  The direct impact of white dwarfs has been suggested as a plausible channel
for type Ia supernovae. In spite of their (a priori) rareness, in highly
populated globular clusters and in galactic centers, where the amount of white
dwarfs is considerable, the rate of violent collisions between two of them
might be non-negligible. Even more, there are indications that binary white
dwarf systems orbited by a third stellar-mass body have an important chance to
induce a clean head-on collision. Therefore, this scenario represents a source
of contamination for the supernova light-curves sample that it is used as
standard candles in cosmology, and it deserves further investigation. Some
groups have conducted numerical simulations of this scenario, but their results
show several differences. In this paper we address some of the possible sources
of these differences, presenting the results of high resolution hydrodynamical
simulations jointly with a detailed nuclear post-processing of the nuclear
abundances, to check the viability of white dwarf collisions to produce
significant amounts of 56Ni. To that purpose, we use a 2D-axial symmetric
smoothed particle hydrodynamic code to obtain a resolution considerably higher
than in previous studies. In this work, we also study how the initial mass and
nuclear composition affect the results. The gravitational wave emission is also
calculated, as this is a unique signature of this kind of events. All
calculated models produce a significant amount of 56Ni, ranging from 0.1 Msun
to 1.1 Msun, compatible not only with normal-Branch type Ia supernova but also
with the subluminous and super-Chandrasekhar subset. Nevertheless, the
distribution mass-function of white dwarfs favors collisions among 0.6-0.7 Msun
objects, leading to subluminous events.
