On the nature of the black-body stars

A selection of 17 stars in the Sloan Digital Sky Survey, previously identified as DC class white dwarfs (WDs), has been reported to show spectra very close to blackbody radiation in the wavelength range from ultraviolet to infrared. Due to the absence of lines and other details in their spectra, the surface gravity of these objects has not been previously well constrained and their effective temperatures have been determined by fits to the continuum spectrum using pure helium atmosphere models. We compute model atmospheres with pure helium and H/He mixtures and use Gaia DR2 parallaxes available for 16 out of the 17 selected stars to analyze their physical properties. We find that the atmospheres of the selected stars are very probably contaminated with a trace amount of hydrogen as $-6 \leq \log{N_H/N_{He}} \leq -5.4$. For the 16 stars with Gaia parallaxes, we calculate a mean stellar mass $0.606\pm 0.076\,M_\odot$, which represents typical mass values and surface gravities ($7.8<\log g<8.3$) for WDs.