| 1. |
- Folsom, C. P., et al.
(författare)
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Magnetic field, chemical composition and line profile variability of the peculiar eclipsing binary star AR Aur
- 2010
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 407:4, s. 2383-2392
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Tidskriftsartikel (refereegranskat)abstract
- AR Aur is the only eclipsing binary known to contain a HgMn star, making it an ideal case for a detailed study of the HgMn phenomenon. HgMn stars are a poorly understood class of chemically peculiar stars, which have traditionally been thought not to possess significant magnetic fields. However, the recent discovery of line profile variability in some HgMn stars, apparently attributable to surface abundance patches, has brought this belief into question. In this paper we investigate the chemical abundances, line profile variability, and magnetic field of the primary and secondary of the AR Aur system, using a series of high-resolution spectropolarimetric observations. We find the primary is indeed a HgMn star, and present the most precise abundances yet determined for this star. We find the secondary is a weak Am star, and is possibly still on the pre-main sequence. Line profile variability was observed in a range of lines in the primary, and is attributed to inhomogeneous surface distributions of some elements. No magnetic field was detected in any observation of either stars, with an upper limit on the longitudinal magnetic field in both stars of 100 G. Modelling of the phase-resolve longitudinal field measurements leads to a 3 Sigma upper limit on any dipole surface magnetic field of about 400 G.
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| 2. |
- Fossati, L., et al.
(författare)
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A detailed spectroscopic analysis of the open cluster NGC 5460
- 2011
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 413:2, s. 1132-1144
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Tidskriftsartikel (refereegranskat)abstract
- Within the context of a large project aimed at studying early F-, A- and late B-type stars, we present the abundance analysis of the photospheres of 21 members of the open cluster NGC 5460, an intermediate-age cluster (log t similar to 8.2) previously not studied with spectroscopy. Our study is based on medium- and high-resolution spectra obtained with the Fibre Large Array Multi Element Spectrograph (FLAMES) instrument of the European Southern Observatory/Very Large Telescope. We show that cluster members have a nearly solar metallicity and that there is evidence that the abundances of magnesium and iron are correlated with the effective temperature, exhibiting a maximum around T-eff = 10 500 K. No correlations are found between abundances and projected equatorial velocity, except for marginal evidence of barium being more abundant in slower than in faster rotating stars. We discovered two He-weak stars and a binary system where the hotter component is an HgMn star. We provide new estimates for the cluster distance (720 +/- 50 pc), age (log t = 8.2 +/- 0.1) and mean radial velocity (-17.9 +/- 5.2 km s-1).
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| 3. |
- Joshi, S., et al.
(författare)
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Time-resolved photometric and spectroscopic analysis of the luminous Ap star HD 103498
- 2010
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 401:2, s. 1299-1307
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of the photometric and spectroscopic monitoring of the luminous Ap star HD 103498. The time-series photometric observations were carried out on 17 nights using a three-channel fast photometer attached to the 1.04-m optical telescope at the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital. The photometric data from five nights in 2007 show a clear signature of 15-min periodicity. However, the follow-up observations during 2007-2009 did not reproduce any such periodicity. To confirm the photometric light variations, time-series spectroscopic observations were carried out with the 2.56-m Nordic Optical Telescope (NOT) at La Palma on 2009 February 2. No radial velocity variations were present in this data set, which is in full agreement with the photometric observations taken around the same date. Model atmosphere and abundance analysis of HD 103498 show that the star is evolved from the main sequence and its atmospheric abundances are similar to those of two other evolved Ap stars, HD 133792 and HD 204411: large overabundances of Si, Cr and Fe and moderate overabundances of the rare-earth elements. These chemical properties and a higher effective temperature distinguish HD 103498 from any known roAp star.
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| 4. |
- Kochukhov, Oleg, et al.
(författare)
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Line profile variations in rapidly oscillating Ap stars : Resolution of the enigma
- 2007
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 376:2, s. 651-672
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Tidskriftsartikel (refereegranskat)abstract
- 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 ratio time-series observations of 10 sharp-lined roAp stars obtained with the high-resolution spectrographs attached to the Very Large Telescope and Canada–France–Hawaii Telescope. 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. We show that, in every investigated star, profile variability of at least one rare-earth ion is characterized by unusual blue-to-red moving features, which we previously discovered in the time-resolved spectra of the roAp star γ Equ. This behaviour is common in rapidly rotating non-radial pulsators but is 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 linewidths. 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 linewidth 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 linewidth 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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| 5. |
- Kochukhov, Oleg, et al.
(författare)
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Magnetic Doppler imaging considering atmospheric structure modifications due to local abundances : a luxury or a necessity?
- 2012
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 421:4, s. 3004-3018
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic Doppler imaging is currently the most powerful method of interpreting high-resolution spectropolarimetric observations of stars. This technique has provided the very first maps of stellar magnetic field topologies reconstructed from time series of full Stokes vector spectra, revealing the presence of small-scale magnetic fields on the surfaces of Ap stars. These studies were recently criticised by Stift et al., who claimed that magnetic inversions are not robust and are seriously undermined by neglecting a feedback on the Stokes line profiles from the local atmospheric structure in the regions of enhanced metal abundance. We show that Stift et al. misinterpreted published magnetic Doppler imaging results and consistently neglected some of the most fundamental principles behind magnetic mapping. Using state-of-the-art opacity sampling model atmosphere and polarized radiative transfer codes, we demonstrate that the variation of atmospheric structure across the surface of a star with chemical spots affects the local continuum intensity but is negligible for the normalized local Stokes profiles except for the rare situation of a very strong line in an extremely Fe-rich atmosphere. For the disc-integrated spectra of an Ap star with extreme abundance variations, we find that the assumption of a mean model atmosphere leads to moderate errors in Stokes I but is negligible for the circular and linear polarization spectra. Employing a new magnetic inversion code, which incorporates the horizontal variation of atmospheric structure induced by chemical spots, we reconstructed new maps of magnetic field and Fe abundance for the bright Ap star a2 CVn. The resulting distribution of chemical spots changes insignificantly compared to the previous modelling based on a single model atmosphere, while the magnetic field geometry does not change at all. This shows that the assertions by Stift et al. are exaggerated as a consequence of unreasonable assumptions and extrapolations, as well as methodological flaws and inconsistencies of their analysis. Our discussion proves that published magnetic inversions based on a mean stellar atmosphere are highly robust and reliable, and that the presence of small-scale magnetic field structures on the surfaces of Ap stars is indeed real. Incorporating horizontal variations of atmospheric structure in Doppler imaging can marginally improve reconstruction of abundance distributions for stars showing very large iron overabundances. But this costly technique is unnecessary for magnetic mapping with high-resolution polarization spectra.
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| 6. |
- Oksala, M. E., et al.
(författare)
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Revisiting the Rigidly Rotating Magnetosphere model for sigma Ori E-I : Observations and data analysis
- 2012
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 419:2, s. 959-970
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Tidskriftsartikel (refereegranskat)abstract
- We have obtained 18 new high-resolution spectropolarimetric observations of the B2Vp star s Ori E with both the Narval and ESPaDOnS spectropolarimeters. The aim of these observations is to test, with modern data, the assumptions of the Rigidly Rotating Magnetosphere (RRM) model of Townsend & Owocki, applied to the specific case of s Ori E by Townsend, Owocki & Groote. This model includes a substantially offset dipole magnetic field configuration, and approximately reproduces previous observational variations in longitudinal field strength, photometric brightness and Ha emission. We analyse new spectroscopy, including H i, He i, C ii, Si iii and Fe iii lines, confirming the diversity of variability in photospheric lines, as well as the double S-wave variation of circumstellar hydrogen. Using the multiline analysis method of least-squares deconvolution (LSD), new, more precise longitudinal magnetic field measurements reveal a substantial variance between the shapes of the observed and RRM model time-varying field. The phase-resolved Stokes V profiles of He i 5876 and 6678 angstrom lines are fitted poorly by synthetic profiles computed from the magnetic topology assumed by Townsend et al.. These results challenge the offset dipole field configuration assumed in the application of the RRM model to s Ori E, and indicate that future models of its magnetic field should also include complex, higher order components.
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| 7. |
- Pandey, Chhavi P., et al.
(författare)
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Abundance and stratification analysis of the chemically peculiar star HD 103498
- 2011
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 417:1, s. 444-452
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Tidskriftsartikel (refereegranskat)abstract
- The slow rotation and the absence of strong mixing processes in the atmospheres of chemically peculiar stars develop the ideal conditions for the appearance of abundance anomalies through the mechanism of microscopic particle diffusion. This makes these objects look spectroscopically and photometrically different from their 'normal' analogues. As a result, it is often difficult to accurately determine the atmospheric parameters of these stars, and special methods are needed for a consistent analysis of their atmospheres. The main aim of the present paper is to analyse atmospheric abundance and stratification of chemical elements in the atmosphere of the chemically peculiar star HD 103498. We find that there are two model atmospheres, computed with individual and stratified abundances, that provide a reasonable fit to the observed spectroscopic and photometric indicators: T(eff) = 9300 K, log g = 3.5 and T(eff) = 9500 K, log g = 3.6. It is shown that Mg has a large abundance gradient in the star's atmosphere with accumulation of Mg ions in the uppermost atmospheric layers, whereas Si demonstrates the opposite behaviour with accumulation in deep layers. In addition, a detailed non-local thermodynamic equilibrium (non-LTE) analysis showed that none of the Mg transitions under consideration is a subject of noticeable non-LTE effects. By comparing the photometry observations after transforming them to physical units, we estimated the radius of HD103498 to be between R = (4.56 +/- 0.77) R(circle dot) for T(eff) = 9300 K, log g = 3.5, and R = (4.39 +/- 0.75) R(circle dot) for T(eff) = 9500 K, log g = 3.6 models, respectively. We note that the lack of suitable observations in absolute units prevents us from uniquely determining the T(eff) of the star at the current stage of analysis.
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| 8. |
- Sachkov, M., et al.
(författare)
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Pulsations in the atmosphere of the rapidly oscillating star 33 Lib
- 2011
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 416:4, s. 2669-2677
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Tidskriftsartikel (refereegranskat)abstract
- In 2009, the rapidly oscillating peculiar A-type (roAp) star 33 Lib was the target of an intense observing campaign, combining ground-based spectroscopy with space photometry obtained with the Microvariability and Oscillation of STars (MOST) satellite. We collected 780 spectra using the Echelle Spectro Polarimetric Device for the Observation of Stars (ESPaDOnS) spectrograph attached at the 3.6-m Canada-France-Hawaii Telescope and 374 spectra with the Fibre-fed Echelle Spectrograph attached at the 2.56-m Nordic Optical Telescope to perform time-resolved spectroscopy of 33 Lib. In addition, we used 111 Ultraviolet and Visual Echelle Spectrograph (UVES) spectra (2004) from the European Southern Observatory archive to check mode stability. Frequency analysis of the new radial velocity (RV) measurements confirms the previously reported frequency pattern (two frequencies and the first harmonic of the main one) and reveals an additional frequency at 1.991 mHz. The new frequency solution perfectly reproduces the RV variations from the 2004 and 2009 observational sets, providing strong support for p mode stability in this roAp star over at least 5 years.
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| 9. |
- Shulyak, D., et al.
(författare)
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Rotation, magnetism and metallicity of M dwarf systems
- 2011
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 418:4, s. 2548-2557
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Tidskriftsartikel (refereegranskat)abstract
- Close M dwarf binaries and higher multiples allow the investigation of rotational evolution and mean magnetic flux unbiased from scatter in inclination angle and age since the orientation of the spin axis of the components is most likely parallel and the individual systems are coeval. Systems composed of an early-type (M0.0-M4.0) and a late-type (M4.0-M8.0) component offer the possibility to study differences in rotation and magnetism between partially and fully convective stars. We have selected 10 of the closest dM systems to determine the rotation velocities and the mean magnetic field strengths based on spectroscopic analysis of FeH lines of Wing-Ford transitions at 1 μm observed with Very Large Telescope/CRIRES. We also studied the quality of our spectroscopic model regarding atmospheric parameters including metallicity. A modified version of the Molecular Zeeman Library (MZL) was used to compute Landég-factors for FeH lines. Magnetic spectral synthesis was performed with the SYNMAST code. We confirmed previously reported findings that less massive M dwarfs are braked less effectively than objects of earlier types. Strong surface magnetic fields were detected in primaries of four systems (GJ 852, GJ 234, LP 717-36 and GJ 3322), and in the secondary of the triple system GJ 852. We also confirm strong 2-kG magnetic field in the primary of the triple system GJ 2005. No fields could be accurately determined in rapidly rotating stars with \upsi sin i \gt 10 km s$^-1$. For slowly and moderately rotating stars, we find the surface magnetic field strength to increase with the rotational velocity \upsi sin i which is consistent with other results from studying field stars. Based on observations made with European Southern Observatory (ESO) Telescopes at the Paranal Observatories under programme ID 81.D-0189.
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| 10. |
- Silvester, J., et al.
(författare)
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Stokes IQUV magnetic Doppler imaging of Ap stars - I. ESPaDOnS and NARVAL observations
- 2012
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 426:2, s. 1003-1030
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we describe and evaluate new spectral line polarization observations obtained with the goal of mapping the surfaces of magnetic Ap stars in great detail. One hundred complete or partial Stokes IQUV sequences, corresponding to 297 individual polarized spectra, have been obtained for seven bright Ap stars using the Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) and NARVAL high-resolution spectropolarimeters. The targets span a range of masses from approximately 1.8 to 3.4 M circle dot, a range of rotation periods from 2.56 to 6.80 ?d and a range of maximum longitudinal magnetic field strengths from 0.3 to over 4 kG. For three of the seven stars, we have obtained dense phase coverage sampling the entire rotational cycle. These data sets are suitable for immediate magnetic and chemical abundance surface mapping using magnetic Doppler imaging. For the remaining four stars, partial phase coverage has been obtained, and additional observations will be required in order to map the surfaces of these stars. The median signal-to-noise ratio of the reduced observations is over 700 per 1.8 km?s-1 pixel. Spectra of all stars show Stokes V Zeeman signatures in essentially all individual lines, and most stars show clear Stokes QU signatures in many individual spectral lines. The observations provide a vastly improved data set compared to previous generations of observations in terms of signal-to-noise ratio, resolving power and measurement uncertainties. Measurement of the longitudinal magnetic field demonstrates that the data are internally consistent within computed uncertainties typically at the 50-100 sigma level. Data are also shown to be in excellent agreement with published observations and in qualitative agreement with the predictions of published surface structure models. In addition to providing the foundation for the next generation of surface maps of Ap stars, this study establishes the performance and stability of the ESPaDOnS and NARVAL high-resolution spectropolarimeters during the period 2006-2010.
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