  We revisit the process of gravitational sedimentation of helium and heavy
elements in the intra-cluster medium. We find that helium applies an inward
drag force on heavy elements, boosting their sedimentation speed to nearly half
its own. This speed is almost independent of the mass and the electric charge
of heavy elements. In the absence of small-scale magnetic fields, helium
sedimentation can increase the He/H abundance ratio in the cores of hot
clusters by three orders of magnitude. It also steepens the baryonic density
profile yielding a higher X-ray luminosity, which offers an explanation of the
observed luminosity-temperature relation. If the primordial He/H ratio is
assumed, then the gas density inferred from the observed X-ray emissivity might
be underestimated by 30% in the cores of clusters and overestimated by 7% in
the outer regions. The dark matter density on the other hand might be
overestimated by a factor of 8/3 in the cores and underestimated by 18% in
outer regions.
