Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...276..415o&link_type=abstract
Astronomy and Astrophysics, Vol.276, NO. 2/SEPII, P. 415, 1993
Astronomy and Astrophysics
Astrophysics
2
Scientific paper
This paper is aimed at modeling the strong OI λ11287 line observed in SN 1987A and deriving information concerning the physical conditions of the regions emitting it. To do this it is necessary to reconsider the O I-Lyβ fluorescence mechanism and include the effects of transfer in λ1304, λ8446 and λ11287, the oxygen lines produced by this process. The observed, slow time evolution of I(λ11287)/I(Hα) cannot be understood using the classical theory where λ11287 is optically thin and is the most direct consequence of the fact that the OI line is quite opaque up to ~400 days. These large optical depths are possible only if the emitting gas is in dense regions with small filling factors; this provides a further evidence that the gas in the envelope of SN 1987A is clumped. An interesting result is that regions with different filling factors must coexist in the envelope, this being a consequence of the fact that the O I emission from a single gas component is expected to last for a time much shorter than observed. Even more intriguing is the interpretation of the profile of λ11287 which, with FWHM ~ 2000 km/s, is much narrower than any other line from the ejecta of SN 1987A. To cope with this fact it is necessary to assume that inside v = 2000 km/s the emitting gas is dense (predominantly f ~ 0.1) and oxygen rich with O/H ~ 1 10^-3^, whilst the regions outside must have f=1 and O/H ~ 5 10^-5^. This picture indicates that the pre-SN oxygen abundance was quite (but not unreasonably) low and that the mechanism which compressed the hydrogen rich regions - probably the inflation of the nickel clumps described by Li et al. (1993) - also induced some mixing between oxygen and hydrogen, with ~10% of the ~1M_sun_ of freshly synthesized oxygen ending up `fluorescently coupled' (i.e. mixed on scales comparable to a Sobolev length δR/R ~ 10^-3^) with the hydrogen rich gas. The presence of unmixed, uniform regions outside 2000 km/s can be understood if the fragments of the nickel core are originally distributed within this radius and overshoot in, but do not compress the outer gas when they inflate. As a by-product of the analysis of IR data it is found that the S/Si abundance ratio is consistent, within fairly large errors, with that predicted by nucleosynthesis models. Other by- products of this work are a detailed analysis of the O I-Lyβ fluorescence mechanism with predictions of line ratios which could be used to distinguish between Lyβ and continuum pumping, analytical relationships applicable to other astrophysical conditions including active galaxy nuclei, and a warning against using the λ1304/λ8446/λ11287 ratios to derive the reddening. Two observational problems, namely the fact that OI λ8446 has never been seen and that Hα was systematically to faint in SN 1987A, are also reconsidered. The first can be quantitatively in terms of absorption in the blue wing of the very opaque Ca II triplet while, for the second, absorption by Paschen b-f in the densest hydrogen clumps is a plausible, but not definite answer.
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