Measurements of General Magnetic Fields on Stars with Vigorous Convective Zones Using High-Accuracy Spectropolarimetry

Abstract

Magnetic fields are studied to understand the nature of activity on stars with convective envelopes. Results of high-accuracy General Magnetic Field (GMF) measurements of different luminosity stars on the right hand side of the Cepheid instability strip of the H-R diagram are reviewed: the presence of a weak general magnetic field for 21 stars with vigorous convection is detected (F9-M3 spectral types and I-V luminosity classes). A substantial (up to some dozens) GMF value was detected on two supergiant stars, three bright giants, twelve giant stars, one subgiant, and three solar-like dwarfs. Furthermore, the variation of global nonaxisymmetric magnetic fields as a function of the stellar rotation is determined for two solar-like stars other than the Sun: the magnetic field of the young solar-like star ξ Boo A shows periodic variations from -10 G to +30 G, and the magnetic field of the old solar-like star 61 Cyg A varies from -10 G up to +4 G. Currently, the nature of this field is unknown. The nonaxisymmetric GMF as a phenomenon is absent in the Babcock’ and Leighton’ phenomenological magneto-kinematic model of the solar cycle. In terms of standard α-Ω dynamo theory, GMF is absent also. There are only two main components of large-scale magnetic fields on the Sun: the toroidal magnetic field and the axisymmetric poloidal field. The coincidence of theoretical conclusions of different authors as well as results of their numerical simulations and new data on the observed magnetic field for solar-like stars (i.e., the presence of a nonaxisymmetric large-scale field) leads to a working hypothesis that GMF reflects properties of a stationary global magnetic field of the Sun’s (or convective star’s) radiative interior onto its surface. There appears to be a third nonaxisymmetric, large-scale component of the magnetic field (Origin Magnetic Field) - the initial magnetic field for dynamo mechanisms; a global magnetic field of radiative interior penetrates into surface of the Sun, and the observed GMF is a time-dependent superposition of all large-scale components: axisymmetric poloidal, nonaxisymmetric toroidal and nonaxisymmetric origin fields.