Spectral Evolution and Accretion Properties of Three Classical Novae from International Ultraviolet Explorer Observations Spectra

We present a comprehensive analysis of ultraviolet (UV) spectra from the International Ultraviolet Explorer (IUE) of three classical novae: OS And (Nova Andromedae 1986), QU Vul (Nova Vulpeculae 1984 No. 2), and V1974 Cyg(Nova Cygni 1992). Using continuum normalization, extinction correction, and Gaussian profile fitting of prominent emission lines (O I λ1306, N III] λ1750, C III] λ1909, [Ne IV] λ1602), we trace the temporal evolution of their spectral phases from outburst to quiescence. The results reveal systematic differences in line evolution and expansion velocities, governed primarily by white dwarf mass and composition. OS And is consistent with a CO2/CO3 nova (MWD ≈ 0.9 M⊙), while QU Vul and V1974 Cyg are ONe novae (MWD ≈ 1.0–1.15 M⊙). Expansion velocities derived from FWHM measurements range from ~1500 to 2200 km s−1, characteristic of fast novae. Quiescent-phase spectra yield mass accretion rates of (1.14 ± 0.20) × 10⁻⁷ M⊙ yr⁻¹ for OS And and (4.44 ± 1.44) × 10⁻⁸ M⊙yr⁻¹ for QU Vul and (9.04 ± 2.11) ×10−10 M⊙yr⁻¹ for V1974 Cyg. The observed evolution of emission features confirms the canonical sequence of initial, pre-nebular, and nebular stages, while the presence of highionization species indicates inhomogeneous ejecta and complex ionization structures. Comparison with thermonuclear runaway models by José & Hernanz (1998) shows general consistency but also highlights discrepancies in dynamical parameters, suggesting the need for refined modelling of late outburst phases. Our results underscore the diagnostic power of UV spectroscopy for probing nova evolution and con straining white dwarf properties in classical novae.