Dense Molecular Clumps in the Envelope of the Yellow Hypergiant IRC+10420

Abstract

The circumstellar envelope of the hypergiant star IRC+10420 has been traced as far out in 28SiO (J = 2-1) as in 12CO J = 1-0 and 12CO J=2-1, in dramatic contrast with the centrally condensed (thermal) SiO- but extended CO-emitting envelopes of giant and supergiant stars. We present an observation of the circumstellar envelope in 28SiO (J = 1 - 0) that, when combined with the previous observation in 28SiO (J = 2 - 1), provides more stringent constraints on the density of the SiO-emitting gas than hitherto possible. The emission in 28SiO (J = 1-0) peaks at a radius of ∼2″ whereas that in 28SiO (J = 2-1) peaks at a smaller radius of ∼1″, giving rise to its ring-like appearance. The ratio of brightness temperature between 28SiO (J = 2-1) and 28SiO (J = 1-0) decreases from a value well above unity at the innermost measurable radius to about unity at a radius of ∼2″, beyond which this ratio remains approximately constant. Dividing the envelope into three zones as in models for the 12CO J = 1-0 and 12CO J = 2Dinh-1 emissions, we show that the density of the SiO-emitting gas is comparable with that of the CO-emitting gas in the inner zone but is at least an order of magnitude higher by comparison in both the middle and the 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.