Rare-earth elements in the atmosphere of the magnetic chemically peculiar star HD 144897

Abstract

Context. The chemically peculiar stars of the upper main sequence represent a natural laboratory for the study of rare-earth elements (REE). Aims. We want to check the reliability of the energy levels and atomic line parameters for the second REE ions currently available in the literature, and obtained by means of experiments and theoretical calculations. Methods. We have obtained a UVES spectrum of a slowly rotating strongly magnetic Ap star, HD 144897, that exhibits very large overabundances of rare-earth elements. Here we present a detailed spectral analysis of this object, taking effects of non-uniform vertical distribution (stratification) of chemical elements into account. Results. We determined the photospheric abundances of 40 ions. For seven elements (Mg, Si, Ca, Ti, Cr, Mn, Fe), we obtained a stratification model that allows us to produce a satisfactory fit to the observed profiles of spectral lines of various strengths. All the stratified elements but Cr show a steep decrease in concentration toward the upper atmospheric layers; for Cr the transition from high to low concentration regions appears smoother than for the other elements. The REEs abundances, which have been determined for the first time from the lines of the first and second ions, have been found typically four dex higher than solar abundances. Our analysis of REE spectral lines provides strong support for the laboratory line classification and determination of the atomic parameters. The only remarkable exception is Nd iii, for which spectral synthesis was found to be inconsistent with the observations. We therefore performed a revision of the Nd iii classification. We confirmed the energies for 11 out of 24 odd energy levels that were classified previously, and derived the energies for additional 24 levels of Nd iii, thereby substantially increasing the number of classified Nd iii lines with corrected wavelengths and atomic parameters.

Magnetic Doppler imaging of the chemically peculiar star HD 125248

“…The Eu iii abundance, unfortunately, is less constrained because we could only determine it from the Eu iii λ 5376 Å and Eu iii λ 6666 Å lines. A similar pattern with the Eu anomaly and the absence of other rare earth anomalies has been observed in the atmosphere of another Ap star HD 144897 (Ryabchikova et al 2006) although more lines of Eu iii were used in abundance analysis of this star. The lack of reliable atomic data (hfs constants, isotopic shifts, collision rates) for Eu iii does not allow us to conclude whether this discrepancy is caused by ignoring the hyperfine splitting and non-LTE effects (Mashonkina et al 2002), or by an actual physical mechanism that produces abundance differences between ionization states of this particular element, atomic diffusion, for example.…”

Model atmospheres of chemically peculiar stars

“…5 of LeBlanc et al 2009). In a simple step function approximation used in the present stratification analysis as well as in most other similar studies (Wade et al 2001;Ryabchikova et al 2005Ryabchikova et al , 2006Ryabchikova et al , 2008, we cannot probe an increase of element abundance in the uppermost atmospheric layers. However, relatively good fitting of the observed line profiles in many stars with different strengths of the magnetic field obtained with a step-like abundance distribution of Si, Ca, Cr, Fe -the main contributors to line opacities -supports the absence of a significant increase of element abundance immediately following the abundance jump.…”

“…Ryabchikova et al 2006Ryabchikova et al , 2005Ryabchikova et al , 2008. In this routine, the vertical abundance distribution of an element is described by four parameters: chemical abundance in the upper atmosphere, abundance in deep layers, the position of the abundance jump and the width of the transition region where the chemical abundance changes between the two values.…”

Realistic model atmosphere and revised abundances of the coolest Ap star HD 101065

“…These data were supplemented by the transition probabilities for the observed and predicted lines of Pr ii−Pr iii ), Nd ii−Nd iii (Mashonkina et al 2005;Ryabchikova et al 2006), Eu iii (Wyart et al 2008), Tb iii and Dy iii (Ryabtsev, private communication), hereafter referred to as "ISAN data". For the latter two ions transition probability calculations were based on the extended term analysis.…”