Astron. Astrophys. 319, 909-922 (1997)

6. Conclusions

Applying Galactic nova population models and recent 1-dim. hydrodynamical nucleosynthesis calculations for ONeMg novae, we have examined the ²⁶ Al production by ONeMg novae in the Galaxy. We summarize our key conclusions below.

(1) The amount of ²⁶ Al produced by our model populations is strongly dependent on the assumed mass ratio distribution in ZAMS binaries. Populations of CVs which evolve from progenitor ZAMS binaries in which the masses of the primary and secondary stars are uncorrelated produce to 10 times more ²⁶ Al than those in which there is a strong correlation between progenitor primary and secondary masses. This is a direct consequence of the fact that populations of ZAMS binaries with un- or weakly-correlated primary and secondary masses produce more CVs than those with strongly correlated primary and secondary masses.

(2) ²⁶ Al production is also a strong function of the amount of mixing which occurs between material accreted from the donor star and material from the underlying white dwarf. For a given population, maximum ²⁶ Al production is achieved for a level of mixing of 50% ().

(3) ²⁶ Al is produced almost exclusively from systems with high mass transfer rates in our model populations and, therefore, systems with orbital periods above the period gap. We note that this is not a trivial result, since such systems comprise only 1% of the entire population.

(4) Choosing optimal models and parameter values, we estimate an upper limit for ²⁶ Al production from ONeMg novae of . In estimating this number, we have taken the fraction of ONeMg novae in our population to be equal to 30%, the current value deduced from observations. This choice constrains the least massive ONeMg WD (at birth) in our population to a value of .

Uncertainties in both theory and observations, particularly our lack of understanding of the physical mechanism responsible for mixing in classical novae, preclude us from placing a lower limit on ²⁶ Al production from ONeMg novae (other than zero) with any confidence at this time.

(5) Selecting models and parameter values that are the most consistent with observational quantities independent of ²⁶ Al production (e.g., fraction of CVs below the period gap, properties of post-CE binaries, mean WD mass in CVs, etc.), we find a value of for ²⁶ Al production from ONeMg novae (again for ). However, we emphasize that this value is strongly dependent on our use of a single value of for the entire population. If the mean amount of mixing in ONeMg novae is approximately , a factor of two greater than the maximum mean amount of mixing for all novae, then our results suggest that ONeMg novae could produce enough ²⁶ Al to account for the entire diffuse 1.8 MeV emission seen by COMPTEL. Ejecta abundances in well-studied novae do not preclude this possibility, and are perhaps (weakly) in support of the possibility that the mean level of mixing in ONeMg novae is higher than that for all novae.

We note that the specific values quoted in points (4) and (5) above are predicated on our assumption that the formation of CVs with ONeMg WDs is quantitatively similar to the formation of CVs with high-mass CO WDs. If detailed models of the formation of CVs with ONeMg WDs suggest otherwise, then our predictions would need to be re-evaluated in light of these more accurate models.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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