Previous simulations predicted the activity-induced radial-velocity (RV)
variations of M dwarfs to range from ∼1 cm/s to ∼1 km/s, depending on
various stellar and activity parameters. We investigate the observed relations
between RVs, stellar activity, and stellar parameters of M dwarfs by analyzing
CARMENES high-resolution visual-channel spectra (0.5−1μm), which were
taken within the CARMENES RV planet survey during its first 20 months of
operation. During this time, 287 of the CARMENES-sample stars were observed
at least five times. From each spectrum we derived a relative RV and a measure
of chromospheric Hα emission. In addition, we estimated the chromatic
index (CRX) of each spectrum, which is a measure of the RV wavelength
dependence. Despite having a median number of only 11 measurements per star,
we show that the RV variations of the stars with RV scatter of >10 m/s and a
projected rotation velocity vsini>2 km/s are caused mainly by activity.
We name these stars `active RV-loud stars' and find their occurrence to
increase with spectral type: from ∼3% for early-type M dwarfs
(M0.0−2.5V) through ∼30% for mid-type M dwarfs (M3.0−5.5V) to
>50% for late-type M dwarfs (M6.0−9.0V). Their RV-scatter amplitude is
found to be correlated mainly with vsini. For about half of the stars, we
also find a linear RV−CRX anticorrelation, which indicates that their
activity-induced RV scatter is lower at longer wavelengths. For most of them we
can exclude a linear correlation between RV and Hα emission. Our results
are in agreement with simulated activity-induced RV variations in M dwarfs. The
RV variations of most active RV-loud M dwarfs are likely to be caused by dark
spots on their surfaces, which move in and out of view as the stars rotate.Comment: A&A accepte