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The circumstellar envelope of the hypergiant star IRC+10420 has been tracedas far out in SiO J=2-1 as in CO J = 1-0 and CO J = 2-1, in dramatic contrastwith the centrally condensed (thermal) SiO- but extended CO-emitting envelopesof giant and supergiant stars. Here, we present an observation of thecircumstellar envelope in SiO J=1-0 that, when combined with the previousobservation in {\sioii}, provide more stringent constraints on the density ofthe SiO-emitting gas than hitherto possible. The emission in SiO peaks at aradius of $\sim$2\arcsec\ whereas that in SiO J=2-1 emission peaks at a smallerradius of $\sim$1\arcsec, giving rise to their ring-like appearances. The ratioin brightness temperature between SiO J=1-0 and SiO J=2-1 decreases from avalue well above unity at the innermost measurable radius to about unity atradius of $\sim$2\arcsec, beyond which this ratio remains approximatelyconstant. 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 gasis comparable with that of the CO-emitting gas in the inner zone, but at leastan order of magnitude higher by comparison in both the middle and outer zones.The SiO-emitting gas therefore originates from dense clumps, likely associatedwith the dust clumps seen in scattered optical light, surrounded by morediffuse CO-emitting interclump gas. We suggest that SiO molecules are releasedfrom dust grains due to shock interactions between the dense SiO-emittingclumps and the diffuse CO-emitting interclump gas.

Dense molecular clumps in the envelope of the yellow hypergiant IRC+10420