  Hydrogen deficiency and a sudden optical light decline by about 6-8 mag are
two principal characteristics of R Coronae Borealis (RCB) stars. The high
latitude carbon star HE 1015-2050 was identified as a hydrogen-deficient carbon
star from low-resolution spectroscopy. Photometric data of the Catalina
Real-Time Transient Survey gathered between 2006 February and 2012 May indicate
that the object exhibits no variability. However, a high-resolution (R ~
50,000) optical spectrum of this object obtained with the 8.2m Subaru telescope
using High Dispersion Spectrograph on the 2012 January 13 offers sufficient
spectral evidences for the object being a cool HdC star of RCB type undergoing
light decline. In contrast to the Na I D broad absorption features, seen in the
low-resolution spectra on several occasions, the high-resolution spectrum
exhibits 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 this
object for the first time. Such emission features combined with neutral and
singly ionized lines of Ca, Ti, Fe, etc., in absorption are reportedly seen in
RCBs 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 of
soot that obscures the visible photosphere. Presence of such circumstellar
material is evident from the polarimetric observations with an estimated V-band
percentage polarization of ~1.7% for this object.
