Age gradient and the second parameter problem in the Galactic halo

Harvey B. Richer, William E. Harris, Gregory G. Fahlman, Roger A. Bell, Howard E. Bond, James E. Hesser, Steve Holland, Carlton Pryor, Peter B. Stetson, Don A. Van Den Berg, Sidney Van Den Bergh

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Abstract

We establish a framework for determining absolute ages of Galactic globular clusters and then use these ages to investigate the age-metallicity and age-Galactocentric distance relations for the 36 clusters with the most reliable age data. The clusters span Galactocentric distances from 4 through 100 kpc and cover a metallicity range from [Fe/H] = -0.6 to -2.3. Adopting currently plausible choices for the relation between cluster metallicity and horizontal-branch luminosity, and α-enhancement ratios, we find that the majority of the globular clusters form an age distribution with a dispersion σ(t) of ∼ 109 yr, and a total age spread smaller than 4 Gyr. Clusters in the lowest metallicity group ([Fe/H] < -1.8) appear to be the same age to well within 1 Gyr at all locations in the Milky Way halo, suggesting that star formation began throughout the halo nearly simultaneously in its earliest stages. We find no statistically significant correlation between mean cluster age and Galactocentric distance (no age gradient) from 4 to 100 kpc. The correlation between cluster age and horizontal-branch type suggests that causes in addition to metallicity and age are required to understand the distribution of stars along the horizontal branches in globular cluster color-magnitude diagrams.

Original languageEnglish (US)
Pages (from-to)602-608
Number of pages7
JournalAstrophysical Journal
Volume463
Issue number2 PART I
DOIs
StatePublished - Jan 1 1996

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Richer, H. B., Harris, W. E., Fahlman, G. G., Bell, R. A., Bond, H. E., Hesser, J. E., Holland, S., Pryor, C., Stetson, P. B., Van Den Berg, D. A., & Van Den Bergh, S. (1996). Age gradient and the second parameter problem in the Galactic halo. Astrophysical Journal, 463(2 PART I), 602-608. https://doi.org/10.1086/177274