Abundance analysis of roAp stars .4. HD 24712

We present the first abundance analysis of the rapidly oscillating chemically peculiar star HD 24712, and determine a T-eff = 7250 K, log g = 4.3, and xi(t) = 1 km s(-1). Microturbulence seems to be entirely simulated by a magnetic field with a polar field strength of 4.4 kG and of dipolar structure, both of which are supported by our polarimetric observations. Rotation of HD 24712 and a spotty surface distribution of the elements result in different mean abundances for different (magnetic) phases. Our results do not support the hypothesis of a monotonic correlation between the amplitude of abundance variations and the atomic number Z, and we present arguments in favour of one of the rotation periods (P-rot = 12(d).4610) discussed in the literature. Rare earth elements are the most overabundant elements relative to the sun, and they have the largest abundance amplitude during a rotation cycle; only Mg has a larger amplitude. For HD 24712 we find a clear overabundance of Co while most of the other iron peak elements are underabundant. A comparison of the abundance pattern with the other three roAp stars analyzed so far by us concludes the paper. A systematic difference between surface gravities obtained from spectroscopy and from both asteroseismology and evolutionary tracks is found for the roAp stars HD 24712, alpha Cir and gamma Equ.