Synthesis of radioactive isotopes and gamma radiation in nova outbursts

Abstract

The range of abundances of radioactive nuclei in material ejected during novae is derived from kinetic calculations of thermonuclear burning in a hydrogen-rich, single-zone envelope at the surface of a CO or ONeMg dwarf. The total amount of radioactive N, O, and F isotopes in the envelope is approximately equal to the mass of CO in the dwarf that is admixed during accretion and/or the outburst. The mass fraction of synthesized 18F is 10−3–10−2. In the case of ONeMg dwarfs, the abundances of 22Na and 26Al are substantially higher and reach several percent and one percent of the mass, respectively. We derived light curves in the annihilation line from short-lived NOF-isotopes and studied the effect of varying the envelope parameters. The light curves are most sensitive to the degree of mixing of the radioactive isotopes in the envelope. Even having as little as 1% of unmixed material in the outer layer results in appreciable suppression of the first luminosity peak due to radioactive NO isotopes. The second luminosity peak, due to 18F decay, is appreciably suppressed only when the relative mass of unmixed material in the outer layer exceeds 10%. We suggest observations of the 18F annihilation-line flux with independent observational estimates of the 18F mass in the ejected envelope as a means to determine the degree of mixing and the mass of synthesized 18O. Calculations of the synthesis of 22Na and 26Al isotopes are compared to estimates of the masses of these isotopes in the envelopes of novae derived from observations of their gamma radiation.