Carbon chemistry in IRC+10216: Infrared detection of diacetylene

TL;DR

This study reports the first infrared detection of diacetylene (C4H2) in the envelope of the carbon-rich AGB star IRC+10216, revealing its distribution and excitation conditions through high-resolution spectroscopy.

Contribution

The paper presents the first infrared detection and analysis of C4H2 in IRC+10216, providing insights into its spatial distribution and excitation temperatures in the star's envelope.

Findings

C4H2 detected with 24 absorption lines

Two excitation populations with different temperatures identified

C4H2 located beyond ~20 stellar radii, in outer shell

Abstract

We present the detection of C4H2 for first time in the envelope of the C-rich AGB star IRC+10216 based on high spectral resolution mid-IR observations carried out with the Texas Echelon-cross-Echelle Spectrograph (TEXES) mounted on the Infrared Telescope Facility (IRTF). The obtained spectrum contains 24 narrow absorption features above the detection limit identified as lines of the ro-vibrational C4H2 band nu6+nu8(sigma_u^+). The analysis of these lines through a ro-vibrational diagram indicates that the column density of C4H2 is 2.4(1.5)E+16 cm^(-2). Diacetylene is distributed in two excitation populations accounting for 20 and 80% of the total column density and with rotational temperatures of 47(7) and 420(120) K, respectively. This two-folded rotational temperature suggests that the absorbing gas is located beyond ~0.4"~20R* from the star with a noticeable cold contribution outwards from ~10"~500R*. This outer shell matches up with the place where cyanoacetylenes and carbon chains are known to form due to the action of the Galactic dissociating radiation field on the neutral gas coming from the inner layers of the envelope.