HD 140283: A STAR IN THE SOLAR NEIGHBORHOOD THAT FORMED SHORTLY AFTER THE BIG BANG

Abstract

HD 140283 is an extremely metal-deficient and high-velocity subgiant in the\nsolar neighborhood, having a location in the HR diagram where absolute\nmagnitude is most sensitive to stellar age. Because it is bright, nearby,\nunreddened, and has a well-determined chemical composition, this star avoids\nmost of the issues involved in age determinations for globular clusters. Using\nthe Fine Guidance Sensors on the Hubble Space Telescope, we have measured a\ntrigonometric parallax of 17.15 +/- 0.14 mas for HD 140283, with an error\none-fifth of that determined by the Hipparcos mission. Employing modern\ntheoretical isochrones, which include effects of helium diffusion, revised\nnuclear reaction rates, and enhanced oxygen abundance, we use the precise\ndistance to infer an age of 14.46 +/- 0.31 Gyr. The quoted error includes only\nthe uncertainty in the parallax, and is for adopted surface oxygen and iron\nabundances of [O/H] = -1.67 and [Fe/H] = -2.40. Uncertainties in the stellar\nparameters and chemical composition, especially the oxygen content, now\ncontribute more to the error budget for the age of HD 140283 than does its\ndistance, increasing the total uncertainty to about +/-0.8 Gyr. Within the\nerrors, the age of HD 140283 does not conflict with the age of the Universe,\n13.77 +/- 0.06 Gyr, based on the microwave background and Hubble constant, but\nit must have formed soon after the big bang.\n