Spatio-kinematic models of five nova remnants: correlations between nova
  shell axial ratio, expansion velocity, and speed class
  We present long-slit intermediate-dispersion spectroscopic observations and
narrow-band direct imaging of four classical nova shells, namely TAur, HRDel,
DQHer and QUVul, and the nova-like source CKVul. These are used to construct
models of their nebular remnants using the morpho-kinematic modelling tool
Shape to reveal their 3D shape. All these nova remnants but CKVul can be
described by prolate ellipsoidal shells with different eccentricity degree,
from the spherical QUVul to the highly elongated shell with an equatorial
component HRDel. On the other hand, CKVul shows a more complex structure, with
two pairs of nested bipolar lobes. The spatio-kinematic properties of the
ellipsoidal nova shells derived from our models include their true axial
ratios. This parameter is expected to correlate with the expansion velocity and
decline time C3 (i.e., their speed class) of a nova as the result the
interaction of the ejecta with the circumstellar material and rotation speed
and magnetic field of the white dwarf. We have compared these three parameters
including data available in the literature for another two nova shells, V533
Her and FH Ser. There is an anti-correlation between the expansion velocity and
the axial ratio and decline time C3 for nova remnants with ellipsoidal
morphology, and a correlation between their axial ratios and decline times C3,
confirming theoretical expectations that the fastest expanding novae have the
smallest axial ratios. We note that the high expansion velocity of the nova
shell HRDel of 615 km/s is inconsistent with its long decline time C3 of 250
days.
