The formation of giant planets is explained by the nucleated instability model, in which a solid core captures a large amount of nebular gas when it grows to critical core mass. It is well known that critical core mass scarcely depends on the boundary conditions of the envelope, i.e., its distance from the central star and the density and temperature of the nebular gas. However, this is not the case when the envelope is wholly convective. Such a situation is realized if we consider the formation of giant planets close to central stars and/or in dense cool nebulae. In the present study, we extensively investigate the dependence of the critical core mass on the distance from the central star and on the density and temperature of the nebular gas; we found that the critical core mass reduces to 2-3 M⊕ at 0.1 AU in dense nebulae with a surface density about 20 times larger than that in the minimum-mass solar nebula model. This result suggests a possibility of in situ formation of the detected extrasolar giant planets close to the central stars.
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