Evidence from zinc abundances for dust fractionation in chemically peculiar stars

Abstract

A SMALL number of intrinsically luminous, low-mass stars have recently been shown to have an extremely peculiar elemental abundance pattern. Their photospheric carbon, nitrogen, oxygen and sulphur abundances are within an order of magnitude of solar values, but all other normally abundant metals are present in only trace amounts; in two stars, iron is deficient by nearly five orders of magnitude. Two possible explanations are that the low iron content is primordial, implying a very great age, whereas the CNO and S abundances have been acquired during evolution, or that the CNO and S abundances reflect the initial stellar composition and the low iron content is the result of chemical separation by dust formation. The latter hypothesis arises mainly because the abundance pattern of these stars is similar to that of interstellar gas1, in which fractionation to dust plays an important part, but it is not easily understood how a process that must occur in the circumstellar envelope can so strikingly affect the photospheric abundances. Here we report the detection of appreciable amounts of zinc in the star HD52961, and argue that, because zinc will condense into dust only at rather low temperatures, its detection in near normal amounts is convincing evidence for the fractionation hypothesis.