Abstract
Based on optical high-resolution spectra obtained with CFHT/ESPaDOnS, we
present new measurements of activity and magnetic field proxies of 442 low-mass
K5-M7 dwarfs. The objects were analysed as potential targets to search for
planetary-mass companions with the new spectropolarimeter and high-precision
velocimeter, SPIRou. We have analysed their high-resolution spectra in an
homogeneous way: circular polarisation, chromospheric features, and Zeeman
broadening of the FeH infrared line. The complex relationship between these
activity indicators is analysed: while no strong connection is found between
the large-scale and small-scale magnetic fields, the latter relates with the
non-thermal flux originating in the chromosphere.
We then examine the relationship between various activity diagnostics and the
optical radial-velocity jitter available in the literature, especially for
planet host stars. We use this to derive for all stars an activity merit
function (higher for quieter stars) with the goal of identifying the most
favorable stars where the radial-velocity jitter is low enough for planet
searches. We find that the main contributors to the RV jitter are the
large-scale magnetic field and the chromospheric non-thermal emission.
In addition, three stars (GJ 1289, GJ 793, and GJ 251) have been followed
along their rotation using the spectropolarimetric mode, and we derive their
magnetic topology. These very slow rotators are good representatives of future
SPIRou targets. They are compared to other stars where the magnetic topology is
also known. The poloidal component of the magnetic field is predominent in all
three stars.
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