Context. Supersonic turbulence in the interstellar mediumplays an important role in the formation of stars. The origin of thisobserved turbulence and its impact on the stellar initial mass function(IMF) still remain open questions. Aims. We investigate the influence of the turbulence forcing onthe mass distributions of gravitationally unstable cores in simulationsof isothermal supersonic turbulence. Methods. Data from two sets of non-selfgravitating hydrodynamicFLASH3 simulations with external stochastic forcing are analysed, eachwith static grid resolutions of 2563, 5123 and 10243grid points. The first set applies solenoidal (divergence-free)forcing, while the second set uses purely compressive (curl-free)forcing to excite turbulent motions. From the resulting density field,we compute the mass distribution of gravitationally unstable cores bymeans of a clump-finding algorithm. Using the time-averaged probabilitydensity functions of the mass density, semi-analytic mass distributionsare calculated from analytical theories. We apply stability criteriathat are based on the Bonnor-Ebert mass resulting from the thermalpressure and from the sum of thermal and turbulent pressure. Results. Although there are uncertainties in applying of theclump-finding algorithm, we find systematic differences in the massdistributions obtained from solenoidal and compressive forcing.Compressive forcing produces a shallower slope in the high-masspower-law regime compared to solenoidal forcing. The mass distributionsalso depend on the Jeans length resulting from the choice of the massin the computational box, which is freely scalable fornon-selfgravitating isothermal turbulence. If the Jeans lengthcorresponding to the density peaks is less than the grid cell size, thedistributions obtained by clump-finding show a strong resolutiondependence. Provided that all cores are numerically resolved and mostcores are small compared to the length scale of the forcing, thenormalised core mass distributions are close to the semi-analyticmodels. Conclusions. The driving mechanism of turbulence has a potentialimpact on the shape of the core mass function. Especially for thehigh-mass tails, the Hennebelle-Chabrier theory implies that theadditional support due to turbulent pressure is important. Key words: hydrodynamics - ISM: clouds - ISM: kinematics and dynamics - methods: numerical - stars: formation - turbulence
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