From Prestellar to Protostellar Cores II. Time Dependence and Deuterium Fractionation

TL;DR

This study models the chemical evolution and deuterium fractionation during star formation, revealing how molecular abundances and D/H ratios change from prestellar to protostellar stages, with implications for understanding organic molecule formation.

Contribution

It introduces a time-dependent gas-grain chemical model applied to a 1-D hydrodynamic framework, extending to circumstellar disk chemistry, highlighting the evolution of complex organics and deuteration.

Findings

Gas-phase methanol and methane are more abundant than CO in early stages.

Complex organic molecules form efficiently during the warm-up phase.

Deuteration is high in large organic molecules due to cold-phase conditions.

Abstract

We investigate the molecular evolution and D/H abundance ratios that develop as star formation proceeds from a dense-cloud core to a protostellar core, by solving a gas-grain reaction network applied to a 1-D radiative hydrodynamic model with infalling fluid parcels. Spatial distributions of gas and ice-mantle species are calculated at the first-core stage, and at times after the birth of a protostar. Gas-phase methanol and methane are more abundant than CO at radii $r\lesssim 100$ AU in the first-core stage, but gradually decrease with time, while abundances of larger organic species increase. The warm-up phase, when complex organic molecules are efficiently formed, is longer-lived for those fluid parcels in-falling at later stages. The formation of unsaturated carbon chains (warm carbon-chain chemistry) is also more effective in later stages; C$^+$, which reacts with CH$_4$ to form carbon chains, increases in abundance as the envelope density decreases. The large organic molecules and carbon chains are strongly deuterated, mainly due to high D/H ratios in the parent molecules, determined in the cold phase. We also extend our model to simulate simply the chemistry in circumstellar disks, by suspending the 1-D infall of a fluid parcel at constant disk radii. The species CH$_3$OCH$_3$ and HCOOCH$_3$ increase in abundance in $10^4-10^5$ yr at the fixed warm temperature; both also have high D/H ratios.