The Global and Small-scale Magnetic Fields of Fully Convective, Rapidly Spinning M Dwarf Pair GJ65 A and B

Abstract

The nearby M dwarf binary GJ65 AB, also known as BL Cet and UV Cet, is a unique benchmark for investigation of dynamo-driven activity of low-mass stars. Magnetic activity of GJ65 was repeatedly assessed by indirect means, such as studies of flares, photometric variability, X-ray and radio emission. Here we present a direct analysis of large-scale and local surface magnetic fields in both components. Interpreting high-resolution circular polarisation spectra (sensitive to a large-scale field geometry) we uncovered a remarkable difference of the global stellar field topologies. Despite nearly identical masses and rotation rates, the secondary exhibits an axisymmetric, dipolar-like global field with an average strength of 1.3 kG while the primary has a much weaker, more complex and non-axisymmetric 0.3 kG field. On the other hand, an analysis of the differential Zeeman intensification (sensitive to the total magnetic flux) shows the two stars having similar magnetic fluxes of 5.2 and 6.7 kG for GJ65 A and B, respectively, although there is an evidence that the field strength distribution in GJ65 B is shifted towards a higher field strength compared to GJ65 A. Based on these complementary magnetic field diagnostic results we suggest that the dissimilar radio and X-ray variability of GJ65 A and B is linked to their different global magnetic field topologies. However, this difference appears to be restricted to the upper atmospheric layers but does not encompass the bulk of the stars and has no influence on the fundamental stellar properties.

Strong dipole magnetic fields in fast rotating fully convective stars

“…From the direct comparison of Zeeman-sensitive lines we see that Gl 65 B has clearly stronger field compared to Gl 65 A since both stars have close spectral types. Considering that the magnetic field in Gl 65 A has complex multipole geometry and in Gl 65 B has more simple dipole-like geometry 24 , our magnetic measurements for these two stars agree with the rest of our conclusions.…”

“…For the binary system Gl 65 AB we measure field of about 4.5 ± 1.0 kG in the primary and 5.8 ± 1.0 kG in the secondary, respectively. Both these values are systematically lower compared to 5.2 ± 0.5 kG and 6.7 ± 0.6 kG reported in Kochukhov & Lavail (2017) 24 . This discrepancy results from the difference in fitting methods utilized in both works.…”

“…Both instruments cover a spectral range between 367 − 1050 nm and have spectral resolution R ≈ 65, 000 (in polarimetric mode). We chose a sample of M dwarfs with known rotational periods and available magnetic maps derived from inversion of Stokes-V spectra 13,24 . The ultimate aim of our work is to measure magnetic flux density from independent spectroscopic diagnostics by utilizing up-to-date radiative transfer modelling.…”

“…In Fig. 3 we plot our magnetic field measurements for all M dwarfs with known magnetic geometries 13,24 . We find the same trend of increasing magnetic field strength as rotation periods decrease to about four days (so-called unsaturated regime), as was found in previous studies 10 .…”

The magnetic early B-type stars – III. A main-sequence magnetic, rotational, and magnetospheric biography

“…2008b; Folsom et al 2016), are generally weak (usually on the order of a few G, although fully convective M dwarfs can reach kG strengths, e.g. Kochukhov & Lavail 2017;Kochukhov & Shulyak 2019), and exhibit frequent topological changes consistent with magnetic activity cycles (e.g. Donati et al 2008a;Mengel et al 2016).…”