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Hydrogen deficiency and a sudden optical light decline by about 6-8 mag aretwo principal characteristics of R Coronae Borealis (RCB) stars. The highlatitude carbon star HE 1015-2050 was identified as a hydrogen-deficient carbonstar from low-resolution spectroscopy. Photometric data of the CatalinaReal-Time Transient Survey gathered between 2006 February and 2012 May indicatethat the object exhibits no variability. However, a high-resolution (R ~50,000) optical spectrum of this object obtained with the 8.2m Subaru telescopeusing High Dispersion Spectrograph on the 2012 January 13 offers sufficientspectral evidences for the object being a cool HdC star of RCB type undergoinglight decline. In contrast to the Na I D broad absorption features, seen in thelow-resolution spectra on several occasions, the high-resolution spectrumexhibits Na I D2 and D1 features in emission. A few emission lines due to Mg I,Sc II, Ti I, Ti II, Fe II and Ba I are also observed in the spectrum of thisobject for the first time. Such emission features combined with neutral andsingly ionized lines of Ca, Ti, Fe, etc., in absorption are reportedly seen inRCBs spectra in the early stage of decline or during the recovery to maximum.Further, the light decline of RCBs is ascribed to the formation of a cloud ofsoot that obscures the visible photosphere. Presence of such circumstellarmaterial is evident from the polarimetric observations with an estimated V-bandpercentage polarization of ~1.7% for this object.

Subaru/HDS study of HE 1015-2050: Spectral evidence of R Coronae Borealis light decline