Measurement and interpretation of deuterium-line emission in the Orion Nebula

We present new observations of the deuterium and hydrogen Balmer lines in t he Orion Nebula. There is a real variation in the deuterium-to-hydrogen lin e ratios across the nebula, being greatest in the emission from the largest proplyd (Orion 244-440). We also present the results of a detailed model f or the emission of these lines, the hydrogen lines being the result of phot oionization and recombination while the deuterium lines are produced by flu orescent excitation of the upper energy states by the far-UV radiation from theta (1) Ori C. Comparison of the observations and predictions of the lin e intensities shows good agreement, both in the strength of the reference l ines at Hb and also in the differences of the Balmer decrement for the two atoms. The fact that both the deuterium and hydrogen emissions arise from m echanisms that count the near-ultraviolet (deuterium) and photoionizing ult raviolet (hydrogen) photons from the dominant star means that there is litt le prospect of similar observations being useful for determination of D/H a bundances in H II regions.