2019A&A...622A.185W

Context. Abundant carbon-chain species have been observed towards lukewarm corinos L1527, B228, and L483. These carbon-chain species are believed to be synthesized in the gas phase after CH₄ desorbs from the dust grain surface at the temperature around 30K.
Aims. We investigate carbon-chain species formation in lukewarm corinos using a more rigorous numerical method and advanced surface chemical models. We also pay attention to the significance of the finite size effect.
Methods. We used the macroscopic Monte Carlo method in our simulations. In addition to the two-phase model, the basic multi-phase model and the new multi-phase models were used for modelling surface chemistry on dust grains. All volatile species can sublime at their sublimation temperatures in the two-phase model while most volatile species are frozen in the ice mantle before water ice sublimes in the basic and the new multi-phase models. The new multi-phase models allow more volatile species to sublime at their sublimation temperatures than the basic multi-phase model does.
Results. The significance of the finite size effect is dependent on the duration of the cold phase. The discrepancies between the rate equation approach and the Monte Carlo method decrease as the duration of the cold phase increases. When T∼30K, the abundances of gaseous CH₄ and CO in the two-phase model are the highest while the basic multi-phase model predicts the lowest CO and CH₄ abundances among all models. The abundances of carbon-chain species in the basic and the new multi-phase models are lower than that in the two-phase model when T∼30K because CH₄ is crucial for the synthesis of carbon-chain species. However, because the abundance of electrons increases as the abundance of H₃O⁺ decreases, some carbon-chain species abundances predicted by the basic multi-phase model may not be lower than that in the new multi-phase models. The two-phase model performs best in predicting carbon-chain species abundances to fit observations while the basic multi-phase model works the worst. The abundances of carbon-chain species predicted by the new multi-phase models agree reasonably well with observations.
Conclusions. The amount of CH₄ can diffuse inside the ice mantle, thus sublime upon warm-up plays a crucial role in the synthesis of carbon-chain species in the gas phase. The carbon-chain species observed in lukewarm corinos may be able to gauge surface chemical models.