Stratification of phosphorus in the atmosphere of the chemically peculiar B-type star HR 1512

Abstract

A number of empirical relationships are shown to indicate an increase in the abundance of phosphorus loge(P) with height in the atmosphere of HR 1512. These include: (a) a correlation of loge(P) with the observed equivalent width Wobs of PII lines; (b) a correlation of loge(P) with the wavelength of the lines; (c) a systematic divergence in the values of loge(P) for lines with different excitation potentials El; in particular, lines with lower El correspond on the average to higher abundances loge(P); and, (d) a distinct dependence of loge(P) on the average geometrical height of formation, Hf. In addition, assuming that loge(P) is constant in the star's atmosphere leads to a systematic discrepancy between the theoretical equivalent widths Wth and the observed values Wobs. By a trial and error method we have found a distribution of the phosphorus abundance loge(P) with height H such that the systematic difference between Wth and Wobs vanishes. It turned out, however, that a simpler, step distribution of loge(P) yields equally good agreement between Wth and Wobs. Although the solution is nonunique, both distributions have some features in common, specifically: (1) a sharp rise in loge(P) occurs in the same range of heights H corresponding to optical thicknesses τ5000 ≈ 10−2–10−3, i.e., stratification of phosphorus takes place in rather high layers of the atmosphere of HR 1512, and (2) the upper bound, logeup(P) = 8.9, is the same in both cases, so that in the region of the rise, loge(P) increases by 3.4 dex. A comparison with available data for HgMn, Am, and Ap type stars shows that similar sharp changes in the abundances loge of several elements occur in other CP stars, at the same optical depths or in even higher layers of their atmospheres.