Magnetic fields of young solar twins^⋆

¹ Department Physics and AstronomyUppsala University, Box 516, 751 20 Uppsala, Sweden
e-mail: lisa.rosen@physics.uu.se
² Department of Physics, PO Box 64, 00014 University of Helsinki, Finland
³ ReSoLVe Centre of Excellence, Aalto University, Department of Computer Science, PO Box 15400, 00076 Aalto, Finland

Received: 7 March 2016
Accepted: 9 May 2016

Abstract

Aims. The goal of this work is to study the magnetic fields of six young solar-analogue stars both individually, and collectively, to search for possible magnetic field trends with age. If such trends are found, they can be used to understand magnetism in the context of stellar evolution of solar-like stars and to understand the past of the Sun and the solar system. This is also important for the atmospheric evolution of the inner planets, Earth in particular.

Methods. We used Stokes IV data from two different spectropolarimeters, NARVAL and HARPSpol. The least-squares deconvolution multi-line technique was used to increase the signal-to-noise ratio of the data. We then applied a modern Zeeman-Doppler imaging code in order to reconstruct the magnetic topology of all stars and the brightness distribution of one of our studied stars.

Results. Our results show a significant decrease in the magnetic field strength and energy as the stellar age increases from 100 Myr to 250 Myr, while there is no significant age dependence of the mean magnetic field strength for stars with ages 250−650 Myr. The spread in the mean field strength between different stars is comparable to the scatter between different observations of individual stars. The meridional field component is weaker than the radial and azimuthal field components in 15 of the 16 magnetic maps. It turns out that 89−97% of the magnetic field energy is contained in l = 1 − 3. There is also no clear trend with age and distribution of field energy into poloidal/toroidal and axisymmetric/non-axisymmetric components within the sample. The two oldest stars in this study show an octupole component that is twice as strong as the quadrupole component. This is only seen in 1 of the 13 maps of the younger stars. One star, χ¹ Ori, displays two field polarity switches during almost 5 yr of observations suggesting a magnetic cycle length of 2, 6, or 8 yr.