The Hybrid Nova V1405 Cas with an Oxygen–Neon or Carbon–Oxygen White Dwarf?

Based on spectrophotometric observations, we have investigated the envelope of Nova V1405 Cas. The spectrophotometric evolution has shown that it belongs in its spectral characteristics to the rare type of hybrid novae. At the optically thick phase in the Balmer lines the profiles had a P Cyg shape; the radial velocities of the absorption components in the profiles pointed to ejecta with velocities reaching \(RV=-1800\) km s\({}^{-1}\). We have determined the maximum envelope expansion velocity from the half-width of the line profiles at the continuum level, which reached 3300 km s\({}^{-1}\) from the H\({\alpha}\) line. We have obtained approximate estimates of the electron density \(N_{e}=10^{7}\) cm\({}^{-3}\), temperature \(T_{e}=2\times 10^{4}\) K, and the abundances of some elements. We show that the neon and iron abundances for such electron density and temperature can differ from the solar ones insignificantly. The low neon abundance does not allow one to assert with confidence that the white dwarf in this nova is an oxygen–neon one. The mass of the envelope has been estimated to be \({\sim}6\times 10^{-5}\;M_{\odot}\).