Abundances determined using Si ii and Si iii in B-type stars: evidence for stratification

Abstract

Context. It is becoming clear that determination of the abundance of Si using lines of Si ii and Si iii can lead to quite discordant results in mid to late B-type stars. The difference between the Si abundances derived from the two ion states can exceed one dex in some cases. Aims. We have carried out a study intended to clarify which kinds of B stars exhibit this discrepancy, to try to identify regularities in the phenomenon, and to explore possible explanations such as abundance stratification by comparing models to observed spectra. Methods. We used spectra from the ESPaDOnS spectropolarimeter and FEROS spectrograph, supplemented with spectra from the ESO and ELODIE archives, of magnetic Bp, HgMn, and normal B-type stars ranging in effective temperature from about 10 500 to 15 000 K. Using these spectra, we derived abundances using the spectrum synthesis program zeeman, which can take the influence of magnetic fields into account. For each star, accurate abundances of Si ii, Si iii, Ti, Cr, and Fe were derived from two separate ∼100 Å windows. Si ii abundances were deduced from multiple lines, and Si iii abundances were found using λλ 4552, 4567, and 4574.Results. All magnetic Bp stars in our sample show a discordance between the derived abundances of the first and second ions of silicon, with the latter being between 0.6−1.7 dex higher. The same behaviour is observed in the non-magnetic stars but to a much smaller extent: Si iii is enhanced by between 0.3−0.8 dex compared to Si ii. We do not detect the discrepancy in three stars, HD 22 136 (normal), HD 57 608 (HgMn) and HD 27 295 (HgMn); these are the only stars in our sample for which the microturbulence parameter is significantly different from zero, and which therefore probably have convection occurring in their atmospheres. Conclusions. We find that vertical stratification of silicon in the atmospheres of B-type stars may provide an explanation of this phenomenon, but our detailed stratification models do not completely explain the discrepancies, which may, in part, be due to non-LTE effects.

A comprehensive analysis of the magnetic standard star HD 94660: Host of a massive compact companion?

“…This line requires an abundance that is more than 10 times that of the mean abundance from the Si ii lines. This is exactly what was reported for a sample of mid to late B-type stars by Bailey & Landstreet (2013a), who argue that the discrepancy between the abundances derived from the first and second ionisation states of silicon is likely the result of strong vertical stratification in the stellar atmosphere. The different abundance values found using stronger and weaker lines of Si ii, and the line of Si iii, indicate that stratification is also likely in HD 94660.…”

“…This is most notable for Fe, where abundances derived from weak and strong lines of Fe ii (as well as weaker neutral Fe lines) differ by factors of order 10. Furthermore, the discrepant abundances derived from lines of Si ii and Si iii reported by Bailey & Landstreet (2013a) are also present in HD 94660. A more detailed analysis of abundance stratification in this star is clearly warranted.…”

Discovery of secular variations in the atmospheric abundances of magnetic Ap stars

“…For the Strömgren uvbyβ photometry we used a version of the fortran program "UVBYBETANEW" (see Napiwotzki et al 1993) that corrects the T eff of the magnetic Bp stars to the appropriate temperature scale (see Landstreet et al 2007). As per the discussions of Landstreet et al (2007) and Bailey & Landstreet (2013b), the uncertainties in T eff and log g were taken to be about ±500 K and 0.2 dex, respectively. Table 2 lists the properties of each star, including its designation, associated cluster, age, mass, T eff , log g, and v sin i.…”

“…We note that the photometrically determined values of T eff and log g vary systematically with increasing age from about 13 500 K and 4.4 to 10 000 K and 3.5, as expected for the evolution of a single star of about 3.5 M from ZAMS to TAMS. Landstreet et al (2007); (2) Bailey & Landstreet (2013b); (3) Bailey et al (2012); ( * ) this work.…”

Photoelectric search for peculiar stars in open clusters

“…Later on, Abt (1985) changed the classification to B6 IVp (Si). Bailey & Landstreet (2013) used this object as a "standard star" for determining Si ii and Si iii elemental abundances in B-type stars. They found no significant deviation from the solar abundance for it.…”