  The exact nature of eta Carinae is still an open issue. In this paper we
assume a binary system to explain the strong X-ray emission, but we also take
into account that, near periastron and because of the highly eccentric orbit,
the wind emerging from eta Carinae accumulates behind the shock and can mimic a
shell-like ejection event. For this process to be effective, at periastron the
secondary star should be located between eta Carinae and the observer, solving
also the discrepancy between the orbital parameters derived from ground and
space based observations. We show that, as the secondary moves in its orbit,
the shell cools down and the number of available stellar ionizing photons is
not enough to maintain the shell temperature at its equilibrium value of about
7500 K. The central part of the shell remains cold and under these conditions
grain formation and growth can take place in timescales of hours. Using
recently published HST data for the optical and UV continuum during the 2003
shell event, we estimated that a fraction of 0.01 of the interstellar dust to H
ratio is enough to explain the observations. We also calculated the neutral gas
column density intercepting the line of sight at each point of the orbit near
periastron, and were able to reproduce the form and duration of the X-ray light
curve without any change in the eta Carinae mass loss rate. This same column
density can explain the observed H$\alpha$ light curve observed during the 2003
event.
