Star Formation Law in the Milky Way
  The Schmidt law (SF law) in the Milky Way was investigated using 3D
distribution maps of HII regions, HI and molecular (\Htwo) gases with spatial
resolutions of $\sim 1$ kpc in the Galactic plane and a few tens of pc in the
vertical direction. HII regions were shown to be distributed in a star-forming
(SF) disk with nearly constant vertical full thickness 92 pc in spatial
coincidence with the molecular gas disk. The vertically averaged volume star
formation rate (SFR) $\rho_{\rm SFR}$ in the SF disk is related to the surface
SFR $\Sigma_{\rm SFR}$ by $\rho_{\rm SFR} /[{\rm M_\odot y^{-1} kpc^{-3}}]
=9.26\times \Sigma_{\rm SFR}/[{\rm M_\odot y^{-1} kpc^{-2}}]$. The SF law
fitted by a single power law of gas density in the form of $\Sigma_{\rm SFR}
\propto \rho_{\rm SFR} \propto \rho_{\rm gas}^\alpha$ and $\propto \Sigma_{\rm
gas}^\beta$ showed indices of $\alpha=0.78 \pm 0.05$ for $\rho_{\rm H_2}$ and
$2.15 \pm 0.08$ for $\rho_{\rm total}$, and $\beta=1.14\pm 0.23$ for
$\Sigma_{\rm total}$, where $\rho$ and $\Sigma$ denote volume and surface
densities, respectively. The star formation rate is shown to be directly
related to the molecular gas, but indirectly to HI and total gas densities. The
dependence of the SF law on the gaseous phase is explained by the phase
transition theory between HI and H$_2$ gases.
