High Precision Photometry of 10,000 Stars in M 3

Abstract

A new color-magnitude diagram for M 3 is presented. 10,000 stars have been measured down to V = 22 with an internal accuracy better than 0.03 mag to get complete and very accurate samples over well defined areas.

More than 10,000 stars have been measured down to V = 22 in two different areas. In the first, with 3.5 < r < 6.0 arcmin, photometric completeness has been achieved down to V = 21.5 and an algorithm to correct for losses due to unrecoverable crowding and blending has been experimentally computed. In the second, within a square field of 15 × 15 arcmin, completeness has been extended only to V = 18, well below the horizontal branch.

Many tests made on the data guarantee an internal photometric accuracy better than 0.03 mag at V = 21. Therefore, both the total population of each branch and the relative star-number ratios are “bona fide” representatives of the corresponding evolutionary time-scales. Here we simply present: 1) the color-magnitude diagram (see Fig. 1) obtained from the reduction of a wide collection of Palomar plates; 2) a table which presents the contribution of the various branches to the integrated cluster light; 3) the preliminary indication that, within the annulus we have considered, the blue stragglers seem to be slightly less centrally concentrated than the subgiants in the same magnitude interval.