An upper limit on the sulphur abundance in HE 1327-2326
  Context: Star HE 1327-2326 is a unique object, with the lowest measured iron
abundance ([Fe/H] ~ -6) and a peculiar chemical composition that includes large
overabundances of C, N, and O with respect to iron. One important question is
whether the chemical abundances in this star reflect the chemical composition
of the gas cloud from which it was formed or if they have been severely
affected by other processes, such as dust-gas winnowing. Aims: We measure or
provide an upper limit to the abundance of the volatile element sulphur, which
can help to discriminate between the two scenarios. Methods: We observed HE
1327-2326 with the high resolution infra-red spectrograph CRIRES at the VLT to
observe the S I lines of Multiplet 3 at 1045 nm. Results: We do not detect the
S I line. A 3sigma$upper limit on the equivalent width (EW) of any line in our
spectrum is EW<0.66 pm. Using either one-dimensional static or
three-dimensional hydrodynamical model-atmospheres, this translates into a
robust upper limit of [S/H]<-2.6. Conclusions: This upper limit does not
provide conclusive evidence for or against dust-gas winnowing, and the evidence
coming from other elements (e.g., Na and Ti) is also inconclusive or
contradictory. The formation of dust in the atmosphere versus an origin of the
metals in a metal-poor supernova with extensive "fall-back" are not mutually
exclusive. It is possible that dust formation distorts the peculiar abundance
pattern created by a supernova with fall-back, thus the abundance ratios in HE
1327-2326 may be used to constrain the properties of the supernova(e) that
produced its metals, but with some caution.
