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Astron. Astrophys. 360, 76-84 (2000)

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2. The code

The simulations we present here have been performed by means of the Tree-SPH code developed by Carraro et al. (1998), Buonomo et al. (2000) and Lia & Carraro (2000). The code, which is able to follow the evolution of a mix of CDM and Baryons (gas and stars), has been successfully checked against standard tests in Carraro et al. (1998), while a fine exploration of the parameters space is presented in Buonomo et al. (2000).

In detail, the gas component is "followed" by means of the smoothed particle hydrodynamics (SPH) technique (Lucy 1977; Gingold & Monaghan 1997; Hernquist & Katz 1989; Steinmetz & Müller 1993), while the gravitational forces are taken into account by means of the hierarchical tree algorithm of Barnes & Hut (1986). We adopt a tolerance parameter [FORMULA], a Plummer softening parameter and expand the tree nodes to quadrupole order.

In SPH each particle represents a fluid element whose position, velocity, energy, density etc. are followed in time and space. The properties of the fluid are locally estimated by an interpolation which involves the smoothing length [FORMULA]. Each particle possesses its own time and space variable smoothing length [FORMULA], and evolves with its own time-step. This renders the code highly adaptive and flexible, and suited for numerical "experiments".

Radiative cooling is considered as a function of temperature and metallicity following Sutherland & Dopita (1993) and Hollenbach & McKee (1979) and the code takes into account the variations in metallicity among the fluid elements as a function of time and position.

Star formation (SF) and feed-back algorithms are described in Buonomo et al. (2000). Specifically SF, following partly Katz (1992), is set to occur when




with the star formation rate of


where [FORMULA] is the dimensionless efficiency of star formation, and [FORMULA] is the characteristic time for the gas to flow, usually set to the maximum between the cooling time and the free-fall time. For the simulations discussed here, we keep [FORMULA]. When formed, stars are distributed in mass according to the Miller & Scalo (1979) initial mass function (IMF).

The effects of energy (and mass) feed-back from supernovae and stellar winds are also taken into account (Chiosi & Maeder 1986, Thornton et al. 1998). In this experiments we deposit all the energy released by SNae ([FORMULA]) and stellar wind in the thermal budget of the bubble, following the kind of arguments discussed in Buonomo et al. (2000).

Finally, the chemical enrichment of the interstellar gas caused by the the stellar wind and ejecta is followed by means of the closed-box model applied to each gas-particle as in Carraro et al. (1998). Metals are then SPH-diluted over the surrounding gas particles.

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000