On turbulent fragmentation and the origin of the stellar IMF

Abstract

Two varieties of the universal stellar initial mass function (IMF) viz., the Kroupa and the Chabrier IMF, have emerged over the last decade to explain the observed distribution of stellar masses. The possibility of the universal nature of the stellar IMF leads us to the interesting prospect of a universal mode of star-formation. It is well-known that turbulent fragmentation of gas in the interstellar medium produces a lognormal distribution of density which is further reflected by the mass-function for clumps at low and intermediate masses. Stars condense out of unstable clumps through a complex interplay between a number of dynamic processes which must be accounted for when tracing the origin of the stellar IMF. In the present work, applying the theory of gravitational fragmentation we first derive the mass function (MF) for clumps. Then a core mass function (CMF) is derived by allowing the clumps to fragment, having subjected each one to a random choice of gas temperature. Finally, the stellar IMF is derived by applying a random core-to-star conversion efficiency, ϵ, in the range of 5–15% to each CMF. We obtain a power-law IMF that has exponents within the error-bars on the Kropua IMF. This derived IMF is preceded by a similar core mass function which suggests, gravoturbulent fragmentation plays a key role in assembling necessary conditions that relate the two mass-functions. In this sense the star-formation process, at least at low redshifts where gas cooling is efficient, is likely to be universal. We argue that the observed knee in the CMF and the stellar IMF may alternatively be interpreted in terms of the characteristic temperature at which gas in potential star-forming clouds is likely to be found. Our results also show that turbulence in star-forming clouds is probably driven on large spatial scales with a power-spectrum steeper than Kolmogorov-type.