Horizontal-Branch Morphology as an Age Indicator

We present high-precision V, B-V color-magnitude diagrams (CMDs) for the classic second parameter globular clusters M3 and M13 from wide-field deep CCD photometry. The data for the two clusters were obtained during the same photometric nights with the same instrument, allowing us to determine accurate relative ages. Based on a differential comparison of the CMDs using the Delta (B-V) method, an age difference of 1.7 +/- 0.7 Gyr is obtained between these two clusters. We compa...

Surface temperature distribution of horizontal-branch (HB) stars is very sensitive to age in old stellar systems, which makes it an attractive age indicator. In this paper, we present the recent revision of our model calculations for the HB morphology of globular clusters. The result is more updated version of our earlier models (Lee, Demarque, & Zinn 1994), which suggests that the HB morphology is more sensitive (\sim 40%) to age. We also present our new model calculations o...

We have applied a new method to analyze the horizontal branch (HB) morphology in relation to the distribution of stars near the red giant branch (RGB) tip for the globular clusters M22, M5, M68, M107, M72, M92, M3 and 47 Tuc. This new method permits determination of the cluster ages to greater accuracy than conventional isochrone fitting. Using the method in conjunction with our new high-quality photometric data for RGB and HB stars in the first five of these clusters, we dis...

We present the results of a first global analysis of the Horizontal Branch morphology of Globular Clusters in the nearby spiral M31, based on their Color Magnitude Diagrams.

After taking into account, for the first time, the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity, our population synthesis models show that the integrated H beta index is significantly affected by the presence of blue HB stars. Our models indicate that the strength of the H beta index increases as much as 0.75 A due to blue HB stars. According to our models, a systematic difference between the globular cluster system in the Milky ...

We review the most outstanding issues related to the study of the morphology of the Horizontal Branch (HB) in the Color-Magnitude Diagrams of Galactic Globular Clusters and its use as age indicator. It is definitely demonstrated (see also Bolte, this meeting) that age cannot be the only 2nd-P driving the HB morphology. Other candidate 2nd-Ps are briefly examined, with special attention to the possible influence of cluster stellar density.

We use deep, high quality colour magnitude diagrams obtained with the Hubble Space Telescope to compute a simplified version of the Mironov index [SMI; B/(B+R)] to parametrize the horizontal branch (HB) morphology for 23 globular clusters in the M31 galaxy (Sample-A), all located in the outer halo at projected distances between 10 kpc and 100 kpc. This allows us to compare them with their Galactic counterparts, for which we estimated the SMI exactly in the same way, in the SM...

Theoretical integrated broad-band colors ranging from far-UV to near-IR have been computed for old stellar systems from our evolutionary population synthesis code. These models take into account, for the first time, the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. Our models show that some temperature-sensitive color indices are significantly affected by the presence of `blue HB stars'. Our models are calibrated in the B-V, V-I,...

We present a new method for measuring the mean age of old/intermediate stellar populations in resolved, metal-poor ($\rm \langle[Fe/H]\rangle \lesssim -1.5$) galaxies using only the morphology of the horizontal branch (HB) and an estimate of the average metallicity. We calculate the ratio of blue-to-red HB stars and the mass-weighted mean ages of 27 M31 satellite galaxies that have star formation histories (SFHs) measured from Hubble Space Telescope-based color-magnitude diag...

The strength of the H$\beta$ index has been computed for the integrated spectra of model globular clusters from the evolutionary population synthesis. These models take into account, for the first time, the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. Our models show that the H$\beta$ index is significantly affected by the presence of blue HB stars. Because of the contribution from blue HB stars, the H$\beta$ does not monotonica...