Zeeman-Doppler imaging of active young solar-type stars

Abstract

Context. By studying young magnetically active late-type stars, i.e. analogues to the young Sun, we can draw conclusions on the evolution of the solar dynamo. Aims. We determine the topology of the surface magnetic field and study the relation between the magnetic field and cool photospheric spots in three young late-type stars. Methods. High-resolution spectropolarimetry of the targets was obtained with the HARPSpol instrument mounted at the ESO 3.6 m telescope. The signal-to-noise ratios of the Stokes IV measurements were boosted by combining the signal from a large number of spectroscopic absorption lines through the least squares deconvolution technique. Surface brightness and magnetic field maps were calculated using the Zeeman-Doppler imaging technique.Results. All three targets show clear signs of magnetic fields and cool spots. Only one of the targets, V1358 Ori, shows evidence of the dominance of non-axisymmetric modes. In two of the targets, the poloidal field is significantly stronger than the toroidal one, indicative of an α 2 -type dynamo, in which convective turbulence effects dominate over the weak differential rotation. In two of the cases there is a slight anti-correlation between the cool spots and the strength of the radial magnetic field. However, even in these cases the correlation is much weaker than in the case of sunspots. Conclusions. The weak correlation between the measured radial magnetic field and cool spots may indicate a more complex magnetic field structure in the spots or spot groups involving mixed magnetic polarities. Comparison with a previously published magnetic field map shows that on one of the stars, HD 29615, the underlying magnetic field changed its polarity between 2009 and 2013.

Magnetic Field of the Eclipsing M-dwarf Binary YY Gem

“…5. These continuum brightness distributions were derived using the modified Tikhonov regularisation method (see Hackman et al 2016;Rosén et al 2018), which numerically stabilises the surface imaging problem by minimising the local map contrast and limiting deviation from the default Owing to a large inclination angle, the Doppler mapping method is unable to distinguish the Northern and Southern stellar hemispheres, except when the surface features are partially obscured during the primary and secondary eclipses. This is why the recovered dark spot geometries are symmetric with respect to the stellar equators everywhere except close to 0 • longitude (phase 0.0) for YY Gem A and 180 • longitude (phase 0.5) for YY Gem B.…”

A sudden change of the global magnetic field of the active M dwarf AD Leo revealed by full Stokes spectropolarimetric observations★

“…The ZDI mapping was performed with the Inver-sLSD code developed by Kochukhov et al (2014) and subsequently used for cool active stars by Rosén et al (2016), Hackman et al (2016) and Kochukhov & Lavail (2017). This code divides the stellar surface into surface elements of roughly equal area, and for each surface element, the local line profiles are computed using the Unno-Rachkovsky analytical solution of the polarised radiative transfer equations (Landi Degl'Innocenti & Landolfi 2004).…”

Transition from axi- to nonaxisymmetric dynamo modes in spherical convection models of solar-like stars

“…In the Sun, the longitudinal distribution of sunspots indicates that the solar large-scale magnetic field is mostly axisymmetric (e.g., Pelt et al 2006). In late-type stars with rapid rotation, by contrast, much larger spots located at high latitudes or even polar regions have been observed using Doppler imaging (DI), Zeeman Doppler imaging (ZDI), and interferometry (e.g., Järvinen et al 2008;Hackman et al 2016;Roettenbacher et al 2016). Many studies have reported on highly nonaxisymmetric spot configurations (e.g., Jetsu 1996;Berdyugina & Tuominen 1998), referred to as active longitudes, while especially the indirect imaging of the surface magnetic field using ZDI tends to yield more axisymmetric configurations See et al 2016).…”