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

4. Summary and conclusions

We have presented the BVI CCD data for about 10,000 stars in the region 0.3' 4' of M 3. These data, along with our previous photographic photometry of additional 10,000 stars in the external regions as far as 7', provide a homogeneous and popolous data base for detailed population studies. Completeness has been achieved for all objects with V 18.6.

Our main results are:

• A new absolute photometric calibration has been obtained. The comparison with the absolute calibration adopted so far for the same field (Sandage and Katem 1982) shows that there is a significant colour term, in the sense that our calibration is 0.07 mag redder than Sandage and Katem (1982) in the blue range e.g. at B-V=-0.2, slightly redder ( 0.03 mag) at the instability strip, and 0.08 mag bluer in the reddest part of the CMD, e.g. at B-V=1.6. This has important implications in the photometric determination of the metallicity, and in other items such as the period-shift effect which will be discussed in a forthcoming paper.
• After completion of our reductions, we became aware of the availability of a new independent photometry of a very wide sample of M 3 stars (about 23,700) carried out by K.A. Montgomery (1995 - M 95), fully overlapping our considered regions. The use of this new catalog has also allowed us to get a link to the photometry presented by Stetson and Harris (1988) for an external area, outside our surveyed region. In synthesis, there are now three independent calibrations referred to observations of Landolt's (1992) standards. They are in marginal agreement, but it is difficult at the present stage to choose which is the best. Since the calibration here adopted seems to be compatible with that obtained by Stetson and Harris (1988) (they actually show a systematic difference of 0.04 both in B and V, yielding however no colour difference), we decided to keep using our new calibration.
• Based on these new data and calibration, the metallicity derived from the photometric indicators and is [Fe/H] -1.45. This value is significantly higher than the value -1.66 commonly used for M 3, and is in very good agreement with the most recent metallicity determinations from high resolution spectroscopy of individual stars.
• Evidence has been found of the presence of a faint and blue extension of the HB, although scarcely populated. These stars, however, could be non-genuine HB members but rather the results of merging or collisions. More photometric and spectroscopic work is needed in order to assess the true nature of these objects.
• From the population ratios of HB, RGB and AGB stars we find a value for the helium content Y=0.23 0.02. This confirms the previous determinations of helium content in globular clusters by this method.
• A significant population of blue straggler stars has been detected and identified. However the exact number of BSS must await HST data for a better space resolution in the most internal regions, for resolving optical blends that may turn pairs of TO stars into BSS, and in general for a better photometric accuracy. Spectroscopic follow-up studies will be extremely important for a better understanding of the formation mechanisms of these stars.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
helpdesk.link@springer.de