We present a careful analysis of 13 high-quality optical spectra of low to moderately active late... more We present a careful analysis of 13 high-quality optical spectra of low to moderately active late-type dwarfs (G1-K5) aimed at determining their magnetic parameters. Among our sample only one star, Eri (spatially averaged field strength ≈ 165 ± 30 G), exhibits the unambiguous signature of a magnetic field, a few are candidates and the remaining show no sign of a magnetic field in the observed spectra. Our analysis is based on an inversion of the spectra using detailed numerical solutions of the Unno-Rachkovsky equations, for multiple spectral lines at different positions on the stellar disk, and including magneto-optical effects. It gives results for Eri which are in good agreement with the detailed analysis of infrared spectra by Valenti et al. (1995). However, the low value of the spatially averaged field strength of these recent analyses imply that most values of the magnetic flux determined previously for moderately active stars are probably too large, often by considerable amounts. We find that the magnetic flux can be reliably determined if considerable care is taken in the analysis, but the magnetic field strength and filling factor cannot be determined separately for moderately active stars with optical spectra of spectral resolution ≤ 10 5 and S/N ≤ 250. In the case of Eri we are able to constrain the temperature of the detected magnetic features, which we find to be similar to or hotter than the non-magnetic surroundings, providing the first direct evidence that the detected field is in the form of plages. We also find that if an inversion approach is used, which determines various line broadening parameters simultaneously in a self-consistent manner, the presence of a magnetic field is not as obvious as some previous analyses have suggested. In addition, we determine fundamental parameters of the stellar sample.
We present a model for the well-known CP star CrB consistent with observations of the mean longit... more We present a model for the well-known CP star CrB consistent with observations of the mean longitudinal field, crossover, mean quadratic field, mean field modulus, and broadband linear polarisation. We find that CrB, like most CP stars, exhibits a complex magnetic structure, which may not be fully accounted for even by a second-order multipolar expansion.
The most common techniques for detecting magnetic fields of chemically peculiar (CP) stars are ba... more The most common techniques for detecting magnetic fields of chemically peculiar (CP) stars are based on the determination of some quantities related to the magnetic morphology, which are obtained through the analysis of Stokes I and V profiles, and on observations of frequency integrated Stokes Q and U. In previous papers of this series we set up a formalism aimed at describing the magnetic field of CP stars in terms of a superposition of a dipole and a quadrupole field, and we proposed a method to recover the magnetic morphology from the combined analysis of those observable quantities which can simply be expressed by means of analytical formulas, namely: the mean longitudinal field, the crossover, and the mean square field. Here we extend this method in order to include those quantities for which numerical integrations are required, i.e., the mean field modulus and the broadband linear polarisation. Estimates of stellar radius and projected equatorial velocity are also explicitly taken into account. We present an application to the well-known CP star β CrB, the only star for which all kinds of magnetic data are available. We find that β CrB, like most CP stars, exhibits a complex magnetic structure, which may not be fully accounted for even by a second-order multipolar expansion. However, we are able to suggest two models which are compatible with all the magnetic data.
The positions in the HR diagram and the kinematic characteristics of rapidly oscillating and non-... more The positions in the HR diagram and the kinematic characteristics of rapidly oscillating and non-oscillating chemically peculiar stars are obtained using new Hipparcos proper motions and parallaxes, and our own radial velocity measurements. We find that rapidly oscillating stars, as a group, are (−0.47±0.34) mag above the zero-age main sequence (ZAMS), while the non-oscillating stars are (−1.20±0.65) mag above the ZAMS and so appear slightly more evolved on average. From the comparison of the kinematical characteristics, we conclude that both groups are very similar. The results of radial velocity measurements indicate that there is a real deficiency of binaries among rapidly oscillating stars. Presently, no such star is known to be a spectroscopic binary.
In a high resolution spectroscopic study of the rapidly oscillating Ap (roAp) star HD 99563 with ... more In a high resolution spectroscopic study of the rapidly oscillating Ap (roAp) star HD 99563 with the Ultraviolet-Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) we have discovered remarkably large amplitude pulsations with some spectral lines showing radial velocity amplitudes up to 5 km s −1 (10 km s −1 peak-to-peak) with a pulsation period of 10.7 min.
We have completed a new study of the formation of hydrogen lines in dense magnetized plasmas. We ... more We have completed a new study of the formation of hydrogen lines in dense magnetized plasmas. We have developed a global formalism including the effects of the magnetic field and of the motional electric field. The latter, resulting from the motion of the radiating hydrogen atom in the magnetic field, modifies the structure of the matrices appearing in the equation of transfer of the Stokes vector and introduces a coupling between microscopic ("intrinsic") and macroscopic (Doppler) contributions to the line profiles. We have used this new treatment of line formation to revisit the interpretation of photopolarimetric (Stokes I and V) observations of Balmer lines in terms of mean longitudinal magnetic field.
The rapidly oscillating Ap stars are of importance for studying the atmospheric structure of star... more The rapidly oscillating Ap stars are of importance for studying the atmospheric structure of stars where the process of chemical element diffusion is significant. We have performed a survey for rapid oscillations in a sample of 9 luminous Ap stars, selected from their location in the colour-magnitude diagram as more evolved main-sequence Ap stars that are inside the instability strip for rapidly oscillating Ap (roAp) stars. Until recently this region was devoid of stars with observed rapid pulsations. We used the VLT UV-Visual Echelle Spectrograph (UVES) to obtain high time resolution spectroscopy to make the first systematic spectroscopic search for rapid oscillations in this region of the roAp instability strip. We report 9 null-detections with upper limits for radial-velocity amplitudes of 20 − 65 ms −1 and precisions of σ = 7 − 20 ms −1 for combinations of Nd and Pr lines. Cross-correlations confirm these null-results. At least six stars are magnetic and we provide magnetic fiel...
The rapidly oscillating Ap (roAp) stars with very strong magnetic fields are of special interest ... more The rapidly oscillating Ap (roAp) stars with very strong magnetic fields are of special interest because the effect of the magnetic field on the structure of their atmospheres can be studied with the greatest detail and accuracy. The cool roAp star HD 154708 possesses the strongest magnetic field among the roAp stars, with a mean magnetic field modulus = 24.5 ± 1.0 kG, which is about a factor of three greater than that of HD 166473, <B〉 ˜ 5.5-9.0 kG, the roAp star with the second-strongest magnetic field.
We present a careful analysis of 13 high-quality optical spectra of low to moderately active late... more We present a careful analysis of 13 high-quality optical spectra of low to moderately active late-type dwarfs (G1-K5) aimed at determining their magnetic parameters. Among our sample only one star, Eri (spatially averaged field strength ≈ 165 ± 30 G), exhibits the unambiguous signature of a magnetic field, a few are candidates and the remaining show no sign of a magnetic field in the observed spectra. Our analysis is based on an inversion of the spectra using detailed numerical solutions of the Unno-Rachkovsky equations, for multiple spectral lines at different positions on the stellar disk, and including magneto-optical effects. It gives results for Eri which are in good agreement with the detailed analysis of infrared spectra by Valenti et al. (1995). However, the low value of the spatially averaged field strength of these recent analyses imply that most values of the magnetic flux determined previously for moderately active stars are probably too large, often by considerable amounts. We find that the magnetic flux can be reliably determined if considerable care is taken in the analysis, but the magnetic field strength and filling factor cannot be determined separately for moderately active stars with optical spectra of spectral resolution ≤ 10 5 and S/N ≤ 250. In the case of Eri we are able to constrain the temperature of the detected magnetic features, which we find to be similar to or hotter than the non-magnetic surroundings, providing the first direct evidence that the detected field is in the form of plages. We also find that if an inversion approach is used, which determines various line broadening parameters simultaneously in a self-consistent manner, the presence of a magnetic field is not as obvious as some previous analyses have suggested. In addition, we determine fundamental parameters of the stellar sample.
We present a model for the well-known CP star CrB consistent with observations of the mean longit... more We present a model for the well-known CP star CrB consistent with observations of the mean longitudinal field, crossover, mean quadratic field, mean field modulus, and broadband linear polarisation. We find that CrB, like most CP stars, exhibits a complex magnetic structure, which may not be fully accounted for even by a second-order multipolar expansion.
The most common techniques for detecting magnetic fields of chemically peculiar (CP) stars are ba... more The most common techniques for detecting magnetic fields of chemically peculiar (CP) stars are based on the determination of some quantities related to the magnetic morphology, which are obtained through the analysis of Stokes I and V profiles, and on observations of frequency integrated Stokes Q and U. In previous papers of this series we set up a formalism aimed at describing the magnetic field of CP stars in terms of a superposition of a dipole and a quadrupole field, and we proposed a method to recover the magnetic morphology from the combined analysis of those observable quantities which can simply be expressed by means of analytical formulas, namely: the mean longitudinal field, the crossover, and the mean square field. Here we extend this method in order to include those quantities for which numerical integrations are required, i.e., the mean field modulus and the broadband linear polarisation. Estimates of stellar radius and projected equatorial velocity are also explicitly taken into account. We present an application to the well-known CP star β CrB, the only star for which all kinds of magnetic data are available. We find that β CrB, like most CP stars, exhibits a complex magnetic structure, which may not be fully accounted for even by a second-order multipolar expansion. However, we are able to suggest two models which are compatible with all the magnetic data.
The positions in the HR diagram and the kinematic characteristics of rapidly oscillating and non-... more The positions in the HR diagram and the kinematic characteristics of rapidly oscillating and non-oscillating chemically peculiar stars are obtained using new Hipparcos proper motions and parallaxes, and our own radial velocity measurements. We find that rapidly oscillating stars, as a group, are (−0.47±0.34) mag above the zero-age main sequence (ZAMS), while the non-oscillating stars are (−1.20±0.65) mag above the ZAMS and so appear slightly more evolved on average. From the comparison of the kinematical characteristics, we conclude that both groups are very similar. The results of radial velocity measurements indicate that there is a real deficiency of binaries among rapidly oscillating stars. Presently, no such star is known to be a spectroscopic binary.
In a high resolution spectroscopic study of the rapidly oscillating Ap (roAp) star HD 99563 with ... more In a high resolution spectroscopic study of the rapidly oscillating Ap (roAp) star HD 99563 with the Ultraviolet-Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) we have discovered remarkably large amplitude pulsations with some spectral lines showing radial velocity amplitudes up to 5 km s −1 (10 km s −1 peak-to-peak) with a pulsation period of 10.7 min.
We have completed a new study of the formation of hydrogen lines in dense magnetized plasmas. We ... more We have completed a new study of the formation of hydrogen lines in dense magnetized plasmas. We have developed a global formalism including the effects of the magnetic field and of the motional electric field. The latter, resulting from the motion of the radiating hydrogen atom in the magnetic field, modifies the structure of the matrices appearing in the equation of transfer of the Stokes vector and introduces a coupling between microscopic ("intrinsic") and macroscopic (Doppler) contributions to the line profiles. We have used this new treatment of line formation to revisit the interpretation of photopolarimetric (Stokes I and V) observations of Balmer lines in terms of mean longitudinal magnetic field.
The rapidly oscillating Ap stars are of importance for studying the atmospheric structure of star... more The rapidly oscillating Ap stars are of importance for studying the atmospheric structure of stars where the process of chemical element diffusion is significant. We have performed a survey for rapid oscillations in a sample of 9 luminous Ap stars, selected from their location in the colour-magnitude diagram as more evolved main-sequence Ap stars that are inside the instability strip for rapidly oscillating Ap (roAp) stars. Until recently this region was devoid of stars with observed rapid pulsations. We used the VLT UV-Visual Echelle Spectrograph (UVES) to obtain high time resolution spectroscopy to make the first systematic spectroscopic search for rapid oscillations in this region of the roAp instability strip. We report 9 null-detections with upper limits for radial-velocity amplitudes of 20 − 65 ms −1 and precisions of σ = 7 − 20 ms −1 for combinations of Nd and Pr lines. Cross-correlations confirm these null-results. At least six stars are magnetic and we provide magnetic fiel...
The rapidly oscillating Ap (roAp) stars with very strong magnetic fields are of special interest ... more The rapidly oscillating Ap (roAp) stars with very strong magnetic fields are of special interest because the effect of the magnetic field on the structure of their atmospheres can be studied with the greatest detail and accuracy. The cool roAp star HD 154708 possesses the strongest magnetic field among the roAp stars, with a mean magnetic field modulus = 24.5 ± 1.0 kG, which is about a factor of three greater than that of HD 166473, <B〉 ˜ 5.5-9.0 kG, the roAp star with the second-strongest magnetic field.
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Papers by Gautier Mathys