Chemical analysis of prestellar cores in Ophiuchus yields short
  timescales and rapid collapse
  Sun-like stars form from the contraction of cold and dense interstellar
clouds. How the collapse proceeds and what are the main physical processes
driving it, however, is still under debate and a final consensus on the
timescale of the process has not been reached. Does this contraction proceed
slowly, sustained by strong magnetic fields and ambipolar diffusion, or is it
driven by fast collapse with gravity dominating the entire process? One way to
answer this question is to measure the age of prestellar cores through
statistical methods based on observations or via reliable chemical
chronometers, which should better reflect the physical conditions of the cores.
Here we report APEX observations of ortho-H$_2$D$^+$ and para-D$_2$H$^+$ for
six cores in the Ophiuchus complex and combine them with detailed
three-dimensional magneto-hydrodynamical simulations including chemistry,
providing a range of ages for the observed cores up to 200 kyr. The outcome of
our simulations and subsequent analysis provides a good match with the
observational results in terms of physical (core masses and volume densities)
and dynamical parameters such as the Mach number and the virial parameter. We
show that models of fast collapse successfully reproduce the observed range of
chemical abundance ratios as the timescales to reach the observed stages is
comparable to the dynamical time of the cores (i.e. the free-fall time) and
much shorter than the ambipolar diffusion time, measured from the electron
fraction in the simulations. To confirm that this ratio can be used to
distinguish between different star-formation scenarios a larger (statistically
relevant) sample of star-forming cores should be explored.
