Chemical analysis of prestellar cores in Ophiuchus yields short timescales and rapid collapse

Abstract

Sun-like stars form from the contraction of cold and dense interstellar clouds. How the collapse proceeds and what the main physical processes are driving it, however, is still under debate and a final consensus on the timescale of the process has not been reached. If the contraction proceeds slowly, supported by strong magnetic fields and mediated by ambipolar diffusion, or is driven by fast collapse with gravity dominating the entire process is still an open question. 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 Atacama Pathfinder EXperiment observations of ortho-H2D+ and para-D2H+ for six cores in the Ophiuchus complex, and we combined them with detailed three-dimensional magneto-hydrodynamical simulations including chemistry, providing a range of ages for the observed cores of up to 200 kyr. The outcome of our simulations and subsequent analysis provides a good matching with the observational results in terms of physical parameters (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 since 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.