2017ApJ...851...65D

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.