A 3mm molecular line survey toward IRC+10216

TL;DR

This paper presents a comprehensive 3mm spectral survey of IRC+10216, identifying 214 emission lines from 43 molecules, including new detections, to better understand its complex circumstellar chemistry.

Contribution

It provides the most sensitive, unbiased 3mm spectral line survey of IRC+10216, expanding molecular inventory and chemical insights with new detections and isotopic ratio estimates.

Findings

214 emission lines identified, including 28 new detections

Line intensity ratios suggest similar chemistry with CIT 6, scaled by distance

Unusually strong C4H and C3N emissions in IRC+10216

Abstract

IRC+10216 is the brightest infrared source in the northern sky, known for its rich chemical composition. It is often used as a standard reference for studying the circumstellar envelope (CSE) of carbon-rich stars. While pioneering 3\,mm spectral surveys have laid foundational datasets, their system temperature limitations rendered spectral line detection thresholds inadequate for probing the source's complex organic molecule inventory at this band, which made superseding observations necessary. We aim to gain an unbiased view regarding circumstellar chemistry and investigate whether IRC+10216 is typical or anomalous in terms of its chemical composition. We carried out an in-depth spectral line survey of the circumstellar envelope of IRC+10216 utilizing the Arizona Radio Observatory 12\,m telescope. We achieved complete spectral sampling across the 90--116 GHz atmospheric window ($\lambda=2.6$--3.3\,mm). A total of 214 emission lines belonging to 43 molecular species are identified in the CSE of IRC+10216, among which 28 lines are newly detected in this object and four emission lines remain unidentified. The excitation temperatures and column densities of 16 molecules are determined through rotation diagrams. We estimate the isotopic ratios of carbon, oxygen, and silicon elements. For the majority of the molecular species, the line intensity ratios between IRC+10216 and CIT\,6 are inversely proportional to the square of their distance, which suggests that the chemical processes occurring within them are similar. Nevertheless, there is evidence suggesting that the emission of C$_{4}$H and C$_{3}$N in IRC+10216 is unusually strong. These observations stand as the most sensitive and unbiased line survey of IRC+10216 within the $\lambda=3$ window carried out by a single-dish telescope. They offer a valuable reference for the astronomical community. (Abridged)