silicon are not high enough to cause significant light variations. The detected variability of many ultraviolet lines is most likely of atmospheric origin and reflects the inhomogeneous elemental surface distribution. The variability of the Civ resonance lines is stronger and it probably results from the dependence of the wind mass-loss rate on the chemical composition and magnetic field orientation. We have not been able to detect a clear signature of the matter trapped in the circumstellar clouds.

In vitro biological response of plasma electrolytically oxidized and plasma‐sprayed hydroxyapatite coatings on Ti–6Al–4V alloy

Plasma electrolytic oxidation (PEO) is a relatively new surface modification process that may be used as an alternative to plasma spraying methods to confer bioactivity to Ti alloy implants. The aim of this study was to compare physical, chemical and biological surface characteristics of two coatings applied by PEO processes, containing different calcium phosphate (CaP) and titanium dioxide phases, with a plasma-sprayed hydroxyapatite (HA) coating. Coating characteristics were examined by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, surface profilometry, and wettability tests. The biological properties were determined using the human osteoblastic cell line MG-63 to assess cell viability, calcium and collagen synthesis. The tests showed that PEO coatings are significantly more hydrophilic (6%) and have 78% lower surface roughness (Ra) than the plasma-sprayed coatings. Cell behavior was demonstrated to be strongly dependent on the phase composition and surface distribution of elements in the PEO coating. MG-63 viability for the TiO2 -based PEO coating containing amorphous CaPs was significantly lower than that for the PEO coating containing crystalline HA and the plasma-sprayed coating. However, collagen synthesis on both the CaP and the TiO2 PEO coatings was significantly higher (92% and 71%, respectively) than on the plasma-sprayed coating after 14 days. PEO has been demonstrated to be a promising method for coating of orthopedic implant surfaces.

Morphology of actinide-rich particles released from the BOMARC accident and collected from soil post remediation

The physical, chemical, and radiological characteristics of material released to the environment from accidents involving nuclear weapon components are dependent upon many factors, especially the manner in which the material is released and delivered to the environment. These characteristics will also be influenced by physical and chemical effects associated with weathering if the material remains exposed to the environment for a long period of time. This study evaluates the morphological characteristics of particles released to the environment as a result of the 1960 BOMARC incident and compares these characteristics to those described following similar incidents at Thule, Greenland (1968) and Palomares, Spain (1966). Each of these incidents involved unique circumstances and conditions that distributed actinide-rich particles to the environment with a range of distinctive morphological characteristics. Morphological and surface elemental analyses were conducted on a set of discrete particles isolated from samples of post-remediated soil collected at McGuire Air Force Base, the site of the BOMARC incident. Scanning electron microscopy and complimentary energy dispersive X-ray spectroscopy were used to perform the analyses. Non-destructive analysis of uranium and plutonium contained in each particle was measured using high-resolution gamma spectrometry. Unique characteristics of the BOMARC samples include some particles exhibiting a smooth, crystalline texture and varying elemental surface distribution of uranium and plutonium, dependent on the particle’s morphology.

Magnetism, chemical spots, and stratification in the HgMn starϕ Phoenicis

Mercury-manganese (HgMn) stars have been considered as non-magnetic and non-variable chemically peculiar (CP) stars for a long time. However, recent discoveries of the variability in spectral line profiles have suggested an inhomogeneous surface distribution of chemical elements in some HgMn stars. From the studies of other CP stars it is known that magnetic field plays a key role in the formation of surface spots. All attempts to find magnetic fields in HgMn stars have yielded negative results. In this study, we investigate the possible presence of a magnetic field in phi Phe (HD 11753) and reconstruct surface distribution of chemical elements that show variability in spectral lines.We also test a hypothesis that a magnetic field is concentrated in chemical spots and look into the possibility that some chemical elements are stratified with depth in the stellar atmosphere. Our analysis is based on high-quality spectropolarimetric time-series observations, covering a full rotational period of the star. Spectra were obtained with the HARPSpol at the ESO 3.6-m telescope. Combining information from all suitable spectral lines, we set an upper limit of 4 G on the mean longitudinal magnetic field. For chemical spots, an upper limit on the longitudinal field varies between 8 and 15 G. We confirmed the variability of Y, Sr, and Ti and detected variability in Cr lines. Stratification analysis showed that Y and Ti are not concentrated in the uppermost atmospheric layers. Our spectropolarimetric observations rule out the presence of a strong, globally-organised magnetic field in phi Phe. This implies an alternative mechanism of spot formation, which could be related to a non-equilibrium atomic diffusion. However, the typical time scales of the variation in stratification predicted by the recent time-dependent diffusion models exceed significantly the spot evolution time-scale reported for phi Phe.

Modelling of the ultraviolet and visual SED variability in the hot magnetic Ap star CU Virginis

The spectral energy distribution (SED) in chemically peculiar stars may be significantly affected by their abundance anomalies. The observed SED variations are usually assumed to be a result of inhomogeneous surface distribution of chemical elements, flux redistribution and stellar rotation. However, the direct evidence for this is still only scarce. We aim to identify the processes that determine the SED and its variability in the UV and visual spectral domains of the helium-weak star CU Vir. We used the model atmospheres to obtain the emergent flux and predict the rotationally modulated flux variability of the star. We show that most of the light variations in the vby filters of the Stromgren photometric system are a result of the uneven surface distribution of silicon, chromium, and iron. Our models are only able to explain a part of the variability in the u filter, however. The observed UV flux distribution is very well reproduced, and the models are able to explain most of the observed features in the UV light curve. The variability observed in the visible is merely a faint gleam of that in the UV. While the amplitude of the light curves reaches only several hundredths of magnitude in the visual domain, it reaches about 1 mag in the UV. The visual and UV light variability of CU Vir is caused by the flux redistribution from the far UV to near UV and visible regions, inhomogeneous distribution of the elements and stellar rotation. Bound-free transitions of silicon and bound-bound transitions of iron and chromium contribute the most to the flux redistribution. This mechanism can explain most of the rotationally modulated light variations in the filters centred on the Paschen continuum and on the UV continuum of the star CU Vir. However, another mechanism(s) has to be invoked to fully explain the observed light variations in the u filter and in the region 2000-2500 A.