Abundances of Classical Cepheids and Evidence for Secondary Star-Formation

Abstract

In addition to the well-known radial abundance gradient in the disc of our Galaxy, there are indications of small-scale variations (Talent and Dufour 1979, Giridhar 1983). We have compiled spectroscopic abundances for long period Cepheids and to eliminate the effect of differences in ages, derived their places of formation by calculating their galactic orbits backwards in time. The stellar orbits were calculated considering the axisymmetric gravitational potential as well as the spiral potential due to the density wave. The parameters related to spiral waves are taken from Yuan (1969). The equations of motion in the (ζ, η, ξ) system were numerically integrated using the Runge-Kutta method and birthsites were transformed to (ζ, η) in the reference frame of the spiral pattern. The details of the computations are described by Giridhar (1985). In addition to the accepted value of 13.5 km s−1 kpc−1 for pattern velocity, birthsites were also calculated for ωp = 11.5 and 15.5 km s−1 kpc−1. Figure 1 shows the birthsites in the galactic plane for the three values of the pattern velocity. The Cepheids in the figure can be identified through the serial number in Table I which contains our compilation of Fe/H and derived birthsites. It is obvious from the figure that the systematic shifts in birthsites for different pattern velocities are not large enough to affect the assignment of birthsites to different arms.