IV-th Great Visible Brightness Minimum of R CrB. II. Spectral Observations

Abstract

Low and high resolution spectral observations during the main stages of a uniquely long and deep minimum in the visible brightness of R CrB during 2007-2015 are reported. As it developed, the spectrum manifested typical tendencies in the variation of line and molecular emission, with replacement of an absorption spectrum by an emission spectrum. The significant, sustained attenuation of the star’s emission aided the detection of rarely observed forbidden emission lines. In particular, the [OII] λλ 3726 – 3729 lines appeared for more than 1500 days during a brightness reduction by 6 ÷9m . The velosity at which the shielding dust was removed to the circumstellar surroundings was reduced by roughly a factor of 1.5 compared to the previously known values. This was interpreted in terms of the formation of a multilayer circumstellar dust shell with an inner radius equal to 1.5 ÷ 3 times the star’s radius. Several possible reasons are proposed for unique Great brightness minima of R CrB with duration and attenuation similar to the minimum of 2007-2015.