Veronique Petit

Context. Magnetic fields can have an important influence on stellar structure and evolution. In intermediate-mass (A- and F-type) stars, there are many known stars with directly measured strong, globally organized magnetic fields, as well as indications of weak and/or small-scale variable fields. However, among the intermediate-mass γ Dor pulsators, there are no known stars with strong surface magnetic fields. Aims. The broad goal of this work is to search for evidence of strong, globally organized fields at the surface of γ Dor pulsators. Methods. We identified objects consistent with being γ Dor pulsators based on an analysis of space photometry from the Transiting Exoplanet Survey Satellite (TESS) mission. A spectropolarimetric survey was then conducted on a subset of 47 of these objects, with a precision sufficient to detect dipolar surface magnetic fields down to a threshold of about 10–100 G. Results. We detected strong magnetism in three targets. However, upon closer inspection, none of these appear to be genuine γ Dor pulsators. We found no evidence of surface magnetism in any of the remaining 44 objects. Conclusions. We conclude that either strong, globally organized magnetic fields and γ Dor pulsation are mutually exclusive, or that such stars are exceedingly rare. A possible explanation is that strong global fields inhibit the excitation mechanism, which prevents γ Dor pulsations from being driven in strongly magnetic intermediate-mass stars. The dipolar surface magnetic field strength upper limits we derive for this sample (≲50–100 G) provide valuable constraints for surface boundary conditions for asteroseismic models that include magnetism for γ Dor stars.

With the growing subset of magnetic massive stars, it is now possible to conduct a systematic survey of radio emission from magnetic hot stars to better understand the underlying emission mechanisms. Previous surveys of radio emission from hot star magnetospheres have focused on high frequencies (>2 GHz). At lower frequencies, additional emission and absorption mechanisms are expected, increasing the complexity of the observed emission. In this work, we survey toward lower frequencies while also increasing the sample size. We report the study of 28 magnetic hot stars with the Giant Metrewave Radio Telescope (GMRT) during cycles 27 and 28 of its operation. Among these, we found 11 detections and 17 nondetections. We also include 16 additional targets observed with GMRT from the literature. We investigated the dependence of low-frequency radio luminosity on different stellar parameters and searched for a scaling relationship at low frequencies. We further test the centrifugal breakout model for gyrosynchrotron emission. The observed low-frequency radio luminosities show a clear dependence of radio emission on magnetic field strength and rotation period, consistent with high-frequency studies. We observe a trend in scaling relationships with frequency and comment on the statistical behavior of gyrosynchrotron spectra. The observed low-frequency behavior likely reflects a combination of free–free absorption and the location of the low-frequency turnover in the gyrosynchrotron spectrum, which may vary among stars depending on their magnetospheric properties and can suppress detectable sub-GHz emission. One of the detected stars, HD 37742, is the first magnetic O-type star detected at sub-GHz frequencies.

NGC 1624-2 has the strongest detected magnetic field of all known main-sequence O-type stars. Recent work reported that existing magnetospheric emission and longitudinal magnetic field measurements did not rule out a rotation period twice as long as the established period. This period suggests a different magnetic configuration with a larger dipolar tilt, making both magnetic poles visible over a single rotation. Because previous spectropolarimetric observations did not have sufficient phase coverage to distinguish between the geometries, both were equally viable. In this paper, we present new spectropolarimetric observations obtained specifically to resolve this ambiguity. Our new magnetic measurements have a strong negative (south) polarity, confirming that the rotational period of NGC 1624-2 is indeed nearly twice as long (306.56 days) as previously thought. Our measurements show that both poles come within a similar angle to our line of sight and likely have roughly the same local magnetic field strength (with a dipolar strength of 15–20 kG or more, depending on the inclination angle).

NGC 1624-2 hosts the strongest surface magnetic field found on an O star thus far. When applied across several epochs of observations, the star’s currently accepted rotation period (157.99 days) does not coherently characterize the variations of spectral lines of magnetospheric origin. We analyze Lomb–Scargle periodograms produced with new and archival multi-instrument spectroscopic time series of Balmer H and He spectral lines. We find that 153.17 ± 0.42 days and 306.56 ± 1.19 days are both equally suitable periods at phasing the spectral and magnetic time series data in a manner consistent with the oblique rotator model. The 306.56-day period implies a magnetic geometry for NGC 1624-2 that is quite different from the previously accepted one, for which both magnetic poles should be observed during a full rotational cycle. If this is the case, the star’s magnetic south pole has yet to be observed, and additional spectropolarimetric observations should be acquired in order to confirm whether or not the south pole is in fact observable.

In this paper, we report auroral radio emission from a magnetic B star HD 142990 using the MeerKAT radio telescope at 900–1670 MHz. This star is known to produce such emission (observed as periodic radio pulses) via electron cyclotron maser emission (ECME). However, past studies on ECME from this star were confined to observations at specific rotational phase ranges where one expects to see such pulses. We, for the first time, observed the star for its one complete rotation cycle and discovered that the star also produces “off-pulse” emission, which we term as secondary enhancements. Two such enhancements were observed, one of which is left circularly polarized (LCP) and the other is right circularly polarized, the latter is confirmed to be persistent. Using simulation, we infer that such pulses are likely related to the large misalignment between the stellar rotation and magnetic dipole axes (>80°), leading to the formation of highly complex magnetospheric plasma distribution. In addition, by extracting dynamic spectra for the primary pulses, we discovered prominent fine structures in one of the LCP pulses, with timescales as small as the instrumental time resolution (8 s). This is the first time that such structures are seen from a magnetic hot star, and has the potential to reveal detailed information about how the emission is driven, and the nature of the elementary sources of radiation. To pinpoint the origin of these fine structures and their significance, higher time and spectral resolution observations should be conducted in the future.

Spectropolarimetry, the observation of polarization and intensity as a function of wavelength, is a powerful tool in stellar astrophysics. It is particularly useful for characterizing stars and circumstellar material, and for tracing the influence of magnetic fields on a host star and its environment. Maintaining modern, flexible, and accessible computational tools that enable spectropolarimetric studies is thus essential. The SpecpolFlow package is a new, completely Pythonic workflow for analyzing stellar spectropolarimetric observations. Its suite of tools provides a user-friendly interface for working with data from an assortment of instruments and telescopes. SpecpolFlow contains tools for spectral normalization and visualization, the extraction of Least-Squares Deconvolution (LSD) profiles, the generation and optimization of line masks for LSD analyses, and the calculation of longitudinal magnetic field measurements from the LSD profiles. It also provides Python classes for the manipulation of spectropolarimetric products. The SpecpolFlow website includes an array of tutorials that guide users through common analysis procedures using the software. SpecpolFlow is distributed as a free, opensource package, with fully documented tools (via an API and command line interface) which are actively maintained by a team of contributors.

We report the results of a comprehensive study of the spectroscopic binary (SB2) system HD 34736 hosting two chemically peculiar (CP) late B-type stars. Using new and archival observational data, we characterise the system and its components, including their rotation and magnetic fields. Fitting of the radial velocities yields |$P_\mathrm{orb}=83\hbox{$.\!\!^{\mathrm{d}}$}219(3)$| and e = 0.8103(3). The primary component is a CP He-wk star with T_effA = 13000 ± 500 K and υ_esin i = 75 ± 3 km s⁻¹ , while the secondary exhibits variability of Mg and Si lines, and has T_effB = 11500 ± 1000 K and υ_esin i = 110–180 km s⁻¹ . TESS and KELT photometry reveal clear variability of the primary component with a rotational period |$P_{\mathrm{rot}A}=1\hbox{$.\!\!^{\mathrm{d}}$}279\, 988\, 5(11)$|⁠, which is lengthening at a rate of 1.26(6) s yr⁻¹. For the secondary, |$P_{\mathrm{rot}B}=0\hbox{$.\!\!^{\mathrm{d}}$}522\, 693\, 8(5)$|⁠, reducing at a rate of −0.14(3) s yr⁻¹. The longitudinal component 〈B_z〉 of the primary’s strongly asymmetric global magnetic field varies from −6 to +5 kG. Weak spectropolarimetric evidence of a magnetic field is found for the secondary star. The observed X-ray and radio emission of HD 34736 may equally be linked to a suspected T Tau-like companion or magnetospheric emission from the principal components. Given the presence of a possible third magnetically active body, one can propose that the magnetic characteristics of the protostellar environment may be connected to the formation of such systems.

Main-sequence radio pulse emitters (MRPs) are magnetic early-type stars that produce coherent radio emission observed in the form of periodic radio pulses. The emission mechanism behind this is the electron-cyclotron maser emission (ECME). Among all kinds of magnetospheric emission, ECME is unique due to its high directivity and intrinsically narrow bandwidth. The emission is also highly circularly polarized and the sign of polarization is opposite for the two magnetic hemispheres. This combination of properties makes ECME highly sensitive to the three-dimensional structures in the stellar magnetospheres. This is especially significant for late-B and A-type magnetic stars that do not emit other types of magnetospheric emission such as Hα, the key probe used to trace magnetospheric densities. In this paper, we use an ultra-wideband observation (0.4–2 GHz) of a late B-type MRP HD 133880 to demonstrate how we can extract information on plasma distribution from ECME. We achieve this by examining the differences in pulse arrival times (“lags”) as a function of frequencies and qualitatively comparing those with lags obtained by simulating ECME ray paths in hot stars’ magnetospheres. This reveals that the stellar magnetosphere has a disk-like overdensity inclined to the magnetic equator with a centrally concentrated density that primarily affects the intermediate frequencies (400–800 MHz). This result, which is consistent with the recent density model proposed for hotter centrifugally supported magnetospheres, lends support to the idea of a unifying model for magnetospheric operations in early-type stars, and also provides further motivation to fully characterize the ECME phenomenon in large-scale stellar magnetospheres.

The ϵ Lupi A (HD 136504) system stands out among magnetic massive binaries as the only short-period binary system in which both components have detectable magnetic fields. The proximity of the magnetospheres of the components leads to magnetospheric interactions, which are revealed as periodic pulses in the radio light curve of this system. In this work, we aim to investigate the magnetospheric interaction phenomenon in the X-ray domain. We observed this system with the XMM-Newton telescope, covering its orbital period. We observe variable X-ray emission with maximum flux near periastron, showing similarity with radio observations. The X-ray spectra show significantly elevated hard X-ray flux during periastron. We attribute the soft X-ray emission to individual magnetospheres, while the hard X-ray emission is explained by magnetospheric interaction, particularly due to magnetic reconnection. However, unlike in the radio, we do not find any significant short-term X-ray bursts. This exotic system may be an ideal target to study magnetospheric interactions in close binaries with organized magnetospheres.