Investigating Anomalous Photochemistry in the Inner Wind of IRC+10216 Through Interferometric Observations of HC$_3$N

TL;DR

This study uses high-resolution ALMA observations to map HC3N in IRC+10216's inner wind, revealing compact, arc-like structures and suggesting enhanced photochemistry driven by a binary companion.

Contribution

It provides the first detailed interferometric mapping of high-energy HC3N lines in the inner envelope of IRC+10216, linking observed abundance enhancements to binary-induced photochemistry.

Findings

HC3N shows compact, arc-like emission structures.

Fractional abundance of HC3N is about 10^-8.

Enhanced HC3N is likely due to binary-driven photochemistry.

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 ALMA 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 AGB star which show extended, hollow emission at 15"-20" radii (e.g. C$_4$H, C$_6$H, 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.