Q(NiI)/Q(FeI) as a function of the heliocentric distance for interstellar and Solar System comets. The solid curve represents the ratio of the sublimation rates of the Ni(CO)₄ and Fe(CO)₅ carbonyls computed assuming T = 280 r_h^−1/2 for both Ni(CO)₄ and Fe(CO)₅. The dotted line represents the sublimation rate ratio assuming T = 237 r_h^−1/3 for the Ni(CO)₄ emitting region, as in Fig. 7.