  Galactic Cepheids in the vicinity of the Sun have a residual line-of-sight
velocity, or gamma-velocity, which shows a systematic blueshift of about 2 km/s
compared to an axisymmetric rotation model of the Milky Way. This term is
either related to the space motion of the star and, consequently, to the
kinematic structure of our Galaxy, or it is the result of the dynamical
structure of the Cepheids' atmosphere. We aim to show that these residual
gamma-velocities are an intrinsic property of Cepheids. We observed nine
galactic Cepheids with the HARPS spectroscope, focusing specifically on 17
spectral lines. For each spectral line of each star, we computed the
gamma-velocity (resp. gamma-asymmetry) as an average value of the interpolated
radial velocity (resp. line asymmetry) curve. For each Cepheid in our sample, a
linear relation is found between the gamma-velocities of the various spectral
lines and their corresponding gamma-asymmetries, showing that residual
gamma-velocities stem from the intrinsic properties of Cepheids. We also
provide a physical reference to the stellar gamma-velocity: it should be zero
when the gamma-asymmetry is zero. Following this definition, we provide very
precise and physically calibrated estimates of the gamma-velocities for all
stars of our sample. To understand this very subtle gamma-asymmetry effect,
further numerical studies are needed. Cepheids' atmosphere are strongly
affected by pulsational dynamics, convective flows, nonlinear physics, and
complex radiative transport. Hence, all of these effects have to be
incorporated simultaneously and consistently into the numerical models to
reproduce the observed line profiles in detail.
