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Stars like our Sun form in self-gravitating dense and cold structures withininterstellar clouds, called pre-stellar cores. Although much is known about thephysical structure of dense clouds just before and soon after the switch-on ofa protostar, the central few thousand astronomical units (au) of pre-stellarcores are unexplored. It is within these central regions that stellar systemsassemble and fragmentation may take place, with the consequent formation ofbinaries and multiple systems. We present ALMA Band 6 observations (ACA and 12marray) 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 zonewith 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 withinRker appears to have fragmented, but detailed analysis shows that similarsubstructure can be reproduced by synthetic interferometric observations of asmooth centrally concentrated dense core with a similar central flat zone. Thepresence of a smooth kernel within a dense core is in agreement with non-idealmagneto-hydro-dynamical simulations of a contracting cloud core with a peaknumber density of 1 x 10^7 cm^{-3}. Dense cores with lower central densitiesare completely filtered out when simulated 12m-array observations are carriedout. These observations demonstrate that the kernel of dynamically evolveddense cores can be investigated at high angular resolution with ALMA.

The central 1000 AU of a pre-stellar core revealed with ALMA. I. 1.3 mm continuum observations