def mass_3d(self, r, rho0, Ra, Rs):
        """
        mass enclosed a 3d sphere or radius r
        :param r:
        :param Ra:
        :param Rs:
        :return:
        """
        m_3d = 4 * np.pi * rho0 * Ra ** 2 * Rs ** 2 / (Rs ** 2 - Ra ** 2) * (Rs * np.arctan(r / Rs) - Ra * np.arctan(r / Ra))
        return m_3d