Atmospheric parameters and chemical composition of the ultra-cool roAp star HD 213637

Abstract

A high-resolution spectrum obtained with the UVES instrument at the ESO VLT is used to determine atmospheric parameters and chemical composition of the roAp star HD 213637. Resolved Zeeman split lines are discovered in the spectrum of this star, indicating a mean field modulus of 5.5 kG. Effective temperature, K determined from the profiles of the hydrogen Hα and Hβ lines, makes HD 213637 one of the coolest (ro)Ap stars known, suggesting that the class of magnetic chemically peculiar stars extends to lower effective temperatures than previously thought. Furthermore, the low temperature of HD 213637 poses serious challenge to some excitation mechanisms suggested as an explanation of the roAp phenomenon. This peculiar star is also unique in its evolutionary stage: it is either located near the very end of its main sequence life or has already ascended the giant branch. Abundance analysis reveals that HD 213637 is iron-deficient and has abundance pattern typical of the cool pulsating Ap stars. An inconsistency between abundances derived from the weak and strong spectral lines and the lines of different ions provide a clear evidence for prominent vertical abundance stratification of many chemical species, notably Na, Mg, Ca, Fe, Pr, Nd, Tb, and Er.