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 o f three parameters, i.e., the white dwarf mass, the envelope mass at igniti on, and the initial composition. A quasi-analytic one-zone approach is used with an up-to-date nuclear reaction network. The nucleosynthesis results s how 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 w hite dwarf mass owing to its smaller gravitational potential. Comparison of the nucleosynthesis results with observations implies that at least two-th irds of the white dwarf masses for the observed ONeMg novae are similar or equal to 1.1 M., which is significantly lower than estimated by previous hy drodynamic studies but consistent with the observations of V1974 Cyg. Moreo ver, the envelope masses derived from the comparison are greater than or si milar to 10(-4) M., which is in good agreement with the ejecta masses estim ated from observations but significantly higher than in previous hydrodynam ic studies. With such a low-mass white dwarf and a high-mass envelope, a no va can produce interesting amounts of the gamma-ray emitters Be-7, Na-22, a nd Al-26. We suggest that V1974 Cyg has produced Na-22 as high as the upper limit derived from the COMPTEL survey. In addition, a nonnegligible part, if not the majority, of the Galactic Al-26 may originate from ONeMg novae. Both the future International Gamma-Ray Astrophysical Laboratory (INTEGRAL) survey for these gamma-ray emitters and abundance estimates derived from u ltraviolet, optical, and near-infrared spectroscopy will impose severe cons traints on the current nova models.