Magnetism, rotation, and nonthermal emission in cool stars

Abstract

Stellar dynamos generate magnetic fields that are of fundamental importance to the variability and evolution of Sun-like and low-mass stars, and for the development of their planetary systems. As a key to understanding stellar dynamos, empirical relations between stellar parameters and magnetic fields are required for comparison to ab initio predictions from dynamo models. We report measurements of surface-average magnetic fields in 292 M dwarfs from a comparison with radiative transfer calculations; for 260 of them, this is the first measurement of this kind. Our data were obtained from more than 15 000 high-resolution spectra taken during the CARMENES project. They reveal a relation between average field strength, ⟨B⟩, and Rossby number, Ro, resembling the well-studied rotation–activity relation. Among the slowly rotating stars, we find that magnetic flux, ΦB, is proportional to rotation period, P, and among the rapidly rotating stars that average surface fields do not grow significantly beyond the level set by the available kinetic energy. Furthermore, we find close relations between nonthermal coronal X-ray emission, chromospheric Hα and Ca H&K emission, and magnetic flux. Taken together, these relations demonstrate empirically that the rotation–activity relation can be traced back to a dependence of the magnetic dynamo on rotation. We advocate the picture that the magnetic dynamo generates magnetic flux on the stellar surface proportional to rotation rate with a saturation limit set by the available kinetic energy, and we provide relations for average field strengths and nonthermal emission that are independent of the choice of the convective turnover time. We also find that Ca H&K emission saturates at average field strengths of ⟨B⟩≈800 G while Hα and X-ray emission grow further with stronger fields in the more rapidly rotating stars. This is in conflict with the coronal stripping scenario predicting that in the most rapidly rotating stars coronal plasma would be cooled to chromospheric temperatures.

Galactic Kinematics and Observed Flare Rates of a Volume-complete Sample of Mid-to-late M Dwarfs: Constraints on the History of the Stellar Radiation Environment of Planets Orbiting Low-mass Stars

“…Our results, and those of others concerning coronal X-ray emission (Stauffer & Hartmann 1987;Pizzolato et al 2003;Wright et al 2018), Hα emission (Douglas et al 2014;Newton et al 2017), and magnetic field strength (Reiners et al 2022), show that fully convective M dwarfs fall largely into two categories: rapidly rotating stars that show a saturated magnetic activity that is independent of rotation up to a critical value, and stars that fall into the unsaturated activity-rotation regime. Theoretical studies posit that stars occupying the saturated regime of magnetic activity have complex multipolar magnetic field topologies, few open field lines (Garraffo et al 2018), and a dynamo that is weakly coupled to the stellar wind (Brown 2014).…”

“…These theoretical studies suggest that stars occupying the unsaturated regime have dipolar fields, many open field lines, and have a dynamo that is strongly coupled to the stellar wind (Brown 2014;Garraffo et al 2018). A recent observational study by Reiners et al (2022) found that saturation of the average surface magnetic field is due to a saturation of the magnetic dynamo rather than a saturation of the stellar surface by magnetic features. They suggest that the surface magnetic field generated is limited by the available kinetic energy and thus can increase only until it reaches the kinetic field.…”

“…In Medina et al (2020), hereafter denoted M20, we studied a sample of 125 fully convective M dwarfs with masses between 0.1 and 0.3 M e that reside within 15 pc and were observed in the southern ecliptic hemisphere during the first year of the TESS mission; we found that this relationship extends to optical flare rates as well. Reiners et al (2022) found that the underlying physical mechanism responsible for observed saturation of magnetic activity can be attributed to saturation of the magnetic dynamo itself. However, the age at which the stars transition from the saturated regime to the unsaturated regime, which could have a substantial impact on the planets orbiting these stars, is not firmly established.…”

A detailed analysis of the Gl 486 planetary system

“…For Table 2, we computed the logarithm of mean R HK of eight HIRES, two ES-PaDOnS, two UVES, one FEROS, and one HARPS measurements collected by Perdelwitz et al (2021) and propagated uncertainties from the standard deviation of the mean (see also: Astudillo-Defru et al 2017a;Houdebine et al 2017;Hojjatpanah et al 2019). Reiners et al (2022) investigated Zeeman-sensitive Ti i and FeH lines and estimated an upper limit of the stellar average magnetic field strength at B = 240 G as in Shulyak et al (2019). We also tabulate an upper limit on the X-ray luminosity from the limit on observed flux of Stelzer et al (2013) and the Gaia EDR3 distance.…”

The HD 260655 system: Two rocky worlds transiting a bright M dwarf at 10 pc

“…We followed the prescriptions in Appendix B of Pérez-Torres et al (2021) to estimate the flux density expected to arise from the interaction between the planets HD 260655 b and HD 260655 c and their host star, at a frequency of ∼504 MHz, which corresponds to the cyclotron frequency of the stellar magnetic field of 180 G, from Reiners et al (2022). We computed the radio emission arising from star-planet interaction for a closed dipolar geometry and for two different models of star-planet interaction, the Zarka-Lanza model and the Saur-Turnpenney model (see Pérez-Torres et al 2021 for details).…”