Chemical composition and stratification of chemical elements in the atmosphere of the Ap star HD 8441

We present the results of our abundance analysis for the atmosphere of the chemically peculiar Ap star HD 8441. HD 8441 is interesting in that the lines of rare-earth elements in its spectrum are weak and the longitudinal magnetic field component is small, being only several hundred gauss. Our estimates of the evolutionary status for HD 8441 have confirmed that it belongs to the group of Ap stars with weak lines of rare-earth elements in their spectra that leave the main sequence. A stratification analysis of the atmosphere of HD 8441 assuming a stepwise distribution of elements with depth reveals a nonuniform distribution of Si, Ca, Cr,Mn, and Fe with a significant increase in elemental abundances in deeper layers. The derived distribution qualitatively agrees with predictions of the theory of diffusive separation of elements under the action of radiation pressure forces and gravity. Comparison of the chemical composition and evolutionary status of HD 8441 with those of the stars HD 66318 and HD 144897 with strong magnetic fields shows that their atmospheres differ mainly by the abundances of rare-earth elements. The iron-peak elements exhibit large overabundances irrespective of the magnetic field strength.     

Temperature behavior of elemental abundances in the atmospheres of magnetic peculiar stars

We analyze the temperature dependence of the abundances of the chemical elements Si, Ca, Cr, and Fe in the atmospheres of normal, metallic-line (Am), magnetic peculiar (Ap), and pulsating magnetic peculiar (roAp) stars in the range 6000–15000 K. The Cr and Fe abundances in the atmospheres of Ap stars increase rapidly as the temperature rises from 6000 to 9000–10000 K. Subsequently, the Cr abundance decreases to values that exceed the solar abundance by an order of magnitude, while the Fe abundance remains enhanced by approximately +1.0 dex compared to the solar value. The temperature dependence of the abundances of these elements in the atmospheres of normal and Am stars is similar in shape, but its maximum is several orders of magnitude lower than that observed for Ap stars. In the range 6000–9500 K, the observed temperature dependences for Ap stars are satisfactorily described in terms of element diffusion under the combined action of gravitational settling and radiative acceleration. It may well be that diffusion also takes place in the atmospheres of normal stars, but its efficiency is very low due to the presence of microturbulence. We show that the magnetic field has virtually no effect on the Cr and Fe diffusion in Ap stars in the range of effective temperatures 6000–9500 K. The Ca abundance and its variation in the atmospheres of Ap stars can also be explained in terms of the diffusion model if we assume the existence of a stellar wind with a variable moderate rate of ～(2–4) × 10^{? 15}M_⊙ yr^?1.     

Superheavy elements in the atmospheres of magnetic stars

The studies of the abundances of superheavy chemical elements in magnetic stars conducted at the Crimean Astrophysical Observatory were facilitated by the launch of the Astron space station in 1983. This spacecraft observed the ultraviolet spectra of such stars. The present brief review is focused on the abundances of superheavy elements (Pt, Au, Hg, Tl, Pb, Bi, Th, and U) in the atmospheres of magnetic Ap and HgMn stars. These results were obtained basing on the visible and ultraviolet spectra of stars. The data accumulated over more than 30 years show that these stars are characterized by significant overabundances (up to 6–7 dex) of such elements. The following important fact is noted: the superheavy element anomalies follow the trend in the anomalies that are characteristic of less heavy elements. Therefore, it may be assumed that all these anomalies (including the significant overabundances of superheavy elements) share a common explanation. Certain unresolved problems are discussed briefly.     

Abundance analysis and searching for nonradial pulsations in the atmosphere of the chemically peculiar star HD 115708

We present the results of our abundance analysis for the magnetic chemically peculiar star HD 115708 based on high-resolution spectra. The atmospheric chemical composition of HD 115708 (T _eff = 7550 K) is shown to be typical of cool Ap stars with a significant ionization disequilibrium for the first and second rare earth ions, which is commonly observed in the atmospheres of pulsating Ap (roAp) stars. Our study of the vertical distribution of elements has shown that Mg, Ca, Cr, and Fe concentrate in deeper atmospheric layers, with their abundances decreasing sharply in the upper layers. The jumps in abundance are 1.5–3 orders of magnitude. Silicon is distributed in depth almost uniformly in the atmosphere of HD 115708. The derived empirical Cr and Fe distributions agree qualitatively with the results of diffusion calculations. Since the atmospheric chemical peculiarities in HD 115708 correspond to roAp stars, we have performed a spectroscopic monitoring to find nonradial pulsations. We have been able to determine only an upper limit for the amplitude of the possible radial velocity pulsations, ～100 m s^?1, due to the insufficient temporal resolution and instability of the main stellar spectrograph (MSS) of the 6-m telescope.     

Nuclear and non-nuclear processes in the production of peculiar A stars

Recent theories of the origin of the abundance anomalies on Ap stars are reviewed. The abundance anomalies are mainly the results of supernova explosions of companion stars and magnetic accretion from the supernova remnants and the interstellar medium. Large rare-earth excesses indicate that magnetic accretion has taken place, while large excesses of Pt and Hg are due to the transfer of material from nearby supernovae. Nuclear reactions involving protons may also have taken place around Ap stars. There are several objections to Michaud's diffusion theory as a comprehensive explanation for Ap abundance anomalies.     

Rusian text ignored

The published data on the abundance determinations for magnetic Ap-stars were considered critically. The traditional methods of curve of growth were shown to be unsufficient for quantitative analysis of the atmospheres of magnetic Ap-stars due to inhomogeneous structure of the surface of these stars. Only qualitative results can be obtained by these methods. Correlations between overabundances of different elements are considered. Very distinct correlations exist between overabundances of Mn and Zr, between Fe and rare earths and others. No correlation could be found between overabundance of elements and T_eff. The methods of determination of local abundances and the results obtained — overabundances of some elements in spots and underabundances of them out of spots - are considered. The significanse of these data for the theory of chemical anomalies in magnetic Ap stars are mentioned.     

Chemical composition of the He-w stars HD 37058, 212454, and 224926

Our spectrophotometric analysis of the atmospheres of HD 37058, HD 212454, and HD 224926 shows these objects to be typical He-w stars with close-to-zero microturbulence velocities, very different magnetic fields, and wide scatter of chemical anomalies. However, one of the main manifestations of separation is that helium moves from the outer layers of the atmosphere into the star’s interior.Our analysis of the stars HD 212454 and 224926 with Be<100 G shows that despite their weak magnetic fields they have the same degree of chemical anomaly as highly magnetized stars. Chemical composition varies over a wide range for stars with the same magnitude of magnetic field. We find the conditions in the temperature interval 13000–16000 K to be the most favorable for the formation of He-w type stars. Helium underabundance is the strongest near the maximum of the distribution and it is observed in stars with weak as well as strong fields. Because of the scatter mentioned above the degree of chemical anomalies is not strictly related to the magnitude of the magnetic field, although the field has an appreciable effect on the formation of chemical inhomogeneities at the star’s surface. Its influence is minimal in stars with very weak magnetic fields and the presence of strong chemical anomalies indicates that microturbulence in these stars is sufficiently weak even without the effect of the magnetic field. It is plausible to assume that the anomalies arise due to slow rotation.The temperature dependences of rotation velocity vsini for stars with weak magnetic fields show no apparent trends associated with the magnitude of magnetic field. The rotation velocities vsini of almost all stars are lower than those of normal stars, except for HD 131120, 142096, 142990, and 143669, which rotate with the same velocity or even faster than normal stars. These objects do not obey the general rule and their example shows that stable atmospheres can also be found among fast rotators and that magnetic field takes no part in the spin-down of CP stars. We believe that CP stars inherited their slow rotation from protostellar clouds.     

UV line profiles as a probe for atomic diffusion theory in stellar atmospheres

In quiet stellar atmospheres that are stabilized, for example, by the presence of strong magnetic fields, a non-uniform distribution of chemical elements (abundance stratification) is predicted by the diffusion theory. Spectroscopic observations of magnetic Ap stars provide a strong support for the diffusion theory. Stratification predictions in the upper atmosphere, in particular, strongly depend on the magnetic field geometry. Optical lines used in stratification analysis of Ap stars do not probe the upper atmospheric layers and, hence, cannot provide reliable information on abundance gradients there. We show the expected effect of stratification on the profiles of strong lines in the UV region and how these lines may be used for constraining diffusion theory.     

A search for magnetic fields in cool sdB stars

Hot cluster horizontal branch (HB) stars and field subdwarf B (sdB) stars are core helium burning stars that exhibit abundance anomalies that are believed to be due to atomic diffusion. Diffusion can be effective in these stars because they are slowly rotating. In particular, the slow rotation of the hot HB stars (T_eff > 11000 K), which show abundance anomalies, contrasts with the fast rotation of the cool HB stars, where the observed abundances are consistent with those of red giants belonging to the same cluster. The reason why sdB stars and hot HB stars are rotating slowly is unknown. In order to assess the possible role of magnetic fields on abundances and rotation, we investigated the occurrence of such fields in sdB stars with T_eff < 30 000 K, whose temperatures overlap with those of the hot HB stars. We conclude that large‐scale organised magnetic fields of kG order are not generally present in these stars but at the achieved accuracy, the possibility that they have fields of a few hundred Gauss remains open. We report the marginal detection of such a field in SB 290; further observations are needed to confirm it (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The atmospheres of helium-deficient Bp stars

Based on spectra taken with a 6-m telescope, we analyzed the abundances of chemical elements in the He-weak stars HD 21699 and HD 217833, estimated their surface magnetic fields (B_s = 4000 and 4500 G, respectively) from the magnetic intensification of spectral lines, and determined their microturbulences (V _t = 0.80 and 0.75 km s⁻¹, respectively). The low values of V _t show that the stellar atmospheres are stabilized by a magnetic field, which explains the presence of diffusion processes that lead to chemical anomalies. Helium is strongly underabundant, and its deficiency is −1.50 and −1.81 dex in HD 21699 and HD 217833, respectively. We used model atmospheres to determine the effective temperatures, T _eff = 16 000 and 15 450 K, and surface gravities, log g = 4.15 and 3.88, for the stars from the Hδ line, implying that they lie on the main sequence near the stars of luminosity class V.     

RARE EARTH ELEMENTS IN Ca-PHOSPHATES OF ALLENDE CARBONACEOUS CHONDRITE

The Ca-phosphate phases in the Allende CV3 meteorite were selectively dissolved in ammoniacal EDTA solution and measured for abundances of the rare earth elements (REE) by radiochemical neutron activation and mass-spectrometric isotope dilution analyses. The REE abundances in CA-phosphates of Allende are remarkably different from those of ordinary chondrites. All the REE except Eu were observed to be enriched by factors of 50–100 relative to the C1 values. This is 3–4 times lower than concentrations of REE in the ordinary-chondrite phosphates. Allende phosphates have a small positive Eu anomaly, in contrast to the large negative Eu anomaly in phosphates from ordinary chondrites. Though the positive Eu anomaly in Allende Ca-phosphates is puzzling, the lack of a negative Eu anomaly in Allende Ca-phosphates suggests that they never have been in equilibrium with Allende coarse-grained Ca, Al-rich inclusions or their precursor materials.     

Helium in stellar atmospheres

Helium, which was first discovered on the sun with the help of spectral analysis, plays, together with hydrogen, a principal role in astrophysics. We consider here two fundamental quantities: primordial helium abundance formed during Big Bang nucleosynthesis and the current initial helium abundances in nearby stars. It is shown that stellar atmospheres are enriched in helium during the main-sequence stage. Observational evidence for helium contamination in close OB-binaries is discussed. Stars with strong abundance anomalies are considered, such as chemically peculiar Ap and Bp helium-deficient stars and some types of objects with helium atmospheres.     

Chemical composition and evolutionary status of the Ap star HD 138633

We present the results of our study of the atmospheric chemical composition and evolutionary status for the chemically peculiar Ap star HD 138633. In contrast to ordinary Ap stars that exhibit strong lines of rare-earth elements in their spectra, these elements are represented very poorly in the spectrum of HD 138633. The magnetic field is estimated to be less than 700 G, which is also atypical of peculiar Ap stars. We have detected a stratification of such elements as Fe, Si, Ca, and Y in the atmosphere of HD 138633 whose pattern agrees qualitatively with the predictions of the theory of diffusive separation of elements under the joint action of gravity and radiation pressure forces. The nonuniform distribution obtained by assuming a stepwise distribution of elements in depth agrees qualitatively with the stratification distribution for β CrB. The search for pulsations points to the possible existence of lowamplitude pulsations with a period of about 17 min.     

A survey of the selenochemistry of major,minor and trace elements

Average data for igneous and/or metaigneous rocks and soils from seven lunar sites are presented. There are compositional similarities between Apollo 11 and Luna 16 eastern maria, Ap 12 and 15 western maria and between Ap 16 and L 20 highlands. Subtle differences do exist between the paired mare sites and the two highland sites and striking differences between the eastern and western maria. Chondritic normalized REE (rare earth element) patterns for igneous rocks and soils from all sites range from 7-350 generally with negative Eu anomalies. Anorthositic gabbroes to anorthosites, presumably highland material, exhibit a positive Eu anomaly. The REE patterns or Sr isotopic ratios suggest two lava flows each for the L 16 and Ap 14 sites, at least four lava flows for the Ap 11 and 12 site and about six for the Ap 15 site. Paucity of lunar andesites suggests rather limited lunar chemical differentiation. Norite-KREEP is a prominent component at Ap 12, 14 and 15, less at Ap 11 and 16 and L 16 and apparently very low at the L 20 highland site. Derivation of lunar soils can be best explained using multi-component mixing systems. Characterization of meteoritic impacting bodies is also observed in addition to a steady state veil of 1.9% carbonaceous Cl like material in soils. Interelement correlations impose constraints on the primitive composition of the Moon and on magmatic processes like selective volatilization.Paper dedicated to Prof. Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.     

Eu III oscillator strengths and europium abundances in Ap stars

We present our calculations of the spectrum and oscillator strengths for the 4f⁷?(4f⁶5d+4f⁶6s) Eu III transitions. The calculations were performed with Cowan's RCN-RCG-RCE codes in the single-configuration approximation. A comparison of computed level lifetimes with experimental data for three levels shows that the scale of theoretical oscillator strengths could be overestimated by a factor of 3. The theoretical oscillator strengths of red Eu III lines are two orders of magnitude smaller than their astrophysical oscillator strengths derived by Ryabchikova et al. (1999) from the condition of ionization balance. The new oscillator strengths were tested by analyzing the Eu abundance using Eu II and Eu III lines in the spectra of hot peculiar stars (α² CVn is a typical representative) and cool peculiar stars (β CrB is a typical representative). First, we computed non-LTE corrections, which proved to be significant for α² CVn. We also analyzed the Eu II λ6645.11-Å line as well as ultraviolet and optical Eu III lines. We show that the new oscillator strengths together with the non-LTE corrections allow the contradiction between the Eu abundances derived by Ryabchikova et al. (1999) separately from optical Eu II and Eu III lines in α² CVn to be resolved. The new Eu abundance, log(Eu/N _tot)=?6.5, also faithfully describes the blended near-ultraviolet resonance Eu III lines. Using the new Eu III oscillator strengths to analyze the spectrum of the cool Ap star β CrB, we found a significant deviation of the n(Eu II)/n(Eu III) ratio from its equilibrium value. For a chemically homogeneous model atmosphere, to obtain the observed intensity of the Eu III λ 6666.35-Å line, the Eu abundance must be increased by two orders of magnitude compared to that required to describe the Eu II λ 6645.11-Å line. We discuss the possibility of explaining the observed intensities of Eu II and Eu III lines in the spectrum of β CrB by the presence of an inhomogeneous atmosphere with Eu concentrated in its uppermost layers. In such atmospheres, the role of non-LTE effects becomes dominant.     

Abundance analysis of the cool extreme helium star LSS 3378

Abundance analysis of the cool extreme helium (EHe) star LSS 3378 is presented. The abundance analysis is done using local thermodynamic equilibrium (LTE) line formation and LTE model atmospheres constructed for EHe stars.
The atmosphere of LSS 3378 shows evidence of H-burning, He-burning, and s -process nucleosynthesis. The derived abundances of iron peak and α-elements indicate the absence of selective fractionation or any other processes that can distort chemical composition of these elements. Hence, the Fe abundance [log ε(Fe) = 6.1] is adopted as an initial metallicity indicator. The measured abundances of LSS 3378 are compared with those of R Coronae Borealis (RCB) stars and with rest of the EHe stars as a group.     

A Study on the Abundance Distribution of Heavy Elements in Metal-poor Stars by the Parameterization Method

Based on a large amount of observed data of element abundances in metal-poor stars, taking the abundance distribution of heavy elements in the solar system as a standard, and selecting Sr, Ba and Eu as the typical elements of the three nucleosynthetic processes in metal-poor stars, namely the weak sprocess, main s-process and r-process, we have studied the contributions of the three kinds of neutron-capture processes to the abundance distribution of heavy elements in metal-poor stars, with the parameterization method. It is found that the higher the metal abundance, the greater the contributions of the weak s-process and the chief s-process to the abundances of lighter neutron-capture elements. The heavier neutron-capture elements are mainly produced by the r-process and the chief s-process; and that at low metallicity, the abundances of heavy neutron-capture elements are mainly produced by the r-process. In the early Galaxy, the weak s-process has almost no contribution to the element abundance.     

Magnetic field and chemical composition of the peculiar star HD 10221

We analyzed the chemical composition of the chemically peculiar (CP) star HD 0221=43 Cas using spectra taken with the NES spectrograph of the 6-m telescope with a spectral resolution of 45 000. The Hβ line profile corresponds most closely to T_eff = 11 900 K and log g = 3.9. The rotational velocity is v_e sin i = 27 ± 2 km s^?1, and the microturbulence is ξ_t = 1 km s^?1. The results of our abundance determination by the method of synthetic spectra show that the star has chemical anomalies typical of SrCrEu stars, although its effective magnetic field is weak, B_e < 100 G. For silicon, we obtained an abundance distribution in atmospheric depth with a sharp jump of 1.5 dex at an optical depth of log τ₅₀₀₀ = ?0.3 and with silicon concentration in deep atmospheric layers. Similar distributions were found in the atmospheres of cooler stars with strong and weak magnetic fields. A comparison of the chemical peculiarities in HD 10221 with known CP stars with magnetic fields of various strengths leads us to conclude that a low rotational velocity rather than amagnetic field is the determining factor in the formation mechanism of chemical anomalies in the atmospheres of CP stars.     

Magnetic activity optimal tracers: from radio to X-ray; the relevance of UV astronomy

Observations and analysis of magnetic activity phenomena in the atmospheres of cool stars—e.g., active regions, flares, stellar cycles—give insight into the fundamental processes in the heating of chromospheres, transition regions (TRs), and coronae. Diagnostics of magnetic activity can be found throughout the whole electromagnetic spectrum; from radio wavelengths, where gyrosynchrotron radiation arises from the quiescent and flaring corona, to optical, where important signatures are the Balmer lines and the Ca ii IRT and H&K lines, eventually to UV and X-rays, the latter mainly due to coronal thermal plasma. The UV and EUV ranges contains a plethora of emission lines that are powerful diagnostics for the warm (10?000 K) chromospheres, hot (100?000–800?000 K) TRs and very hot (1–10 MK) coronae. Also very weak coronal winds from cool stars have been identified and characterized thanks to high resolution UV spectra. Here I review the main results from UV observations of cool stars atmospheres and outline what can be expected from future UV imaging and spectroscopy measurements.     

Main-sequence magnetic CP stars: II. Physical parameters and chemical composition of the atmosphere

This paper continues a series of reviews dedicated to magnetic CP stars. The occurrence frequency of CP stars among B5–F0-type main-sequence stars is shown to be equal to about 15–20%. The problems of identification and classification of these objects are addressed. We prefer the classification of Preston, which subdivides chemically peculiar stars into the following groups: Am, λ Boo, Ap/Bp, Hg-Mn, He-weak, and He-strong stars. The main characteristic features of objects of each group are briefly analyzed. The rotation velocities of CP stars are shown to be about three times lower than those of normal stars of the same spectral types (except for λ Boo and He-strong objects). The rotation periods of CP stars range from 0.5 to 100 days, however, there is also a small group of objects with especially long (up to several tens of years) variability periods. All kinds of peculiar stars can be found in visual binaries, with Am-and Hg-Mn-type stars occurring mostly in short-period binaries with P < 10 days, and the binary rate of these stars is close to normal. The percentage of binaries among magnetic stars (20%) is lower than among normal stars. A rather large fraction of CP1-and CP2-type stars was found to occur in young clusters (with ages smaller than 10⁷ years). Photometric and spectral variability of peculiar stars of various types is discussed, and it is shown that only objects possessing magnetic fields exhibit light and spectral variations. The chemical composition of the atmospheres of CP stars of various types is considered. The abundances of various elements are usually determined by comparing the line profiles in the observed spectrum with those of the synthetic spectra computed for various model atmospheres. Different mechanisms are shown to contribute to chemical inhomogeneity at the star’s surface, and the hypothesis of selective diffusion of atoms in a stable atmosphere is developed. Attention is also paid to the problems of the determination of local chemical composition including the stratification of elements. Some of the coolest SrCrEu peculiar stars are found to exhibit fast light variations with periods ranging from 6 to 15 min. These variations are unassociated with rotation, but are due to nonradial pulsations. The final part of the the review considers the fundamental parameters of CP stars. The effective temperatures, luminosities, radii, and masses of these objects are shown to agree with the corresponding physical parameters of normal main-sequence stars of the same spectral types.