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

A set of high-resolution optical spectra of R CrB acquired before, during a nd after its 1995-96 decline is discussed. All of the components reported f rom earlier declines are seen. This novel data set provides new information on these components including several aspects not previously seen in decli nes 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 abs orption lines of high-excitation lines, and quickly followed by emission in these and in low-excitation lines. This 'photospheric trigger' implies tha t dust causing the decline is formed close to the star. These emission line s fade quickly. After 1995 November 2, low-excitation narrow (FWHM similar to 12 km s(-1)) emission lines remain. These appear to be a permanent featu re, 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. Absorpt ion lines unaffected by overlying sharp emission are greatly broadened, wea kened and redshifted at the faintest magnitudes when scattered light from t he star is a greater contributor than direct light transmitted through the fresh soot cloud. A few broad lines (FWHM similar or equal to 300 km s(-1)) are seen at and near minimum light with approximately constant flux: promi nent among these are the He I triplet series, Na I D and [N II] lines. Thes e lines are blueshifted by about 30 km s(-1) relative to the systemic veloc ity, 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, ar ise 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-asympto tic giant branch star 89 Her.