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In this work we analyze the motion of a thin shell composed of a scalar field. We study the equations of motion resulting from the matching conditions of the two surrounding vacuum spacetimes which are described by the Schwarzschild metric. It is shown that there exist numerical solutions which describe shells which collapse, expand, and oscillate. The conditions for gravitational collapse are obtained by analyzing certain effective potentials. In particular, for a massless scalar field we show that the collapse occurs only if the parameters determining the gravitational mass and the pressure of the shell are proportional. The case of a static configuration is also investigated.
Núñez et al. (Wed,) studied this question.