2015MNRAS.449.3183Z

Inspired by Barnard 68, a Bok globule, that undergoes stable oscillations, we perform multiphase hydrodynamic simulations to analyse the stability of Bok globules. We show that a high-density soft molecular core, with an adiabatic index γ = 0.7 embedded in a warm isothermal diffuse gas, must have a small density gradient to retain the stability. Despite being stable, the molecular core can still collapse spontaneously as it will relax to develop a sufficiently large density gradient after tens of oscillations, or a few 10⁷ yr. However, during its relaxation, the core may abruptly collapse triggered by the impingement of small-amplitude, long-wavelength ( ∼ 6-36 pc) sound waves in the warm gas. This triggered collapse mechanism is similar to a sonoluminescence phenomenon, where underwater ultrasounds can drive air bubble coalescence. The collapse configuration is found to be different from both inside-out and outside-in models of low-mass star formation; none the less the mass flux is close to the prediction of the inside-out model. The condition and the efficiency for this core collapse mechanism are identified. Generally speaking, a broad-band resonance condition must be met, where the core oscillation frequency and the wave frequency should match each other within a factor of several. A consequence of our findings predicts the possibility of propagating low-mass star formation, for which collapse of cores, within a mass range short of one order of magnitude, takes place sequentially tracing the wavefront across a region of few tens of pc over 10⁷ yr.