The Relative Age of the Thin and Thick Galactic Disks

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Abstract
We determine the relative ages of the open cluster NGC 188 and selected Hipparcos field stars by isochrone fitting and compare them to the age of the thick-disk globular cluster 47 Tuc. The best-fit age for NGC 188 was determined to be 6.5 ± 1.0 Gyr. The solar-metallicity Hipparcos field stars yielded a slightly older thin-disk age, 7.5 ± 0.7 Gyr. Two slightly metal-poor ([Fe/H] = -0.22) field stars whose kinematic and orbital parameters indicate that they are members of the thin disk were found to have an age of 9.7 ± 0.6 Gyr. The age for 47 Tuc was determined to be 12.5 ± 1.5 Gyr. All errors are internal errors due to the uncertainty in the values of metallicity and reddening. Thus, the oldest stars dated in the thin disk are found to be 2.8 ± 1.6 Gyr younger than 47 Tuc. Furthermore, as discussed by Chaboyer, Sarajedini, & Armandroff, 47 Tuc has a similar age to three globular clusters located in the inner part of the Galactic halo, implying that star formation in the thin disk started within 2.8 ± 1.6 Gyr of star formation in the halo.
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