code: def sigma2(self,R,log=False):
          
          if log:
              return 2.*sc.log(self._params[2])-2.*(R-1.)/self._params[1]
          else:
              return self._params[2]**2.*sc.exp(-2.*(R-1.)/self._params[1])
docs: NAME:
             sigma2
          PURPOSE:
             return the radial velocity variance at this R
          INPUT:
             R - Galactocentric radius (/ro)
             log - if True, return the log (default: False)
          OUTPUT:
             sigma^2(R)
          HISTORY:
             2010-03-26 - Written - Bovy (NYU)