| 1. |
- Folsom, C. P., et al.
(författare)
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Magnetic fields and chemical peculiarities of the very young intermediate-mass binary system HD 72106
- 2008
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 391:2, s. 901-914
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Tidskriftsartikel (refereegranskat)abstract
- The recently discovered magnetic Herbig Ae and Be stars may provide qualitatively new information about the formation and evolution of magnetic Ap and Bp stars. We have performed a detailed investigation of one particularly interesting binary system with a Herbig Ae secondary and a late B-type primary possessing a strong, globally ordered magnetic field. 20 high-resolution Stokes V spectra of the system were obtained with the ESPaDOnS instrument mounted on the Canada-France-Hawaii Telescope. In these observations we see clear evidence for a magnetic field in the primary, but no evidence for a magnetic field in the secondary. A detailed abundance analysis was performed for both stars, revealing strong chemical peculiarities in the primary and normal chemical abundances in the secondary. The primary is strongly overabundant in Si, Cr and other iron-peak elements, as well as Nd, and underabundant in He. The primary therefore appears to be a very young Bp star. In this context, line profile variations of the primary suggest non-uniform lateral distributions of surface abundances. Interpreting the 0.639 95 +/- 0.000 09 d variation period of the Stokes I and V profiles as the rotational period of the star, we have modelled the magnetic field geometry and the surface abundance distributions of Si, Ti, Cr and Fe using magnetic Doppler imaging. We derive a dipolar geometry of the surface magnetic field, with a polar strength B-d = 1230 G and an obliquity beta = 57 degrees. The distributions Ti, Cr and Fe are all qualitatively similar, with an elongated patch of enhanced abundance situated near the positive magnetic pole. The Si distribution is somewhat different, and its relationship to the magnetic field geometry less clear.
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| 2. |
- Grunhut, H, et al.
(författare)
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Discovery of a magnetic field in the O9 sub-giant star HD 57682 by the MiMeS Collaboration
- 2009
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966 .- 1745-3925 .- 1745-3933. ; 400:1, s. L94-L98
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Tidskriftsartikel (refereegranskat)abstract
- We report the detection of a strong, organized magnetic field in the O9IV star HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the 3.6-m Canada-France-Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Programme. From the fitting of our spectra using non-local thermodynamic equilibrium model atmospheres, we determined that HD 57682 is a 17(-9)(+19)M(circle dot) star with a radius of 7.0(-1.8)(+2.4)R(circle dot) and a relatively low mass-loss rate of 1.4(-0.95)(+3.1) x 10(-9) M-circle dot yr(-1). The photospheric absorption lines are narrow, and we use the Fourier transform technique to infer v sin i = 15 +/- 3 km s(-1). This v sin i implies a maximum rotational period of 31.5 d, a value qualitatively consistent with the observed variability of the optical absorption and emission lines, as well as the Stokes V profiles and longitudinal field. Using a Bayesian analysis of the velocity-resolved Stokes V profiles to infer the magnetic field characteristics, we tentatively derive a dipole field strength of 1680(-356)(+134)G. The derived field strength and wind characteristics imply a wind that is strongly confined by the magnetic field.
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| 3. |
- Hareter, M., et al.
(författare)
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MOST discovers a multimode delta Scuti star in a triple system : HD 61199
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 492:1, s. 185-195
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Tidskriftsartikel (refereegranskat)abstract
- Context. A field star, HD61199 (V approximate to 8), simultaneously observed with Procyon by the MOST (Microvariability & Oscillations of STars) satellite in continuous runs of 34, 17, and 34 days in 2004, 2005, and 2007, was found to pulsate in 11 frequencies in the delta Scuti range with amplitudes from 1.7 down to 0.09 mmag. The photometry also showed variations with a period of about four days. To investigate the nature of the longer period, 45 days of time-resolved spectroscopy was obtained at the Thuringer Landessternwarte Tautenburg in 2004. The radial velocity measurements indicate that HD61199 is a triple system. Aims. A delta Scuti pulsator with a rich eigenspectrum in a multiple system is promising for asteroseismology. Our objectives were to identify which of the stars in the system is the delta Scuti variable and to obtain the orbital elements of the system and the fundamental parameters of the individual components, which are constrained by the pulsation frequencies of the delta Scuti star.Methods. Classical Fourier techniques and least-squares multi-sinusoidal fits were applied to the MOST photometry to identify the pulsation frequencies. The groundbased spectroscopy was analysed with least-squares-deconvolution (LSD) techniques, and the orbital elements derived with the KOREL and ORBITX routines. Asteroseismic models were also generated. Results. The photometric and spectroscopic data are compatible with a triple system consisting of a close binary with an orbital period of 3.57 days and a d Scuti companion (HD61199 A) as the most luminous component. The d Scuti star is a rapid rotator with about v sin i = 130 km s(-1) and an upper mass limit of about 2.1 M-circle dot. For the close binary components, we find they are of nearly equal mass, with lower mass limits of about 0.7 M-circle dot. Comparisons to synthetic spectra indicate these stars have a late-F spectral type. The observed oscillation frequencies are compared to pulsation models to further constrain the evolutionary state and mass of HD61199 A. The orbit frequency of the close binary corresponds to the difference of the two d Scuti frequencies with the highest amplitudes a coincidence that is remarkable, but not explained.
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| 4. |
- Kochukhov, O, et al.
(författare)
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A self-consistent empirical model atmosphere, abundance and stratification analysis of the benchmark roAp star alpha Circini
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 499:3, s. 851-863
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Tidskriftsartikel (refereegranskat)abstract
- Context. Chemically peculiar (CP) stars are unique natural laboratories for the investigation of the microscopic diffusion processes of chemical elements. The element segregation under the influence of gravity and radiation pressure leads to the appearance of strong abundance gradients in the atmospheres of CP stars. Consequently, the atmospheric temperature-pressure structure of these objects could deviate significantly from the atmospheres of normal stars with homogeneous abundances. Aims. In this study we performed a self-consistent, empirical model atmosphere study of the brightest rapidly oscillating Ap star alpha Cir. We account for chemical stratification in the model atmosphere calculations and assess the importance of non-uniform vertical element distribution on the model structure, energy distribution and hydrogen line profiles. Methods. For the chemical stratification analysis we use the DDAFIT minimization tool in combination with a magnetic spectrum synthesis code. The model atmospheres with inhomogeneous vertical distributions of elements are calculated with the LLMODELS stellar model atmosphere code. Results. Based on an iterative procedure of the chemical abundance analysis of 52 ions of 35 elements, stratification modeling of 4 elements (Si, Ca, Cr and Fe) and subsequent re-calculations of the atmospheric structure, we derived a new model atmosphere of alpha Cir which is consistent with the inferred atmospheric chemistry of the star. We find T-eff = 7500 K, log g = 4.1, and demonstrate that chemical stratification has a noticeable impact on the model structure and modifies the formation of the hydrogen Balmer lines. At the same time, the energy distribution appears to be less sensitive to the presence of large abundance gradients. Conclusions. Our spectroscopically determined T-eff of alpha Cir agrees with the fundamental effective temperature of this star. This shows that temperatures inferred in detailed spectroscopic analyses of cool magnetic CP stars are not affected by a large systematic bias.
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| 5. |
- Kochukhov, O., et al.
(författare)
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Discovery of very low amplitude 9-minute multiperiodic pulsations in the magnetic Ap star HD75445
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 493:3, s. L45-L48
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Tidskriftsartikel (refereegranskat)abstract
- We present our discovery of pulsational radial-velocity variations in the cool Ap star HD75445, an object spectroscopically similar to the bright, rapidly-oscillating Ap (roAp) star gamma Equ. Based on high-resolution time-series spectroscopy obtained with the HARPS spectrometer at the European Southern Observatory 3.6-m telescope, we detected oscillations in Nd II and Nd III lines with a period close to 9 min and amplitudes of 20-30 m s(-1). Substantial variation in the pulsational amplitude during our 3.8 h observing run reveals the presence of at least three excited non-radial modes. The detection of extremely low amplitude pulsations in HD75445 indicates that the roAp excitation mechanism produces variability in the radial velocity amplitude of between a few tens ms(-1) and several km s(-1). This supports the idea that many, if not all, cool Ap stars occupying the roAp instability strip may harbour non-radial pulsations, which currently remain undetected due to their small photometric and radial-velocity amplitudes.
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| 6. |
- Kochukhov, O., et al.
(författare)
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The discovery of high-amplitude, 10.9-minute oscillations in the cool magnetic Ap star HD 115226
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 479:2, s. L29-L32
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Tidskriftsartikel (refereegranskat)abstract
- We present the discovery of pulsational variations in the cool magnetic Ap star HD115226 - the first high-amplitude rapidly oscillating Ap (roAp) star discovered with time-series spectroscopy. Using high-resolution spectra obtained with the HARPS instrument at the European Southern Observatory 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(-1) 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 HD115226 exhibits a typical roAp spectroscopic signature, notably ionization anomaly of Pr, Nd, and Dy. We discuss the discovery of pulsations in HD115226 in the context of recent spectroscopic studies of roAp stars and point to the existence of a correlation between spectroscopic pulsational amplitude and the stellar rotation rate.
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| 7. |
- Nesvacil, N., et al.
(författare)
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Element stratification in roAp stars 10 Aquilae (HD 176232)
- 2008
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Ingår i: Contributions of the Astronomical Observatory Skalnaté Pleso. - 1335-1842. ; 38:2, s. 329-334
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of our analysis of the vertical element stratification in the atmospheres of 9 roAp stars. Using high resolution observations (R approximate to 100000) obtained with the ESO-VLT spectrograph UVES and state of the art model atmosphere and spectral synthesis codes, we analysed the inhomogeneous vertical distribution of Mg, Si, Ca, Cr, Fe and Sr in 10 Aquilae (HD 176232) and Fe stratification in the whole sample. We present the derived vertical abundance map of 10 Aquilae's atmosphere and compare our findings with Fe stratification profiles of all roAp stars.
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| 8. |
- Ryabchikova, T., et al.
(författare)
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Isotopic anomaly and stratification of Ca in magnetic Ap stars
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 480:3, s. 811-823
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We have completed 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. Methods. With the UVES spectrograph at the 8 m ESO VLT, we have obtained high-resolution spectra of Ap stars in the wavelength range 3000-10 000 angstrom. 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, and have deduced the relative isotopic composition and its dependence on height using the profile of the IR-triplet Ca II line at lambda 8498 angstrom. Results. 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) approximate to -1, and strongly depleted above log tau(5000) = -1.5. The IR-triplet Ca II line at lambda 8498 angstrom reveals a significant contribution of the heavy isotopes Ca-46 and Ca-48, 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 Ca-40 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. Conclusions. 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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| 9. |
- Sachkov, M., et al.
(författare)
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Pulsations in the atmosphere of the rapidly oscillating Ap star 10 Aquilae
- 2008
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 389:2, s. 903-918
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Tidskriftsartikel (refereegranskat)abstract
- The rapidly oscillating Ap (roAp) star 10 Aquilae (10 Aql) shows one of the lowest photometric pulsation amplitudes and is characterized by an unusual spectroscopic pulsational behaviour compared to other roAp stars. In summer 2006 this star became target of an intense observing campaign, that combined ground-based spectroscopy with space photometry obtained with the MOST (Microvariability & Oscillations Stars) satellite. More than 1000 spectra were taken during seven nights over a time-span of 21 d with high-resolution spectrographs at the 8-m European Southern Observatory (ESO) Very Large Telescope (VLT) and 3.6-m Telescopio Nazionale Galileo (TNG) 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. Taking advantage of the clear oscillation patterns seen in a number of rare earth ions and using the contemporaneous MOST photometry to resolve aliasing in the radial velocity measurements, we improve also the determination of pulsation frequencies. 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 H alpha core is formed, reach a maximum of 200-300 m s(-1) in the layers probed by Ce, Sm, Dy lines and then decrease to 20-50 m s(-1) in the layers where Nd III and Pr III lines are formed. A unique pulsation feature of 10 Aql is a second pulsation maximum indicated by Tb III lines which form in the uppermost atmospheric layers and oscillate with amplitudes of up to 350 m s(-1). The dramatic decline of pulsations in the atmospheric layers probed by the strong Pr III and Nd III lines accounts for the apparent peculiarity of 10 Aql when compared to other roAp stars. The phase-amplitude diagrams and bisector measurements of the Nd III 5102 angstrom 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. Finally, we report the discovery of a puzzling asymmetry of the strong Nd III lines with their blue wing extending up to -50 km s(-1), which is about 25 times the estimated value of nu(e) sin i.
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| 10. |
- Shulyak, D, et al.
(författare)
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Model atmospheres of chemically peculiar stars Self-consistent empirical stratified model of HD 24712
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 499:3, s. 879-890
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Tidskriftsartikel (refereegranskat)abstract
- Context. High-resolution spectra of some chemically peculiar stars clearly demonstrate the presence of strong abundance gradients in their atmospheres. However, these inhomogeneities are usually ignored in the standard scheme of model atmosphere calculations, breaking the consistency between model structure and spectroscopically derived abundance pattern. Aims. In this paper we present the first empirical self-consistent stellar atmosphere model of the roAp star HD 24712 with stratification of chemical elements included, and which is derived directly from the observed profiles of spectral lines without time-consuming simulations of physical mechanisms responsible for these anomalies. Methods. We used the LLmodels stellar model atmosphere code and DDAFIT minimization tool for analysis of chemical element stratification and construction of a self-consistent atmospheric model. Empirical determination of Pr and Nd stratification in the atmosphere of HD 24712 is based on NLTE line formation for Pr II/III and Nd II/III with the use of the DETAIL code. Results. Based on an iterative procedure of stratification analysis and subsequent re-calculation of model atmosphere structure, we constructed a self-consistent model of HD 24712, i.e. the model whose temperature-pressure structure is consistent with the results of the stratification analysis. It is shown that stratification of chemical elements leads to considerable changes in model structure compared to the non-stratified homogeneous case. We find that accumulation of rare earth elements (REE) allows for the inverse temperature gradient to be present in the upper atmosphere of the star with a maximum temperature increase of about 600 K. Conclusions.
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