Deeply Embedded Protostellar Population in the Central Molecular Zone
  Suggested by H$_2$O Masers and Dense Cores
  The Central Molecular Zone (CMZ), usually referring to the inner 500 pc of
the Galaxy, contains a dozen of massive ($\sim10^5$ $M_\odot$) molecular
clouds. Are these clouds going to actively form stars like Sgr B2? How are they
affected by the extreme physical conditions in the CMZ, such as strong
turbulence? Here we present a first step towards answering these questions.
Using high-sensitivity, high angular resolution radio and (sub)millimeter
observations, we studied deeply embedded star formation in six massive clouds
in the CMZ, including the 20 and 50 km s$^{-1}$ clouds, Sgr B1 off (as known as
dust ridge clouds e/f), Sgr C, Sgr D, and G0.253-0.016. The VLA water maser
observations suggest a population of deeply embedded protostellar candidates,
many of which are new detections. The SMA 1.3 mm continuum observations reveal
peaks in dust emission associated with the masers, suggesting the existence of
dense cores. While our findings confirm that clouds such as G0.253-0.016 lack
internal compact substructures and are quiescent in terms of star formation,
two clouds (the 20 km s$^{-1}$ cloud and Sgr C) stand out with clusters of
water masers with associated dense cores which may suggest a population of
deeply embedded protostars at early evolutionary phases. Follow-up observations
with VLA and ALMA are necessary to confirm their protostellar nature.
