The infancy of core-collapse supernova remnants
  We present 3D hydrodynamic simulations of neutrino-driven supernovae (SNe)
with the PROMETHEUS-HOTB code, evolving the asymmetrically expanding ejecta
from shock breakout until they reach the homologous expansion phase after
roughly one year. Our calculations continue the simulations for two red
supergiant (RSG) and two blue supergiant (BSG) progenitors by Wongwathanarat et
al., who investigated the growth of explosion asymmetries produced by
hydrodynamic instabilities during the first second of the explosion and their
later fragmentation by Rayleigh-Taylor instabilities. We focus on the late time
acceleration and inflation of the ejecta caused by the heating due to the
radioactive decay of $^{56}$Ni to $^{56}$Fe and by a new outward-moving shock,
which forms when the reverse shock from the He/H-shell interface compresses the
central part of the ejecta. The mean velocities of the iron-rich ejecta
increase between 100 km/s and 350 km/s ($\sim$8-30\%), and the fastest one
percent of the iron accelerates by up to $\sim$1000 km/s ($\sim$20-25\%). This
'Ni-bubble effect', known from 1D models, accelerates the bulk of the nickel in
our 3D models and causes an inflation of the initially overdense Ni-rich
clumps, which leads to underdense, extended fingers, enveloped by overdense
skins of compressed surrounding matter. We also provide volume and surface
filling factors as well as a spherical harmonics analysis to characterize the
spectrum of Ni-clump sizes quantitatively. Three of our four models give volume
filling factors larger than 0.3, consistent with what is suggested for SN 1987A
by observations.
