G. Mathys

Magnetic Ap Stars in the Hertzsprung-Russell Diagram*

The evolutionary state of magnetic Ap stars is rediscussed using the recently released Hipparcos data. The distribution of the magnetic Ap stars of mass below 3 M☉ in the H-R diagram differs from that of the normal stars in the same temperature range at a high level of significance. Magnetic stars are concentrated toward the center of the main-sequence band. This is shown in two forms of the H-R diagram: one where log L is plotted against log Teff and a version more directly tied to the observed quantities, showing the astrometry-based luminosity (Arenou & Luri) against the (B2-G)0 index of Geneva photometry. In particular, it is found that magnetic fields appear only in stars that have already completed at least approximately 30% of their main-sequence lifetime. No clear picture emerges as to the possible evolution of the magnetic field across the main sequence. Hints of some (loose) relations between magnetic field strength and other stellar parameters are found: stars with shorter periods tend to have stronger fields, as do higher temperature and higher mass stars. A marginal trend of the magnetic flux to be lower in more slowly rotating stars may possibly be seen as suggesting a dynamo origin for the field. No correlation between the rotation period and the fraction of the main-sequence lifetime completed is observed, indicating that the slow rotation in these stars must already have been achieved before they became observably magnetic.

Discovery of magnetic fields in the βCephei star ξ1 CMa and in several slowly pulsating B stars

We present the results of a magnetic survey of a sample of eight β Cephei stars and 26 slowly pulsating B (SPBs) stars with the FOcal Reducer low dispersion Spectrograph at the Very Large Telescope. A weak mean longitudinal magnetic field of the order of a few hundred Gauss is detected in the β Cephei star ξ 1 CMa and in 13 SPB stars. The star ξ 1 CMa becomes the third magnetic star among the β Cephei stars. Before our study, the star ζ Cas was the only known magnetic SPB star. All magnetic SPB stars for which we gathered several magnetic field measurements show a field that varies in time. We do not find a relation between the evolution of the magnetic field with stellar age in our small sample. Our observations imply that β Cephei and SPB stars can no longer be considered as classes of non-magnetic pulsators, but the effect of the fields on the oscillation properties remains to be studied.

Measurements of magnetic fields over the pulsation cycle in six roAp stars with FORS 1 at the VLT

Received xx/Accepted yy Abstract. With FORS1 at the VLT we have tried for the first time to measure the magnetic field variation over the pulsation cycle in six roAp stars to begin the study of how the magnetic field and pulsation interact. For the star HD101065, which has one of the highest photometric pulsation amplitudes of any roAp star, we found a signal at the known photometric pulsation frequency at the 3� level in one data set; however this could not be confirmed by later observations. A preliminary simple calculation of the expected magnetic variations over the pulsation cycle suggests that they are of the same order as our current noise levels, leading us to expect that further observations with increased S/N have a good chance of achieving an unequivocal detection.

New measurements of magnetic fields of roAp stars with FORS 1 at the VLT

Magnetic fields play a key role in the pulsations of rapidly oscillating Ap (roAp) stars since they are a necessary in- gredient of all pulsation excitation mechanisms proposed so far. This implies that the proper understanding of the seismological behaviour of the roAp stars requires knowledge of their magnetic fields. However, the magnetic fields of the roAp stars are not well studied. Here we present new results of measurements of the mean longitudinal field of 14 roAp stars obtained from low resolution spectropolarimetry with FORS 1 at the VLT.

Detection of an extraordinarily large magnetic field in the unique ultra-cool Ap star HD 154708

We have discovered an extraordinarily large mean longitudinal magnetic field of 7.5 kG in the ultra-cool low mass Ap star HD 154708 using FORS 1 in spectropolarimetric mode. From UVES spectra, we have measured a mean magnetic field modulus of 24.5 kG. This is the second-largest mean magnetic field modulus ever measured in an Ap star. Furthermore, it is very likely that this star is one of the coolest and least massive among the Ap stars and is located in the H-R diagram in the same region in which rapidly oscillating Ap stars have been detected. We note that all known roAp stars have much smaller magnetic fields, by at least a factor of three.

Discovery of kilogauss magnetic fields in three DA white dwarfs

We have detected longitudinal magnetic fields between 2 and 4 kG in three (WD 0446−790, WD 1105−048, WD 2359−434) out of a sample of 12 normal DA white dwarfs by using optical spectropolarimetry done with the VLT Antu 8 m telescope equipped with FORS1. With the exception of 40 Eri B (4 kG) these are the first positive detections of magnetic fields in white dwarfs below 30 kG. Although suspected, it was not clear whether a significant fraction of white dwarfs contain magnetic fields at this level. These fields may be explained as relics from magnetic fields in the main-sequence progenitors considerably enhanced by magnetic flux conservation during the shrinkage of the core. A detection rate of 25% (3/12) may indicate now for the first time that a substantial fraction of white dwarfs have a weak magnetic field. This result, if confirmed by future observations, would form a cornerstone for our understanding of the evolution of stellar magnetic fields.

A statistical analysis of the magnetic structure of CP stars

We present the results of a statistical study of the magnetic structure of upper main sequence chemically peculiar stars. We have modelled a sample of 34 stars, assuming that the magnetic morphology is described by the superposition of a dipole and a quadrupole field, arbitrarily oriented. In order to interpret the modelling results, we have introduced a novel set of angles that provides one with a convenient way to represent the mutual orientation of the quadrupolar component, the dipolar component, and the rotation axis. Some of our results are similar to what has already been found in previous studies, e.g., that the inclination of the dipole axis to the rotation axis is usually large for short-period stars and small for long-period ones - see Landstreet & Mathys (2000). We also found that for short-period stars (approximately P< 10 days) the plane containing the two unit vectors that characterise the quadrupole is almost coincident with the plane containing the stellar rotation axis and the dipole axis. Long-period stars seem to be preferentially characterised by a quadrupole orientation such that the planes just mentioned are perpendicular. There is also some loose indication of a continuous transition between the two classes of stars with increasing rotational period.

Evolution of magnetic fields in stars across the upper main sequence: I. Catalogue of magnetic field measurements with FORS 1 at the VLT

To properly understand the physics of Ap and Bp stars it is particularly important to identify the origin of their magnetic fields. For that, an accurate knowledge of the evolutionary state of stars that have a measured magnetic field is an important diagnostic. Previous results based on a small and possibly biased sample suggest that the distribution of magnetic stars with mass below 3M⊙ in the H-R diagram differs from that of normal stars in the same mass range (Hubrig et al. 2000). In contrast, higher mass magnetic Bp stars may well occupy the whole main-sequence width (Hubrig, Scholler & North 2005b). In order to rediscuss the evolutionary state of upper main sequence magnetic stars, we define a larger and bias-free sample of Ap and Bp stars with accurate Hipparcos parallaxes and reliably determined longitudinal magnetic fields. We used FORS 1 at the VLT in its spectropolarimetric mode to measure the magnetic field in chemically peculiar stars where it was unknown or poorly known as yet. In this first paper we present our results of the mean longitudinal magnetic field measurements in 136 stars. Our sample consists of 105 Ap and Bp stars, two PGa stars, 17 HgMn stars, three normal stars, and nine SPB stars. A magnetic field was for the first time detected in 57 Ap and Bp stars, in four HgMn stars, one Pga star, one normal B-type star and four SPB stars. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Discovery of 17 new sharp-lined Ap stars with magnetically resolved lines ⋆

Chemically peculiar A stars (Ap) are extreme examples of the interaction of atomic element diffusion processes with magnetic fields in stellar atmospheres. The rapidly oscillating Ap stars provide a means for studying these processes in three dimensions and are at the same time important for studying the pulsation excitation mechanism in A stars. As part of the first comprehensive, uniform, high-resolution spectroscopic survey of Ap stars, which we are conducting in the Southern hemisphere with the Michigan Spectral Catalogues as the basis of target selection, we report here the discovery of 17 new magnetic Ap stars having spectroscopically resolved Zeeman components from which we derive magnetic field moduli in the range 3–30 kG. Among these are (1) the current second strongest known magnetic A star, (2) a double-lined Ap binary with a magnetic component and (3) an A star with particularly peculiar and variable abundances. Polarimetry of these stars is needed to constrain their field geometries and to determine their rotation periods. We have also obtained an additional measurement of the magnetic field of the Ap star HD 92499.

A 3D study of the photosphere of HD 99563 – I. Pulsation analysis★

We have used high-speed spectroscopy of the rapidly oscillating Ap (roAp) star HD 99563 to study the pulsation amplitude and phase behaviour of elements in its stratified atmosphere over one 2.91-d rotation cycle. We identify spectral features related to patches in the surface distribution of chemical elements and study the pulsation amplitudes and phases as the patches move across the stellar disc. The variations are consistent with a distorted non-radial dipole pulsation mode. We measure a 1.6 km s ―1 rotational variation in the mean radial velocities of Hα and argue that this is the first observation of Hα abundance spots caused by He settling through suppression of convection by the magnetic field on an oblique rotator, in support of a prime theory for the excitation mechanism of roAp star pulsation. We demonstrate that HD 99563 is the second roAp star to show aspect dependence of blue-to-red running wave line profile variations in Nd m spots.