Skip to main content
Log in

The Milky Way globular cluster system within the context of a closed box

  • Original Article
  • Published:
Astrophysics and Space Science Aims and scope Submit manuscript

Abstract

Metallicities of the Milky Way globular cluster (GC) system are interpreted within the framework of a closed box model of chemical evolution. Model age-metallicity relations normalised to the Sun are compared with an observed age-metallicity relation constructed from literature data for the Galactic globular clusters and solar neighbourhood dwarf stars. One branch of the GC age-metallicity relation is seen to form a natural extension of the solar neighbourhood relation to low metallicities. Such clusters may fit in situ into the same Galactic closed-box model as many disk stars. Timescales for halo and disk GC formation are computed based on this assumption. However, a single closed box cannot account for a second branch in the GC age-metallicity data. This problem is addressed by assuming that some GCs did not form in situ within the closed box of the Galaxy, but rather were formed within separate parent objects whose chemical evolution was characterised by longer star formation timescales than the Milky Way box. These parent objects were eventually acquired by the Milky Way. Estimates are made of the initial mass and star formation timescale of those objects that contributed GCs to the accreted component of the Milky Way.

This is a preview of subscription content, log in via an institution to check access.

Access this article

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Instant access to the full article PDF.

Fig. 1
Fig. 2
Fig. 3
Fig. 4

Similar content being viewed by others

Notes

  1. This estimate of the GC formation rate would be a lower limit if the original Galactic GC system has been significantly reduced in number by disruptive processes, such as stellar dynamical evaporation and tidal shocking, as predicted by theoretical models such as those of Aguilar et al. (1988), Chernoff and Weinberg (1990), Gnedin and Ostriker (1997), Murali and Weinberg (1997), Vesperini (1998), Gnedin et al. (1999), Takahashi and Portegies Zwart (2000), Fall and Zhang (2001), Vesperini and Zepf (2003), McLaughlin and Fall (2008), Prieto and Gnedin (2008), Whitehead et al. (2013), and Webb et al. (2017).

  2. This conclusion does depend upon whether the oldest GC ages in Leaman et al. (2013) also match the oldest Population II (and III) stars in the Galaxy. If the oldest field stars predated the first GCs by 0.5 Gyr then the solid curve in Fig. 1 for a Galactic age of 13.5 Gyr would be applicable, and it could be concluded that the majority, if not all, of the GCs with \(\mathrm{[Fe/H]} < -1.6\) are lower branch objects.

  3. The disk models of Bekki (2014) for systems with baryonic masses of \(6.6 \times 10^{9}\) \(M_{\odot }\) or less (halo masses of \(10^{11}\) \(M_{\odot }\) or less) are mostly characterised by a fraction of gas in molecular hydrogen that does not exceed 0.3, and is typically less than 0.1 for baryonic disk masses of \(6.6 \times 10^{8}\) \(M_{\odot }\) or less.

  4. As with the globular clusters, however, the field stars appear to have originated both in situ within the proto-Galaxy and within systems that were accreted by the Galaxy (e.g., Deason et al. 2016; Conroy et al. 2019).

  5. We thank Andreas Burkert for suggesting this point to the author.

References

Download references

Author information

Authors and Affiliations

Authors

Additional information

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Smith, G.H. The Milky Way globular cluster system within the context of a closed box. Astrophys Space Sci 365, 193 (2020). https://doi.org/10.1007/s10509-020-03897-0

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: https://doi.org/10.1007/s10509-020-03897-0

Keywords

Navigation