The 1995-96 decline of R Coronae Borealis: high-resolution optical spectroscopy

Abstract

A set of high-resolution optical spectra of R CrB acquired before, during and after its 1995–96 decline is discussed. All of the components reported from earlier declines are seen. This novel data set provides new information on these components including several aspects not previously seen in declines of R CrB and other R Coronae Borealis stars. In the latter category is the discovery that the onset of the decline is marked by distortions of absorption lines of high-excitation lines, and quickly followed by emission in these and in low-excitation lines. This ‘photospheric trigger’ implies that dust causing the decline is formed close to the star. These emission lines fade quickly. After 1995 November 2, low-excitation narrow (FWHM ∼12 km s−1) emission lines remain. These appear to be a permanent feature, slightly blueshifted from the systemic velocity, and unaffected by the decline except for a late and slight decrease of flux at minimum light. The location of the warm dense gas providing these lines is uncertain. Absorption lines unaffected by overlying sharp emission are greatly broadened, weakened and redshifted at the faintest magnitudes when scattered light from the star is a greater contributor than direct light transmitted through the fresh soot cloud. A few broad lines (FWHM ≃300 km s−1) are seen at and near minimum light with approximately constant flux: prominent among these are the He i triplet series, Na i D and [N ii] lines. These lines are blueshifted by about 30 km s−1 relative to the systemic velocity, with no change in velocity over the several months for which the lines were seen. It is suggested that these lines, especially the He i lines, arise from an accretion disc around an unseen compact companion which may be a low-mass white dwarf. If so, R CrB is similar to the unusual post-asymptotic giant branch star 89 Her.