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ASTR 5610, Majewski [SPRING 2018]. Lecture Notes

ASTR 5610 (Majewski) Lecture Notes


Star clusters, and especially globular cluster ages, play a central role in understanding the evolution of the Milky Way and other galaxies.

As an example of the central role that clusters play in stellar population studies, recall Vatican Conference table of Galactic stellar populations and the ages therein, which were driven by the best stellar evolution model/cluster age dating of the time:

Summary tables from the 1957 Vatican Conference proceedings discussed in Lecture 1.

In general, before the 1990s, the general consensus was that the halo globular clusters were coeval to within a few Gyr.

A number of techniques have been used to obtain cluster ages.

These rely heavily on our models of stellar evolution and how it is reflected by the color-magnitude diagram of clusters.

MSTO Ages and Isochrone Fitting

That clusters have well-defined MSTOs points to their approximation to a Simple Stellar Population.

The position of the MSTO in a star cluster -- and the fact that it burns down lick a wick with age -- is central to age determination.

From Binney & Merrifield, Figure 6.6.
For the Galactic globular cluster system, we are restricted to a small range of ages and, in turn, a smaller range of MSTO colors and absolute magnitudes:

From Binney & Merrifield, Figure 6.10.
Thus, we require reasonably good photometry at the MSTO.

The position of the MSTO is also related to the metallicity of the cluster.

From Binney & Merrifield, Figure 6.3.

There are several important problems relavant to this and related calibrations of age to MSTO brightness:

The ΔV Method

A method that is free of distance, photometric calibration and reddening uncertainties is one first suggested by Iben & Faulker (1968, ApJ, 153, 101) based on the magnitude difference between the HB and the MSTO.

The Δ(B-V) (Color-Difference) Method

VandenBerg, Bolte & Stetson (1990) and Sarajedini & Demarque (1990) presented a new technique for getting age estimates that has the advantages of the ΔV(HB-MSTO) method and overcomes some of the disadvantages of that method.

However, nothing comes for free! There are some disadvantages of the method:

White Dwarf Cooling Ages

Not useful for globulars yet, because need to get to bottom of WD sequence.

But has been tried on open clusters, like the example below on M37.

From Kalirai, J.~S. et al. (2001, AJ, 122, 3239).
An attempt on the nearby globular cluster M4, but the HST imaging is still not deep enough.

White Dwarf sequence in the globular cluster M4 by Richer et al. (1997, ApJ, 484, 741). A theoretical cooling sequence for C-core WDs is shown on the right, as is an age scale. Note that the end of the WD sequence is not yet discerned in this very deep imaging, but can be seen in the even deeper HST imaging study by Hansen et al. (2002, ApJ, 574, L155) which shows M4 to have an age of at east 12.7 ± 0.7 Gyr (2 sigma error).

Age Range Within the Milky Way Globular Cluster System

In general, MW GC system is old -- but apparently not equally old, as suggested by the ΔV method given above.

From Binney & Merrifield, Figure 6.12.
But the ability to get ages in individual clusters has traditionally been only as good as about 2 Gyr, so tricky.

From the above plot (assuming a modest MV(RR)-[Fe/H] slope), Chaboyer et al. (1996) conclude:

[Among the youngest globulars?: Gratton & Ortolani (1988, A&AS, 73, 137) first showed that the wimpy cluster Palomar 12 had a ΔV(HB-MSTO) that was significantly smaller than any other MWGC.

Differential Comparison of Clusters with Same Metallicity

As shown by many groups, there is a big advantage in making detailed studies of clusters with exactly same metallicity and doing differential comparisons.

Age Range in the Milky Way Globular Cluster System From HST Data

Are the above suggestions of an age range in the MW GC system real? They seem to hold up with the very latest HST photometry on a large sample of 64 globular clusters from the HST Globular Cluster Treasury Project.

The analysis by this team takes advantage of:

  • Many of the young clusters in this analysis are in more associated with the more disant halo...
  • Globular cluster metallicities as a function of Galactocentric radius. Clusters in the "old group" are closer to the Galactic center and show a metallicity gradient with age, while the "young group" is more predominant at larger radius. From Marin-Franch et al. (2009, ApJ, 694, 1498).

  • Unfortunately, HST data only exist for a fraction of the entire MW GC system, and the deep imaging treasury program is limited to only relatively nearby clusters (as shown in the figure above) and avoids those that are heavily extinguished by dust. So it is not a "complete" view of the MW system -- though enough to show interesting phenomena among nearby halo clusters.

    Thus, still interesting to look at ground-based work that samples the MW GC system more broadly.

    Key take away messages:

    • There is a definitively proven age spread among the Milky Way globulars. As much as 6 Gyr.

    • The oldest clusters are old (assumed to be 12.8 Gyr in the above analysis) and show very little age spread.

    • The youngest identified globular clusters appear mostly in the outer halo and include clusters assumed to be accreted or to be parts of existing dSph satellites.

    Cluster Ages and Horizontal Branches

    We have already discussed horizontal branch morphology in the context of age differences.
  • Robert Zinn (1993) popularized a way of thinking about the globular cluster system based on the HB parameter, (B-R)/(B+V+R), of Lee et al. (1994) versus [Fe/H]:

    From Zinn (1993, in The Globular Cluster-Galaxy Connection, eds. G.H. Smith and J.P. Brodie, ASP Conf. Ser. Vol. 48, p. xxx).
    As we shall see, the issue of cluster HB types and ages, and their correlation (or lack of correlation) to other cluster properties plays an important role in our understanding of the evolution of the Galactic halo.

    Cluster Ages and Oosterhoff Classes

    In 1939, Oosterhoff pointed out a curious phenomenon in the Milky Way globular cluster system:

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    All material copyright © 2003,2006,2008,2010,2012,2014,2016,2018 Steven R. Majewski. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 551 and Astronomy 5610 at the University of Virginia.