def mass_3d(self, r, rho0, gamma):
        """
        mass enclosed a 3d sphere or radius r
        :param r:
        :param a:
        :param s:
        :return:
        """
        mass_3d = 4 * np.pi * rho0 /(-gamma + 3) * r ** (-gamma + 3)
        return mass_3d