2017A&A...597A..40J

Context. Cyanopolyynes are chains of carbon atoms with an atom of hydrogen and a CN group on either side. They are detected almost everywhere in the interstellar medium (ISM), as well as in comets. In the past, they have been used to constrain the age of some molecular clouds, since their abundance is predicted to be a strong function of time. Finally, cyanopolyynes can potentially contain a large portion of molecular carbon.
Aims. We present an extensive study of the cyanopolyynes distribution in the solar-type protostar IRAS 16293-2422. The goals are (i) to obtain a census of the cyanopolyynes in this source and of their isotopologues; (ii) to derive how their abundance varies across the protostar envelope; and (iii) to obtain constraints on the history of IRAS 16293-2422 by comparing the observations with the predictions of a chemical model.
Methods. We analysed the data from the IRAM-30 m unbiased millimeter and submillimeter spectral survey towards IRAS 16293-2422 named TIMASSS. The derived spectral line energy distribution (SLED) of each detected cyanopolyyne was compared with the predictions from the radiative transfer code GRenoble Analysis of Protostellar Envelope Spectra (GRAPES) to derive the cyanopolyyne abundances across the envelope of IRAS 16293-2422. Finally, the derived abundances were compared with the predictions of the chemical model UCL_CHEM.
Results. We detect several lines from cyanoacetylene (HC₃N) and cyanodiacetylene (HC₅N), and report the first detection of deuterated cyanoacetylene, DC₃N, in a solar-type protostar. We found that the HC₃N abundance is roughly constant (∼1.3x10^–11) in the outer cold envelope of IRAS 16293-2422, and it increases by about a factor 100 in the inner region where the dust temperature exceeds 80 K, namely when the volcano ice desorption is predicted to occur. The HC₅N has an abundance similar to HC₃N in the outer envelope and about a factor of ten lower in the inner region. The comparison with the chemical model predictions provides constraints on the oxygen and carbon gaseous abundance in the outer envelope and, most importantly, on the age of the source. The HC₃N abundance derived in the inner region, and where the increase occurs, also provide strong constraints on the time taken for the dust to warm up to 80 K, which has to be shorter than ∼10³-10⁴yr. Finally, the cyanoacetylene deuteration is about 50% in the outer envelope and ≤5% in the warm inner region. The relatively low deuteration in the warm region suggests that we are witnessing a fossil of the HC₃N abundantly formed in the tenuous phase of the pre-collapse and then frozen into the grain mantles at a later phase.
Conclusions. The accurate analysis of the cyanopolyynes in IRAS 16293-2422 unveils an important part of its past story. It tells us that IRAS 16293-2422 underwent a relatively fast (≤10⁵yr) collapse and a very fast (≤10³-10⁴yr) warming up of the cold material to 80K.