Aluminum Abundances , Deep Mixing and the Blue - Tail Second - Parameter Effect in the Globular Clusters M 3 and M 13

We analyze high resolution, high signal-to-noise spectra of six red-giant-branch (RGB) stars in the globular cluster M 3 (NGC 5272) and three in M 13 (NGC 6205) that were obtained with the Mayall 4-meter telescope and echelle spectrometer on Kitt Peak. The spectra include lines of O, Na, Mg, Al, Si, Ca, Ti, V, Mn, Fe and Ni. We also analyze the [Al/Fe] values of 96 RGB stars in M 13 covering the brightest 3.5 magnitudes, which include 66 measurements that were derived from moderate resolution, low signal-to-noise spectra obtained with the WIYN 3.5-meter telescope and Hydra multi-object spectrograph, also on Kitt Peak. In addition, we compile from the literature and inspect the [Na/Fe] values of 119 RGB stars in M 13. We test for bimodality in the [Al/Fe] and [Na/Fe] distributions using the KMM algorithm and find that the [Al/Fe] values in M 13 are distributed bimodally at all points along the RGB that were observed, while the [Na/Fe] values are bimodal only over the brightest two magnitudes. The ratios of Alenhanced to Al-normal and Na-enhanced to Na-normal giants increase towards the tip of the RGB in M 13, which is suggestive of deep mixing in this cluster. The limited M 3 data exhibit a bimodal distribution of [Al/Fe] values and are suggestive of no deep mixing; however, they are too few to be conclusive. We further test for a relationship between deep mixing on the RGB and a second parameter that can create the extended blue tail seen along the horizontal-branches of some clusters by using an “instantaneous” mixing algorithm, which we develop here. We Visiting Astronomer, Kitt Peak National Observatory. KPNO is operated by AURA, Inc. under contract to the National Science Foundation. NRC Research Associate