On the determination of temperatures of ionizing stars in H II regions

Following earlier work by Shields and Searle and by Mathis, we define a radiation softness parameter $$\eta=(\text O^+/\text O^{++})(\text S^{++}/\text S^+)$$, which we recommend as a criterion of effective temperature of the ionizing star(s) of H II regions that is relatively insensitive to chemical composition and ionization conditions found in optically observed nebulae, particularly when measurements of the nebular lines of [S III] in the far red are available. A survey of the available data for giant H II regions confirms previous suggestions that, at any given oxygen abundance, the effective temperature has an upper limit (corresponding to the youngest observed main sequence) which increases with diminishing abundance.