  The physical mechanism driving mass ejection during a nova eruption is still
poorly understood. Possibilities include ejection in a single ballistic event,
a common envelope interaction, a continuous wind, or some combination of these
processes. Here we present a study of 12 Galactic novae, for which we have
pre-maximum high-resolution spectroscopy. All 12 novae show the same spectral
evolution. Before optical peak, they show a slow P Cygni component. After peak
a fast component quickly arises, while the slow absorption remains superimposed
on top of it, implying the presence of at least two physically distinct flows.
For novae with high-cadence monitoring, a third, intermediate-velocity
component is also observed.
  These observations are consistent with a scenario where the slow component is
associated with the initial ejection of the accreted material and the fast
component with a radiation-driven wind from the white dwarf. When these flows
interact, the slow flow is swept up by the fast flow, producing the
intermediate component. These colliding flows may produce the gamma-ray
emission observed in some novae. Our spectra also show that the transient heavy
element absorption lines seen in some novae have the same velocity structure
and evolution as the other lines in the spectrum, implying an association with
the nova ejecta rather than a pre-existing circumbinary reservoir of gas or
material ablated from the secondary. While this basic scenario appears to
qualitatively reproduce multi-wavelength observations of classical novae,
substantial theoretical and observational work is still needed to untangle the
rich diversity of nova properties.
