A magnetic field study of two fast-rotating, radio bright M dwarfs. StKM 1-1262 and V374 Peg

TL;DR

This study leverages SPIRou spectropolarimetry to map both total and large-scale magnetic fields in two fast-rotating M dwarfs, StKM 1-1262 and V374 Peg, detected by LOFAR at low radio frequencies. By combining Zeeman broadening analyses with Zeeman-Doppler imaging and Doppler imaging, the authors derive atmospheric parameters, total surface fields ($ angle B_I angle$ ≈ 3.53 kG and 5.46 kG) and large-scale fields ($ angle B_V angle$ ≈ 300 G and 780 G), revealing predominantly poloidal, dipolar, axisymmetric topologies (StKM moderately axisymmetric; V374 Peg highly axisymmetric). They find a strong anti-correlation between total magnetic flux and effective temperature for StKM ($ ho=-0.96$) and a moderate anti-correlation for V374 Peg ($ ho=-0.43$), linking surface inhomogeneities to magnetic flux. The brightness maps show dark spots consistent with the magnetic maps, with long-term stability, particularly for V374 Peg, underscoring the value of time-series spectropolarimetry for interpreting stellar magnetism and its radio signatures in exoplanetary environments.

Abstract

Radio observations at low frequencies are sensitive to the magnetic activity of stars and the plasma environment surrounding them. The accurate interpretation of the processes underlying the radio signatures requires a detailed characterisation of the stellar magnetism. We study two M dwarfs, StKM 1-1262 (M0 type, P$_\mathrm{rot}=1.24$ d) and V374 Peg (M4 type, P$_\mathrm{rot}=0.4455$ d), which were detected with the LOw Frequency ARray (LOFAR). StKM 1-1262 exhibited a type-II radio burst, potentially resulting from a coronal mass ejection event. V374 Peg manifested low-frequency radio emission typical of an electron-cyclotron maser instability emission mechanism. We analysed spectropolarimetric observations of these M dwarfs collected with the SpectroPolarimètre InfraRouge (SPIRou). Firstly, we refined the stellar parameters such as effective temperature, surface gravity, and metallicity, and measured the average surface magnetic flux via modelling of Zeeman broadening in unpolarised spectra. We then applied Zeeman-Doppler imaging to least-squares deconvolution line profiles in circular polarisation to reconstruct their large-scale magnetic fields. StKM 1-1262 has a total, unsigned magnetic field of $3.53\pm0.06$ kG on average and the large-scale magnetic field topology is dipolar and moderately axisymmetric, with an average strength of 300 G. V374 Peg has an unsigned magnetic field of $5.46\pm0.09$ kG and the large-scale field is dipolar and axisymmetric, with an average strength of 800 G. For StKM 1-1262, we found a strong anti-correlation between the total magnetic field and the effective temperature which is reminiscent of the tight link between small-scale magnetic fields and surface inhomogeneities. For V374 Peg, we found a moderate anti-correlation, possibly due to a more even distribution of surface features. (Abridged)

Figures (16)

• Figure 1: Distribution of the magnetic flux on the different magnetic components. The top panel refers to StKM 1-1262 and the bottom panel to V374 Peg.
• Figure 2: Total magnetic field measurements of StKM 1-1262 and V374 Peg as a function of rotational phase. The data points are colour-coded with the effective temperature obtained from spectral modelling (see Sect. \ref{['sec:ZBro']}). The rotational phases are computed according to Eq. \ref{['eq:ephemeris']}, using the stellar rotation period listed in Table \ref{['tab:stars_properties']}.
• Figure 3: Longitudinal magnetic field measurements of StKM 1-1262 and V374 Peg. The panels show B$_\ell$ as a function of rotational phase for StKM 1-1262 and V374 Peg. The rotational phases are computed according to Eq. \ref{['eq:ephemeris']}, using the rotational period listed in Table \ref{['tab:stars_properties']}.
• Figure 4: Long-term measurements of V374 Peg's longitudinal magnetic field. The leftmost epochs (couloured with light brown) represent the August 2005, August 2006, and September 2009 time series analysed by Morin2008a. The rightmost (dark brown) time series corresponds to the data analysed in this work.
• Figure 5: Reconstructed large-scale magnetic field maps in flattened polar view. StKM 1-1262 is shown in the top row and V374 Peg in the bottom row. From the left, the radial, azimuthal, and meridional components of the magnetic field vector are illustrated. Concentric circles represent different stellar latitudes: -30 $^{\circ}$, +30 $^{\circ}$, and +60 $^{\circ}$ (dashed lines), as well as the equator (solid line). The radial ticks are located at the rotational phases when the observations were collected. The rotational phases are computed with Eq. \ref{['eq:ephemeris']} using the first observation of each individual epoch (see Table \ref{['tab:log']}). The colour bar indicates the polarity and strength (in G) of the magnetic field.