Carbon Chemistry in IRC+10216: Infrared Detection of Diacetylene

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 ${\nu }_6+{\nu }_8\left({\sigma }_u^{+}\right).$ The analysis of these lines through a ro-vibrational diagram indicates that the column density of C₄H₂ is (2.4 ± 1.5) × 10¹⁶ 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 $\simeq 0_{·}^{"}4\simeq 20R_{\star }$ 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.