The dependence of the substellar initial mass function on the initial conditions for star formation

Abstract

We have undertaken a series of hydrodynamical simulations of multiple star formation in small turbulent molecular clouds. Our goal is to determine the sensitivity of the properties of the resulting stars and brown dwarfs to variations in the initial conditions imposed. In this paper we report on the results obtained by applying two different initial turbulent velocity fields. The slope of the turbulent power-law spectrum a is set to -3 in half of the calculations and to -5 in the other half. We find that, whereas the stellar mass function seems to only be weakly dependent on the value of a, the substellar mass function turns out to be more sensitive to the initial slope of the velocity field. We argue that, because the role of turbulence is to create substructure from which gravitational instabilities may grow, variations in other initial conditions that also determine the fragmentation process are likely to affect the shape of the substellar mass function as well. The absence of many planetary-mass 'free-floaters' in our simulations, especially in the mass range 1-10 M J , suggests that, if these objects are abundant, they are likely to form by similar mechanisms to those thought to operate in quiescent accretion discs, instead of via instabilities in gravitationally unstable discs. We also show that the distribution of orbital parameters of the multiple systems formed in our simulations is not very sensitive to the initial conditions imposed. Finally, we find that multiple and single stars share comparable kinematical properties, both populations being able to attain velocities in the range 1-10 km s -1 . From these values we draw the conclusion that only low-mass star-forming regions such as Taurus-Auriga or Ophiuchus, where the escape speed is low, might have suffered some depletion of their single and binary stellar population.