摘要：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 (~1.3 ×
10-11) 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 ~103−104 yr. 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 (≤105 yr) collapse and a very fast (≤103−104 yr) warming up of the cold material
to 80 K.
