The bright O I λ11287 line observed in SN1987A is produced by the Bowen fluorescence with Lyβ and comes from regions that lie within a Sobolev length (δR ∼ 10−3RSN, the maximum distance over which fluorescence can work) from hydrogen rich gas ionized by the 56Co decay. Its strength relative to hydrogen lines (e.g. Brγ) depends on the O/H relative abundance in the ‘fluorescent region’ and on the density (i.e. the filling factor) of the gas. The observed evolution of λ11287 can be successfully understood using a relatively simple theory which takes into account the effects of transfer in the O I lines and is the generalization of the classical theory of Bowen fluorescence.

The most important result is that the time evolution of the relative intensities and profiles of O Iλ11287 and Brγ is a powerful diagnostic to determine:

1. – The filling factor of the hydrogen rich gas;

2. – The pre-SN O/H relative abundance;

3. – The amount of small scale mixing between hydrogen and oxygen rich regions and its radial stratification.

In SN1987A the results are the following:

1. –Inside 2000 km/s the hydrogen rich material is clumped with f ≃ 0.1

2. – Outside 2000 km/s the gas has f ≃ 1 and the oxygen relative abundance is quite low: O/H≃ 5 × 10−5, indicating that only the pre-SN oxygen is fluorescently coupled with hydrogen.

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