THE NOVA SHELL AND EVOLUTION OF THE RECURRENT NOVA T PYXIDIS

T Pyxidis (T Pyx) is the prototypical recurrent nova (RN), with five eruptions from 1890 to 1967 and a mysterious nova shell. We report new observations of the nova shell with the Hubble Space Telescope (HST) in the year 2007, which provides a long time baseline to compare with HST images from 1994 and 1995. We find that the knots in the nova shell are expanding with velocities ranging from roughly 500 km s(-1) to 715 km s(-1), assuming a distance of 3500 pc. The fractional expansion of the knots is constant, which implies no significant deceleration of the knots, which must have been ejected by an eruption close to the year 1866. We see knots that have turned on after 1995; this shows that the knots are powered by shocks from the collision of the "1866" ejecta with fast ejecta from later RN eruptions. We derive that the "1866" ejecta has a total mass of similar to 10(-4.5) M(circle dot), which with the low ejection velocity shows that the "1866" event was an ordinary nova eruption, not an RN eruption. This also implies that the accretion rate before the ordinary nova event must have been low (around the 4 x 10(-11) M(circle dot) yr(-1) expected for gravitational radiation alone), and that the matter accumulated on the surface of the white dwarf for similar to 750,000 years. The current accretion rate (> 10(-8) M(circle dot) yr(-1)) is similar to 1000x higher than expected for a system below the period gap, with the plausible reason being that the "1866" event started a continuing supersoft source that drives the accretion. The accretion rate has been declining since before the 1890 eruption, with the current rate being only 3% of its earlier value. The decline in the observed accretion rate shows that the supersoft source is not self-sustaining; we calculate that the accretion in T Pyx will effectively stop in upcoming decades. With this, T Pyx will enter a state of hibernation lasting for an estimated 2,600,000 years before gravitational radiation brings the system into contact again. Thus, T Pyx has an evolutionary cycle going from an ordinary CV state (lasting 750,000 years), to its current RN state (lasting little longer than a century), to a future hibernation state (lasting 2,600,000 years), and then repeating this cycle.