An observational limit to the evolutionary time scale of the 13,000 K white dwarf G117-B15A

Abstract

Nature has provided us with a way of measuring the evolutionary time scale of some stars through aster­ oseismology, i.e., the study of stellar interiors through their intrinsic variations in brightness. By measuring the slow, secular change in one or more of its periodicities, we can measure the rate at which a luminosity­ variable white dwarf is cooling. Using 204 hr ofhigh-speed photometric observations ofthe DAV star G117-B15A during the last 14 yr, we have obtained an observational limit to the rate of period change for its dominant pulsation at 215.2 s of dPjdt = (8.3 ± 5.0) x 10-15 s s-ˡ. This rate of period change corresponds to an evolutionary time scale of r = (8.2 ± 5.0) x 108 yr, consistent with the change expected due to cooling of the white dwarf undergoing nonradial g-mode pulsations. The observed limit on the rate of period change makes G117-B15A the most accurate optical clock known, with a stability comparable to that of the atomic clocks used by the National Bureau of Standards, and exceeded only by a few ms radio pulsars. Since ali DA white dwarfs are thought to cool through the DAV instability strip, and there are no known differences between the properties of variables and nonvariable white dwarfs, the measurement of an evolu­ tionary time scale, suitably scaled by mass and surface temperature, should apply to ali DA white dwarfs.