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We have performed survey-type observations in 1 mm continuum and molecularlines toward dense cores (32 prestellar + 7 protostellar) with an averagedensity of $\gtrsim$10$^5$ cm$^{-3}$ in the Taurus molecular clouds using theAtacama Large Millimeter/submillimeter Array-Atacama Compact Array (ALMA-ACA)stand-alone mode with an angular resolution of 6.$''$5 ($\sim$900 au). Theprimary purpose of this study is to investigate the innermost part of densecores toward understanding the initial condition of star formation. In theprotostellar cores, contributions from protostellar disks dominate the observedcontinuum flux with a range of 35-90% except for the very low-luminosityobject. For the prestellar cores, we have successfully confirmed continuumemission from dense gas with a density of $\gtrsim$3 $\times$10$^5$ cm$^{-3}$toward approximately one-third of the targets. Thanks to the lower spatialfrequency coverage with the ACA-7 m array, the detection rate is significantlyhigher than that of the previous surveys, which have 0 or 1 continuum detectedsources among large number of starless samples using the ALMA Main array. Thestatistical counting method tells us that the lifetime of the prestellar coresuntil protostar formation therein approaches the free-fall time as the densityincreases. Among the prestellar cores, at least two targets have possibleinternal substructures, which are detected in continuum emission with the sizescale of $\sim$1000 au if we consider the molecular line (C$^{18}$O andN$_2$D$^{+}$) distributions. These results suggest that small-scalefragmentation/coalescence processes occur in a region smaller than 0.1 pc,which may determine the final core mass associated with individual protostarformation before starting the dynamical collapse of the core with centraldensity of $\sim$(0.3-1) $\times$ 10$^6$ cm$^{-3}$.

astrophysical journal/the astrophysical journal

FRagmentation and Evolution of dense cores Judged by ALMA (FREJA). I  (Overview). Inner $\sim$1000 au structures of prestellar/protostellar cores  in Taurus