2025ApJ...988..112K

V1674 Her is one of the fastest novae, of which the very early phase is well observed, including optical rise to the peak over 10 mag. We present a full theoretical light-curve model of V1674 Her. Our 1.35 M_⊙ white dwarf (WD) model with the mass accretion rate of 1 × 10^–11M_⊙ yr^–1 explains overall properties including a very fast rise and decay of the optical V light curve. The WD photosphere expands up to 21 R_⊙, thus a 0.26 M_⊙ companion star orbiting the WD every 3.67 hr is engulfed 2.7 hr after the onset of thermonuclear runaway and appears 5.3 days after that. The duration of X-ray flash is only 0.96 hr. The evolution of the expanding envelope and temporal change of the photospheric radius are very consistent with observed optical and X-ray modulations with the orbital and spin (501 s) periods. We confirm that the decay phase of the nova light curve is well approximated by a sequence of steady-state envelope solutions. Using the time-stretching method of nova light curves, we obtain the V-band distance modulus of (m - M)_V = 16.3 ± 0.2 and determine the distance to be d = 8.9 ± 1 kpc for the interstellar extinction of E(B - V) = 0.5 ± 0.05.