Markus Schöller

Massive stars are predominantly found in binary and multiple systems that interact over the course of their lifetime. As several studies suggest that the origin of magnetic fields in massive stars involves a merging event, mass transfer, or a common envelope evolution, magnetic field measurements in stripped star systems, blue supergiants (BSGs), and Be stars, all previously suggested as binary interaction products, are of special interest. We present new magnetic field measurements for three systems, the stripped star system HR 6819, the BSG k Sco, and the Be star o Aqr, and discuss the appearance of the Zeeman features frequently observed in complex systems.

Magnetic fields are considered to be key components of massive stars, with a far-reaching impact on their evolution and ultimate fate. A magnetic mechanism was suggested for the collimated explosion of massive stars, relevant for long-duration gamma-ray bursts, X-ray flashes, and asymmetric core collapse supernovae. However, the origin of the observed stable, globally organized magnetic fields in massive stars is still a matter of debate: it has been argued that they can be fossil, dynamo generated, or generated by strong binary interactions or merging events. Taking into account that multiplicity is a fundamental characteristic of massive stars, observational evidence is accumulating that the magnetism originates through interaction between the system components, both during the initial mass transfer or when the stellar cores merge.

Context. While about two dozen Herbig Ae/Be stars have been reported to be magnetic, only two of them, HD 101412 and HD 190073, have had their magnetic field geometries studied in the past. The knowledge of the magnetic field structure is important to understanding how magnetospheric accretion works in these stars. Aims. We aim to study in detail the spectral and magnetic variability of HD 179218, which is necessary to put constraints on its magnetic field geometry. Methods. We measured the mean longitudinal magnetic field, ⟨ B _z ⟩, from newly acquired and archival high-resolution spectropolarimetric observations of HD 179218 using the least-squares deconvolution technique. Additionally, we studied the spectral variability of the hydrogen lines using dynamical spectra. Results. Based on our analysis of the Stokes V spectra of HD 179218, we report for the first time the definite detection of a magnetic field. Using a slightly refined rotation period of P _rot = 1.34102 d, we constrained its geometry as follows: an estimated magnetic obliquity angle of β = 79.9 ± 0.7° and a dipole strength of B _d = 2142 ± 52 G. The ⟨ B _z ⟩ variation is best fitted by the superposition of a sine wave and of its first harmonic, but more spectropolarimetric observations are necessary to test the impact of the limited measurement precision and the uneven coverage of the rotation cycle. The strongest emission in the H α and H β line profiles in the medium-resolution spectra acquired in 2025 was detected close to the phases of the best visibility of the magnetic poles. HD 179218 is the second Herbig Ae/Be star after HD 190073 for which a first snapshot of a magnetosphere is presented.

Context. Recent observational studies suggest that Be stars most likely are formed through the process of mass transfer in binary systems. In view of the wide consensus that the origin of the magnetic field in stars with radiative envelopes involves binary interaction processes, searching for magnetic fields in Be stars appears especially promising. Aims. As a pilot project, we searched for the presence of magnetic fields in a sample of seven well-known Be stars. Methods. We used high-resolution HARPSpol spectra to measure the mean longitudinal magnetic field, employing the least squares deconvolution technique. A dedicated measurement procedure introduced by our group in recent years was applied. Results. Opposite to previous spectropolarimetric studies reporting that magnetic fields in Be stars are weak and usually below 100 G, our study presents the first observational evidence that magnetic fields in Be stars can be as strong as a few hundred gauss. Magnetic fields are detected in all studied Be stars, with the strongest magnetic field being about −460 G for the B0.5 III star HD 184915. Magnetic fields in the range between 338 and 380 G (in absolute values) are detected in three other Be stars, HD 209409, HD 209522, and HD 224686. Due to the fact that magnetic fields in Be stars are stronger than previously believed, we must re-evaluate our understanding of the initial conditions of massive binaries to be able to determine the origin of such systems.

Context . Despite the importance of magnetic fields in massive stars, their origin is widely debated and still not well understood. Aims. With the mounting evidence for the importance of studying magnetic fields in interacting massive binary and multiple systems, it appears necessary to investigate the presence of magnetic fields in semi-detached systems with ongoing mass transfer, and in contact systems where mass is actively being exchanged. Methods . We present an analysis of 53 high-resolution HARPSpol spectropolarimetric observations of a sample of 14 massive binary and multiple systems using the least-squares deconvolution technique. The majority of the studied systems are classified as semi-detached or contact binaries. Results . Definite detections of the presence of a magnetic field are achieved in all studied systems apart from the rather faint system SV Cen, for which only a marginal detection was obtained. The fact that the presence of magnetic fields is detected in all but one of the studied systems strongly suggests that interaction between the system components plays a definite role in the generation of magnetic fields in massive stars. The measured mean longitudinal magnetic field strength for all targets is of the order of a few hundred Gauss to a few kiloGauss. The strongest longitudinal magnetic fields of 4–5 kG are discovered in the massive O-type triple system MY Ser in both components of the contact binary. kiloGauss-order magnetic fields are also detected in two other systems, V1294 Sco and V606 Cen. It is possible that there is an implication of some system characteristics, such as multiplicity, the mass ratio between the components, and a large fillout factor, on the measured magnetic field strength. Our results for the magnetic field measurements in interacting binaries present the first assessment of the occurrence rate of magnetic fields in a representative sample of such systems.

Context. Recent observations of the accretion disk around the Herbig Ae/Be star HD 169142 have revealed its complex and asymmetric morphology that indicates the presence of planets. The knowledge of the magnetic field structure in host stars is indispensable for our understanding of the magnetospheric interaction between the central stars, the circumstellar (CS) environment, and the planetary companions. Aims. We want to study the geometry of the magnetic field of HD 169142. Methods. We measured the mean longitudinal magnetic field from high-resolution ESPaDOnS and HARPSpol spectra of HD 169142 using the least-squares deconvolution technique. Additionally, the spectral variability of hydrogen lines is studied using dynamical spectra. Results. Our analysis of the Stokes V spectra reveals the presence of definitely detected narrow Zeeman features observed using line masks with neutral iron lines. On two observing epochs, we also obtain marginally detected broad Zeeman features. To explain the simultaneous appearance of narrow and broad Zeeman features, we discuss different scenarios, including one scenario related to a non-photospheric origin of the narrow Zeeman features due to magnetospheric interaction with warm CS matter. In an environment such as a wind or an accretion disk, spectral lines may form over a relatively large volume, and the field topology may therefore be complex not only in latitude and azimuth, but in radius as well. Dynamical plots of the Hβ line show an intriguing very complex structure with appearing and disappearing absorption features, which can be related to the complex morphology of the CS matter with asymmetric dust clump structures. The profiles of spectral lines belonging to different elements are variable, indicating the presence of chemical spots.

Context. The Herbig Ae/Be star HD 190073 is one of the very few magnetic Herbig Ae/Be stars for which close low-mass companions have been reported. Previously published magnetic field measurements indicated an annual change in the field configuration. Aims. We aim to study in detail the spectral and magnetic variability of this star and characterise its magnetosphere for the first time. Methods. Newly acquired and archival spectropolarimetric observations are combined to determine a more precise magnetic period and to constrain the geometry of the magnetic field. The variability of hydrogen line profiles is studied using dynamical spectra. Archival X-shooter observations of the He I 10 830 Å triplet are used to characterise its variability over the rotation cycle. Further, we carry out 2D magnetohydrodynamical (MHD) simulations of the magnetosphere using the NIRVANA MHD code. Results. From the spectropolarimetric observations, we determine for HD 190073 a magnetic period P = 51.70 ± 0.06 d. We estimate a magnetic obliquity angle β = 82.9 ± 6.4° and a dipole strength B_d = 222 ± 66 G. Our dynamical spectra constructed for the hydrogen line profiles observed during 2011 clearly reveal a ringlike magnetospheric structure appearing at the rotation phase of best visibility of the positive magnetic pole. These spectra present the first snapshot of a magnetosphere around a Herbig Ae/Be star. 2D MHD simulations involving nonisothermal gas show that the magnetosphere is compact, with a radius of about 3 R_*, and that the wind flow extends over tens of R_*. With a reported radius of the accretion disk of 1.14 au around HD 190073, the distance between the star and the disk is about 25 R_*. The detection of a magnetosphere around HD 190073, and the possible presence of lower-mass companions at different distances, make this system a valuable laboratory for studying the magnetic interaction between the host star, its companions, and the accretion disk.

Context. Studies of the magnetic characteristics of massive stars have recently received significant attention because they are progenitors of highly magnetised compact objects. Stars initially more massive than about 8 M_⊙ leave behind neutron stars and black holes by the end of their evolution. The merging of binary compact remnant systems produces astrophysical transients detectable by gravitational wave observatories. Studies of magnetic fields in massive stars with low metallicities are of particular interest because they provide important information on the role of magnetic fields in the star formation of the early Universe. Aims. While several detections of massive Galactic magnetic stars have been reported in the last few decades, the impact of a low-metallicity environment on the occurrence and strength of stellar magnetic fields has not yet been explored. Because of the similarity between Of?p stars in the Magellanic Clouds (MCs) and Galactic magnetic Of?p stars, which possess globally organised magnetic fields, we searched for magnetic fields in Of?p stars in the MCs. Additionally, we observed the massive contact binary Cl NGC 346 SSN7 in the Small Magellanic Cloud to test the theoretical scenario that the origin of magnetic fields involves a merger event or a common envelope evolution. Methods. We obtained and analysed measurements of the magnetic field in four massive Of?p stars in the MCs and the binary Cl NGC 346 SSN7 using the ESO/VLT FORS2 spectrograph in spectropolarimetric mode. Results. We detected kilogauss-scale magnetic fields in two Of?p-type stars and in the contact binary Cl NGC 346 SSN7. These results suggest that the impact of low metallicity on the occurrence and strength of magnetic fields in massive stars is low. However, because the explored stellar sample is very small, additional observations of massive stars in the MCs are necessary.

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.

Tight relationships exist in the local Universe between the central stellar properties of galaxies and the mass of their supermassive black hole (SMBH)^1–3. These suggest that galaxies and black holes co-evolve, with the main regulation mechanism being energetic feedback from accretion onto the black hole during its quasar phase^4–6. A crucial question is how the relationship between black holes and galaxies evolves with time; a key epoch to examine this relationship is at the peaks of star formation and black hole growth 8–12 billion years ago (redshifts 1–3)⁷. Here we report a dynamical measurement of the mass of the black hole in a luminous quasar at a redshift of 2, with a look back in time of 11 billion years, by spatially resolving the broad-line region (BLR). We detect a 40-μas (0.31-pc) spatial offset between the red and blue photocentres of the Hα line that traces the velocity gradient of a rotating BLR. The flux and differential phase spectra are well reproduced by a thick, moderately inclined disk of gas clouds within the sphere of influence of a central black hole with a mass of 3.2 × 10⁸ solar masses. Molecular gas data reveal a dynamical mass for the host galaxy of 6 × 10¹¹ solar masses, which indicates an undermassive black hole accreting at a super-Eddington rate. This suggests a host galaxy that grew faster than the SMBH, indicating a delay between galaxy and black hole formation for some systems.