Investigating Anomalous Photochemistry in the Inner Wind of IRC+10216 through Interferometric Observations of HC 3 N

Abstract

In recent years, many questions have arisen regarding the chemistry of photochemical products in the carbon-rich winds of evolved stars. To address them, it is imperative to constrain the distributions of such species through high-angular-resolution interferometric observations covering multiple rotational transitions. We used archival Atacama Large Millimeter/submillimeter Array observations to map rotational lines involving high energy levels of cyanoacetylene (HC 3 N) toward the inner envelope (radius < 8″/1000 au) of the carbon star IRC+10216. The observed lines include the J = 28 − 27, J = 30 − 29, and J = 38 − 37 transitions of HC 3 N in its ground vibrational state. In contrast to previous observations of linear carbon chains toward this asymptotic giant branch (AGB) star that show extended, hollow emission at 15″–20″ radii (e.g., C 4 H, C 6 H, and HC 5 N), the maps of the HC 3 N lines here show compact morphologies comprising various arcs and density enhancements, with significant emission from gas clumps at an angular distance of ∼3″ (350 au) from the central AGB star. We compared visibility sampled non-LTE radiative transfer models with the observed brightness distributions, and derive a fractional abundance with respect to H 2 of 10 −8 for HC 3 N at the radii probed by these lines. These results are consistent with enhanced photochemistry occurring in warm (∼200 K) regions of the circumstellar envelope. After application of a specialized chemical model for IRC+10216, we find evidence that the enhanced HC 3 N abundances in the inner wind are most likely due to a solar-type binary companion initiating photochemistry in this region.