Hydrodynamical-Chemical Models from Prestellar Cores to Protostellar Cores

Abstract

We investigate the molecular evolution in star forming cores from dense cloud cores (n_H ~ 10⁴ cm⁻³, T ~ 10 K) to protostellar cores. A detailed gas-grain reaction network is solved in infalling fluid parcels in 1-D radiation hydrodynamic model. Large organic molecules are mainly formed via grain-surface reaction at T ~ several 10 K and sublimated to the gas-phase at ~ 100 K, while carbon-chain species are formed at a few 10 K after the sublimation of CH₄ ice. The former accounts for the high abundance of large organic molecules in hot corinos such as IRAS16293, and the latter accounts for the carbon chain species observed toward L1527. The relative abundance of carbon chain species and large organic species would depend on the collapse time scale and/or temperature in the dense core stage. The large organic molecules and carbon chains in the protostellar cores are heavily deuterated; although they are formed in the warm temperatures, their ingredients have high D/H ratios, which are set in the cold core phase and isothermal collapse phase. HCOOH is formed by the gas-phase reaction of OH with the sublimated H₂CO, and is further enriched in Deuterium due to the exothermic exchange reaction of OH + D → OD + H.

In the fluid parcels of the 1-D collapse model, warm temperature T. ~ several 10 K lasts for only ~ 10⁴ yr, and the fluid parcels fall to the central star in ~ 100 yr after the temperature of the parcel rises to T ≥ 100 K. These timescales are determined by the size of the warm region and infall (~ free-fall) velocity: r_warm/t_ff. In reality, circum stellar disk is formed, in which fluid parcels stay for a longer timescale than the infall timescale. We investigate the molecular evolution in the disk by simply assuming that a fluid parcel stays at a constant temperature and density (i.e. a fixed disk radius) for 10⁴ − 10⁵ yrs. We found that some organic species which are underestimated in our 1-D collapse model, such as CH₃OCH₃ and HCOOCH₃, become abundant in the disk. We also found that these disk species have very high D/H ratio as well, since their ingredients are highly deuterated.

Finally we investigate molecular evolution in a 3D hydrodynamic simulation of star forming core. We found CH₃OH are abundant in the vicinity of the first core. The abundances of large organic species are determined mainly by the local temperature (sublimation), because of the short lifetime of the first core and the efficient mass accretion via angular momentum transfer.