Formation of giant planets in dense nebulae: Critical core mass revisited

The formation of giant planets is explained by the nucleated instability mo del, in which a solid core captures a large amount of nebular gas when it g rows to critical core mass. It is well known that critical core mass scarce ly depends on the boundary conditions of the envelope, i.e., its distance f rom the central star and the density and temperature of the nebular gas. Ho wever, 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 t o central stars and/or in dense cool nebulae. In the present study, we exte nsively investigate the dependence of the critical core mass on the distanc e from the central star and on the density and temperature of the nebular g as; 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 centra l stars.