74 MHz Nonthermal Emission from Molecular Clouds: Evidence for a Cosmic
  Ray Dominated Region at the Galactic Center
  We present 74 MHz radio continuum observations of the Galactic center region.
These measurements show nonthermal radio emission arising from molecular clouds
that is unaffected by free-free absorption along the line of sight. We focus on
one cloud, G0.13--0.13, representative of the population of molecular clouds
that are spatially correlated with steep spectrum
(alpha^{74MHz}_{327MHz}=1.3\pm0.3) nonthermal emission from the Galactic center
region. This cloud lies adjacent to the nonthermal radio filaments of the Arc
near l~0.2^0 and is a strong source of 74 MHz continuum, SiO (2-1) and FeI
Kalpha 6.4 keV line emission. This three-way correlation provides the most
compelling evidence yet that relativistic electrons, here traced by 74 MHz
emission, are physically associated with the G0.13--0.13 molecular cloud and
that low energy cosmic ray electrons are responsible for the FeI Kalpha line
emission. The high cosmic ray ionization rate ~10-13 s-1 H-1 is responsible for
heating the molecular gas to high temperatures and allows the disturbed gas to
maintain a high velocity dispersion. LVG modeling of multi-transition SiO
observations of this cloud implies H2 densities ~104-5 cm-3 and high
temperatures. The lower limit to the temperature of G0.13-0.13 is ~100K,
whereas the upper limit is as high as 1000K. Lastly, we used a time-dependent
chemical model in which cosmic rays drive the chemistry of the gas to
investigate for molecular line diagnostics of cosmic ray heating. When the
cloud reaches chemical equilibrium, the abundance ratios of HCN/HNC and
N2H+/HCO+ are consistent with measured values. In addition, significant
abundance of SiO is predicted in the cosmic ray dominated region of the
Galactic center. We discuss different possibilities to account for the origin
of widespread SiO emission detected from Galactic center molecular clouds.
