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 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.
