Mean Longitudinal Magnetic Field Research Articles

The mean longitudinal magnetic field and its uses in radial-velocity surveys

ABSTRACT This work focuses on the analysis of the mean longitudinal magnetic field as a stellar activity tracer in the context of small exoplanet detection and characterization in radial-velocity (RV) surveys. We use Solar Dynamics Observatory/Helioseismic and Magnetic Imager filtergrams to derive Sun-as-a-star magnetic field measurements, and show that the mean longitudinal magnetic field is an excellent rotational period detector and a useful tracer of the solar magnetic cycle. To put these results into context, we compare the mean longitudinal magnetic field to three common activity proxies derived from HARPS-N Sun-as-a-star data: the full width at half-maximum, the bisector span, and the S-index. The mean longitudinal magnetic field does not correlate with the RVs and therefore cannot be used as a one-to-one proxy. However, with high cadence and a long baseline, the mean longitudinal magnetic field outperforms all other considered proxies as a solar rotational period detector, and can be used to inform our understanding of the physical processes happening on the surface of the Sun. We also test the mean longitudinal magnetic field as a ‘stellar proxy’ on a reduced solar data set to simulate stellar-like observational sampling. With a Gaussian Process regression analysis, we confirm that the solar mean longitudinal magnetic field is the most effective of the considered indicators, and is the most efficient rotational period indicator over different levels of stellar activity. This work highlights the need for polarimetric time series observations of stars.

Statistical relations between spectropolarimetric observables and the polar strength of the stellar dipolar magnetic field

Global magnetic fields of early-type stars are commonly characterised by the mean longitudinal magnetic field 〈Bz〉 and the mean field modulus 〈B〉, derived from the circular polarisation and intensity spectra, respectively. Observational studies often report a root mean square (rms) of 〈Bz〉 and an average value of 〈B〉. In this work, I used numerical simulations to establish statistical relationships between these cumulative magnetic observables and the polar strength, Bd, of a dipolar magnetic field. I show that in the limit of many measurements randomly distributed in rotational phase, 〈Bz〉rms = 0.179−0.043+0.031 Bd and 〈B〉avg = 0.691−0.023+0.020 Bd. The same values can be recovered with only three measurements, provided that the observations are distributed uniformly in the rotational phase. These conversion factors are suitable for ensemble analyses of large stellar samples, where each target is covered by a small number of magnetic measurements.

Extreme magnetic field modulus variability of the Bp star HD 57372

In chemically peculiar Ap/Bp stars with large-scale organised magnetic fields of simple centred dipole configuration, the ratio between the maximum and the minimum of the mean magnetic field modulus is on the order of 1.25. Values of two or more are observed only for very few Ap/Bp stars and are indicative of a very unusual magnetic field geometry. Determining the magnetic field structure of Ap/Bp stars is bound to provide a different insight into the physics and the origin of the magnetic fields in early-type stars. In this respect, the Bp star HD\,57372 is of particular interest because strongly variable magnetically split lines have been observed in HARPS and APOGEE spectra. We obtained and analysed measurements of the mean magnetic field modulus and of the mean longitudinal magnetic field using near-infrared spectra and optical polarimetric spectra distributed over the stellar rotation period. The mean magnetic field modulus bm of HD\,57372, as estimated from absorption lines that are split via the Zeeman effect and resolved in both optical and near-infrared spectra, is found to vary by an extraordinary amount: about 10\,kG. The exceptional value of three for the ratio between the maximum and the minimum of the field modulus is indicative of a very unusual geometry for HD\,57372's magnetic field. All observable quantities were found to vary in phase with the photometric period of 7.889\,days. This includes the longitudinal magnetic field bz , which varies from $-6$\,kG up to $1.7$\,kG in FORS2 spectra, as well as the metal line strengths, whose equivalent widths change by up to 50<!PCT!> of their mean values over the course of the rotation period. The B8 temperature class of HD\,57372 also places it among the hottest stars known to exhibit resolved, magnetically split lines.

Characterizing the dynamical magnetosphere of the extremely slowly rotating magnetic O9.7 V star HD 54879 using rotational modulation of the Hα profile

AbstractThe magnetic field in the O9.7 V star HD 54879 has been monitored for almost a decade. Spectropolarimetric observations reveal a rather strong mean longitudinal magnetic field that varies with a period of about 7.41 years. Observations in the H line show a variation with the same period, while the H line shows only little variation. Assuming the periodic variation to be caused by a slow rotation and a dipolar magnetic field, we find a magnetic field strength of 2 kG at the magnetic poles. With the relatively low mass loss rate of year−1, this star is a case of extremely strong magnetic confinement. Both theoretical arguments and numerical simulations indicate the presence of an extended disk of increased gas density in the equatorial plane of the magnetic field, where gas from the line‐driven stellar wind is trapped. This disk is likely to be the origin of the observed H emission, which peaks together with the strongest line‐of‐sight magnetic field. The profile of the H line is resolved in several components and shows a remarkable variability with the rotation period.

Spectropolarimetry of magnetic Chemically Peculiar stars in the Orion OB1 association

ABSTRACT We summarize the results of a spectropolarimetric survey of 56 chemically peculiar (CP) stars in the association of Orion OB1. We uniformly collected the observational material with the 6-m telescope BTA of the Special Astrophysical Observatory in 2013–2021. We identify 14 new magnetic CP stars with a longitudinal magnetic field exceeding approximately 500 G. The studied sample contains 31 magnetic stars or 55 per cent of the whole CP population in Orion OB1. We show that the percentage of the magnetic CP stars and the field strength drop sharply with age. The mean longitudinal magnetic field in the young subgroup OB1b (log t = 6.23) is confidently almost three times stronger than in the older subgroups OB1a (log t = 7.05) and OB1c (log t = 6.66). In the Orion Nebula, a place with the youngest stellar population (log t < 6.0), we detect the magnetic field only in 20 per cent of CP stars. Such an occurrence drastically differs from 83 per cent of magnetic CP stars in the nearby subgroup OB1c. We consider this effect an observational bias caused by a significant portion of a very young population with the signatures of Herbig Ae/Be stars. The technique we used for magnetic measurements and the quality of available data do not allow us to detect weak fields in the case of stars with a limited number of lines and emissions in spectra.

Magnetic field geometry and magnetospheric environment of the strongly magnetic Of?p star NGC 1624-2

ABSTRACT NGC 1624-2 is an O7f?p star with a reported probable polar magnetic field strength ≥20 kG, which is the strongest magnetic field ever measured in an O-type star. We study the variability of the mean longitudinal magnetic field 〈Bz〉 and the mean field modulus 〈B〉 to obtain constraints on its field geometry. Only one magnetic pole is observable over the rotation cycle. The approximately sinusoidal variation of 〈Bz〉 and the ratio of the values of the extrema of 〈B〉 indicate that there is an important component of the field that is dipolar. The 〈Bz〉 values measured over the rotation cycle are in the range from −0.2 to 4.5 kG, whereas the values for 〈B〉 vary between 9 and 12 kG. The 〈Bz〉 values obtained using the O iii λ7455 emission line are in the range from 0.4 to 2.3 kG and show a variability pattern similar to that detected for the absorption lines. The fact that the phase of the 〈Bz〉 minimum coincides with the phase of the 〈B〉 maximum, indicates that the field structure must significantly depart from a centred dipole. Further, we discuss the nature of the observed variable Stokes V profiles corresponding to a longitudinal field of negative polarity detected in the emission He i lines and present the first magnetohydrodynamical numerical simulations of the gas flow in the magnetosphere of this star.

The search for magnetic fields in two Wolf–Rayet stars and the discovery of a variable magnetic field in WR 55

ABSTRACT Magnetic fields in Wolf–Rayet (WR) stars are not well explored, although there is indirect evidence, e.g. from spectral variability and X-ray emission, that magnetic fields should be present in these stars. Being in an advanced stage of their evolution, WR stars have lost their hydrogen envelope, but their dense winds make the stellar core almost unobservable. To substantiate the expectations on the presence of magnetic fields in the most-evolved massive stars, we selected two WR stars, WR 46 and WR 55, for the search of the presence of magnetic fields using FORS 2 spectropolarimetric observations. We achieve a formally definite detection of a variable mean longitudinal magnetic field of the order of a few hundred gauss in WR 55. The field detection in this star, which is associated with the ring nebula RCW 78 and the molecular environment, is of exceptional importance for our understanding of star formation. No field detection at a significance level of 3σ was achieved for WR 46, but the variability of the measured field strengths can be rather well phased with the rotation period of 15.5 h previously suggested by FUSE(Far Ultraviolet Spectroscopic Explorer) observations.

The anomalous atmospheric structure of the strongly magnetic Ap star HD 166473

ABSTRACT High-resolution spectropolarimetric observations of the strongly magnetic, superslowly rotating rapidly oscillating Ap star HD 166473 are used to investigate the implications of the presence of a variable strong magnetic field on the vertical and surface horizontal distribution of various chemical elements. The analysis of the calculated least-squares deconvolution Stokes I and V profiles confirms the previously reported detection of non-uniform horizontal surface distribution of several chemical elements. To test the vertical abundance stratification of iron peak and rare earth elements, magnetic field measurements were carried out using spectral lines of these elements belonging to neutral and ionized stages. We find clear indication of the existence of a relation between the magnetic field strength and its orientation and vertical element stratification: magnetic field values obtained for elements in different stages close to the magnetic equator are rather similar, whereas the dispersion in field strengths is remarkably large in the regions close to magnetic field poles. At the phases of negative and positive extrema the mean longitudinal field strength determined from the analysis of the rare-earth element lines is usually stronger than when using Fe and Cr. The strongest mean longitudinal magnetic field, up to −4160 ± 226 G, is detected using the La ii line list at the negative extremum, followed by the measurements using the Pr iii lines with 〈Bz〉=−3740 ± 343 G and the Ce ii lines with 〈Bz〉 = −3372 ± 247 G. The strongest mean longitudinal magnetic field of positive polarity, up to 3584 ± 354 G is detected using the Pr iii lines, followed by the measurement 〈Bz〉 = 2517 ± 249 G using the Ce ii lines.

Spin-orbit alignment and magnetic activity in the young planetary system AU Mic

We present high-resolution near-infrared spectropolarimetric observations using the SPIRou instrument at Canada-France-Hawaii Telescope (CFHT) during a transit of the recently detected young planet AU Mic b, with supporting spectroscopic data from iSHELL at NASA InfraRed Telescope Facility. We detect Zeeman signatures in the Stokes V profiles and measure a mean longitudinal magnetic field of ¯Bℓ = 46.3 ± 0.7 G. Rotationally modulated magnetic spots likely cause long-term variations of the field with a slope of dBℓ/dt = −108.7 ± 7.7 G d−1. We apply the cross-correlation technique to measure line profiles and obtain radial velocities through CCF template matching. We find an empirical linear relationship between radial velocity and Bℓ, which allows us to estimate the radial-velocity induced by stellar activity through rotational modulation of spots for the five hours of continuous monitoring of AU Mic with SPIRou. We model the corrected radial velocities for the classical Rossiter-McLaughlin effect, using MCMC to sample the posterior distribution of the model parameters. This analysis shows that the orbit of AU Mic b is prograde and aligned with the stellar rotation axis with a sky-projected spin-orbit obliquity of λ = 0°−15°+18°. The aligned orbit of AU Mic b indicates that it formed in the protoplanetary disk that evolved into the current debris disk around AU Mic.

A Magnetic Snapshot Survey of F-Type Stars

Abstract We present a spectropolarimetric magnetic snapshot survey of 55 stars which includes 53 F-type stars ranging from spectral types F0 to F9 plus 2 chemically peculiar stars β CrB, and δ Cap. We look for magnetic fields in stars spanning a range of effective temperatures where the transition from fossil to dynamo magnetic fields is believed to occur. High-resolution spectropolarimetry using circularly polarized spectra is used to look for a magnetic detection in the Stokes V profile, determine the mean longitudinal magnetic field (Bl), and to look for correlations with stellar parameters. Surface magnetic fields are detected on 14 F-stars, and present in every spectral class from F3V-F9V ranging in strength from 0.3 ± 0.1 G (36 UMa, F8V) to 8.3 ± 0.9 G (h Dra, F8V). Thus we find photospheric magnetic fields are present in stars as early as spectral type F3V with an outer convection zone thickness less than a few per cent of the stellar radius.

The 10.5 year rotation period of the strongly magnetic rapidly oscillating Ap star HD 166473

How magnetic fields contribute to the differentiation of the rotation rates of the Ap stars and affect the occurrence of non-radial pulsation in some of them are important open questions. Valuable insight can be gained into these questions by studying some of the most extreme examples of the processes at play. The super-slowly rotating rapidly oscillating Ap (roAp) star HD 166473 is such an example. We performed the first accurate determination of its rotation period, Prot = (3836 ± 30) d, from the analysis of 56 measurements of the mean magnetic field modulus ⟨B⟩ based on high-resolution spectra acquired between 1992 and 2019 at various observatories and with various instrumental configurations. We complemented this analysis with the consideration of an inhomogeneous set of 21 determinations of the mean longitudinal magnetic field ⟨Bz⟩ spanning the same time interval. This makes HD 166473 one of only four Ap stars with a period longer than 10 years for which magnetic field measurements have been obtained over more than a full cycle. The variation curves of ⟨B⟩ and of ⟨Bz⟩ are well approximated by cosine waves. The magnetic field of HD 166473 only seems to deviate slightly from axisymmetry, but it definitely involves a considerable non-dipolar component. Among the stars with rotation periods longer than 1000 d for which magnetic field measurements with full phase coverage are available, HD 166473 has the strongest field. Its magnetic field is also one of the strongest known among roAp stars. Overall, the magnetic properties of HD 166473 do not seem fundamentally distinct from those of the faster-rotating Ap stars. However, considering as a group the eight Ap stars that have accurately determined periods longer than 1000 d and whose magnetic variations have been characterised over a full cycle suggests that the angles between their magnetic and rotation axes tend to be systematically large.

The very slow rotation of the magnetic O9.7 V star HD 54879

ABSTRACT The first FOcal Reducer low dispersion Spectrograph (FORS 2) spectropolarimetric observation of the longitudinal magnetic field of HD 54879 of the order of −600 G with a lower limit of the dipole strength of ∼2 kG dates back to 2014. Since then, observations showed a gradual decrease of the absolute value of the mean longitudinal magnetic field. In the course of the most recent monitoring of HD 54879 using FORS 2 spectropolarimetric observations from 2017 October to 2018 February, a longitudinal magnetic field strength change from about −300 G down to about −90 G was reported. A sudden increase of the absolute value of the mean longitudinal magnetic field and an accompanying spectral variability was detected on 2018 February 17. New FORS 2 spectropolarimetric data obtained from 2018 December to 2019 February confirm the very slow magnetic field variability, with the field decreasing from about 150 G to −100 G over 2 months. Such a slow magnetic field variability, related to the extremely slow rotation of HD 54879, is also confirmed using high-resolution High Accuracy Radial velocity Planet Searcher in polarimetric mode and Echelle SpectroPolarimetric Device for the Observation of Stars spectropolarimetry. The re-analysis of the FORS 2 polarimetric spectra from 2018 February indicates that the previously reported field increase and the change of the spectral appearance was caused by improper spectra extraction and wavelength calibration using observations obtained at an insufficient signal-to-noise ratio. The presented properties of HD 54879 are discussed in the context of the Of?p spectral classification.

Using machine learning algorithms to measure stellar magnetic fields

Context.Regression methods based on machine learning algorithms (MLA) have become an important tool for data analysis in many different disciplines.Aims.In this work, we use MLA in an astrophysical context; our goal is to measure the mean longitudinal magnetic field in stars (Heff) from polarized spectra of high resolution, through the inversion of the so-called multi-line profiles.Methods.Using synthetic data, we tested the performance of our technique considering different noise levels: In an ideal scenario of noise-free multi-line profiles, the inversion results are excellent; however, the accuracy of the inversions diminish considerably when noise is taken into account. We therefore propose a data pre-process in order to reduce the noise impact, which consists of a denoising profile process combined with an iterative inversion methodology.Results.Applying this data pre-process, we find a considerable improvement of the inversions results, allowing to estimate the errors associated to the measurements of stellar magnetic fields at different noise levels.Conclusions.We have successfully applied our data analysis technique to two different stars, attaining for the first time the measurement ofHefffrom multi-line profiles beyond the condition of line autosimilarity assumed by other techniques.

Magnetic field detections in Herbig Ae SB2 systems

AbstractStudies of the presence of magnetic fields in Herbig Ae/Be stars are extremely important because they enable us to improve our insight into how the magnetic fields of these stars are generated and how they interact with their environment, including their impact on the planet formation process and the planet-disk interaction. We report new detections of weak mean longitudinal magnetic fields in the close Herbig Ae double-lined spectroscopic binary AK Sco and in the presumed spectroscopic Herbig Ae binary HD 95881 (Järvinen et al. 2018) based on observations obtained with HARPSpol attached to ESO’s 3.6 m telescope. Such studies are important because only very few close spectroscopic binaries with orbital periods below 20 d are known among Herbig Ae stars. Our detections favour the conclusion that the previously suggested low incidence (5-10%) of magnetic Herbig Ae stars can be explained by the weakness of these fields and the limited accuracy of the published measurements. The search for magnetic fields and the determination of their geometries in close binary systems will play an important role for understanding the mechanisms that are responsible for the magnetic field generation.

Weak Magnetic Fields in Two Herbig Ae Systems: The SB2 AK Sco and the Presumed Binary HD 95881

Abstract We report the detection of weak mean longitudinal magnetic fields in the Herbig Ae double-lined spectroscopic binary AK Sco and in the presumed spectroscopic Herbig Ae binary HD 95881 using observations with the High Accuracy Radial velocity Planet Searcher polarimeter (HARPSpol) attached to the European Southern Observatory’s (ESO’s) 3.6 m telescope. Employing a multi-line singular value decomposition method, we detect a mean longitudinal magnetic field G in the secondary component of AK Sco on one occasion. For HD 95881, we measure G and G at two different observing epochs. For all the detections the false alarm probability is smaller than 10−5. For AK Sco system, we discover that accretion diagnostic Na i doublet lines and photospheric lines show intensity variations over the observing nights. The double-lined spectral appearance of HD 95881 is presented here for the first time.

Magnetic field topology and chemical spot distributions of the Ap star HD 119419

Context.Analysis of high-resolution spectropolarimetric time-series observations of early-type magnetic stars is currently the most advanced method of obtaining detailed information on their surface magnetic field topologies and horizontal spot distributions.Aims.In this study we analyse a new set of high-quality full Stokes vector observations of the magnetic Ap star HD 119419 – a member of the 14 Myr old Lower Cen-Cru association – for the purpose of studying the surface field topology and mapping the chemical abundance spots.Methods.We made use of the circular and linear polarisation data collected for HD 119419 with the HARPSpol instrument at the ESO 3.6-m telescope. These observations were analysed with a multi-line magnetic diagnostic technique and modelled in detail with a Magnetic Doppler imaging (MDI) code.Results.We present a new set of high-precision mean longitudinal magnetic field measurements and derive a revised stellar rotational period by comparing our measurements with the literature data. We also redetermine the basic stellar atmospheric parameters. Our four Stokes parameter magnetic inversions reveal a moderately complex surface field topology with a mean field strength of 18 kG and a maximum local strength of 24 kG. A poloidal dipolar component dominates the magnetic energy spectrum of the surface field in HD 119419. However, significant contributions of the higher-order spherical harmonic components are also present. We show that the dipole plus quadrupole part of the reconstructed field geometry is incapable of reproducing the observed amplitudes and shapes of the StokesQandUprofiles. The chemical abundance distributions of Fe, Cr, Ti, and Nd, derived self-consistently with the magnetic field geometry, are characterised by large abundance gradients and a lack of clear correlation with the magnetic field structure.Conclusions.This full Stokes vector analysis of HD 119419 extends the modern hot-star magnetic mapping investigations to an open cluster Ap star with a well-determined age. Further, MDI studies of cluster members will allow us to study the field topologies and chemical abundance spots as a function of stellar age.

Mixed poloidal–toroidal magnetic configuration and surface abundance distributions of the Bp star 36 Lyn★

Abstract Previous studies of the chemically peculiar Bp star 36 Lyn revealed a moderately strong magnetic field, circumstellar material and inhomogeneous surface abundance distributions of certain elements. We present in this paper an analysis of 33 high signal-to-noise ratio, high-resolution Stokes IV observations of 36 Lyn obtained with the Narval spectropolarimeter at the Bernard Lyot Telescope at Pic du Midi Observatory. From these data, we compute new measurements of the mean longitudinal magnetic field, Bℓ, using the multiline least-squares deconvolution (LSD) technique. A rotationally phased Bℓ curve reveals a strong magnetic field, with indications for deviation from a pure dipole field. We derive magnetic maps and chemical abundance distributions from the LSD profiles, produced using the Zeeman–Doppler imaging code InversLSD. Using a spherical harmonic expansion to characterize the magnetic field, we find that the harmonic energy is concentrated predominantly in the dipole mode (ℓ = 1), with significant contribution from both the poloidal and toroidal components. This toroidal field component is predicted theoretically, but not typically observed for Ap/Bp stars. Chemical abundance maps reveal a helium enhancement in a distinct region where the radial magnetic field is strong. Silicon enhancements are located in two regions, also where the radial field is stronger. Titanium and iron enhancements are slightly offset from the helium enhancements, and are located in areas where the radial field is weak, close to the magnetic equator.

Rotationally modulated variability and pulsations of the He-rich star CPD −62°2124 with an extraordinarily strong magnetic field

A longitudinal magnetic field with a strength of 5.2 kG was recently detected in CPD −62°2124, which has a fractional main-sequence lifetime of about 60 per cent. Strongly magnetic early-B type chemically peculiar stars in an advanced evolutionary state are of special interest to understand the evolution of the angular momentum and spin-down time-scales in the presence of a global magnetic field. We made use of 17 FORS 2 low-resolution spectropolarimetric observations and 844 ASAS3 photometric measurements for the determination of the rotation period, pulsationsand the magnetic field geometry of the star. We calculated periodograms and applied phenomenological models of photometric, spectral and spectropolarimetric variability. We found that all quantities studied, specifically equivalent widths, the mean longitudinal magnetic field 〈Bz〉 and the flux in the V filter, vary with the same period P = 2.628 d, which was identified as the rotation period. The observed variations can be fully explained by a rigidly rotating main-sequence star with an uneven distribution of chemical elements, photometric spots and a stable, nearly dipolar magnetic field with a polar field strength of about 21 kG, frozen into the body of the star. The magnetic field of CPD −62°2124 is tilted to the rotation axis by β = 28° ± 7°, while the inclination of the rotation axis towards the line of sight is only i = 20° ± 5°. In the acquired FORS 2 spectra, we detect short-term line profile variations indicating the presence of β Cephei type pulsations. As of today, no other pulsating star of this type is known to possess such a strong magnetic field.

Magnetic field geometry and chemical abundance distribution of the He-strong star CPD −57°3509

The magnetic field of CPD -57 3509 was recently detected in the framework of the BOB (B fields in OB stars) collaboration. We acquired low-resolution spectropolarimetric observations of CPD -57 3509 with FORS2 and high-resolution UVES observations randomly distributed over a few months to search for periodicity, to study the magnetic field geometry, and to determine the surface distribution of silicon and helium. We also obtained supplementary photometric observations at a timeline similar to the spectroscopic and spectropolarimetric observations. A period of 6.36d was detected in the measurements of the mean longitudinal magnetic field. A sinusoidal fit to our measurements allowed us to constrain the magnetic field geometry and estimate the dipole strength in the range of 3.9-4.5kG. Our application of the Doppler imaging technique revealed the presence of He I spots located around the magnetic poles, with a strong concentration at the positive pole and a weaker one around the negative pole. In contrast, high concentration Si III spots are located close to the magnetic equator. Further, our analysis of the spectral variability of CPD -57 3509 on short time scales indicates distinct changes in shape and position of line profiles possibly caused by the presence of beta Cep-like pulsations. A small periodic variability in line with the changes of the magnetic field strength is clearly seen in the photometric data.

Ap stars with resolved magnetically split lines: Magnetic field determinations from StokesIandVspectra

We present the results of a systematic study of the magnetic fields and other properties of the Ap stars with resolved magnetically split lines. This study is based on new measurements of the mean magnetic field modulus, the mean longitudinal magnetic field, the crossover, the mean quadratic magnetic field, and the radial velocity of 43 stars, complemented by magnetic data from the literature for 41 additional stars. Stars with resolved magnetically split lines represent a significant fraction, of the order of several percent, of the whole population of Ap stars. Most of them are genuine slow rotators, whose consideration provides new insight into the long-period tail of the distribution of the periods of the Ap stars. Emerging correlations between rotation periods and magnetic properties provide important clues for the understanding of the braking mechanisms that have been at play in the early stages of stellar evolution. The geometrical structures of the magnetic fields of Ap stars with magnetically resolved lines appear in general to depart slightly, but not extremely, from centred dipoles. However, there are a few remarkable exceptions, which deserve further consideration. We suggest that pulsational crossover can be observed in some stars; if confirmed, this would open the door to the study of non-radial pulsation modes of degree $\ell$ too high for photometric or spectroscopic observations. How the lack of short orbital periods among binaries containing an Ap component with magnetically resolved lines is related with their (extremely) slow rotation remains to be fully understood, but the very existence of acorrelation between the two periods lends support to the merger scenario for the origin of Ap stars.