Résumé
Stellar activity may induce Doppler variability at the level of a few m/s
which can then be confused by the Doppler signal of an exoplanet orbiting the
star. To first order, linear correlations between radial velocity measurements
and activity indices have been proposed to account for any such correlation.
The likely presence of two super-Earths orbiting Kapteyn's star was reported in
Anglada et al. (2014, MNRAS 443L, 89A), but this claim was recently challenged
by Robertson et al. (2015, ApJ 805L, 22R) arguing evidence of a rotation period
(143 days) at three times the orbital period of one of the proposed planets
(Kapteyn's b, P=48.6 days), and the existence of strong linear correlations
between its Doppler signal and activity data. By re-analyzing the data using
global optimization methods and model comparison, we show that such claim is
incorrect given that; 1) the choice of a rotation period at 143 days is
unjustified, and 2) the presence of linear correlations is not supported by the
data. We conclude that the radial velocity signals of Kapteyn's star remain
more simply explained by the presence of two super-Earth candidates orbiting
it. We also advocate for the use of global optimization procedures and
objective arguments, instead of claims lacking of a minimal statistical
support.