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Exact Solutions and Scalar Fields in Gravity - Instituto Avanzado de ...

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228 EXACT SOLUTIONS AND SCALAR FIELDS IN GRAVITY<br />

grid we impose an outflow boundary condition such that no disturbances<br />

can propagate <strong>in</strong>to the doma<strong>in</strong> of computation from the boundary surface.<br />

This boundary condition is naturally implemented because the gas<br />

flows out supersonically from the doma<strong>in</strong> of computation. The supersonic<br />

outflow is due to the apparent motion with respect to the zoom<strong>in</strong>g<br />

coord<strong>in</strong>ates.<br />

The present calculation provi<strong>de</strong>s a further <strong>in</strong>tercomparison test at high<br />

resolution for the isothermal Gaussian cloud us<strong>in</strong>g a numerical framework<br />

that is completely different to those employed <strong>in</strong> Refs. [10, 17].<br />

With the present zoom<strong>in</strong>g grid resolution, the calculation took about<br />

timesteps to complete the evolution, correspond<strong>in</strong>g to about<br />

30 hr of CPU time on an Orig<strong>in</strong> 2000 mach<strong>in</strong>e.<br />

3. RESULTS<br />

The Gaussian cloud mo<strong>de</strong>l can <strong>in</strong>crease <strong>in</strong> <strong>de</strong>nsity by about 4 or<strong>de</strong>rs<br />

of magnitu<strong>de</strong> before the isothermal approximation beg<strong>in</strong>s to fail. In<br />

real molecular cloud cores significant non-isothermal heat<strong>in</strong>g will take<br />

place once ren<strong>de</strong>r<strong>in</strong>g the isothermal approximation<br />

<strong>in</strong>valid. However, for the purposes of this paper we ignore the effects

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