Servicio de Información Comunitario sobre Investigación y Desarrollo - CORDIS

Final Activity and Management Report Summary - JEANSMASS (The Role of Thermal Physics in Determining the Masses of Stars)

Stars form from molecular cloud cores by gravoturbulent fragmentation. The evolution of a star forming molecular cloud depends critically on the thermodynamic processes which determine the temperature, and hence the pressure, of the gas. The aim of this project was to explore the influence of these thermodynamic processes by developing much more realistic algorithms of heating, cooling and chemistry in star-forming clouds, and then combing them with high-resolution numerical simulations of star formation. In particular we wanted to investigate the crucial properties of star forming regions, i.e., the stellar initial mass function, the star formation efficiency and the spatial distribution, and their dependence on epoch, metallicity, level of turbulence and background radiation field.

We have used the publicly available smoothed particle hydrodynamics (SPH) code GADGET2, augmented by an algorithm to include the microphysics of heating, cooling and chemistry appropriate for the early epochs of star formation in the universe. We have also added a particle splitting routine to increase the resolution of the SPH calculations. We have run the simulations on the new MERLIN super-computer of Cardiff University's ARCCA service (Advanced Research Computing Cardiff). Firstly we have investigated star formation in the early universe. Numerical simulations have indicated that the metal-free first stars, the so-called Population III, are expected to be very massive. In contrast, the stellar mass spectrum in the present-day universe is dominated by low-mass stars.

It has been proposed that the transition from primordial to modern initial mass functions occurs due to the onset of effective metal-line cooling at a critical metallicity. However, these simulations neglected molecular hydrogen cooling. We performed simulations using a complex network that follows molecular hydrogen formation and also directly follows carbon monoxide and water. We have found that the mass spectrum of fragments formed in our high-resolution simulations has very little dependence on the metallicity of the gas at low densities. Our current results show no evidence for a critical metallicity. However, the degree of fragmentation occurring in these simulations appears to be a consequence of the initial conditions chosen: simulations using substantially different initial conditions find no evidence for any fragmentation.

We also found that this is true for increased turbulence or rotation of the gas. Our results call attention to an alternative scenario for the transition to the present-day low-mass IMF. They stress the importance of dust-induced cooling at higher densities. Secondly we have investigated the formation of giant dense cloud complexes and of stars within them by means of SPH simulations of the collision of gas streams in the warm neutral medium at moderately supersonic velocities. The collisions cause compression, cooling and turbulence generation in the gas, forming a cloud that then becomes self-gravitating and begins to collapse globally. Simultaneously, the turbulent, nonlinear density fluctuations induce fast, local collapse events.

We have compared the statistical distributions of the physical properties of the dense cores appearing in the central region of massive collapse with those from a recent survey of the massive star forming region in the Cygnus X molecular cloud, finding that the observed and simulated distributions are in general very similar. We have found that the star formation efficiency (SFE) decreases with increasing inflow velocity of the streams. Increasing levels of background turbulence similarly reduce the SFE, because the turbulence disrupts the coherence of the colliding streams, fragmenting the cloud, and producing small-scale clumps scattered through the numerical box, which have low SFEs.

Finally, the SFE is very sensitive to the mass of the inflows with SFE decreasing as the mass in the colliding streams decreases. We conclude that the SFE is a highly sensitive function of the parameters of the cloud formation process, and may be responsible for significant intrinsic scatter in observational determinations of the SFE.

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United Kingdom
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