Deeply Embedded Protostellar Population in the 20 km s-1 Cloud of the
  Central Molecular Zone
  We report the discovery of a population of deeply embedded protostellar
candidates in the 20 km s$^{-1}$ cloud, one of the massive molecular clouds in
the Central Molecular Zone (CMZ) of the Milky Way, using interferometric
submillimeter continuum and H$_2$O maser observations. The submillimeter
continuum emission shows five 1-pc scale clumps, each of which further
fragments into several 0.1-pc scale cores. We identify 17 dense cores, among
which 12 are gravitationally bound. Among the 18 H$_2$O masers detected, 13
coincide with the cores and probably trace outflows emanating from the
protostars. There are also 5 gravitationally bound dense cores without H$_2$O
maser detection. In total the 13 masers and 5 cores may represent 18 protostars
with spectral types later than B1 or potential growing more massive stars at
earlier evolutionary stage, given the non-detection in the centimeter radio
continuum. In combination with previous studies of CH$_3$OH masers, we conclude
that the star formation in this cloud is at an early evolutionary phase, before
the presence of any significant ionizing or heating sources. Our findings
indicate that star formation in this cloud may be triggered by a tidal
compression as it approaches pericenter, similar to the case of G0.253+0.016
but with a higher star formation rate, and demonstrate that high angular
resolution, high sensitivity maser and submillimeter observations are a
promising technique to unveil deeply embedded star formation in the CMZ.
