arXiv subjects

T. Ryabchikova

Publications and source records attributed to T. Ryabchikova.

At least 55 records · Page 3Linked to original sources

arXiv2008

Non-LTE line formation for Pr II and Pr III in A and Ap stars

Non-LTE line formation for Pr II and Pr III is considered through a range of effective temperatures between 7250 K and 9500 K. A comprehensive model atom for Pr II/III is based on the measured and the predicted energy levels, in total, 6708 levels of Pr II and Pr III. We describe calculations of the Pr II energy levels and oscillator strengths for the transitions in Pr II and Pr III. The influence of departures from LTE on Pr abundance determinations is evaluated. At Teff >= 8000 K departures from LTE lead to overionization of Pr II and to systematically depleted total absorption in the line and positive abundance corrections. At the lower temperatures, different lines of Pr II may be either weakened or amplified depending on the line strength. The non-LTE effects strengthen the Pr III lines and lead to negative abundance corrections. Non-LTE corrections grow with effective temperature for the Pr II lines, and, in contrast, they decline for the Pr III lines. The Pr II/III model atom is applied to determine the Pr abundance in the atmosphere of the roAp star HD 24712 from the lines of two ionization stages. In the chemically uniform atmosphere with [Pr/H] = 3, the departures from LTE may explain only small part (0.3 dex) of the difference between the LTE abundances derived from the Pr II and Pr III lines (2 dex). We find that the lines of both ionization stages are described for the vertical distribution of the praseodymium where the Pr enriched layer with [Pr/H] > 4 exists in the outer atmosphere at log tau_5000 < -4. The departures from LTE for Pr II/III are strong in the stratified atmosphere and have the opposite sign for the Pr II and Pr III lines. Using the revised partition function of Pr II and experimental transition probabilities, we determine the solar non-LTE abundance of Pr as log (Pr/H) = -11.15\pm0.08.

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arXiv2008

Pulsations in the atmosphere of the rapidly oscillating Ap star 10 Aquilae

The rapidly oscillating Ap (roAp) star 10 Aql shows one of the lowest photometric pulsation amplitudes and is characterized by an unusual spectroscopic pulsational behavior compared to other roAp stars. More than 1000 spectra were taken during 7 nights over a time span of 21 days with high-resolution spectrographs at the 8-m ESO VLT and 3.6-m TNG telescopes giving access to radial velocity variations of about 150 lines from different chemical species. A comparison of pulsation signatures in lines formed at different atmospheric heights allowed us to resolve the vertical structure of individual pulsation modes in 10 Aql which is the first time for a multiperiodic roAp star. The inferred propagation of pulsation waves in 10 Aql is qualitatively similar to other roAp stars: pulsation amplitudes become measurable in the layers where Y and Eu are concentrated, increase in layers where the Halpha core is formed, reach a maximum of 200-300 m/s in the layers probed by Ce, Sm, Dy lines and then decrease to 20-50 m/s in the layers where NdIII and PrIII lines are formed. A unique pulsation feature of 10 Aql is a second pulsation maximum indicated by TbIII lines which form in the uppermost atmospheric layers and oscillate with amplitudes of up to 350 m/s. The dramatic decline of pulsations in the atmospheric layers probed by the strong PrIII and NdIII lines accounts for the apparent peculiarity of 10 Aql when compared to other roAp stars. The phase-amplitude diagrams and bisector measurements of the NdIII 5102 A line reveal a rapid change of phase and amplitude with height for all three main pulsation modes, indicating the presence of a pulsation node in the stellar atmosphere. (abridged)

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arXiv2008

Isotopic anomaly and stratification of Ca in magnetic Ap stars

We have carried out an accurate investigation of the Ca isotopic composition and stratification in the atmospheres of 23 magnetic chemically peculiar (Ap) stars of different temperature and magnetic field strength. With the UVES spectrograph at the 8m ESO VLT, we have obtained high-resolution spectra of Ap stars in the wavelength range 3000-10000 A. Using a detailed spectrum synthesis calculations, we have reproduced a variety of Ca lines in the optical and ultraviolet spectral regions, inferring the overall vertical distribution of Ca abundance, then we have deduced the relative isotopic composition and its dependence on height using the profile of the the IR-triplet Ca II line at lambda 8498 A. In 22 out of 23 studied stars, we found that Ca is strongly stratified, being usually overabundant by 1.0-1.5 dex below log tau_5000 ~ -1, and strongly depleted above log tau_5000 = -1.5. The IR-triplet Ca II line at lambda 8498 A reveals a significant contribution of the heavy isotopes 46Ca and 48Ca, which represent less than 1% of the terrestrial Ca isotopic mixture. We confirm our previous finding that the presence of heavy Ca isotopes is generally anticorrelated with the magnetic field strength. Moreover, we discover that in Ap stars with relatively small surface magnetic fields (<=4-5 kG), the light isotope 40Ca is concentrated close to the photosphere, while the heavy isotopes are dominant in the outer atmospheric layers. This vertical isotopic separation, observed for the first time for any metal in a stellar atmosphere, disappears in stars with magnetic field strength above 6-7 kG. We suggest that the overall Ca stratification and depth-dependent isotopic anomaly observed in Ap stars may be attributed to a combined action of the radiatively-driven diffusion and light-induced drift.

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arXiv2007

The discovery of high-amplitude 10.9-minute oscillations in the cool magnetic Ap star HD 115226

We present the discovery of pulsational variations in the cool magnetic Ap star HD 115226 -- the first high-amplitude rapidly oscillating Ap star discovered with time-series spectroscopy. Using high-resolution spectra obtained with the HARPS instrument at the ESO 3.6-m telescope, we detect radial velocity variations with a period of 10.86 min in Pr III, Nd III, Dy III lines and in the narrow cores of hydrogen lines. Pulsational amplitudes exceed 1 km/s in individual lines of Nd III. The presence of running waves in the stellar atmosphere is inferred from a phase shift between the radial velocity maxima of rare-earth and hydrogen lines. Our abundance analysis demonstrates that HD 115226 exhibits typical roAp spectroscopic signature, notably ionization anomaly of Pr, Nd and Dy. We discuss the discovery of pulsations in HD 115226 in the context of recent spectroscopic studies of roAp stars and point to the existence of correlation between spectroscopic pulsational amplitude and the stellar rotation rate.

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arXiv2007

Spectroscopic study of pulsations in the atmosphere of roAp star 10 Aql

We present the analysis of spectroscopic time-series observations of the roAp star 10 Aql. Observations were carried out in July 2006 with the UVES and SARG spectrographs simultaneously with the MOST mini-satellite photometry. All these data were analysed for radial velocity (RV) variations. About 150 lines out of the 1000 measured reveal clear pulsation signal. Frequency analysis of the spectroscopic data gives four frequencies. Three highest amplitude frequencies in spectroscopy coincide with the photometric ones. Phase-amplitude diagrams created for the lines of different elements/ions show that atmospheric pulsations may be represented by a superposition of the standing and running wave components, similar to other roAp stars. The highest RV amplitudes, 300-400 m/s, were measured for Ce II, Dy III, Tb III, and two unidentified lines at lambda 5471, 5556 A. We discovered ~0.4 period phase jump in the RV measurements across the Nd III line profiles. It indicates the presence of the pulsation node in stellar atmosphere. The phase jump occurs at nearly the same atmospheric layers for the two main frequencies. There is no rotational modulation in the average spectra for the 6 different nights we analysed.

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arXiv2007

A search of very low amplitude magnetoacoustic pulsations with HARPS

We have obtained time-resolved spectroscopic observations for a sample of 10 cool Ap stars using the ultra-stable spectrograph HARPS at the ESO 3.6-m telescope. The aim of our study was to search for low-amplitude oscillations in Ap stars with no or inconclusive evidence of pulsational variability. Here we report initial results of our investigation. We confirm the presence of ~16-min period pulsations in beta CrB (HD 137909) and demonstrate multiperiodic character of oscillations in this star. Furthermore, we discovered very low amplitude 9-min pulsations in HD 75445 - an object spectroscopically very similar to known roAp stars.

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arXiv2007

Pulsation tomography of rapidly oscillating Ap stars. Resolving the third dimension in peculiar pulsating stellar atmospheres

We present detailed analysis of the vertical pulsation mode cross-section in ten rapidly oscillating Ap (roAp) stars based on spectroscopic time-series observations. The aim of this analysis is to derive from observations a complete picture of how the amplitude and phase of magnetoacoustic waves depend on depth. We find common features in the pulsational behaviour of roAp stars. Within a sample of representative elements the lowest amplitudes are detected for Eu II (and Fe in 33 Lib and in HD 19918), then pulsations go through the layers where Halpha core, Nd, and Pr lines are formed. There RV amplitude reaches its maximum, and after that decreases in most stars. The maximum RV of the second REE ions is always delayed relative to the first ions. The largest phase shifts are detected in Tb III and Th III lines. Pulsational variability of the Th III lines is detected here for the first time. The Y II lines deviate from this picture, showing even lower amplitudes than Eu II lines but half a period phase shift relative to other weakly pulsating lines. The roAp stars exhibit similarity in the depth-dependence of pulsation phase and amplitude, indicating similar chemical stratification and comparable vertical mode cross-sections. In general, pulsations waves are represented by a superposition of the running and standing wave components. In the atmospheres of roAp stars with the pulsation frequency below the acoustic cut-off frequency, pulsations have a standing-wave character in the deeper layers and behave like a running wave in the outer layers. Cooler roAp stars develop a running wave higher in the atmosphere. In stars with pulsation frequency close to the acoustic cut-off one, pulsation waves have a running character starting from deep layers. (Abridged)

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arXiv2007

Ca isotopic anomaly in the atmospheres of Ap stars

We present results of the Ca stratification analysis in the atmospheres of 21 magnetic chemically peculiar (Ap) stars. This analysis was based on the spectral observations carried out with the UVES spectrograph attached to the 8-m VLT telescope. Ca was found to be strongly stratified in all stars with different effective temperatures and magnetic field strengths. This element is overabundant by 1-1.5 dex below logtau_5000~-1 and strongly depleted above logtau_5000=-1.5. Based on the overall Ca abundance distributions, we modelled a profile of the IR-triplet Ca II 8498 line. It shows a significant contribution of the heavy isotopes 46Ca and 48Ca, which represent less than 1% of the solar Ca isotopic mixture. In Ap stars with the relatively small surface magnetic fields (<= 4-5 kG) the light 40Ca isotope is concentrated close to the photosphere, while the heavy isotopes are pushed towards the outer layers. Isotopic separation disappears in the atmospheres of stars with magnetic fields above 6-7 kG. The observed overall Ca stratification and isotopic anomalies may be explained by a combined action of the radiatively-driven diffusion and the light-induced drift.

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arXiv2006

Line profile variations in rapidly oscillating Ap stars: resolution of the enigma

We have carried out the first survey of the pulsational line profile variability in rapidly oscillating Ap (roAp) stars. We analysed high signal-to-noise time-series observations of ten sharp-lined roAp stars obtained with the high-resolution spectrographs attached to the VLT and CFHT telescopes. We investigated in detail the variations of Pr III, Nd II, Nd III and Tb III lines and discovered a prominent change of the profile variability pattern with height in the atmospheres of all studied roAp stars. In every investigated star profile variability of at least one rare-earth ion is characterized by unusual blue-to-red moving features - a behaviour inexplicable in the framework of the standard oblique pulsator model of slowly rotating roAp stars. Using analysis of the line profile moments and spectrum synthesis calculations, we demonstrate that unusual oscillations in spectral lines of roAp stars arise from the pulsational modulation of line widths. This variation occurs approximately in quadrature with the radial velocity changes, and its amplitude rapidly increases with height in stellar atmosphere. We propose that the line width modulation is a consequence of the periodic expansion and compression of turbulent layers in the upper atmospheres of roAp stars. Thus, the line profile changes observed in slowly rotating magnetic pulsators should be interpreted as a superposition of two types of variability: the usual time-dependent velocity field due to an oblique low-order pulsation mode and an additional line width modulation, synchronized with the changes of stellar radius. Our explanation of the line profile variations of roAp stars solves the long-standing observational puzzle and opens new possibilities for constraining geometric and physical properties of the stellar magnetoacoustic pulsations.

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arXiv2006

Pulsation in the atmosphere of the roAp star HD 24712. I. Spectroscopic observations and radial velocity measurements

We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712. For this purpose we analyzed spectra collected during 2001-2004. An extensive data set was obtained in 2004 simultaneously with the photometry of the Canadian MOST mini-satellite. This allows us to connect directly atmospheric dynamics observed as radial velocity variations with light variations seen in photometry. We directly derived for the first time and for different chemical elements, respectively ions, phase shifts between photometric and radial velocity pulsation maxima indicating, as we suggest, different line formation depths in the atmosphere. This allowed us to estimate for the first time the propagation velocity of a pulsation wave in the outer stellar atmosphere of a roAp star to be slightly lower than the sound speed. We confirm large pulsation amplitudes (150-400 m/s) for REE lines and the Halpha core, while spectral lines of the other elements (Mg, Si, Ca, and Fe-peak elements) have nearly constant velocities. We did not find different pulsation amplitudes and phases for the lines of rare-earth elements before and after the Balmer jump, which supports the hypothesis of REE concentration in the upper atmosphere above the hydrogen line-forming layers. We also discuss radial velocity amplitudes and phases measured for individual spectral lines as tools for a 3D tomography of the atmosphere of HD 24712.

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arXiv2006

Chemical stratification in the atmosphere of Ap star HD 133792. Regularized solution of the vertical inversion problem

High spectral resolution studies of cool Ap stars reveal conspicuous anomalies of the shape and strength of many absorption lines. This is a signature of large atmospheric chemical gradients produced by the selective radiative levitation and gravitational settling of chemical species. Here we present a new approach to mapping the vertical chemical structures in stellar atmospheres. We have developed a regularized chemical inversion procedure that uses all information available in high-resolution stellar spectra. The new technique for the first time allowed us to recover chemical profiles without making a priori assumptions about the shape of chemical distributions. We have derived average abundances and applied the vertical inversion procedure to the high-resolution VLT UVES spectra of the weakly magnetic, cool Ap star HD 133792. Our analysis yielded improved estimates of the atmospheric parameters of HD 133792. We show that this star has negligible vsini and the mean magnetic field modulus =1.1+/-0.1 kG. We have derived average abundances for 43 ions and obtained vertical distributions of Ca, Si, Mg, Fe, Cr, and Sr. All these elements except Mg show high overabundance in the deep layers and solar or sub-solar composition in the upper atmosphere of HD 133792. In contrast, the Mg abundance increases with height. We find that transition from the metal-enhanced to metal-depleted zones typically occurs in a rather narrow range of depths in the atmosphere of HD 133792. Based on the derived photospheric abundances, we conclude that HD 133792 belongs to the rare group of evolved cool Ap stars, which possesses very large Fe-peak enhancement, but lacks a prominent overabundance of the rare-earth elements.

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arXiv2006

Rare-earth elements in the atmosphere of the magnetic chemically peculiar star HD 144897. New classification of the Nd III spectrum

We have obtained a UVES spectrum of a slowly rotating strongly magnetic Ap star, HD 144897, that exhibits very large overabundances of rare-earth elements. Here we present a detailed spectral analysis of this object, also taking into account effects of non-uniform vertical distribution (stratification) of chemical elements. We have determined the photospheric abundances of 40 ions. For seven elements (Mg, Si, Ca, Ti, Cr, Mn, Fe), we have obtained a stratification model that allow us to produce a satisfactory fit to the observed profiles of spectral lines of various strength. REEs abundances, that for the first time in the literature have been determined from the lines of the first and second ions, have been found typically four dex larger than solar abundances. Our analysis of REE spectral lines provide a strong support to the laboratory line classification and determination of the atomic parameters. The only remarkable exception is Nd III, for which spectral synthesis was found to be inconsistent with the observations. We have therefore performed a revision of the Nd III classification. We have confirmed the energies for 11 out of 24 odd energy levels classified previously, and we have derived the energies for additional 24 levels of Nd III, thereby increasing substantially the number of classified Nd III lines with corrected wavelengths and atomic parameters.

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arXiv2005

HD 178892 - a cool Ap star with extremely strong magnetic field

We report a discovery of the Zeeman resolved spectral lines, corresponding to the extremely large magnetic field modulus =17.5 kG, in the cool Ap star HD 178892. The mean longitudinal field of this star reaches 7.5 kG, and its rotational modulation implies the strength of the dipolar magnetic component Bp>=23 kG. We have revised rotation period of the star using the All Sky Automated Survey photometry and determined P=8.2478 d. Rotation phases of the magnetic and photometric maxima of the star coincide with each other. We obtained Geneva photometric observation of HD 178892 and estimated Teff=7700+/-250 K using photometry and the hydrogen Balmer lines. Preliminary abundance analysis reveals abundance pattern typical of rapidly oscillating Ap stars.

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arXiv2005

Spectroscopy of roAp star pulsation: HD24712

We present results of the radial velocity (RV) analysis of spectroscopic time-series observations of the roAp star HD24712 (HR1217) which were carried out simultaneously with the Canadian MOST mini-satellite photometry. Only lines of the rare-earth elements (REE) show substantial amplitudes of RV pulsations. Based on new Zeeman measurements we found different shapes of the magnetic curves derived by using Fe-peak and REE separately. Frequency analysis of the spectroscopic data showed that the highest amplitude frequencies are the same in photometry and spectroscopy. Photometric and spectroscopic pulsation curves are shifted in phase, and the phase shift depends on the atomic species. The observed distribution of RV pulsation amplitudes and phases with the optical depth as well as the observed phase lag between luminosity and radius variations are explained satisfactorily by the model of nonadiabatic nonradial pulsations of a magnetic star.

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arXiv2005

On the influence of Stark broadening on Cr I lines in stellar atmospheres

Using the semiclassical perturbation method, electron-, proton-, and ionized helium-impact line widths and shifts for the nine Cr I spectral lines from the $4p^7P^0-4d^7D$ multiplet, have been calculated for a perturbers density of 10$^{14}$ cm$^{-3}$ and temperatures T =2,500 - 50,000 K. The obtained results have been used to investigate the influence of Stark broadening effect in the Cr-rich Ap star $β$ CrB atmosphere on line shapes of these lines. It has been found that the contribution of the proton and He II collisions to the line width and shift is significant, and it is comparable and sometimes (depending of the electron temperature) even larger than electron-impact contribution. Moreover, not only the Stark line width, but, depending on the electron-, proton-, and He\ii density in stellar atmosphere also the Stark shift may contribute to the blue as well as to the red asymmetry of the same line. The obtained results have been used to investigate the influence of Stark broadening effect on line shapes of Crılines in the atmosphere of Cr-rich Ap star $β$ CrB.

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arXiv2004

The null result of a search for pulsational variations of the surface magnetic field in the roAp star gamma Equulei

We describe an analysis of the time-resolved measurements of the surface magnetic field in the roAp star gamma Equ. We have obtained a high-resolution and high S/N spectroscopic time-series, and the magnetic field was determined using Zeeman resolved profiles of the Fe II 6149.25 A and Fe I 6173.34 A lines. Contrary to recent reports we do not find any evidence of magnetic variability with pulsation phase, and derive an upper limit of 5-10 G for pulsational modulation of the surface magnetic field in gamma Equ.

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arXiv2004

No magnetic field variation with pulsation phase in the roAp star gamma Equulei

We present an analysis of 210 high-resolution time-resolved spectropolarimetric observations of the roAp star gamma Equ obtained over three nights in August and September 2003. Radial velocity variations due to p-mode non-radial pulsations are clearly detected in the lines of rare-earth elements, in particular Pr III, Nd II and Nd III. In contrast, we find absolutely no evidence for the variation of the mean longitudinal magnetic field over the pulsation period in gamma Equ at the level of 110-240 G which was recently reported by Leone & Kurtz (2003). Our investigation of the variability of circularly polarized profiles of 13 Nd III lines demonstrates that, at the 3 sigma confidence level, no magnetic field variation with an amplitude above 40-60 G was present in gamma Equ during our monitoring of this star.

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arXiv2003

On the influence of Stark broadening on Si I lines in stellar atmospheres

We study the influence of Stark broadening and stratification effects on Siılines in the rapidly oscillating (roAp) star 10 Aql, where the Siı6142.48 Åand 6155.13 Ålines are asymmetrical and shifted. First we have calculated Stark broadening parameters using the semiclassical perturbation method for three Siılines: 5950.2 Å, 6142.48 Åand 6155.13 Å. We revised the synthetic sp$ calculation code taking into account both Stark width and shift for these lines. From the comparison of our calculations with the observations we found that Stark broadening + the stratification effect can explain asymmetry of the Siı6142.48 Åand 6155.13 Ålines in the atmospere of roAp star 10 Aql.

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