V.3 No 1

61

Chapter 2. Hypothesis of origin of planetary system (part 1)

At the considered stage the most important is the gathering ("fall") of substance towards the centre or centres of gravity of the cloud, so this stage is called "free fall". Though in reality it will be not a free fall, since as densifying, the gas in central region heats and this is a powerful counteraction to the compression. Besides, with the pressure growing, the viscosity of medium also grows, and this also impedes the process. Factually, the free-fall stage is the comparably fast (in astrophysical scale) formation of the compact region of high pressure and temperature.

Fig. 2.8.2 - 2.8.4

At the next (conventionally third) stage of protostar formation (see Fig. 2.8.3) the "free-fall" goes on, but to the forefront comes the issue of heat exchange between the hot centre and cold periphery of star. The compressed substance impedes the radiative heat exchange denoted in Fig. 2.8.4 by orange arrows. This causes the energy accumulating within the protostellar substance and to the convective motion of the substance outward (blue arrows in Fig. 2.8.4) where it loses the excessive energy and comes back cooled. This means, the process transits to its fourth stage. This stage has been studied and calculated by Japan scientist Chusiro Hayashi and it is called by his name. At this stage practically all substance of protostar is involved into convective heat exchange. Only in the exterior, quite rarefied region the radiation is possible. Dependently on the protostar mass ? and luminosity L, the temperature in this layer is estimated as

In numerical estimation it amounts 2500- 3500 K [1, p.86]. From this, whatever would be the mass and luminosity, the temperature in the layer where the convection emerges to the surface depends on them slightly and remains constant during the entire Hayashi stage. And this is natural, because the intensity of convection emergence to the surface depends on the gradient of temperature and density. And though factually the temperature of the inner region will grow, for the exterior observer the luminosity will diminish with the protostar compression.

At this stage of the protostar formation, Shklovsky selects several typical regions whose location along the radius of protostellar nebula is shown in Fig. 2.10. Shklovsky thinks, the free fall forms the shock front necessary for relatively fast compression of the central region of protostar that creates the conditions for thermonuclear "reactor" to initiate. But since, as we said before, free fall is relatively smooth process, we would rather say of gradual compression reduction that rises the hydrostatic pressure in the centre of cloud, with some asymmetry of the compression wave to the centre of protostar. But the shock front in the conventional understanding we could attribute to the external affection in presence, for example, a nearby supernova.

Fig. 2.10. Schematic structure of compressing protostellar cloud [1, p. 102, Fig. 35].