We have performed survey-type observations in 1 mm continuum and molecular lines toward dense cores ( 32 prestellar + 7 protostellar ) with an average density of \gtrsim 10 ^ { 5 } cm ^ { -3 } in the Taurus molecular clouds using the Atacama Large Millimeter/submillimeter Array–Atacama Compact Array ( ALMA-ACA ) stand-alone mode with an angular resolution of 6 \farcs 5 ( \sim 900 au ) .
The primary purpose of this study is to investigate the innermost part of dense cores with view to understanding the initial condition of star formation .
In the protostellar cores , contributions from protostellar disks dominate the observed continuum flux with a range of 35 % –90 % except for the very low-luminosity object .
For the prestellar cores , we have successfully confirmed continuum emission 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 spatial frequency coverage with the ACA 7 m array , the detection rate is significantly higher than that of the previous surveys , which have zero or one continuum-detected sources among a large number of starless samples using the ALMA Main Array .
The statistical counting method tells us that the lifetime of the prestellar cores until protostar formation therein approaches the freefall time as the density increases .
Among the prestellar cores , at least two targets have possible internal substructures , which are detected in continuum emission with the size scale of \sim 1000 au if we consider the molecular line ( C ^ { 18 } O and N _ { 2 } D ^ { + } ) distributions .
These results suggest that small-scale fragmentation/coalescence processes occur in a region smaller than 0.1 pc , which may determine the final core mass associated with individual protostar formation before starting the dynamical collapse of the core with central density of \sim ( 0.3–1 ) \times 10 ^ { 6 } cm ^ { -3 } .