Modeling Carbon Chain Anions in L1527

TL;DR

This study models the chemistry of carbon-chain molecules and negative ions in the protostellar region L1527, successfully reproducing observed abundances and highlighting the significance of anions in the chemical evolution of star-forming regions.

Contribution

It extends existing chemical networks to include negative ions and models their formation post-methane evaporation, providing insights into molecular abundances in L1527.

Findings

Most observed molecular abundances are reproduced at ~5000 years.

Anion abundance surpasses electrons at later times, affecting organic chemistry.

The model matches observed C6H- ratios but overestimates C4H- ratios, indicating need for further reaction rate studies.

Abstract

The low-mass protostellar region L1527 is unusual because it contains observable abundances of unsaturated carbon-chain molecules including CnH radicals, H2Cn carbenes, cyanopolyynes, and the negative ions C4H- and C6H-, all of which are more associated with cold cores than with protostellar regions. Sakai et al. suggested that these molecules are formed in L1527 from the chemical precursor methane, which evaporates from the grains during the heat-up of the region. With the gas-phase osu.03.2008 network extended to include negative ions of the families Cn-, and CnH-, as well as the newly detected C3N-, we modeled the chemistry that occurs following methane evaporation at T~ 25-30 K. We are able to reproduce most of the observed molecular abundances in L1527 at a time of ~5000 yr. At later times, the overall abundance of anions become greater than that of electrons, which has an impact on many organic species and ions. The anion-to-neutral ratio in our calculation is in good agreement with observation for C6H- but exceeds the observed ratio by more than three orders of magnitude for C4H-. In order to explain this difference, further investigation is needed on the rate coefficients for electron attachment and other reactions regarding anions.