Strong Surface Magnetic Fields Research Articles

The escape of fast radio burst emission from magnetars

ABSTRACT We reconsider the escape of high-brightness coherent emission of fast radio bursts (FRBs) from magnetars’ magnetospheres, and conclude that there are numerous ways for the powerful FRB pulse to avoid non-linear absorption. Sufficiently strong surface magnetic fields, $\ge 10{{\ \rm per\ cent}}$ of the quantum field, limit the waves’ non-linearity to moderate values. For weaker fields, the electric field experienced by a particle is limited by a combined ponderomotive and parallel-adiabatic forward acceleration of charges by the incoming FRB pulse along the magnetic field lines newly opened during FRB/coronal mass ejection. As a result, particles surf the weaker front part of the pulse, experiencing low radiative losses, and are cleared from the magnetosphere for the bulk of the pulse to propagate. We also find that initial mildly relativistic radial plasma flow further reduces losses.

Testing pulsation diagnostics in the rapidly oscillating magnetic Ap star γ Equ using near-infrared CRIRES+ observations

Context. Pulsations of rapidly oscillating Ap stars and their interaction with the stellar magnetic field have not been studied in the near-infrared (near-IR) region despite the benefits these observations offer compared to visual wavelengths. The main advantage of the near-IR is the quadratic dependence of the Zeeman effect on the wavelength, as opposed to the linear dependence of the Doppler effect. Aims. To test pulsation diagnostics of roAp stars in the near-IR, we aim to investigate the pulsation behaviour of one of the brightest magnetic roAp stars, γ Equ, which possesses a strong surface magnetic field of the order of several kilogauss and exhibits magnetically split spectral lines in its spectra. Methods. Two magnetically split spectral lines belonging to different elements, the triplet Fe I at 1563.63 nm and the pseudo-doublet Ce III at 1629.2 nm, were recorded with CRIRES+ over about one hour in the H band with the aim of understanding the character of the line profile variability and the pulsation behaviour of the magnetic field modulus. Results. The profile shapes of both studied magnetically split spectral lines vary in a rather complex manner probably due to a significant decrease in the strength of the longitudinal field component and an increase in the strength of the transverse field components over the last decade. A mean magnetic field modulus of 3.9 kG was determined for the Zeeman triplet Fe I at 1563.63 nm, whereas for the pseudo-doublet Ce III at 1629.2 nm we observe a much lower value of only about 2.9 kG. For comparison, a mean field modulus of 3.4kG was determined using the Zeeman doublet Fe II at 6249.25 in optical PEPSI spectra recorded just about two weeks before the CRIRES+ observations. Different effects that may lead to the differences in the field modulus values are discussed. Our measurements of the mean magnetic field modulus using the line profiles recorded in different pulsational phase bins suggest a field modulus variability of 32 G for the Zeeman triplet Fe I at 1563.63 nm and 102 G for the pseudo-doublet Ce III at 1629.2 nm.

Using helium 10 830 Å transits to constrain planetary magnetic fields

ABSTRACT Planetary magnetic fields can affect the predicted mass-loss rate for close-in planets that experience large amounts of ultraviolet irradiation. In this work, we present a method to detect the magnetic fields of close-in exoplanets undergoing atmospheric escape using transit spectroscopy at the 10 830 Å line of helium. Motivated by previous work on hydrodynamic and magnetohydrodynamic photoevaporation, we suggest that planets with magnetic fields that are too weak to control the outflow’s topology lead to blueshifted transits due to dayside-to-nightside flows. In contrast, strong magnetic fields prevent this day-to-night flow, as the gas is forced to follow the magnetic field’s roughly dipolar topology. We post-process existing 2D photoevaporation simulations, computing synthetic transit profiles in helium to test this concept. As expected, we find that hydrodynamically dominated outflows lead to blueshifted transits of the order of the sound speed of the gas. Strong surface magnetic fields lead to unshifted or slightly redshifted transit profiles. High-resolution observations can distinguish between these profiles; however, eccentricity uncertainties generally mean that we cannot conclusively say that velocity shifts are due to the outflow for individual planets. The majority of helium observations are blueshifted, which could be a tentative indication that close-in planets generally have surface dipole magnetic field strengths $\lesssim \!\! 0.3$ G. More 3D hydrodynamic and magnetohydrodynamic simulations are needed to confirm this conclusion robustly.

Gamma Ray Pulsars and Opportunities for the MACE Telescope

Rapidly rotating neutron stars with very strong surface magnetic fields are observed to emit pulsed emission in the whole range of electromagnetic spectrum from radio to high-energy gamma rays. These so-called pulsars are known for their exceptional rotational stability. The radio emission from pulsars is generally believed to be powered by the rotational energy of neutron stars. More than 3000 pulsars have been currently known from radio observations; however, only about 10% are observed in the high-energy gamma ray band. The Fermi-LAT observations in the energy range above 100 MeV have discovered more than 300 pulsars. However, the origin of high-energy non-thermal radiation from pulsars is not completely understood and remains an active area of research. In this contribution, we report a summary of observational features of the gamma ray pulsars and briefly discuss observability for the MACE gamma ray telescope, which has just started its regular science operation at Hanle in India. Six gamma ray pulsars, other than the well-known Crab and Geminga, are identified as probable candidates for MACE observations.

Nature of Intense Magnetism and Differential Rotation in Convective Dynamos of M-dwarf Stars with Tachoclines

Many of the M-dwarf stars, though they are tiny and dim, are observed to possess strong surface magnetic fields and exhibit remarkably intense flaring. Such magnetism may severely impact habitability on the exoplanets now discovered nearby. The origin of the magnetism must rest with dynamo action achieved by turbulent convection coupled to rotation within the M-dwarfs. To further explore the nature and diversity of the magnetism that can result, we turn here to an extensive set of 45 global MHD simulations to explore dynamos operating within deep convective envelopes of rapidly rotating M2 (0.4 M ⊙) stars. We observe a wide range of cycle periods present in the convection zones, whose durations we find to scale with the Rossby number as Ro−1.66±0.07 in concurrence with scalings identified in simulations of more massive stars. We find a unifying relationship between the ratio of magnetic to convective kinetic energy (ME/CKE) and the degree to which the differential rotation is quenched by magnetic fields. We show that the presence of a tachocline in these model stars enhances their axisymmetric magnetic field components, leading to a surface dipole fraction on average 78% greater than an equivalent star with only a CZ, potentially shedding light on the nature of the tachocline divide through resultant effects on the spin-down rate.

Monte Carlo Simulations on Possible Collimation Effects of Outflows to Fan Beam Emission of Ultraluminous Accreting X-Ray Pulsars

Pulsating ultraluminous X-ray sources (PULXs) are accreting pulsars with apparent X-ray luminosity exceeding 1039 erg s−1. We perform Monte Carlo simulations to investigate whether a high collimation effect (or strong beaming effect) is dominant in the presence of accretion outflows, for the fan beam emission of the accretion column of the neutron stars in PULXs. We show that the three nearby PULXs (RX J0209.6−7427, Swift J0243.6+6124, and SMC X-3), namely, the Three Musketeers here, have their main pulsed emission not strongly collimated even if strong outflows exist. This conclusion can be extended to the current sample of extragalactic PULXs, if accretion outflows are commonly produced from them. This means that the observed high luminosity of PULXs is indeed intrinsic, which can be used to infer the existence of very strong surface magnetic fields of ∼1013–14 G, possibly multipole fields. However, if strong outflows are launched from the accretion disks in PULXs as a consequence of disk spherization by radiation pressure, regular dipole magnetic fields of ∼1012 G may be required, comparable to that of the Three Musketeers, which have experienced large luminosity changes from well below their Eddington limit (2 × 1038 erg s−1 for an NS) to super-Eddington and whose maximum luminosity fills the luminosity gap between Galactic pulsars and extragalactic PULXs.

High-energy Photon Opacity in the Twisted Magnetospheres of Magnetars

Magnetars are neutron stars characterized by strong surface magnetic fields generally exceeding the quantum critical value of 44.1 TG. High-energy photons propagating in their magnetospheres can be attenuated by QED processes like photon splitting and magnetic pair creation. In this paper, we compute the opacities due to photon splitting and pair creation by photons emitted anywhere in the magnetosphere of a magnetar. Axisymmetric, twisted dipole field configurations embedded in the Schwarzschild metric are treated. The paper computes the maximum energies for photon transparency that permit propagation to infinity in curved spacetime. Special emphasis is given to cases where photons are generated along magnetic field loops and/or in polar regions; these cases directly relate to resonant inverse Compton scattering models for the hard X-ray emission from magnetars and Comptonized soft gamma-ray emission from giant flares. We find that increases in magnetospheric twists raise or lower photon opacities, depending on both the emission locale and the competition between field-line straightening and field strength enhancement. Consequently, given the implicit spectral transparency of hard X-ray bursts and persistent “tail” emission of magnetars, photon splitting considerations constrain their emission region locales and the twist angle of the magnetosphere; these constraints can be probed by future soft gamma-ray telescopes such as COSI and AMEGO. The inclusion of twists generally increases the opaque volume of pair creation by photons above its threshold, except when photons are emitted in polar regions and approximately parallel to the field.

Starspots, chromospheric emission lines, and flares of zero-age main-sequence stars

Abstract Zero-age main-sequence (ZAMS) stars are considered to have enormous starspots and show strong chromospheric emission lines because of their strong surface magnetic field. We discuss the dynamo activities of ZAMS stars with respect to their periodic light variation caused by a starspot and with respect to the strength of the chromospheric emission lines. The light curves of 33 ZAMS stars in IC 2391 and IC 2602 were obtained from TESS (Transiting Exoplanet Survey Satellite) photometric data. The light curves can be grouped into the following four categories: single frequency, possible shape changer, beater, and complex variability. The amplitudes of the light curves are 0.001–0.145 mag, similar to those of ZAMS stars in Pleiades. The starspot coverages are $0.1\%$–$21\%$. We found that the light variations and Ca ii emission line strength of ZAMS stars in IC 2391, IC 2602, and the Pleiades cluster are as large as those of the most active superflare stars and two orders larger than those of the Sun, and are located on the extensions of the superflare stars. These results suggest that superflare stars link the properties of the Sun to those of the ZAMS stars of ages between 30 and 120 Myr. ZAMS stars with a single frequency or possible shape change in the light curve tend to have both large light variation, indicating large spot coverage, and saturated Ca ii emission line strength. ZAMS stars with beat or complex variability have small spot coverage and a faint Ca ii emission line. We also detected 21 flares in the TESS light curves of 12 ZAMS stars in IC 2391 and IC 2602, where most of these stars have saturated chromospheric Ca ii emission lines. The energies of the flares are estimated to be ∼1033–1035 erg, which is comparable with the energy of a superflare.

News from Gaia on σ Ori E: A case study for the wind magnetic braking process

Context.σ Ori E, a massive helium B-type star, shows high surface rotation and a strong surface magnetic field, potentially challenging the process of wind magnetic braking. Aims. The Gaia satellite provides an accurate distance to σ Ori E and confirms its membership to the σ Ori cluster. We account for these two key pieces of information in order to investigate whether single star models can reproduce the observed properties of σ Ori E and provide new estimates for its metallicity, mass, and age. Methods. We computed rotating stellar models accounting for wind magnetic braking and magnetic quenching of the mass loss. We considered two metallicities (Z = 0.014, with a helium mass fraction Y = 0.273 and Z = 0.020 with Y = 0.266), four initial masses between 8 and 9 M⊙, three initial rotations between 250 and 450 km s−1, and three initial surface equatorial magnetic field between 3 and 7 kG. Differential rotation is assumed for the internal rotation in all models. We looked for models simultaneously accounting for the observed radius, position in the HR diagram, surface velocity, and braking timescale. Results. We obtain that σ Ori E is a very young star (age less than 1 Myr) with an initial mass of around 9 M⊙, a surface equatorial magnetic field of around 7 kG, and a metallicity Z (mass fraction of heavy elements) of around 0.020. No solution is obtained with the present models for a metallicity of Z = 0.014. The initial rotation of the models fitting σ Ori E is not highly constrained and could be anywhere in the range studied here. Because of its very young age, models predict no observable changes of the surface abundances due to rotational mixing. Conclusions. The simultaneous high surface rotation and high surface magnetic field of σ Ori E may simply be a consequence of its young age. This young age implies that the processes responsible for producing the chemical inhomogeneities that are observed at its surface should be rapid. Therefore, for explaining the properties of σ Ori E, there is no necessity to invoke a merging event, although such a scenario cannot be discarded. Other stars (HR 5907, HR 7355, HR 345439, HD 2347, CPD –50°3509) showing similar properties to σ Ori E (fast rotation and strong surface magnetic field) may also be very young stars, although determination of the braking timescales is needed to confirm such a conclusion.

Brown dwarfs as ideal candidates for detecting UV aurora outside the Solar System: Hubble Space Telescope observations of 2MASS J1237+6526

Context. Observations of auroral emissions are powerful means to remotely sense the space plasma environment around planetary bodies and ultracool dwarfs. Therefore successful searches and characterization of aurorae outside the Solar System will open new avenues in the area of extrasolar space physics. Aims. We aim to demonstrate that brown dwarfs are ideal objects to search for UV aurora outside the Solar System. We specifically search for UV aurora on the late-type T6.5 brown dwarf 2MASS J12373919+6526148 (in the following 2MASS J1237+6526). Methods. Introducing a parameter referred to as auroral power potential, we derive scaling models for auroral powers for rotationally driven aurora applicable to a broad range of wavelengths. We also analyze Hubble Space Telescope observations obtained with the STIS camera at near-UV, far-UV, and Ly-α wavelengths of 2MASS J1237+6526. Results. We show that brown dwarfs, due to their typically strong surface magnetic fields and fast rotation, can produce auroral UV powers on the order of 1019 watt or more. Considering their negligible thermal UV emission, their potentially powerful auroral emissions make brown dwarfs ideal candidates for detecting extrasolar aurorae. We find possible emission from 2MASS J1237+6526, but cannot conclusively attribute it to the brown dwarf due to low signal-to-noise values in combination with nonsystematic trends in the background fluxes. The observations provide upper limits for the emission at various UV wavelength bands. The upper limits for the emission correspond to a UV luminosity of ~1 × 1019 watt, which lies in the range of the theoretically expected values. Conclusions. The possible auroral emission from the dwarf could be produced by a close-in companion and/or magnetospheric transport processes.

NGC 6611 601: a hot pre-main-sequence spectroscopic binary containing a centrifugal magnetosphere host star

ABSTRACT W 601 (NGC 6611 601) is one of the handful of known magnetic Herbig Ae/Be stars. We report the analysis of a large data set of high-resolution spectropolarimetry. The star is a previously unreported spectroscopic binary, consisting of two B2 stars with a mass ratio of 1.8, masses of 12 and 6.2 M⊙, in an eccentric 110-d orbit. The magnetic field belongs to the secondary, W 601 B. The H α emission is consistent with an origin in W 601 B’s centrifugal magnetosphere; the star is therefore not a classical Herbig Be star in the sense that its emission is not formed in an accretion disc. However, the low value of log g = 3.8 determined via spectroscopic analysis and the star’s membership in the young NGC 6611 cluster are most consistent with it being on the pre-main sequence. The rotational period inferred from the variability of the H α line and the longitudinal magnetic field 〈Bz〉 is 1.13 d. Modelling of Stokes V and 〈Bz〉 indicates a surface dipolar magnetic field Bd between 6 and 11 kG. With its strong emission, rapid rotation, and strong surface magnetic field, W 601 B is likely a precursor to H α-bright magnetic B-type stars such as σ Ori E. By contrast, the primary is an apparently non-magnetic (Bd < 300 G) pre-main-sequence early B-type star. In accordance with expectations from magnetic braking, the non-magnetic primary is apparently more rapidly rotating than the magnetic star.

Strong surface magnetic field generation in relativistic short pulse laser–plasma interaction with an applied seed magnetic field

While plasma often behaves diamagnetically, we demonstrate that the laser irradiation of a thin opaque target with an embedded target-transverse seed magnetic field Bseed can trigger the generation of an order-of-magnitude stronger magnetic field with opposite sign at the target surface. Strong surface field generation occurs when the laser pulse is relativistically intense and results from the currents associated with the cyclotron rotation of laser-heated electrons transiting through the target and the compensating current of cold electrons. We derive a predictive scaling for this surface field generation, Bgen ∼ −2πBseedΔx/λ0 (in the large spot size limit), where Δx is the target thickness and λ0 is the laser wavelength, and conduct 1D and 2D particle-in-cell simulations to confirm its applicability over a wide range of conditions. We additionally demonstrate that both the seed and surface-generated magnetic fields can have a strong impact on application-relevant plasma dynamics, for example substantially altering the overall expansion and ion acceleration from a μm-thick laser-irradiated target with a kilotesla-level seed magnetic field.

Line-drag damping of Alfvén waves in radiatively driven winds of magnetic massive stars

ABSTRACT Line-driven stellar winds from massive (OB) stars are subject to a strong line-deshadowing instability. Recently, spectropolarimetric surveys have collected ample evidence that a subset of Galactic massive stars hosts strong surface magnetic fields. We investigate here the propagation and stability of magnetoradiative waves in such a magnetized, line-driven wind. Our analytic, linear stability analysis includes line-scattering from the stellar radiation, and accounts for both radial and non-radial perturbations. We establish a bridging law for arbitrary perturbation wavelength after which we analyse separately the long- and short-wavelength limits. While long-wavelength radiative and magnetic waves are found to be completely decoupled, a key result is that short-wavelength, radially propagating Alfvén waves couple to the scattered radiation field and are strongly damped due to the line-drag effect. This damping of magnetic waves in a scattering-line-driven flow could have important effects on regulating the non-linear wind dynamics, and so might also have strong influence on observational diagnostics of the wind structure and clumping of magnetic line-driven winds.

Parameter Comparison of Several Types of Pulsars

We have carried out a comparative analysis of the parameters of two samples of radio pulsars. The first of them is characterized by periods $$P > 2$$ s, the second one—by the surface magnetic fields $${{B}_{s}} > 4.4 \times $$ 1013 G. The purpose for selecting these values is for the understanding the origin of the difference between normal pulsars and anomalous X-ray pulsars (AXP) and gamma-repeaters (SGR) with similar values of the periods and magnetic fields. It is shown that the derivative dependence of the period on its value $$P$$ in the long-period pulsars does not agree with the model of magneto-dipole braking and requires other mechanisms or the search for specific distributions of their parameters. In the pulsars with large magnetic fields and in AXP/SGR, this dependence does not contradict the magnetic dipole model. For pulsars with $$P > 2$$ s, the transformation efficiency of the rotation energy into the observed radiation turns out to be several times higher than for the objects with shorter periods. The pulsars with long periods have, on average, two orders of magnitude larger age than the sources with $${{B}_{s}} > 4.4 \times $$ 1013 G and AXP/SGR, and are, on average, located higher above the Galactic plane. It is concluded that the main physical parameters of the pulsars with large surface magnetic fields and their spatial distribution are close to the parameters and distribution of AXP/SGR. The comparison shows that it is not the rotation period, but the strong surface magnetic field is the decisive factor for the AXP/SGR peculiarity.

Massive Black Holes Regulated by Luminous Blue Variable Mass Loss and Magnetic Fields

Abstract We investigate the effects of mass loss during the main-sequence (MS) and post-MS phases of massive star evolution on black hole (BH) birth masses. We compute solar metallicity Geneva stellar evolution models of an 85 star with mass-loss rate ( ) prescriptions for MS and post-MS phases and analyze under which conditions such models could lead to very massive BHs. Based on the observational constraints for of luminous stars, we discuss two possible scenarios that could produce massive BHs at high metallicity. First, if a massive BH progenitor evolves from the observed population of massive MS stars known as WNh stars, we show that its average post-MS mass-loss rate has to be less than . However, this is lower than the typical observed mass-loss rates of luminous blue variables (LBV). Second, a massive BH progenitor could evolve from a yet undetected population of 80–85 stars with strong surface magnetic fields, which could quench mass loss during the evolution. In this case, the average mass-loss rate during the post-MS LBV phase has to be less than 5 × 10−5 to produce 70 BHs. We suggest that LBVs that explode as SNe have large envelopes and small cores that could be prone to explosion, possibly evolving from binary interaction (either mergers or mass gainers that do not fully mix). Conversely, LBVs that directly collapse to BHs could have evolved from massive single stars or binary-star mergers that fully mix, possessing large cores that would favor BH formation.

Chandra X-ray study confirms that the magnetic standard Ap star KQ Vel hosts a neutron star companion

Context. KQ Vel is a peculiar A0p star with a strong surface magnetic field of about 7.5 kG. It has a slow rotational period of nearly 8 years. Bailey et al. (A&A, 575, A115) detected a binary companion of uncertain nature and suggested that it might be a neutron star or a black hole. Aims. We analyze X-ray data obtained by the Chandra telescope to ascertain information about the stellar magnetic field and/or interaction between the star and its companion. Methods. We confirm previous X-ray detections of KQ Vel with a relatively high X-ray luminosity of 2 × 1030 erg s−1. The X-ray spectra suggest the presence of hot gas at > 20 MK and, possibly, of a nonthermal component. The X-ray light curves are variable, but data with better quality are needed to determine a periodicity, if any. Results. We interpret the X-ray spectra as a combination of two components: the nonthermal emission arising from the aurora on the A0p star, and the hot thermal plasma filling the extended shell that surrounds the “propelling” neutron star. Conclusions. We explore various alternatives, but a hybrid model involving the stellar magnetosphere along with a hot shell around the propelling neutron star seems most plausible. We speculate that KQ Vel was originally a triple system and that the Ap star is a merger product. We conclude that KQ Vel is an intermediate-mass binary consisting of a strongly magnetic main-sequence star and a neutron star.

Single-pulse Modeling and the Bi-drifting Subpulses of Radio Pulsar B1839-04

Abstract We study the bi-drifting pulsar B1839-04, where the observed subpulse drift direction in the two leading pulse components is opposite from that in the two trailing components. Such diametrically opposed apparent motions challenge our understanding of an underlying structure. We find that for the geometry spanned by the observer and the pulsar magnetic and rotation axes, the observed bi-drifting in B1839-04 can be reproduced assuming a nondipolar configuration of the surface magnetic field. Acceptable solutions are found to either have relatively weak (∼1012 G) or strong (∼1014 G) surface magnetic fields. Our single-pulse modeling shows that a global electric potential variation at the polar cap that leads to a solid-body-like rotation of spark forming regions is favorable in reproducing the observed drift characteristics. This variation of the potential additionally ensures that the variability is identical in all pulse components resulting in the observed phase locking of subpulses. Thorough and more general studies of pulsar geometry show that a low ratio of impact factor to opening angle (β/ρ) increases the likelihood of bi-drifting to be observed. We thus conclude that bi-drifting is visible when our line of sight crosses close to the magnetic pole.

A New Parameter of the Photospheric Magnetic Field to Distinguish Eruptive-flare Producing Solar Active Regions

Abstract Solar flares and coronal mass ejections (CMEs) are eruptive phenomena caused by coronal magnetic fields. In particular, large eruptive events originate in active regions (AR) with strong surface magnetic fields. However, it is still unclear what determines the capability of an AR to specifically produce eruptive flares and CMEs, and this hinders our knowledge of the initiation mechanism for the eruptive component of these phenomena. In this study, we propose a new parameter r m to measure the possibility that a flare that occurs in an AR can be eruptive and produce a CME. The parameter r m is defined by the ratio of the magnetic flux of twist higher than a threshold T c to the surrounding—and specifically, the overlying—magnetic flux. The value of r m for each AR can be estimated using nonlinear force-free field extrapolation models of the coronal magnetic field. Based on the data obtained by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, we calculated the values of r m for 29 ARs at 51 times prior to flares larger than M5.0 class. We find that the footpoints of field lines with twist higher than 0.2 can represent the subsequent flare ribbons well, and field lines that overlie and “fence in” the highly twisted region will work to confine the eruption, generating confined flares. Discriminant function analysis is used to show that r m is moderately well able to distinguish ARs that have the capability of producing eruptive flares.

Magnetic fields in M dwarfs from the CARMENES survey

Context. M dwarfs are known to generate the strongest magnetic fields among main-sequence stars with convective envelopes, but we are still lacking a consistent picture of the link between the magnetic fields and underlying dynamo mechanisms, rotation, and activity. Aims. In this work we aim to measure magnetic fields from the high-resolution near-infrared spectra taken with the CARMENES radial-velocity planet survey in a sample of 29 active M dwarfs and compare our results against stellar parameters. Methods. We used the state-of-the-art radiative transfer code to measure total magnetic flux densities from the Zeeman broadening of spectral lines and filling factors. Results. We detect strong kG magnetic fields in all our targets. In 16 stars the magnetic fields were measured for the first time. Our measurements are consistent with the magnetic field saturation in stars with rotation periods P < 4 d. The analysis of the magnetic filling factors reveal two different patterns of either very smooth distribution or a more patchy one, which can be connected to the dynamo state of the stars and/or stellar mass. Conclusions. Our measurements extend the list of M dwarfs with strong surface magnetic fields. They also allow us to better constrain the interplay between the magnetic energy, stellar rotation, and underlying dynamo action. The high spectral resolution and observations at near-infrared wavelengths are the beneficial capabilities of the CARMENES instrument that allow us to address important questions about the stellar magnetism.

The accelerating rotation of the magnetic He-weak star HD 142990

ABSTRACT HD 142990 (V 913 Sco; B5 V) is a He-weak star with a strong surface magnetic field and a short rotation period (Prot ∼ 1 d). Whilst it is clearly a rapid rotator, recent determinations of Prot are in formal disagreement. In this paper, we collect magnetic and photometric data with a combined 40-yr baseline in order to re-evaluate Prot and examine its stability. Both period analysis of individual data sets and O − C analysis of the photometric data demonstrate that Prot has decreased over the past 30 yr, violating expectations from magnetospheric braking models, but consistent with behaviour reported for 2 other hot, rapidly rotating magnetic stars, CU Vir and HD 37776. The available magnetic and photometric time series for HD 142990 can be coherently phased assuming a spin-up rate $\dot{P}$ of approximately −0.6 s yr−1, although there is some indication that $\dot{P}$ may have slowed in recent years, possibly indicating an irregular or cyclic rotational evolution.