We adopt the Sedov-Taylor dynamical model of an adiabatically expanding
spherical blast wave (Taylor, 1950; Sedov, 1959) appropriate for
middle-aged SNR's. In such a model the solution for the temperature and
density has a self-similar form, and the radius of the shell and the
shock velocity evolve with time as 
and 
,
respectively. The Sedov solution is a good approximation when the swept
up interstellar mass exceeds the mass of the SN ejecta (
, Woltjer 1972). The association between the Vela SNR and
the old pulsar PSR0833-45 (Weiler & Panagia 1980, Weiler & Sramek
1988) suggests that the SNR is well in its adiabatic phase. To our
purposes, a useful relation linking the shock temperature 
with
the shock wave velocity 
is the following (McKee & Hollenbach
1980):
In the following, we shall assume instantaneous energy equipartition
between ions and electrons at the shock front. In this case, eq.
1 allows us to derive the shock speed from X-ray observations;
in fact, the electron temperature derived from fitting the X-ray data
is equal to in equation 1.