The central 1000 AU of a pre-stellar core revealed with ALMA. I. 1.3 mm
  continuum observations
  Stars like our Sun form in self-gravitating dense and cold structures within
interstellar clouds, called pre-stellar cores. Although much is known about the
physical structure of dense clouds just before and soon after the switch-on of
a protostar, the central few thousand astronomical units (au) of pre-stellar
cores are unexplored. It is within these central regions that stellar systems
assemble and fragmentation may take place, with the consequent formation of
binaries and multiple systems. We present ALMA Band 6 observations (ACA and 12m
array) of the dust continuum emission of the 8 Msun pre-stellar core L1544,
with angular resolution of 2'' x 1.6'' (linear resolution 270 au x 216 au).
Within the primary beam, a compact region of 0.1 Msun, which we call a
"kernel", has been unveiled. The kernel is elongated, with a central flat zone
with radius Rker ~ 10'' (~ 1400 au). The average number density within Rker is
~1 x 10^6 cm^{-3}, with possible local density enhancements. The region within
Rker appears to have fragmented, but detailed analysis shows that similar
substructure can be reproduced by synthetic interferometric observations of a
smooth centrally concentrated dense core with a similar central flat zone. The
presence of a smooth kernel within a dense core is in agreement with non-ideal
magneto-hydro-dynamical simulations of a contracting cloud core with a peak
number density of 1 x 10^7 cm^{-3}. Dense cores with lower central densities
are completely filtered out when simulated 12m-array observations are carried
out. These observations demonstrate that the kernel of dynamically evolved
dense cores can be investigated at high angular resolution with ALMA.
