def mass_2d(self, R, Rs, rho0):
        """
        mass enclosed a 3d sphere or radius r
        :param r:
        :param Ra:
        :param Rs:
        :return:
        """
        x = R/Rs
        gx = self.g_(x)
        m_2d = 4*rho0*Rs*R**2*gx/x**2 * np.pi
        return m_2d
mass enclosed a 3d sphere or radius r
        :param r:
        :param Ra:
        :param Rs:
        :return: