  This paper presents the first high-resolution SiO map made in an external
galaxy. The nucleus of the nearby barred spiral NGC253 has been observed
simultaneously in the v=0, J=2-1 line of SiO and in the J=1-0 line of H13CO+
with the IRAM interferometer, with a resolution of 7.5"x2.6". Emission from SiO
and H13CO+ is extended in the nucleus of NGC253. The bulk of the SiO/H13CO+
emission arises from a 600pcx250pc circumnuclear disk (CND) with a double
ringed structure. The inner ring, of radius r=60pc, viewed edge-on along
PA=51deg, hosts the nuclear starburst; the outer pseudo-ring opens out as a
spiral-like arc up to r=300pc. The kinematics of the gaseous disk,
characterized by strong non-circular motions, is interpreted in terms of the
resonant response of the gas to the barred potential. The inner ring would
correspond to the inner Inner Lindblad Resonance(iILR), whereas the outer
region is linked to the onset of a trailing spiral wave across the outer ILR
(oILR). Most notably, we report the detection of a molecular gas counterpart of
the giant outflow of hot gas, previously seen in X-ray and optical lines, and
tentatively identified as a dust chimney in the the 450mu continuum band.The
SiO shows a high average fractional abundance in the CND of X(SiO)=1.5x10E-10.
This is more than an order of magnitude above the predicted value of a PDR.
Moreover, X(SiO) varies at least by an order of magnitude between the inner
starburst region, which dominates the global emission, where we derive
X(SiO)=1--2x10E-10, and the outer region, where X(SiO) reaches a few 10E-9. SiO
abundance is also significantly enhanced in the outflow (X(SiO)=3--5x10E-10).
Different mechanisms are explored to explain the unlike chemical processing of
molecular gas within the nucleus.
