Simulating star formation in molecular cloud cores
Attwood, Rhianne Elizabeth
arxiv: Astrophysics::Galaxy Astrophysics | Astrophysics::Earth and Planetary Astrophysics | Astrophysics::Solar and Stellar Astrophysics
In this thesis we investigate the influence of certain physical effects on the collapse and fragmentation of isolated, low-mass, low-turbulence cores, in particular on the mass distribution, binary statistics and kinematics of the resulting stars. We perform numerical simulations using a Smoothed Particle Hydrodynamics code to model this mode of star formation. Firstly we model acoustic oscillations of a self-gravitating isentropic monatomic gas sphere using our SPH code and find that if the smoothing lengths are adjusted so as to keep the number of neighbours in the range AAu, NNE1B should be set to zero, to reduce the level of numerical dissipation and diffusion. We suggest that this should become a standard test for codes simulating star formation, since pressure waves generated by the switch from approximate isothermality to approximate adiabaticity play a crucial role in the fragmentation of collapsing cores. We perform a large ensemble of SPH simulations of cores having different levels of turbulence, using a new, more realistic treatment of thermodynamics, developed by Stamatellos et al. (2007), which takes into account the thermal history of protostellar gas and captures the thermal inertia effects. We compare the results with simulations using a standard barotropic equation of state. We find that increasing the level of turbulence generally tends to reduce the fraction of the core mass which is converted into stars, and increase the number of stars formed by a single core. Using the new treatment results in more protostellar objects being formed, and a higher proportion of brown dwarfs. Of the multiple systems that form, they tend to have shorter periods, higher eccentricities and higher mass ratios. We also note that in our simulations the process of fragmentation is often bimodal, in the following sense. The first protostar to form is usually, at the end, the most massive, i.e. the primary. However, frequently a disc-like structure subsequently forms round this primary, and then, once it has accumulated sufficient mass, quickly fragments to produce several secondaries. We believe that this delayed fragmentation of a disc-like structure is likely to be an important source of very low-mass stars in nature (both low-mass hydrogen-burning stars and brown dwarf stars). We also model the evolution of an ensemble of prestellar cores in the Ophiuchus Main Cloud using initial conditions for the sizes and levels of turbulence constrained by the observations of Motte et al. (1998) and Andre' et al. (2007), and the recently revised core masses of Stamatellos et al. (2007). We find that star formation in these core is extremely efficient with typically the formation of a single star, but we also see the formation of multiple systems in a number of cores. We find that the number of stars formed by a core is highest if the core has high mass, and/or if it has a high initial level of turbulence, and/or if it starts from a low initial density. We explain why. Finally we explore the effect metallicity has on the mass distribution and binary statistics of stars formed from low-mass low-turbulence cores. We find that reducing the metallicity decreases the number of stars formed from a single core and reduces the number of brown dwarfs formed. It also reduces the binary frequency.
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