THE RATE OF CHANGE OF THE GRAVITATIONAL CONSTANT AND THE COOLING OF WHITE-DWARFS

In this paper we examine the effects introduced in the luminosity func tion of white dwarf stars by a change of the gravitational constant du ring the life of the Universe. A decrease of G induces the expansion o f the star and accelerates the cooling process. This effect has notice able effects on the luminosity function and can be used to set upper b ounds on the rate of change of G. The values of the derived upper limi t depend on the adopted age of the galactic disc, the position of the cut-off of the white dwarf luminosity function, and the physics of coo ling - such as the degree of previous stratification of the chemical c onstituents of white dwarfs and the phase diagram of the carbon/oxygen (C/O) mixture. For reasonable ages of the solar neighbourhood this me thod provides G/G less than or equal to -(1 +/- 1) x 10(-11) yr(-1) if previously stratified models are adopted and G/G less than or equal t o -(3(-3)(+1)) x 10(-11) yr(-1) if the distribution of carbon and oxyg en is assumed to be constant through the white dwarf. These values are comparable with the upper bound obtained from the binary pulsar, whic h is considered to be the best constraint on the rate of variation of the gravitational constant.