arXiv subjects

James A. Barron

Publications and source records attributed to James A. Barron.

6 recordsLinked to original sources

arXiv2026

The rotational and magnetic properties of Polaris from long-term spectropolarimetric monitoring

Polaris is a highly unusual Cepheid with observed properties that are difficult to reconcile with stellar evolutionary models. Since the initial detection of Polaris' magnetic field in 2020, we have conducted a magnetic monitoring campaign with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope. We compute Stokes $V$ least-squares deconvolution profiles and measure the associated mean longitudinal magnetic field strengths $\langle B_{z}\rangle$. The surface magnetic field has remained remarkably stable over five years of observations, with $\langle B_{z}\rangle$ varying between approximately $-3$ G and $+0.6$ G. From the periodic modulation of $\langle B_{z}\rangle$ we infer a stellar rotation period of $P_{\mathrm{rot}}=100.29\pm0.19$ days. This is the first direct measurement of $P_{\mathrm{rot}}$ for a classical Cepheid. Previous interferometric radius measurements and $P_{\mathrm{rot}}$ imply an equatorial rotation velocity of $v_{\mathrm{eq}}=23.3\pm0.2$ km s$^{-1}$. We set a conservative upper bound on the projected equatorial rotational velocity of $v_{\mathrm{eq}}\sin i_{\star} < 13.5$ km s$^{-1}$ and constrain the stellar inclination angle to be $i_{\star}<37^{\circ}$. Using the previously determined orbital solution, we find a high likelihood of a strong spin-orbit misalignment. We determine the lower bound on the obliquity angle between the stellar rotation and orbital axes to be $β>18.7^{\circ}$ at 99% confidence. We discuss the challenges in interpreting the origin and properties of the surface magnetic field in the context of Polaris' uncertain evolutionary history and the merger hypothesis.

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arXiv2025

Discovery of a rapidly evolving global magnetic field in the M-dwarf YZ Cet and constraints on the magnetic field of its planet YZ Cet b

We present a spectropolarimetric study of the nearby M4.5V exoplanet host star YZ Cet, based on near-infrared observations obtained with the SpectroPolarimètre InfraRouge (SPIRou) at the Canada--France--Hawaii Telescope. We detect striking changes in the large-scale magnetic field strength and geometry over the course of just a few stellar rotations, a level of short-term global magnetic field evolution rarely reported in M dwarfs. We modeled the temporal variation of the longitudinal magnetic field using a Gaussian regression process, which allowed us to robustly determine the stellar rotation period and quantify the evolution timescale of the magnetic field. Independent Zeeman Doppler Imaging reconstructions of the two epochs confirm a significant reconfiguration of the star's global magnetic strength and topology. The detection of a weaker, complex, axisymmetric magnetic field (mean $|B| \sim 201$~G), which changes into a stronger, non-axisymmetric, dipole-dominated field (mean $|B| \sim 276$~G) over a few rotation cycles, is in contrast to results from similar fully convective M-dwarf stars. YZ Cet is known to exhibit polarized radio bursts potentially driven by auroral radio emission from star--planet interaction (SPI). By combining our magnetic maps with recent radio observations, we refine the constraints on the magnetic field strength of the innermost planet, YZ Cet b. These results underscore the importance of monitoring stellar magnetic variability to interpret multi-wavelength SPI signatures and to characterize the magnetospheres of potentially habitable exoplanets.

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arXiv2025

A magnetic field detection in the massive O-type bright giant 63 Oph

Surface magnetic fields are detected in less than $10\%$ of the massive O-type star population and even less frequently among `old' massive stars approaching the terminal-age main sequence (TAMS). It is unclear to what extent the rarity of magnetic detections in massive stars near the TAMS is due to magnetic field decay or observational biases. We report the detection of a weak surface magnetic field in the O-type giant 63~Oph ($T_{\mathrm{eff}}=35.0\pm0.3\,$kK, $\log g=3.51\pm0.03$) from new ESPaDOnS circularly polarized spectra. The mean longitudinal field strength associated with the magnetic detection is $\langle B_{z}\rangle=84\pm14\,$G, which we use to set a lower limit on the dipolar field strength of $B_{\mathrm{p}}\geq300\pm50\,$G. We report Balmer line equivalent widths (EW) and radial velocity (RV) measurements from the analysis of spectra primarily obtained by the IACOB project with the FEROS, FIES and HERMES spectrographs. We identify a dominant period of $\sim19.8\,$d in the EWs which we attribute to the effects of a rotating magnetosphere under the Oblique Rotator Model. We do not identify any coherent signals in a time-series analysis of archival Hipparcos, ASAS-SN and K2 photometry. Our findings show that 63~Oph may be a rare link between strongly magnetic massive stars detected on or near the zero-age main sequence and weakly-magnetic O-type supergiants. Additional observations are needed to fully constrain 63~Oph's magnetic field geometry and magnetospheric properties.

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arXiv2025

A Non-Stop Aurora? The Intriguing Radio Emission from the Rapidly Rotating Magnetic Massive Star HR 5907

HR 5907 (HD 142184) stands out among magnetic OB stars for its rapid rotation, exceptionally hard X-ray emission, and strong magnetic field. High-frequency (>5 GHz) radio emission from the star exhibits an approximately flat spectrum that can be attributed to gyrosynchrotron emission from a dense centrifugal magnetosphere. In a survey of radio emission from massive stars at sub-GHz frequencies, we noticed remarkable low-frequency radio emission from this star, characterized by high circular polarization and brightness temperature, which is inconsistent with the gyrosynchrotron model. We present a follow-up low-frequency radio study of this star with the upgraded Giant Metrewave Radio Telescope (uGMRT) in search of emission mechanisms that can go undiagnosed at higher frequencies. We detect variable radio emission characterized by varying degrees of circular polarization (15-45%) throughout the rotation cycle. The broad-band spectral fitting also suggests additional emission components at lower frequencies. We show that the observed emission is likely auroral emission via electron cyclotron maser emission (ECME), and identify this star as a Main-sequence Radio Pulse emitter (MRP). For MRPs, ECME is usually observed as short polarized enhancements near the magnetic nulls of the star. The detection of a high degree of circular polarization (>15%) at all times makes HR 5907 unique among MRPs. This is only the second MRP after $ρ$ Oph C (detected polarization fraction: 0-60%) that exhibits persistent coherent radio emission attributed to the nearly aligned stellar magnetic and rotational axes.

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arXiv2022

First results of a magnetic survey of classical Cepheids

We report recent ESPaDOnS and HARPSpol spectropolarimetric observations from our ongoing magnetic survey of the brightest twenty-five classical Cepheids. Stokes $V$ magnetic signatures are detected in eight of fifteen targets observed to date. The Stokes $V$ profiles show a diversity of morphologies with weak associated longitudinal field measurements of order 1 G. Many of the Stokes $V$ profiles are difficult to interpret in the context of the normal Zeeman effect. They consist of approximately unipolar single or double lobe(s) of positive or negative circular polarization. We hypothesize that these unusual signatures are due to the Zeeman effect modified by atmospheric velocity or magnetic field gradients. In contrast, the Stokes $V$ profiles of Polaris and MY Pup appear qualitatively similar to the complex magnetic signatures of non-pulsating cool supergiants, possibly due to the low pulsation amplitudes of these two stars.

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arXiv2022

Finding magnetic north: an extraordinary magnetic field detection in Polaris and first results of a magnetic survey of classical Cepheids

Classical Cepheids are essential objects in the study of stellar evolution and cosmology; however, we know little about their magnetic properties. We report the detection of Stokes $V$ features interpreted as Zeeman signatures in four classical Cepheids using high-resolution spectropolarimetric observations obtained with ESPaDOnS at CFHT. Eight observations of $η$ Aql were acquired in 2017 covering its 7.2 d pulsation period, and single observations of Polaris, $ζ$ Gem, $δ$ Cep and RT Aur were obtained in 2020 as part of our ongoing systematic survey. We use mean circular polarization Stokes $V$ profiles generated using the Least-Squares Deconvolution procedure to diagnose Zeeman signatures and measure mean longitudinal field strengths $\langle B_{z}\rangle$. We detect magnetic signatures across all pulsation phases of $η$ Aql ($-0.89\pm0.47$ G$\,<\langle B_{z}\rangle<1.27\pm 0.40$ G), as well as in the single observations of Polaris ($0.59\pm0.16$ G), $ζ$ Gem ($0.41\pm0.16$ G) and $δ$ Cep ($0.43\pm0.19$ G). The Stokes $V$ profile of Polaris is detected at extremely high S/N and implies a complex magnetic field topology. It stands in stark contrast to all other detected Stokes $V$ profiles, which show unusual approximately unipolar positive circular polarization lobes analogous to those observed in some Am stars.

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