Spectroscopic and photometric observations of the R Coronae Borealis stars S Apodis and RZ Normae throughout their recent declines

Abstract

Spectroscopic and photometric observations of two R Coronae Borealis (RCB) stars, S Apodis and RZ Normae, have been obtained during the decline phase of each of these stars. Although the declines have very different global characteristics -- the depth of the decline in brightness is respectively 6 and 2 mag in S Aps and RZ Nor -- there are some strong similarities in the spectroscopic and photometric characteristics. The photometry shows strong reddening during the initial phase of these declines. The spectroscopy indicates that similar characteristics are observed in the Na I D region, with very few other spectroscopic changes. In both declines Na I D lines show strong emission components and high-velocity (blueshifted by up to 300 km s-1) absorption lines. The centre of the absorption features appears to be shifted gradually during the declines, indicating the acceleration of the dust cloud.