The mass accretion rate in quiescent cataclysmic variables (CVs) and ex-novae.

Pierluigi Selvelli - OATS-INAF
Elena Mason - OATS-INAF
Roberto Gilmozzi - ESO-Garching.

CVs are semi-detached binaries  in which the white dwarf (WD) primary star accretes
material from the  low main sequence companion. Novae are a class of CVs that displays
thermonuclear runaway driven outbursts. Other different classes of CVs are identified
depending on the mechanism driving and controlling the transfer of material between the
companion and the WD. Theory predicts that, during their lifetime, all CVs evolve from
long to short orbital periods and, as they do so, the mass accretion rate decline. Hence,
the mass accretion rate is a very important parameter since it characterizes both the
evolutionary phase of a system and the recurrence time of nova outbursts.

While the period -- mass-accretion relation appears almost univocal in theoretical models,
observationally a large range of mass accretion rates is reported for a given orbital
period. Observationally, the determination of the mass accretion rate heavily relays on
the knowledge of the system distance and the  UV spectral energy distibution,  where the
bulk of the accretion luminosity is emitted (at least for the case of non strongly
magnetic CVs). Unlike other CVs, novae  have more reliable distances determined from
empirical methods.

Searching archival (IUE and STIS) UV data of old novae and nova distance determination in
the literature, we obtained a fiducial sample of 18 novae for which we could estimate the
UV luminosity and the mass accretion rate with relatively high accuracy. Our preliminary
results suggest that, contrarily to the expectation, there is no correlation between the
mass accretion rate and the orbital period in old novae. While a number of speculations
can be brought up to explain the discrepancy,  the analysis should be extended to a larger
sample of novae as well as to other CVs. This will be possible  in the near future with
the accurate distances that will be soon available from the  GAIA project,  in combination
with archive  UV observations of CVs by IUE, GALEX, HST and other experiments.

Contact person: Pierluigi Selvelli