  The circumstellar envelope of the hypergiant star IRC+10420 has been traced
as far out in SiO J=2-1 as in CO J = 1-0 and CO J = 2-1, in dramatic contrast
with the centrally condensed (thermal) SiO- but extended CO-emitting envelopes
of giant and supergiant stars. Here, we present an observation of the
circumstellar envelope in SiO J=1-0 that, when combined with the previous
observation in {\sioii}, provide more stringent constraints on the density of
the SiO-emitting gas than hitherto possible. The emission in SiO peaks at a
radius of $\sim$2\arcsec\ whereas that in SiO J=2-1 emission peaks at a smaller
radius of $\sim$1\arcsec, giving rise to their ring-like appearances. The ratio
in brightness temperature between SiO J=1-0 and SiO J=2-1 decreases from a
value well above unity at the innermost measurable radius to about unity at
radius of $\sim$2\arcsec, beyond which this ratio remains approximately
constant. Dividing the envelope into three zones as in models for the CO J =
1-0 and CO J = 2-1 emission, we show that the density of the SiO-emitting gas
is comparable with that of the CO-emitting gas in the inner zone, but at least
an order of magnitude higher by comparison in both the middle and outer zones.
The SiO-emitting gas therefore originates from dense clumps, likely associated
with the dust clumps seen in scattered optical light, surrounded by more
diffuse CO-emitting interclump gas. We suggest that SiO molecules are released
from dust grains due to shock interactions between the dense SiO-emitting
clumps and the diffuse CO-emitting interclump gas.
