The dynamics of collapsing cores and star formation

Abstract

Low-mass stars are understood to form by the gravitational collapse of the dense molecular clouds known as starless cores. However, it has proven impossible to use continuum observations to distinguish among the different hypotheses describing the collapse because the predicted density distributions for all spherical self-gravitating clouds are quite similar. However, the predicted velocities are quite different. We use two different molecular line transitions, H2O (110-101) and C18O (1-0), that are excited at different densities, 108 and 103 cm-3, to measure the velocities at large and small radii in the contracting core L1544. We compare the observed spectra against those predicted for several different models of gravitational collapse including the Larson-Penston flow, the inside-out collapse of the singular isothermal sphere, the quasi-equilibrium contraction of an unstable Bonnor- Ebert sphere, and the non-equilibrium collapse of an overdense Bonnor-Ebert sphere. Only the model of the unstable quasi-equilibrium Bonnor-Ebert sphere is able to produce the observed shapes of both spectral lines. With this model, we interpret other molecular line observations of L1544 in the literature to find that the extended inward velocities seen in lines of CS(2-1) and N2H+ are located within the starless core itself, in particular in the region where the density profile follows an inverse square law. If these conclusions were to hold in the analysis of other starless cores, this would imply that the formation of hydrostatic clouds within the turbulent interstellar medium is not only possible but also not exceptional and may be an evolutionary phase in low-mass star formation.