A 235 star sample sodium, magnesium, and aluminum abundance study in the globular clusters M3 (NGC 5272) and M13 (NGC 6205)

We have derived sodium, magnesium, and aluminum abundances for more than 100 red giant branch (RGB) stars in each of the Galactic globular clusters M3 and M13 ( 235 total stars), using moderate resolution (R similar to 10,000) spectra obtained with the 3.5 m WIYN telescope and Hydra multifiber spectrograph. Temperatures for the cluster samples are based on calibrations of photometric indices, in particular. V - K Gravities, microturbulent velocities, and the overall M3 and M13 metallicities ([Fe/H] similar to -1.50) are based on the results of previous work by the Lick-Texas group. Na and Mg abundances have been determined from observed versus synthetic spectrum matches of the Na I lambda lambda 5682, 5688 lines and the Mg I lambda 5711 line. Al abundances have been determined from equivalent-width measurements of the Al I (lambda lambda 6696, 6698) doublet. The resulting M3 and M13 analyses are compared with the more detailed high-resolution analyses of smaller samples of RGB stars. We conclude that on average, M13 tends to have higher abundances of Na, Mg, and Al than M3 by roughly 0.10 - 0.20 dex, a likely by-product of additional proton-capture synthesis. Compared to halo field stars of comparable [Fe/H] metallicity, M3 and M13 giants tend to have a greater abundance of Na for a given Mg. Unlike M3, the most luminous M13 giants (M-v(o) < -1.70) show a strong inclination toward having [Na/Fe] values enhanced above the cluster median. In the last 0.70 mag interval before M13' s RGB tip, there is a large number of low-O, high-Na, and high-Al stars. This is not seen in M3. Compared with halo field giants, the anomalous abundances of O, Na, Mg, and Al seen in both M3 and M13 stars appear to have a primordial (pollution?) origin. However, a second stage of proton-capture synthesis within the M13 giants themselves seems required to distinguish their pattern of O, Na, Mg, and Al abundances from those of the brightest giants of M3.