2009ApJ...698.1924S

We used the Extended Submillimeter Array (eSMA) in its most extended configuration to investigate the innermost (within a radius of ∼290 R_* from the star) circumstellar envelope (CSE) of IRC+10216 where acceleration of gas and dust due to strong stellar radiation is taking place. We imaged the CSE using HCN and other molecular lines with a beam size of 0".22x0".46, deeply into the very inner edge (∼15 R_*) of the envelope where the expansion velocity is only ∼3 km/s. The excitation mechanisms of hot HCN and KCl lines are discussed. HCN maser components are spatially resolved for the first time on an astronomical object. We identified two discrete regions in the envelope: a region with a radius of ≲ 15 R_*, where molecular species have just formed and the gas has begun to be accelerated (Region I) and a shell region (Region II) with a radius of 23 R_* and a thickness of 15 R_*, whose expansion velocity has reached up to 13 km/s, nearly the terminal velocity of 15 km/s. The Si³⁴S line detected in Region I shows a large expansion velocity of 16 km/s due to strong wing components, indicating that the emission may arise from a shock region in the innermost envelope. In Region II, the position angle of the most copious mass-loss direction was found to be ∼ 120°±10°, which may correspond to the equatorial direction of the star. Region II contains a torus-like feature. These two regions may have emerged due to significant differences in the size distributions of the dust particles in the two regions.