Nucleosynthesis in ONeMg novae: Models versus observations to constrain the masses of ONeMg white dwarfs and their envelopes

Nucleosynthesis in ONeMg novae has been investigated with the wide ranges of three parameters, i.e., the white dwarf mass, the envelope mass at ignition, and the initial composition. A quasi-analytic one-zone approach is used with an up-to-date nuclear reaction network. The nucleosynthesis results show correlation with the peak temperatures or the cooling timescales during outbursts. Among the combinations of white dwarf and envelope masses that give the same peak temperature, the explosion is more violent for a lower white dwarf mass owing to its smaller gravitational potential. Comparison of the nucleosynthesis results with observations implies that at least two-thirds of the white dwarf masses for the observed ONeMg novae are ≃1.1 M_⊙, which is significantly lower than estimated by previous hydrodynamic studies but consistent with the observations of V1974 Cyg. Moreover, the envelope masses derived from the comparison are ≳10^-4 M_⊙, which is in good agreement with the ejecta masses estimated from observations but significantly higher than in previous hydrodynamic studies. With such a low-mass white dwarf and a high-mass envelope, a nova can produce interesting amounts of the γ-ray emitters ⁷Be, ²²Na, and ²⁶Al. We suggest that V1974 Cyg has produced ²²Na as high as the upper limit derived from the COMPTEL survey. In addition, a nonnegligible part, if not the majority, of the Galactic ²⁶Al may originate from ONeMg novae. Both the future International Gamma-Ray Astrophysical Laboratory (INTEGRAL) survey for these γ-ray emitters and abundance estimates derived from ultraviolet, optical, and near-infrared spectroscopy will impose severe constraints on the current nova models.