A Census of Early Phase High-Mass Star Formation in the Central
  Molecular Zone
  We present new observations of C-band continuum emission and masers to assess
high-mass ($>$8 $M_\odot$) star formation at early evolutionary phases in the
inner 200 pc of the Central Molecular Zone (CMZ) of the Galaxy. The continuum
observation is complete to free-free emission from stars above 10-11 $M_\odot$
in 91% of the covered area. We identify 104 compact sources in the continuum
emission, among which five are confirmed ultracompact H II regions, 12 are
candidates of ultra-compact H II regions, and the remaining 87 sources are
mostly massive stars in clusters, field stars, evolved stars, pulsars,
extragalactic sources, or of unknown nature that is to be investigated. We
detect class II CH$_3$OH masers at 23 positions, among which six are new
detections. We confirm six known H$_2$CO masers in two high-mass star forming
regions, and detect two new H$_2$CO masers toward the Sgr C cloud, making it
the ninth region in the Galaxy that contains masers of this type. In spite of
these detections, we find that current high-mass star formation in the inner
CMZ is only taking place in seven isolated clouds. The results suggest that
star formation at early evolutionary phases in the CMZ is about 10 times less
efficient than expected by the dense gas star formation relation, which is in
line with previous studies that focus on more evolved phases of star formation.
This means that if there will be any impending, next burst of star formation in
the CMZ, it has not yet begun.
