Formation of planets around stars of various masses - II. Stars of two and three solar masses and the origin and evolution of circumstellar dust clouds

Abstract

Planet formation around stars of middle mass is investigated. It is shown that within the lifetime of the central stars with 2 and 3 M⊙ the planets form only within 35 and 20 AU from the star respectively, even with the maximum admissible amount of the initial gaseous nebula. The observed parts of the dust clouds around α Lyr and β Pic are far outside these planet-forming regions. We propose a model for the dust disc in which dust is formed by the collision of planetesimals. With this model the surface brightness of an edge-on disc in the scattered light is proportional to R−35/8, where R is the projected distance from the star on the sky. The exponent to R, 35/8 = 4.375, is very close to the observed value 4.3 for the β Pic disc. The surface brightness, the geometrical and optical thicknesses and the intensity of the thermal radiation of the β Pic disc are well reproduced with the model in which the amount of the initial gaseous nebula is slightly greater than that around the Protosun. The thermal radiation of the dust cloud around α Lyr is reproduced with the model in which the amount of the initial gaseous nebula is about 0.1 times that around the Protosun.