Context :

Aims : We report a detailed attempt to model the ortho-to-para abundance ratio of c -C _ { 3 } H _ { 2 } so as to reproduce observed values in the cores of the well-known source TMC-1 .
According to observations , the ortho-to-para ratios vary , within large uncertainties , from a low of near unity to a high of approximately three depending on the core .

Methods : We used the osu.2003 network of gas-phase chemical reactions augmented by reactions that specifically consider the formation , depletion , and interconversion of the ortho and para forms of the c -C _ { 3 } H _ { 2 } and its precursor ion c -C _ { 3 } H _ { 3 } ^ { + } .
We investigated the sensitivity of the calculated ortho-to-para ratio for c -C _ { 3 } H _ { 2 } to a large number of factors .

Results : For the less evolved cores C , CP , and D , we had no difficulty reproducing the observed ortho-to-para ratios of 1-2 .
In order to reproduce observed ortho-to-para ratios of near three , observed for the evolved cores A and B , it was necessary to include rapid ion-catalyzed interconversion processes .

Conclusions :