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| 2. |
- Folsom, C. P., et al.
(författare)
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Magnetic, chemical and rotational properties of the Herbig Ae/Be binary system HD 72106
- 2008
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Ingår i: Contributions of the Astronomical Observatory Skalnaté Pleso. - 1335-1842. ; 38:2, s. 245-250
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Tidskriftsartikel (refereegranskat)abstract
- Recently, strong, globally-ordered magnetic fields have been detected in some Herbig Ae and Be (HAeBe) stars, suggesting a possible evolutionary connection to main sequence magnetic chemically peculiar Ap and Bp stars. We have undertaken a detailed study of the binary system HD 72106, which contains a B9 magnetic primary and a HAeBe secondary, using the ESPaDOnS spectropolarimeter mounted on the CFHT. A careful analysis of the very young primary reveals that it has an approximately dipolar magnetic field geometry, strong chemical peculiarities, and strong surface chemical abundance inhomogeneities. Thus the primary is very similar to an Ap/Bp star despite having completed less then 1.5% of its main sequence life, and possible still being on the pre-main sequence. In contrast, a similar analysis of the secondary reveals solar chemical abundances and no magnetic field.
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| 3. |
- Fossati, L., et al.
(författare)
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Chemical evolution of A- and B-type stars in open clusters : observed abundances vs. diffusion models
- 2008
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Ingår i: Contributions of the Astronomical Observatory Skalnaté Pleso. - 1335-1842. ; 38:2, s. 123-128
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Tidskriftsartikel (refereegranskat)abstract
- We have decided to address the problem of how abundances and peculiarities change during main sequence evolution. We have setup a program to measure the atmospheric abundance patterns from tens of A-type star members of clusters of different ages, and compare the results with theory predictions. In this paper we present the overall project and we focus on the results obtained for a sample of Am stars of the Praesepe cluster (log t = 8.85 +/- 0.15; Gonzalez-Garcia et al., 2006). We have obtained spectra for eight Am stars, two normal A-type stars and one blue straggler, that are probable members of the Praesepe cluster. For all of these stars we have determined fundamental parameters and photospheric abundances for a large number of chemical elements. For seven stars we also obtained spectra in circular polarisation and applied the LSD technique to measure the mean longitudinal magnetic field. We have found good agreement between abundance predictions of diffusion models and measured abundances, except for Na and S. Li appears to be overabundant in three stars of our sample. No magnetic field was detected in any of the analysed stars.
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| 4. |
- Fossati, L., et al.
(författare)
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Late stages of the evolution of A-type stars on the main sequence : Comparison between observed chemical abundances and diffusion models for 8 Am stars of the Praesepe cluster
- 2007
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 476:2, s. 911-925
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We aim to provide observational constraints on diffusion models that predict peculiar chemical abundances in the atmospheres of Am stars. We also intend to check if chemical peculiarities and slow rotation can be explained by the presence of a weak magnetic field. Methods. We have obtained high resolution, high signal-to-noise ratio spectra of eight previously-classified Am stars, two normal A-type stars and one Blue Straggler, considered to be members of the Praesepe cluster. For all of these stars we have determined fundamental parameters and photospheric abundances for a large number of chemical elements, with a higher precision than was ever obtained before for this cluster. For seven of these stars we also obtained spectra in circular polarization and applied the LSD technique to constrain the longitudinal magnetic field. Results. No magnetic field was detected in any of the analysed stars. HD 73666, a Blue Straggler previously considered as an Ap (Si) star, turns out to have the abundances of a normal A-type star. Am classification is not confirmed for HD 72942. For HD 73709 we have also calculated synthetic Aa photometry that is in good agreement with the observations. There is a generally good agreement between abundance predictions of diffusion models and values that we have obtained for the remaining Am stars. However, the observed Na and S abundances deviate from the predictions by 0.6 dex and ≥0.25 dex respectively. Li appears to be overabundant in three stars of our sample.
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| 5. |
- Fossati, L., et al.
(författare)
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The chemical abundance analysis of normal early A- and late B-type stars
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 503:3, s. 945-962
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Forskningsöversikt (refereegranskat)abstract
- Context. Modern spectroscopy of early-type stars often aims at studying complex physical phenomena such as stellar pulsation, the peculiarity of the composition of the photosphere, chemical stratification, the presence of a magnetic field, and its interplay with the stellar atmosphere and the circumstellar environment. Comparatively less attention is paid to identifying and studying the "normal" A- and B-type stars and testing how the basic atomic parameters and standard spectral analysis allow one to fit the observations. By contrast, this kind of study is paramount for eventually allowing one to correctly quantify the impact of the various physical processes that occur inside the atmospheres of A- and B-type stars. Aims. We wish to establish whether the chemical composition of the solar photosphere can be regarded as a reference for early A- and B-late B-type stars. Methods. We have obtained optical high-resolution, high signal-to-noise ratio spectra of three slowly rotating early-type stars (HD 145788, 21 Peg and pi Cet) that show no obvious sign of chemical peculiarity, and performed a very accurate LTE abundance analysis of up to 38 ions of 26 elements (for 21 Peg), using a vast amount of spectral lines visible in the spectral region covered by our spectra. Results. We provide an exhaustive description of the abundance characteristics of the three analysed stars with a critical review of the line parameters used to derive the abundances. We compiled a table of atomic data for more than 1100 measured lines that may be used in the future as a reference. The abundances we obtained for He, C, Al, S, V, Cr, Mn, Fe, Ni, Sr, Y, and Zr are compatible with the solar ones derived with recent 3D radiative-hydrodynamical simulations of the solar photosphere. The abundances of the remaining studied elements show some degree of discrepancy compared to the solar photosphere. Those of N, Na, Mg, Si, Ca, Ti, and Nd may well be ascribed to non-LTE effects; for P, Cl, Sc and Co, non-LTE effects are totally unknown; O, Ne, Ar, and Ba show discrepancies that cannot be ascribed to non-LTE effects. The discrepancies obtained for O (in two stars) and Ne agree with very recent non-LTE abundance analysis of early B-type stars in the solar neighbourhood.
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| 6. |
- Fossati, L., et al.
(författare)
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The effect of rotation on the abundances of the chemical elements of the A-type stars in the Praesepe cluster
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 483:3, s. 891-902
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We study how chemical abundances of late B-, A-, and early F-type stars evolve with time, and we search for correlations between the abundance of chemical elements and other stellar parameters, such as effective temperature and upsilon sin i. Methods. We observed a large number of B-, A-, and F-type stars belonging to open clusters of different ages. In this paper we concentrate on the Praesepe cluster (log t = 8.85), for which we have obtained high-resolution, high signal-to-noise ratio spectra of sixteen normal A- and F-type stars and one Am star, using the SOPHIE spectrograph of the Observatoire de Haute-Provence. For all the observed stars, we derived fundamental parameters and chemical abundances. In addition, we discuss another eight Am stars belonging to the same cluster, for which the abundance analysis had been presented in a previous paper. Results. We find a strong correlation between the peculiarity of Am stars and upsilon sin i. The abundance of the elements underabundant in Am stars increases with upsilon sin i, while it decreases for the overabundant elements. Chemical abundances of various elements appear correlated with the iron abundance.
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| 7. |
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| 8. |
- Heiter, Ulrike, et al.
(författare)
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VALD — an atomic and molecular database for astrophysics
- 2008
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 130, s. 012011-
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Tidskriftsartikel (refereegranskat)abstract
- The VALD database of atomic and molecular data aims to ensure a robust and consistent analysis of astrophysical spectra. We offer a convenient e-mail and web-based user interface to a vast collection of spectral line parameters for all chemical elements and in the future also for molecules. An international team is working on the following tasks: collecting line parameters from relevant theoretical and experimental publications, computing line parameters, evaluating the data quality by comparison of similar data from different sources and by comparison with astrophysical observations, and incorporating the data into VALD. A unique feature of VALD is its capability to provide the most comprehensive spectral line lists for specific astrophysical plasma conditions defined by the user.
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| 9. |
- Khomenko, E., et al.
(författare)
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Simulations of Magnetoacoustic Pulsations in Atmospheres of Rapidly Oscillating Ap Stars
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 704:2, s. 1218-1238
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Forskningsöversikt (refereegranskat)abstract
- Rapidly oscillating Ap (roAp) stars exhibit an astrophysically interesting combination of strong, dipolar-like magnetic fields and high-overtone p-mode pulsations similar to the Sun. Recent time-resolved spectroscopy of these stars unravelled a complex picture of propagating magnetoacoustic pulsation waves, with amplitude and phase strongly changing as a function of atmospheric height. To interpret these observations and gain a new insight into the atmospheric dynamics of roAp stars we have carried out two-dimensional time-dependent, non-linear magnetohydrodynamical simulations of waves for a realistic atmospheric stratification of a cool Ap star. We explore a grid of simulations in a wide parameter space, treating oscillations of the velocity, magnetic field, and thermodynamic quantities in a self-consistent manner. Our simulations foster a new understanding of the influence of the atmosphere and the magnetic field on the propagation and reflection properties of magnetoacoustic waves, formation of node surfaces, and relative variation of different quantities. Our simulations reproduce all main features of the observed pulsational behavior of roAp stars. We show, for the first time, that the overall depth dependence of the pulsations in roAp atmospheres is strongly influenced by the density inversion at the photospheric base.
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| 10. |
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