Measuring the magnetic fields in the chromospheres of low-mass stars

Abstract

Magnetic fields in the upper atmospheres of solar-like stars are believed to provide an enormous amount of energy to power the hot coronae and drive large-scale eruptions that could impact the habitability of planetary systems around these stars. However, these magnetic fields have never been routinely measured on stars beyond the solar system. Through decade-long spectropolarimetric observations, we have now achieved the measurements of magnetic fields in the lower and middle chromospheres of three M-dwarfs. Our results indicate that the line-of-sight component of the chromospheric magnetic fields can reach up to hundreds of Gauss, whose sign frequently opposes that of the photospheric field. The measurements highlight the magnetic field complexity and the variation with height close to the surface of these M-dwarfs. They provide critical constraints on the energy budget responsible for heating and eruptions of stellar upper atmospheres, and enable assessments of how stellar magnetic activity may affect exoplanet environments.

Figures (12)

• Figure 1: Examples of spectropolarimetric spectral lines of three stars. The profiles are sampled from the observations of 2007Jun25 (AD Leo), 2007Jan27(YZ CMi), 2005Sep18 (EV Lac), with above-average signal-to-noise. a. The spectral profiles of LSD line profile (photosphere). b. H$\alpha$ (lower chromosphere). c. Ca IRT (middle chromosphere). The gray lines show the unpolarized intensity profiles normalized to the unpolarized continuum intensity $I_c$. The color lines represent Stokes $V$ profiles normalized to $I_c$, and the corresponding Null ($N/I_c$) profile with no expected signal from the targets. Both $V/I_c$ and $N/I_c$ are multiplied by 100 for the purpose of illustration. The error bars of $V/I_c$ indicate the $\pm1\sigma$ uncertainties. The velocities have been corrected from the stellar system radial velocity to shift the line center of photospheric LSD to 0 km/s. The inferred magnetic field is printed in each panel.
• Figure 2: Phase-folded mean longitudinal magnetic field strengths in the photospheres and chromospheres.a.AD Leo. b.YZ CMi. c.EV Lac. a-c.The rotational phases were derived from the same ephemera. Red triangles are photospheric fields $\langle B_{\mathrm{p}}\rangle$ inferred from LSD profiles; yellow squares are lower chromospheric fields $\langle B_{\mathrm{cL}}\rangle$ from H$\alpha$, blue pentagons are middle chromospheric fields $\langle B_{\mathrm{cM}}\rangle$ from Ca IRT. Filled markers are taken from a marked epoch with full phase coverage for each star. For these epochs, the linear Pearson correlation coefficients between the field strengths of two layers are also listed in the left block. For each marked epoch, we give a two-harmonic Fourier series to fit the pattern, showing the periodic signal of the magnetic field variation.
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