HD 140283: A Star in the Solar Neighborhood that Formed Shortly After the Big Bang

HD 140283 is an extremely metal-deficient subgiant in the solar neighborhood, having a location in the HR diagram where absolute magnitude is most sensitive to stellar age. Because it is bright, nearby, unreddened, and has an well-determined metallicity, this star avoids most of the issues involved in age determinations for globular clusters. Using the Fine Guidance Sensors on the Hubble Space Telescope, we have measured a trigonometric parallax of 17.14 ± 0.22 mas for HD 140283, with an error one-third of that determined by the Hipparcos mission. Employing modern theoretical isochrones, which include effects of helium diffusion and enhanced oxygen abundance, we use the precise distance to infer an age of 13.30 ± 0.30 Gyr, where the quoted error is the random error due to the parallax uncertainty. Within the errors, the age of HD 140283 is slightly less than the age of the Universe, 13.76 ± 0.11 Gyr, based on the microwave background and Hubble constant. However, there is a larger contribution to the systematic error in the age of HD 140283 arising from remaining uncertainties in the stellar parameters and chemical composition, in particular the oxygen abundance. These systematic uncertainties are now considerably larger than the random error due to the distance uncertainty.