Measuring the evolution of the most stable optical clock G 117-B15A

Abstract

We report our measurement of the rate of change of period with time (˙P ) for the 215 s periodicity in the pulsating white dwarf G 117-B15A, the most stable optical clock known. After 31 years of observations, we have finally obtained a 4σ measurement ˙Pobserved = (4.27± 0.80) x 10 15 s s-ˡ. Taking into account the proper-motion effect of ˙P proper = (7.0 ± 2.0) x 10 16 s s-ˡ,we obtain a rate of change of period with time of ˙P =(3.57 ± 0.82) ; 10 15 s s-ˡ. This value is consistent with the cooling rate in our white dwarf models only for cores of C or C/O. With the refinement of the models, the observed rate of period change can be used to accurately measure the ratio of C/O in the core of the white dwarf.