We have investigated the evolution and distribution of molecules in collapsing prestellar cores via numerical chemical models , adopting the Larson-Penston solution and its delayed analogues to study collapse .
Molecular abundances and distributions in a collapsing core are determined by the balance among the dynamical , chemical and adsorption time scales .
When the central density n _ { H } of a prestellar core with the Larson-Penston flow rises to 3 \times 10 ^ { 6 } cm ^ { -3 } , the CCS and CO column densities are calculated to show central holes of radius 7000 AU and 4000 AU , respectively , while the column density of N _ { 2 } H ^ { + } is centrally peaked .
These predictions are consistent with observations of L1544 .
If the dynamical time scale of the core is larger than that of the Larson-Penston solution owing to magnetic fields , rotation , or turbulence , the column densities of CO and CCS are smaller , and their holes are larger than in the Larson-Penston core with the same central gas density .
On the other hand , N _ { 2 } H ^ { + } and NH _ { 3 } are more abundant in the more slowly collapsing core .
Therefore , molecular distributions can probe the collapse time scale of prestellar cores .
Deuterium fractionation has also been studied via numerical calculations .
The deuterium fraction in molecules increases as a core evolves and molecular depletion onto grains proceeds .
When the central density of the core is n _ { H } = 3 \times 10 ^ { 6 } cm ^ { -3 } , the ratio DCO ^ { + } /HCO ^ { + } at the center is in the range 0.06-0.27 , depending on the collapse time scale and adsorption energy ; this range is in reasonable agreement with the observed value in L1544 .