  Aims: We investigated the masses and spatial distributions of pre-stellar and
protostellar candidates in the young, low-mass star forming region Serpens
South, where active star formation is known to occur along a predominant
filamentary structure. Previous observations used to study these distributions
have been limited by two important observational factors: (1) sensitivity
limits that leave the lowest-mass sources undetected, or (2) resolution limits
that cannot distinguish binaries and/or cluster members in close proximity.
Methods: Recent millimeter-wavelength interferometry observations can now
uncover faint and/or compact sources in order to study a more complete
population of protostars, especially in nearby ($D<500$ pc) clusters. Here we
present ALMA observations of 1 mm (Band 6) continuum in a $3 \times 2$
arcminutes region at the center of Serpens South. Our angular resolution of
$\sim1$ arcsec is equivalent to $\sim400$ au, corresponding to scales of
envelopes and/or disks of protostellar sources. Results: We detect 52 sources
with 1 mm continuum, and we measure masses of $0.002 - 0.9$ solar masses
corresponding to gas and dust in the disk and/or envelope of the protostellar
system. For the deeply embedded (youngest) sources with no IR counterparts, we
find evidence of mass segregation and clustering according to: the Minimum
Spanning Tree method, distribution of projected separations between unique
sources, and concentration of higher-mass sources near to the dense gas at the
cluster center. Conclusions: The mass segregation of the mm sources is likely
primordial rather than dynamical given the young age of this cluster, compared
with segregation time. This is the first case to show this for mm sources in a
low-mass protostellar cluster environment.
