White dwarf constraints on a varying $G$

Abstract

A secular variation of $G$ modifies the structure and evolutionary time scales of white dwarfs. Using an state-of-the-art stellar evolutionary code, an up-to-date pulsational code, and a detailed population synthesis code we demonstrate that the effects of a running $G$ are obvious both in the properties of individual white dwarfs, and in those of the white dwarf populations in clusters. Specifically, we show that the white dwarf evolutionary sequences depend on both the value of $\dot G/G$, and on the value of $G$ when the white dwarf was born. We show as well that the pulsational properties of variable white dwarfs can be used to constrain $\dot G/G$. Finally, we also show that the ensemble properties of of white dwarfs in clusters can also be used to set upper bounds to $\dot G/G$. Precisely, the tightest bound --- $\dot G/G \sim -1.8 10^{-12}$ yr$^{-1}$ --- is obtained studying the population of the old, metal-rich, well populated, open cluster NGC 6791. Less stringent upper limits can be obtained comparing the theoretical results obtained taking into account the effects of a running $G$ with the measured rates of change of the periods of two well studied pulsating white dwarfs, G117--B15A and R548. Using these white dwarfs we obtain $\dot G/G\sim -1.8\times 10^{-10}$ yr$^{-1}$, and $\dot G/G\sim -1.3\times 10^{-10}$ yr$^{-1}$, respectively, which although less restrictive than the previous bound, can be improved measuring the rate of change of the period of massive white dwarfs.