Impact of the shape of the prestellar density fluctuations on the core mass function

ABSTRACT It is well known that departure from sphericity in the geometry of primordial dark matter haloes modifies their mass function. The ellipsoidal collapse model yields a better agreement with simulations of hierarchical clustering than the original spherical model. In this paper, we examine the same issue in the context of star formation by studying the impact of non-sphericity of density perturbations in a gravoturbulent medium on the prestellar core mass function (CMF). An important question, notably, is to find out how ellipsoidal, instead of spherical, initial density fluctuations modify both the high-mass and low-mass tails of the CMF. Our study shows that triaxial density fluctuations indeed depart from a purely spherical form but the deformation (prolateness and ellipticity) remains modest, suggesting that the usual hypothesis of spherical collapse in existing theories of the IMF is reasonable. We find that, as in the cosmological case, the departure from sphericity increases the collapse barrier, stabilizing the prestellar cores. The striking difference between the stellar case and the cosmological one for the ellipsoidal collapse model is that, although in both cases the less dense structures are the most deformed, they correspond to small scales, thus low-mass haloes in cosmology but to large scales, thus large mass cores in star formation. As a result, the high-mass range of the CMF is the most affected by the ellipsoidal collapse, resulting in a slightly less steep slope than the one predicted with the spherical hypothesis and a peak slightly shifted towards lower masses.