THE STELLAR POPULATION OF THE GLOBULAR-CLUSTER-M3 .1. PHOTOGRAPHIC PHOTOMETRY OF 10000 STARS

A new photographic colour-magnitude diagram (CMD) for the Galactic glo bular cluster M 3 is presented. This constitutes the preliminary part of a long-term project aiming at using the M 3 stellar population as a powerful testbench for the predictions of the stellar evolution theor y and for cosmological purposes via the population synthesis methods. In this first step, more than 10 000 stars were measured on the Mt. Pa lomar and Mt. Wilson original plates (Sandage 1953) down to V > 21.5 w ith very high internal accuracy to obtain complete samples over well d efined areas. The list of magnitudes, colours and positions for all th e measured stars is available from the CDS at Strasbourg upon request. From the analysis of the present CMD the following results are derive d: (a) The overall morphology of the main branches and their normal po ints reproduce in essence the previous results. In particular, the exi stence of a sizeable population of blue straggler stars (BSS) detected by Sandage (1953) is confirmed. (b) The Red Giant Branch (RGB) and As ymptotic Giant Branch (AGB) are clearly separated, and the RGB metalli city indicators lead to ([Fe/H]) = -1.66 +/- 0.10 with an upper limit sigma[Fe/H] similar to 0.09. The narrow Sub-Giant Branch (SGB) and Tur n-Off (TO) region suggest that also the dispersion in the abundance of C,N,O should be very low. (c) The Horizontal Branch (HB) is narrow an d the bulk of KB stars covers a wide range in effective temperature (l og T-e = 3.70 to 4.55). Six groups of stars have been tentatively iden tified based on their location on the HB, namely: R (red), V (variable s), B (blue), EB (extremely blue) HEB (hot extremely blue) ER (extreme ly red) - some of these stars may be non-genuine HB stars. The various population ratios computed lead: (i) to confirm the full extension of semiconvection (or, alternatively, to calibrate the overshooting para meter) and to exclude the existence of a ''long'' breathing pulses pha se; and (ii) to determine the primordial helium abundance Y-p via the R-method, Y-p = 0.23 +/- 0.02. (d) The distance modulus is estimated a s (m - M)(0) = 14.94 and the age is t = 18.7 +/- 3.5 Gyr. The effects of different assumptions and the results previously obtained by variou s authors using the present data are also discussed. It is interesting to note the wide range of values estimated for the age (from similar to 13 to similar to 20 Gyr). (e) 65 BSS candidates have been identifie d. Within the considered areas, BSS seem to be less centrally concentr ated than the subgiants in the same magnitude interval, but further ch ecks are needed. Speculations on the possible detection of a BSS proge ny located in the red HB are also presented. Future papers of this ser ies will deal with the analysis of CCD data for the inner regions of t he cluster.