Spot system and stratification of some chemical elements in the atmosphere of He-weak star HD 21699

Surface helium and silicon distribution of a He-weak CP star HD21699 was examined using the spectra obtained at different phases of the star’s rotational period. The abundance of helium over the whole surface is weak, but in the magnetic polar regions it is strong, apparently due to the impact of stellar wind. Silicon accumulates in the region with a horizontal alignment of the magnetic lines, as follows from the theory. The abundance of helium and silicon increases with depth. Moreover, the boundary of the abundance variation is very sharp and its depth practically does not change with the rotational period phase. Accounting for the stratification of chemical elements considerably changes the deduced temperature distribution with depth as well as the form of the hydrogen line profile.     

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.     

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.     

Spectrophotometry of HD 182255

We investigate the HD 182255, a star earlier suspected to be helium-weak. We show it to be a SiHe-w-type CP object. Our study revealed four regions with anomalous elemental abundances on the stellar surface near the equator of rotation. Its magnetic field could not be measured, apparently due to the unfavorable orientation of the star. The star is seen almost (rotation) pole on, the inclination of its rotation axis is i = 12°. The magnitude of HD 182255 varies as a result of its rotation owing to the nonuniform distribution of chemical elements on its surface, which leads to the variable blanketing effect. The helium and silicon abundances increase with depth, i.e., the stratification typical of CP stars is observed. The star undergoes nonradial pulsations and nonstationary variations of the structure of its upper atmosphere.     

Composite model for the magnetic field of HD 21699

A model is constructed for the magnetic field of the He-weak CP star HD 21699. This star has the field structure of a dipole displaced by 0.4 radii from the center perpendicular to its axis. The magnetic poles appear to be close to one another on the surface; they are separated by 55°, not by 180° as in the case of a central dipole. The phase dependences of the equivalent widths of He and Si lines have extrema at the phases corresponding to passage through the visible meridian of zero magnetic field between the magnetic poles. At the magnetic poles, the intensity of the helium lines is maximal and of the silicon lines, minimal. The silicon abundance is maximal in the regions where the magnetic field is predominantly tangential to the star’s surface. Because of averaging over the visible hemisphere and owing to the closeness of the magnetic poles, only one wave of variation in the intensity of the spectral lines of these chemical elements, one wave of photometric variability, and an average surface magnetic field Bs are observed. __________ Translated from Astrofizika, Vol. 50, No. 3, pp. 441–451 (August 2007).     

Detailed photospheric abundances of 28 Peg and HD 202240

The atmospheric parameters and chemical abundances of two neglected A-type stars, 28 Peg and HD 202240, were derived using high resolution spectra obtained at the TÜBİTAK National Observatory. We determined the photospheric abundances of 11 elements for 28 Peg and 20 for HD 202240, using equivalent-width measurement and spectral synthesis methods. Their abundance patterns are in good agreement with those of chemically normal A-type stars having similar atmospheric parameters. We pinpoint the position of these stars on the H-R diagram and estimate their masses and ages as; 2.60 ± 0.10 M_⊙ and 650 ± 50 Myr for 28 Peg and 4.50 ± 0.09 M_⊙ and 150 ± 10 Myr for HD 202240. To compare our abundance determinations with those of stars having similar ages and atmospheric parameters, we select members of open clusters. We notice that our target stars exhibit similar abundance patterns with these members.     

Surface mapping in the CP4 star HD 21699

Spectra obtained on photographic plates with the Coudé-spectrograph of the 2-m telescope of BNAO - Rozhen have been used to map the helium surface distribution of the He-w star HD 21699. One-spot model has been determined to fit the observed periodic variations of the equivalent widths. The results show one large He-weak spot situated around the positive magnetic pole.     

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.     

A peculiar metal-rich star, HD 135485

Local thermodynamic equilibrium (LTE) absolute and differential abundances are presented for a peculiar metal-rich B-type star, HD 135485. These suggest that HD 135485 has a general enrichment of ∼0.5 dex in all the metals observed (C, N, O, Ne, Mg, Al, Si, P, S, Cl, Ar, Sc, Ti, Cr, Mn, Fe and Sr), except for nickel. The helium enhancement and hence hydrogen deficiency can account for ≤ 0.2 dex of this enhancement of metals, with the additional enhancement probably being representative of the progenitor gas. However, some of the metals appear to have greater enhancements, which may have occurred during the star's evolution. The significantly larger nitrogen abundance coupled with a modest helium enhancement observed in HD 135485 indicates that carbon–nitrogen (CN) processed material has possibly contaminated the stellar surface. Neon and carbon enhancements may indicate that helium core flashes have also occurred in HD 135485. Some of the iron-group elements (viz. Mn and Ni) appear to have similar abundance patterns to that of silicon Ap stars, but it is uncertain how these abundance patterns formed if they were not present in the progenitor gas. From a kinematical investigation it is unclear whether this star formed in a metal-rich region as implied by its chemical composition. From its position in the Hertzsprung–Russell diagram, HD 135485 would appear to be an evolved star lying close to or on the horizontal branch.     

A model atmosphere analysis of HD 25823

An abundance analysis using photographic region spectrograms and fully line-blanketed model atmospheres has been performed for the chemically peculiar (CP) star HD 25823 model parameters were determined from H profile and photometric data. Its pattern of abundance shows that the heavier elements in HD 25823 are overabundant.     

Photometric and spectroscopic study of silicon Ap-star HD193722

The period of light variationP=1 _. ^d 13316 has been found for the silicon B9 IVp star HD 193722. Spectroscopic study of this star was based on 35 spectrograms with dispersion 4 Å mm^–1 well distributed in phase. The measurements of radial velocities of spectral line components for SiII, HeI, EuII, FeII and SrII allowed us to localize several regions on the surface of the star with enhanced abundance of these elements. The phase of maximum light inU, B andV was found to be the same as the phase of maximum Eu abundance. The coincidence of the regions with larger abundance of Si and He in HD 193 722 disagree with the hypothesis of diffusion in the presence of a magnetic field developed by Michaud (1971), to explain the peculiar chemical composition of Ap-stars.HD 193722 is a silicon B9 IVp star for which the magnetic field has not been measured. In the list by Palmeret al. (1962) its rotational velocityV sini is given as 250 kms^–1. As will be seen below, this value is too high. Megessier (1971) determined from hydrogen line profiles and continuous spectrumT _eff=13 000° and lgg=3.5.The results of photometric and spectroscopic study of HD193722 are given below.     

贫金属星HD203608和HD211998的重元素丰度分布

根据贫金属星个别重元素丰度的观测值和太阳系重核素的ｒ过程和ｓ过程分量的丰度分布,计算了贫金属星ＨＤ２０３６０８和ＨＤ２１１９９８的重元素丰度并与观测进行了比较。结果表明,太阳系纯ｒ过程和ｓ过程元素丰度均不能拟合出这二颗样品星的丰度观测值,而应同时考虑ｒ过程和ｓ过程的贡献     

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.     

The evolutionary state of magnetic Ap stars

The characteristics of two stars, 25 Sex and HD 21699, as additional candidates for the sample of magnetic stars belonging to superclusters, are discussed. For 25 Sex, which was already accepted as a probable member of the Hyades supercluster in a previous study, arguments supporting the view that this star indeed is a magnetic star are presented. In the case of HD 21699, the radial velocity derived from our observations is not inconsistent with membership. But from the determinations of its proper motion found in the literature, this star cannot be regarded as a probable member of the α Per cluster. On the basis of recent evolutionary models, all the well established Ap cluster members appear to be close to the end of their main-sequence life. This suggests that A stars possibly become magnetic at the end of core hydrogen burning.     

The Lithium‐rich supergiant HD172365

The F‐type supergiant HD172365 was discovered by Luck (1982) to be a high Li abundance star. We find that the lithium abundance in this star is log A(Li) = 2.9 from a careful spectroscopic study. This value is larger than the abundance derived by Luck. We discuss the properties of this supergiant and tentatively conclude that HD172365 may, in fact, be a post‐blue straggler star with a mass ∼2 times greater than the “turn‐off point” mass for the open cluster IC 4756, of which HD172365 is a member. The star is formed by a merger of a close binary system. This conclusion is supported by, for example, its large projected rotational velocity, solar carbon abundance and high lithium content.     

Structure of the atmosphere of the magnetic star hd21699

The temperature distribution with depth in the atmosphere of the magnetic star HD21699 is modelled for each observed rotational phase using Hδ line profiles obtained with a signal to noise ratio S/N = 1000. The observed temperature distributions were averaged within and outside the region of the magnetic spots. It appears that these average temperatures are equal to within the limits of error. This shows that the magnetic field does not affect the physical condition of the matter in the star and does not disturb its thermodynamic state.     

Analysis of spectra of V471 Tau and HD 115404

We analyze the chemical composition of the atmospheres of a single K-type star HD 115404 and the secondary component of the V471 Tau variable. We use the technique of modeling of synthetic spectra to analyze the high-resolution spectra of these stars, taken with the RTT 150 Russian-Turkish telescope and find the abundances of 23 and 17 elements in the atmospheres of HD 115404 and V471 Tau, respectively. We demonstrate the lack of composition anomalies in the HD 115404 and show it to be consistent with the published data, inferred from equivalent widths of spectral lines. We find the abundances of 15 elements from Na to Ba to be consistent with the metallicity of the atmosphere of V471 Tau ([Fe/H] = −0.22 ± 0.12dex), which differs significantly from the average metallicity of the Hyades cluster. We show the existence of strong carbon and oxygen overabundances (by more than 1dex) due to the enrichment of the secondary by the nucleosynthesis products during the common-envelope stage of the system. On the whole, we demonstrate that V471 Tau and the other precataclysmic variables share similar composition anomalies.     

Application of a probabilistic neural network in radial‐velocity curve analysis of the spectroscopic binary stars HD 152218, HD 143511, HD 27149, and ER Vul

We use an Artificial Neural Network (ANN) to derive the orbital parameters of spectroscopic binary stars. Using measured radial velocity data of four double‐lined spectroscopic binary systems HD 152218, HD 143511, HD 27149, and ER Vul, we find corresponding orbital and spectroscopic elements. Our numerical results are in good agreement with those obtained by others using more traditional methods (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comprehensive study of the magnetic stars HD 5797 and HD 40711 with large chromium and iron overabundances

We present the results of a comprehensive study of the chemically peculiar stars HD 5797 and HD 40711. The stars have the same effective^temperature, T _eff = 8900 K, and a similar chemical composition with large iron (+1.5 dex) and chromium (+3 dex) overabundances compared to the Sun. The overabundance of rare-earth elements typically reaches +3 dex. We have measured the magnetic field of HD 5797. The longitudinal field component B _e has been found to vary sinusoidally between −100 and +1000 G with a period of 69 days. Our estimate of the evolutionary status of the stars suggests that HD 5797 and HD 40711, old objects with an age t ≈ 5 × 10⁸ yr, are near the end of the core hydrogen burning phase.     

Magnetic field of CP stars in the Ori OB1 association. II. HD36540, HD36668, HD36916, HD37058

We present the results of magnetic field measurements of four chemically peculiar (CP) stars with helium abundance anomalies which are the members of the Orion stellar association OB1. The stars under study were classified as magnetic by other authors earlier. The present paper contains the results of the extensive study of the stars. Magnetic field measurements allowed us to conclude that HD36540 has a weak field and the longitudinal component B _e does not exceed 500 G. The longitudinal field of HD36668 varies with the period P = 2_. ^d 11884 and the amplitude from ?2 to +2 kG. The magnetic field of HD36916 has mainly negative polarity and varies within the range from 0 to ?1 kG with the period P = 1.d 565238. HD37058 is a magnetic star, the longitudinal field of which varies from ?1.2 to +0.8 kG with the period P = 14_. ^d 659. The B _e field variability pattern for the stars HD36916 and HD37058 is of a simple harmonic type. The longitudinal field of HD36668 is best described with two combined harmonic functions (“a doublewave”). The variability period of HD36540 is still undetermined. For all the stars from this paper, we measured radial velocities V _r, axial rotation rates v _e sin i, and determined basic parameters of atmospheres (effective temperatures T _eff and gravity acceleration log g). We also estimated masses M, luminosities L, and radii R of the stars.