also markedly different from the rest of the species as it extends out to radial distances beyond 30$''$. As for HC$_5$N, it appears at the same radial distance than CN and disappears with HC$_3$N, while the radical C$_3$N appears somewhat farther than the cyanopolyynes HC$_3$N and HC$_5$N. All this information is of great value in understanding the formation mechanism of each of these species in IRC+10216 and will be the subject of further discussion in Section \[sec:model\].

Model {#sec:model}
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The ALMA emission maps of carbon chains obtained toward IRC+10216 provides an excellent opportunity to determine the main chemical processes responsible for their formation along the circumstellar outflow. To this end we have constructed a simple model of IRC+10216 that consists of a spherical envelope of gas and dust which expands around an AGB star with a constant velocity. The parameters of the star and envelope have been taken from [@agu2012] (2012), i.e., the adopted mass loss rate is $2\times10^{-5}$ $M_{\odot}$ yr$^{-1}$. For the distance, we use the recent value of 123 pc determined by [@gro2012] (2012). First we constructed a chemical model to describe the chemical composition of the gas as it expands, and then we carried out radiative 