In previous studies we identified two classes of starless cores , thermally subcritical and supercritical , distinguished by different dynamical behavior and internal structure .
Here we study the evolution of the dynamically-unstable , thermally-supercritical cores by means of a numerical hydrodynamic simulation that includes radiative equilibrium and simple molecular chemistry .
From an initial state as an unstable Bonnor-Ebert ( BE ) sphere , a contracting core evolves toward the configuration of a singular isothermal sphere ( SIS ) by inside-out collapse .
We follow the gas temperature and abundance of CO during the contraction .
The temperature is predominantly determined by radiative equilibrium , but in the rapidly contracting center of the core , compressive heating raises the gas temperature by a few degrees over its value in static equilibrium .
The time scale for the equilibration of CO depends on the gas density and is everywhere shorter than the dynamical timescale .
The result is that the dynamics do not much affect the abundance of CO which is always close to that of a static sphere of the same density profile , and CO can not be used as a chemical clock in starless cores .
We use our non-LTE radiative transfer code MOLLIE to predict observable CO and N _ { 2 } H ^ { + } line spectra , including the non-LTE hyperfine ratios of N _ { 2 } H ^ { + } , during the contraction .
These are compared against observations of the starless core L1544 .
The comparison indicates that the dust in L1544 has an opacity consistent with ice-covered rather than bare grains , the cosmic ray ionization rate is about 1 \times 10 ^ { -17 } s ^ { -1 } , and the density structure of L1544 is approximately that of a Bonnor-Ebert sphere with a maximum central density of 2 \times 10 ^ { 7 } cm ^ { -3 } , equivalent to an average density of 3 \times 10 ^ { 6 } cm ^ { -3 } within a radius of 500 AU .
The observed CO line widths and intensities are reproduced if the CO desorption rate is about 30 times higher than the rate expected from cosmic-ray strikes alone , indicating that other desorption processes are also active .