The Collapse Of The Molecular Cloud Core In The Formation Of The Star And Planet

The collapse of molecular cloud, the formation of star & planets and the accretion disk are important in the astrophysics, meanwhile the coincidence of these two areas is of so many. These types of astrophysical phenomena are followed in time, but also difficult to completely separate in the evolution process.At the fields of molecular cloud collapse, star formation and accretion disk, there have been a great deal of excellent work. In recent years, The study of planet formation by using accretion disk theory has became hot. We believe that careful consideration between the collapse of molecular clouds and accretion disk will be conducive to a better understanding of star and planet formation of the physical processes, and to identify some of these important parameters. The self-similar solution which is found by Frank Shu is the basic work in molecular clouds collapse. By the role of its own gravity, the collapse of molecular cloud is"inside-out", and the material fall in the protostar by the way of free-fall in the region near the core; And Bondi described the steady accretion equations. The Bondi's accretion solutions also behave like the collapse solutions if the comparison is made to the basis of core properties. For instance, the essential features of Shu's solution, such as the mass accretion rate, position of the sonic point, and radial dependences of velocity and density near the core can be obtained approximately from the steady Bondi solutions simply by using Shu's instantaneous value of the core mass in the appropriate Bondi formulas. In fact, with the knowledge the"free-fall"conditions prevail near the core, this conclusion is very natural. And because the"free-fall"conditions prevail near the core, there is the relation between the trajectory of the material and the angular velocity of gas molecules;meanwhile , the trajectory of the material will directly affect the accretion rate in the different radius. By the above conclusion, we know that the collapse of molecular cloud is closely related to the formation of the accretion disk, and the two physical processes directly affect the conditions of planet formation.Astronomers centre to consider only the gravitational effects of the central celestial bodies in the previous studies. when the collapse conditions are Triggered, the cloud collapse to format stars and planets by the dominant role of gravity. And we get the flat disk model. But the formation of stars in the universe is not isolated. It will be influence by the gravity outside. Although these gravitational field is much smaller than the gravitational field of celestial body, the effect can't be ignored in the long evolution of star formation process.In this article, we consider the effect to the disk by Large-scale gravitational field outside. By calculation, we know that the gravitational field of 1 kpc scale is the largest and will evidently impact on the stars which are away from the regional centre of molecular cloud. Before the collapse of molecular cloud, there is full of material in the core region, and the gas pressure is in support of the outside world gravitational effect. However, by the process of accretion, Material gradually gathered at the accretion disk and the core. Then, the pressure become lower. The accretion disk and the core are away from the original plane by the gravity of the outside. The brink of the accretion disk extends to the surrounding gas layer. Then, it would be supported by the pressure. In the entire star formation process of evolution, the star would evidently departure from the location of gravitational center. Moreover, the movement of the disk in the external gravitational field isn't isokinetic. On the edge of the disk, the velocity field gradient of the Vertical direction would be great. And the viscous role produce heat which would increase the luminescence of the region. The increasing source of energy is not the accretion of the core, but the external gravitational field.