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Astron. Astrophys. 320, 757-775 (1997)

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The stellar population of the globular cluster M 3

II. CCD photometry of additional 10,000 stars

F.R. Ferraro 1, 2, E. Carretta 1, 2, C.E. Corsi 3, F. Fusi Pecci 1, C. Cacciari 1, R. Buonanno 3, B. Paltrinieri 4 and D. Hamilton 5

1 Osservatorio Astronomico di Bologna, Via Zamboni 33, I-40126 Bologna, Italy
2 Visiting Astronomer, German-Spanish Astronomical Center, Calar Alto, Spain
3 Osservatorio Astronomico di Roma, Roma, Italy
4 Dipartimento d'Astronomia, Via Zamboni 33, I-40126 Bologna, Italy
5 Max-Planck Institut f. Astrophysik, Heidelberg, Germany

Received 5 July 1996 / Accepted 1 October 1996


We present BVI CCD photometry for more than 10,000 stars in the innermost region (0.3' [FORMULA] r [FORMULA] 4') of the globular cluster M 3. When added to the previous photographic photometry by Buonanno et al.  (1994) reaching as far as r [FORMULA] 7', this results in an homogeneous data-set including about 19,000 stars measured in this cluster, which can be now regarded as one of the main templates for stellar population studies.

Our main results, from the new colour-magnitude diagrams (CMD), are:

1. Completeness has been achieved for all objects at V [FORMULA] 18.6 and [FORMULA].

2. Our new independent photometric calibration is redder than the old calibration (Sandage and Katem 1982) in the blue range, and bluer in the reddest part of the CMD. This colour term has important consequences on some issues (e.g. metallicity, period-shift effect, etc.).

3. The metallicity derived from the photometric indicators [FORMULA] and [FORMULA] is [Fe/H] [FORMULA] -1.45, i.e. significantly higher than the value -1.66 generally used so far. This is in very good agreement with the most recent high resolution spectroscopic studies of individual stars.

4. There is indication of the presence of a very faint and blue extension of the HB, although scarcely populated.

5. The population ratios of HB, RGB and AGB stars confirm the value previously found for the helium content, i.e. Y=0.23 [FORMULA] 0.02.

6. There is a significant population of blue straggler stars, although the exact number must await HST data for a better resolution in the very central regions.

Key words: Galaxy: globular clusters: individual: M 3 – stars: Population II – HR diagram

Send offprint requests to: F.R. Ferraro

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998