Anomalous gallium line profiles in HgMn stars: possible evidence for chemically stratified atmospheres.

Abstract

An analysis of co-added high-resolution IUE spectra is presented for three mercury-manganese stars (μ Lep, HR 2844, and HR 7143) which exhibit anomalous Gaiii λ1495 resonance line profiles. This line appears to have unusually weak wings in these stars when compared with synthetic profiles computed under the conventional assumptions of atmospheric chemical homogeneity and local thermodynamic equilibrium. It is shown that these anomalous line profiles can be reproduced by calculations in which gallium is stratified in the upper atmosphere. A detailed analysis of each star indicates that schematic stratified models can yield consistent fits to the anomalous λ1495 profiles simultaneously with the (apparently unaffected) Gaiii λ1534 and Gaii λ1414 resonance lines. Moreover, the best-fit gallium abundance derived from these three lines is, in each case, in excellent agreement with the predicted maximum which can be supported in the photosphere by radiation pressure. Although other explanatory mechanisms for the anomalous λ1495 profiles -for example non-LTE effects- cannot be excluded, this preliminary study provides tentative evidence for the presence of chemically stratified atmospheres in HgMn stars.