The age of the oldest Open Clusters

Abstract

Abstract. We determine ages of 71 old Open Clusters by a two-step method: we use main-squence fitting to 10 selected clusters, in order to obtain their distances, and derive their ages from comparison with our own isochrones used before for Globular Clusters. We then calibrate the morphological age indicator δ(V), which can be obtained for all remaining clusters, in terms of age and metallicity. Particular care is taken to ensure consistency in the whole procedure. The resulting Open Cluster ages connect well to our previous Globular Cluster results. From the Open Cluster sample, as well as from the combined sample, questions regarding the formation process of Galactic components are addressed. The age of the oldest open clusters (NGC 6791 and Be 17) is of the order of 10 Gyr. We determine a delay by 2.0 ± 1.5 Gyr between the start of the halo and thin disk formation, whereas thin and thick disk started to form approximately at the same time. We do not find any significant agemetallicity relationship for the open cluster sample. The cumulative age distribution of the whole open cluster sample shows a moderately significant (∼2σ level) departure from the predictions for an exponentially declining dissolution rate with timescale of 2.5 Gyr. The cumulative age distribution does not show any trend with galactocentric distance, but the clusters with larger height to the Galactic plane have an excess of objects between 2-4 and 6 Gyr with respect to their counterpart closer to the plane of the Galaxy.

CCD UBV(RI)C photometry of twenty open clusters

“…13 also shows little correlation. Carraro et al (1998), Chen et al (2003, and Salaris et al (2004) also did not find any relation between the ages and metallicities for their OCs. The significant scatter in the ages and metallicities dominates and hides any possible small correlation.…”

“…The morphological ages are estimated from the equation, logA = 0.04δV 2 + 0.34δV + 0.07[F e/H] + 8.76 of Salaris et al (2004), applying the metal abundances, [Fe/H], of the OCs in Table 6. These ages of 12 OCs together with their uncertainties have been listed in Column 8 of Table 9.…”

The Red Giant Branch luminosity function bump

“…According to Salaris et al (1993), as long as the α-elements have approximately the same enhancement, scaled-solar models mimic α-enhanced models computed with the same global metallicity [M/H]. This is, however, strictly true only for [M/H] values up to ∼-1.0, as shown by Salaris & Weiss (1998) and Vandenberg et al (2000). For higher global metallicities this equivalence is not well satisfied anymore.…”

“…Therefore we investigated how different would be the ∆F555W bump HB parameter at a given [M/H] larger than −1, for a scaled-solar and an α-enhanced metal mixture. We used isochrones by Salaris & Weiss (1998) with [M/H] = −0.3 (approximately the upper end of the metallicity range spanned by our GGC sample), both scaled-solar and with [α/Fe] = 0.4; we found that the ∆F555W bump HB values are changed by only 0.05 mag for typical GGC ages, the α-enhanced ones being larger. This is however just an upper limit to the real difference, since these metal rich clusters seem to show an α-enhancement lower than [α/Fe] = 0.4.…”

Age and Metallicity of a Globular Cluster in the Dwarf Spheroidal Galaxy DDO 78

“…As for the estimates of the absolute age of NGC 362, they range from 8.7 Gyr (Salaris & Weiss 1998) and 9.9 Gyr (Carretta et al 2000) to 11-12 Gyr (Vandenberg 2000). Since the differences of Hβ, Mg 2 , Mgb, and Fe indices between NGC 362 and GC in DDO 78…”