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On the post main sequence expansion of low mass stars
Received 29 June 1998 / Accepted 20 October 1998
The post main sequence expansion of stars is investigated by means of a simple composite configuration: an isothermal He-core (allowing for non-relativistic electron degeneracy) is surrounded by a H-envelope of constant density (polytrope ). Solving the equations of hydrostatic equilibrium for fixed values of total mass and temperature at the interface a one dimensional sequence of models is obtained with the mass of the core as parameter. As soon as the main part of the core becomes fully degenerate, the model stars expand rapidly. This behaviour is in good agreement with that of models obtained by numerical simulations.
The expansion is caused by an intermediate non-degenerate layer of large extension (but of very small mass content) just below the interface. It shifts the envelope to larger distances from the center and thus reduces the gravitational pull on it due to the highly contracted part of the core. Without this layer the thermal forces of the envelope - determined by the hydrogen burning temperatures at the interface - would be much too small to balance gravity. Such a loosely bound envelope extends to the large radii in question. Hence, the model suggests the fixed temperature required by hydrogen burning to be the ultimate reason for the post main sequence expansion.
Key words: stars: evolution stars: interiors
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Online publication: February 23, 1999