G-0.02-0.07, the Compact HII Region Complex nearest to the Galactic
  Center with ALMA
  We have observed the compact HII region complex nearest to the dynamical
center of the Galaxy, G-0.02-0.07, using ALMA in the H42a recombination line,
CS J=2-1, H13CO+ J=1-0, and SiO v=0, J=2-1 emission lines, and 86 GHz continuum
emission. The HII regions HII-A to HII-C in the cluster are clearly resolved
into a shell-like feature with a bright-half and a dark-half in the
recombination line and continuum emission. The absorption features in the
molecular emission lines show that HII-A, B and C are located on the near side
of the 50 km/s Molecular Cloud (50MC) but HII-D is located on the far side. The
electron temperatures and densities range Te=5150-5920 K and ne=950-2340 cm-3,
respectively. The electron temperatures on the bright-half are slightly lower
than those on the dark-half, while the electron densities on the bright-half
are slightly higher than those on the dark-half. The HII regions are located on
the molecular filaments in the 50MC. They have already broken through the
filaments and are growing in the surrounding molecular gas. There are some
shocked molecular gas components around the HII regions. From line width of the
H42a recombination line, the expansion velocities from HII-A to HII-D are
estimated to be Vexp=16.7, 11.6, 11.1, and 12.1 km/s, respectively. The
expansion timescales from HII-A to HII-D are estimated to be Tage~1.4x0^4,
1.7x10^4, 2.0x10^4, and 0.7x10^4 years, respectively. The spectral types of the
central stars from HII-A to HII-D are estimated to be O8V, O9.5V, O9V, and B0V,
respectively. The positional relation among the HII regions, the SiO molecule
enhancement area, and Class-I maser spots suggest that the shock wave caused by
a cloud-cloud collision propagated along the line from HII-C to HII-A in the
50MC. The shock wave would trigger the massive star formation.
