2005PASP..117.1173K

We have added data for 16 distant Cepheids observed with the echelle spectrograph of the Apache Point 3.5 m telescope to improve the correlations of abundance parameters with pulsation period and distance from the Galactic center (R_G). While we see no important trends with pulsation period, some important ratios are closely related to the evolution of stars through the instability strip. The most interesting is the N/C ratio, which is predicted to be enhanced by a factor of 4 when an evolving supergiant cools to T_eff=4000 K and the outer convection zone reaches the depth at which CN processing has converted C to N. We find a Gaussian distribution around a value of [N/C]=0.6, just as predicted. The ratio of about 30:1 mixed to unmixed stars can be taken as the ratio of evolutionary time spent on the blue loop to the time spent on the first crossing through the instability strip. According to stellar models, the main mixing event takes place when T_effdrops below 4000 K, which is outside the red edge of the instability strip; i.e., after stars have crossed the instability strip for the first time. We have confirmed previously known correlations of [Fe/H] with R_G. We find that [Fe/H] shows a gradient of -0.06/Kpc. The only other significant gradient is that of [Eu/Fe], which increases with R_G, thereby showing an increasing ratio of SN II/SN Ia in the outer Galaxy.