[C I] and [C II] emission in the circumstellar envelope of IRC +10216

I. Observational data and NLTE modelling of the [C I] emission^★

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: mjeste@mpifr.de

Received: 31 January 2023
Accepted: 22 May 2023

Abstract

Context. The envelopes of evolved late-type stars on the asymptotic giant branch are characterised by a complex chemistry that is close to thermochemical equilibrium near the stellar surface; however, in the outer envelope, it is dominated by radical reactions, assisted by a photo-chemistry driven by the interstellar radiation field.

Aims. The study at hand aims to describe the distribution of atomic carbon, C⁰, throughout the envelope, in support of an improved understanding of its photo-chemistry. Additionally, we also briefly discuss the observation of [C II] emission towards the star.

Methods. We obtain spectra of the [C I]³P₁ → ³P₀ fine structure line (at 492.160700 GHz) at projected distances of up to 78″ from the star. The line profiles are characterised by both direct fitting of Gaussian components, and by modelling the observed line of the [C I] triplet. We also report the detection of the ²P_3/2 → ²P_1/2 line (at 1900.5369 GHz) from the C⁺ fine structure singlet at the central position and its non-detection at 32″ from the star.

Results. The overall picture of the [C I] emission from IRC +10216 agrees with more limited previous studies. The satisfying agreement between the observed and modelled line profiles, with emission at the systemic velocity appearing beyond one beam (13″ HPBW) from the star, rules out that the C⁰ is located in a thin shell. Given that the bond energy of CO falls only 0.1 eV below the ionisation threshold of C⁰, the absence of observable [C II] emission from sightlines beyond a projected distance of ~10¹⁷ cm (≳20″–30″) from the star (adopting a distance of 130 pc) does not contradict a scenario where the majority of C⁰ is located between that of CO and C⁺, as expected for an external far-ultraviolet radiation field. This conjecture is also corroborated by a model in which the C⁰ shell is located farther outside, failing to reproduce the [C I] line profiles at intermediate sky-plane distances from the star. Comparing a photochemical model adopted from literature with the simplifying assumption of a constant C⁰ abundance with respect to the H₂ density (with the 1/r² fall-off of a mass-conserving expansion flow), we constrain the inner boundary of the [C I] emitting shell, located at ~10¹⁶ cm from the star.