def surfacemass(self,R,log=False):
        """
        NAME:
           surfacemass
        PURPOSE:
           return the surface density profile at this R
        INPUT:
           R - Galactocentric radius (/ro)
           log - if True, return the log (default: False)
        OUTPUT:
           Sigma(R)
        HISTORY:
           2010-03-26 - Written - Bovy (NYU)
        """
        if log:
            return -R/self._params[0]
        else:
            return sc.exp(-R/self._params[0])