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 ofthree 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 anup-to-date nuclear reaction network. The nucleosynthesis results showcorrelation with the peak temperatures or the cooling timescales duringoutbursts. Among the combinations of white dwarf and envelope masses which givethe same peak temperature, the explosion is more violent for a lower whitedwarf mass owing to its smaller gravitational potential. Comparison of thenucleosynthesis results with observations implies that at least two-third ofthe white dwarf masses for the observed ONeMg novae are $\simeq 1.1 M_\odot$,which are significantly lower than estimated by previous hydrodynamic studiesbut consistent with the observations of V1974 Cyg. Moreover, the envelopemasses derived from the comparison are $\gtrsim 10^{-4} M_\odot$, which are ingood agreement with the ejecta masses estimated from observations butsignificantly higher than in previous hydrodynamic studies. With such a lowmass white dwarf and a high mass envelope, the nova can produce interestingamounts of $\gamma$-ray emitters $^7$Be, $^{22}$Na, and $^{26}$Al. We suggestthat V1974 Cyg has produced $^{22}$Na as high as the upper limit derived fromthe COMPTEL survey. In addition, a non-negligible part of the Galactic$^{26}$Al may originate from ONeMg novae, if not the major contributors. Boththe future INTEGRAL survey for these $\gamma$-ray emitters and abundanceestimates derived from ultraviolet, optical, and near infrared spectroscopieswill impose a severe constraint on the current nova models.