Photometry of Faint Stars in Globular Clusters using the Six Meter Telescope

Our program is based on photographic and photoelectric UBV photometry of globular cluster stars with the Soviet 6 m telescope. M 10 = NGC 6254 remains, regretfully, the only cluster for which we were able to gain photoelectric observational material sufficient for calibration of the photographic photometry for faint stars. Samus and Shugarov (1983) presented for M 10 a V, (B - V) diagram showing an unusually large magnitude difference between the main sequence turn-off point and the horizontal branch, V(TO) - V(HB) ≍ 3.8 mag. It seemed of interest to compare M 10 and M 12 by their values of V(TO) - V(HB). A preliminary calibration of the faint star photographic photometry in M 12 leads to V(TO) - V(HB) ≍ 4 mag (Mironov et al., 1984). One may notice that in the classification introduced by Mironov and Samus (1974, 79), which is based mainly on the horizontal-branch morphology, both M 10 and M 12 belong to group I, presumably the older group. V, (B-V) diagrams were also published by us for NGC 288 and M 2 = NGC 7089 (Samus and Shugarov 1978, 79).

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Horizontal branch morphology: A new photometric parametrization

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935995 ◽

2019 ◽

Vol 629 ◽

pp. A53 ◽

Cited By ~ 3

Author(s):

M. Torelli ◽

G. Iannicola ◽

P. B. Stetson ◽

I. Ferraro ◽

G. Bono ◽

...

Keyword(s):

Metal Content ◽

Globular Clusters ◽

Linear Trend ◽

Cumulative Number ◽

Horizontal Branch ◽

Imaging Data ◽

Galactic Globular Clusters ◽

Absolute Age ◽

Advanced Camera For Surveys ◽

Branch Morphology

Context. Theory and observations indicate that the distribution of stars along the horizontal branch of Galactic globular clusters mainly depends on the metal content. However, the existence of globular clusters with similar metal content and absolute age but different horizontal branch morphologies, suggests the presence of another parameter affecting the star distribution along the branch. Aims. To investigate the variation of the horizontal branch morphology in Galactic globular clusters, we define a new photometric horizontal branch morphology index, overcoming some of the limitations and degeneracies affecting similar indices available in the literature. Methods. We took advantage of a sample of 64 Galactic globular clusters, with both space-based imaging data (Advanced Camera for Surveys survey of Galactic globular clusters) and homogeneous ground-based photometric catalogues in five different bands (U, B, V, R, I). The new index, τHB, is defined as the ratio between the areas subtended by the cumulative number distribution in magnitude (I) and in colour (V − I) of all stars along the horizontal branch. Results. This new index shows a linear trend over the entire range in metallicity (−2.35 ≤ [Fe/H] ≤ −0.12) covered by our Galactic globular cluster sample. We found a linear relation between τHB and absolute cluster ages. We also found a quadratic anti-correlation with [Fe/H], becoming linear when we eliminate the age effect on τHB values. Moreover, we identified a subsample of eight clusters that are peculiar according to their τHB values. These clusters have bluer horizontal branch morphology when compared to typical ones of similar metallicity. These findings allow us to define them as the ’second parameter’ clusters in the sample. A comparison with synthetic horizontal branch models suggests that they cannot be entirely explained with a spread in helium content.

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Star clusters as tracers of galactic subsystems

Symposium - International Astronomical Union ◽

10.1017/s0074180900123435 ◽

1993 ◽

Vol 153 ◽

pp. 323-324

Author(s):

B. Barbuy ◽

E. Bica ◽

S. Ortolani

Keyword(s):

Spatial Distribution ◽

Galaxy Formation ◽

Formation Process ◽

Globular Clusters ◽

Star Clusters ◽

Open Clusters ◽

Horizontal Branch ◽

The Galaxy ◽

Magnitude Difference

We have obtained CCD BVRI colour-magnitude diagrams for a series of disk globular clusters, improving parameters and detecting a new one: Lyngå 7. Using the magnitude difference between turn-off and horizontal branch Δ(TO-HB) as an age discriminator, and their spatial distribution we compare old disk open clusters, young halo globular clusters, and metal-rich disk globular clusters, obtaining clues to the Galaxy formation process.

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Relative Ages from Horizontal-Branch Morphology: Revisited

Symposium - International Astronomical Union ◽

10.1017/s0074180900223449 ◽

2002 ◽

Vol 207 ◽

pp. 110-112

Author(s):

Young-Wook Lee ◽

Hyun-chul Lee ◽

Suk-Jin Yoon ◽

Soo-Chang Rey ◽

Brian Chaboyer

Keyword(s):

High Precision ◽

Globular Clusters ◽

Age Dating ◽

Age Difference ◽

Stellar Systems ◽

Horizontal Branch ◽

Ccd Photometry ◽

Relative Age ◽

New Models ◽

Branch Morphology

We present our recent revision of model constructions for the horizontal-branch (HB) morphology of globular clusters, which suggests the HB morphology is more sensitive to age compared to our earlier models. We also present our high precision CCD photometry for the classic second parameter pair M3 and M13. The relative age dating based on this photometry indicates that M13 is indeed older than M3 by 1.7 Gyr. This is consistent with the age difference predicted from our new models, which provides a further support that the HB morphology is a reliable age indicator in most population II stellar systems.

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