Globular cluster ages

Abstract

We review two new methods to determine the age of globular clusters (GCs). These two methods are more accurate than the classical isochrone fitting technique. The first method is based on the morphology of the horizontal branch and is independent of the distance modulus of the globular cluster. The second method uses a careful binning of the stellar luminosity function and determines simultaneously the distance and age of the GC. We find that the oldest galactic GCs have an age of 13.5 ؎ 2 gigayears (Gyr). The absolute minimum age for the oldest GCs is 10.5 Gyr (with 99% confidence) and the maximum 16.0 Gyr (with 99% confidence). Therefore, an Einstein-De Sitter Universe (⍀ ‫؍‬ 1) is not totally ruled out if the Hubble constant is about 65 ؎ 10Galactic globular clusters (GCs) are the best stellar clocks to establish a lower limit to the age of the universe. Nevertheless, a robust age for the oldest GCs is yet to be determined. GCs are excellent stellar clocks because they fulfill the following properties:• All the stars were born at the same time.• The population is chemically homogeneous.• There has not been any further episodes of star formation that gave birth to new stars, which could cover up the oldest population.Support for GCs being old comes from two facts: their metallicity is as low as 1͞100 of solar and the characteristics of their color magnitude diagram (CMD) are those corresponding to ages larger than 10 gigayears (Gyr), i.e., the stars around the main sequence turn-off (MSTO) have masses lower than 1 M J .Despite the continuous effort carried out during more than 30 years to give a precise value for the age of GCs, the uncertainty in their age still remains at about 4 Gyr. The problem is particularly complicated because age and distance have the same effect on the morphology of the MSTO point. Deficiencies in the input physics combined with the uncertainties in cluster distances and interstellar reddening have made it difficult to determine GC ages with an accuracy better than about 25%.In this review I present two alternative methods used to derive ages of GCs that are independent of the traditional MSTO fitting. The first method is based on the morphology of the horizontal branch (HB) and uses the reddest points in the HB to determine the mass of the stars in the red giant branch (RGB) (1). This method is independent of the distance modulus and therefore is a useful tool to compute systematics in ages determinations by using the traditional MSTO fitting technique. The second of the methods is based on a careful binning of the luminosity function (LF) and determines simultaneously the age and distance of a GC. It therefore is a very useful technique to determine distances to GC independently of the subdwarf fitting and RR-Lyrae techniques. It is also very powerful to determine relative ages of GCs with little error.In the next sections I describe the traditional isochrone fitting method and the HB morphology and LF methods. I finish with a discussion on the uncertainties of the different...

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