Confirming the Magnetic Field Detection at the Surface of $χ$ Cyg
Alexis Lavail, Arturo López Ariste, Quentin Pilate, Philippe Mathias, Fabrice Herpin, Agnès Lèbre
TL;DR
The paper reports a second detection of a photospheric magnetic field in χ Cyg using Neo-Narval spectropolarimetry across three epochs in 2025. By applying the Least Squares Deconvolution technique to co-added spectra, the authors measure a mean longitudinal field of $B_l = 3.4 ± 0.6$ G near maximum light in 2025 Aug, with non-detections at other phases. The results imply a strong pulsation-phase dependence of magnetic signatures, likely linked to shocks in the atmosphere, and support the existence of surface magnetic fields amplified during maximum luminosity. This finding helps bridge the gap between photospheric magnetism and circumstellar maser fields, informing models of magnetism and mass loss in Mira stars.
Abstract
We present spectropolarimetric observations of $χ$ Cygni obtained with Neo-Narval at Télescope Bernard Lyot in 2025. We obtained observations across three epochs (2025 Jul, Aug, and Oct) near maximum light to search for magnetic field signatures at the stellar photosphere. We detected a clear circular polarization signal in the 2025 Aug observations (pulsation phases $0.99$ to $0.01$). We measure a mean longitudinal magnetic field of $B_l = 3.4 \pm 0.6$ G. No detections were obtained for the 2025 Jul and Oct epochs. The pulsation-phase dependence suggests that field detection is tied to specific shock conditions near maximum light.