3. Discussion

The derived turn-off times (or limits) for the novae in our sample are listed in Table 3. The combination of X-ray and UV data presented here shows that most novae decline in bolometric luminosity in about 1-5 years after the outburst to the level where hydrogen shell burning is likely to have ceased. GQ Mus remains the long-lived exception to this trend. Other novae seem to reach UV spectroscopic stages similar to that observed in GQ Mus after nearly a decade within only a few years after outburst, and thereafter turn off completely.

 

Nova UV Turn-off X-ray Turn-off Turn-off

time (years) time (years) time (years)

V1668 Cyg < 1.3 < 1.3

PW Vul 1.6- 1.7 < 7

QU Vul > 4.7 < 6.5 4.7- 6.5

V842 Cen < 6

OS And < 6

QV Vul < 3.5 < 4 < 3.5

V443 Sct 0.9 - 2.5 < 3 0.9 -2.5

V838 Her 0.3 - 2.3

V351 Pup 2.1 - 2.9

V705 Cas > 1.8 > 1.8

LMC 88 No. 1 > 1.2 < 4 1.2 - 4

LMC 88 No. 2 0.7-2.1 < 3 0.7 - 2.1

LMC 91 > 0.9 > 0.9

Furthermore, the turn-off times do not appear to correlate with the properties of the system. For instance V838 Her and QU Vul, both classified as Neon novae, had relatively short and long turn-off times, respectively. These are puzzling results when compared with the theoretical predictions. Classical novae with high mass white dwarfs are expected to shut down in a year or less, while low mass white dwarfs should sustain nuclear burning for a decade or more. Because this pattern depends on white dwarf mass, it also is expected to correlate with the peak luminosity and thus with the speed classes of classical novae (see Prialnik & Kovetz 1985). Neon novae are often theoretically attributed to outbursts on massive O-Ne-Mg white dwarfs and should therefore have short nuclear burning lifetimes. Thus far this trend has not been observed.

The lack of a pronounced correlation between turn-off times and neon abundances suggests either that only a subset of Neon novae are due to the presence of a high mass O-Ne-Mg white dwarf (Livio & Truran 1994) or possibly none at all (Shara & Prialnik 1994), or that factors other than the white dwarf mass control the turn-off times.

Since there is a spread in turn-off times, but not a clear dependence on the white dwarf mass, as expected from the basic theory, we suspect that the real turn-off times of novae are influenced by a variety of factors, including the white dwarf temperature at the onset of the outburst, the chemical composition, the white dwarf magnetic field, the number of prior outbursts or post-outburst irradiation induced mass transfer (e.g. Schwartzman et al. 1994; Kovetz & Prialnik 1994).

All the available data in the UV and X-rays ranges indicate that typical Galactic classical novae run out of nuclear fuel in the course of a few years after the outburst. In the light of the theoretical predictions summarized in the Introduction, such a conclusion implies either that a highly efficient wind mechanism has to be active to remove material rapidly from the nova remnant, or that nova eruptions can occur with low mass accreted envelopes.

The mass of material converted to helium during a nova outburst is typically (assuming a bolometric luminosity of ), where is the turn-off time in years. The ratio of mass burned to mass ejected in a typical classical nova outburst then is . Therefore, for the observed of a few years, only a small fraction of the accreted envelope mass can remain on the white dwarf. The observed nuclear burning times agree with other data in suggesting that novae do not retain sufficient mass to evolve toward becoming Type I supernovae or neutron stars born by accretion-induced collapse.

Acknowledgements

M.O. acknowledges Support of the Italian Space Agency (ASI) and a grant of the University of Wisconsin Graduate School. Undertaking this work was suggested and inspired by Luciana Bianchi, to whom we are very thankful. We also acknowledge very useful and interesting conversations with Angelo Cassatella and Hakki Ögelman.