AAS Meeting #193 - Austin, Texas, January 1999
Session 47. Supernovae
Display, Thursday, January 7, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[47.02] IUE Observations of the Light Echo of SN 1987A

S.S. Lawrence, A.P.S. Crotts (Columbia), R. Gilmozzi (ESO-Garching)

Supernova 1987A was discovered in the optical roughly one day after core collapse, and the first ultraviolet observations were taken 1.6~days after the explosion. This was too late to observe directly the far-UV flux at the time of shock breakout through the progenitor's atmosphere: both the total flux and effective photospheric temperature dropped very rapidly in the first day. Such observations would have provided strong constraints on models of the progenitor star and early development of the explosion. This far-UV flux is still available to us, however, reflected off interstellar dust clouds in the form of the light echos.

We have used the International Ultraviolet Explorer (IUE) to observe the reflected spectrum of SN~1987A in the vacuum ultraviolet at the time of shock breakout. A bright patch of reflection nebulosity was identified in the optical late in 1994, and observed deeply with IUE in January~1995. The same location was then observed one year later, after the light echo had passed. A subtraction of the two spectra allowed us to remove the ``background'' of scattered UV photons from surrounding hot stars in the LMC. The resulting SN spectrum was then de-reddened using an empirical UV~extinction curve derived from the adjacent Star~#2. The reflected spectrum of SN~1987A from 1300--1750Å\ is well-fit with a power law of the form f\lambda \propto \lambda-7.5. This is consistent with emission from an extremely hot source in the Rayleigh-Jeans limit (f\lambda \propto \lambda-4) modified by Rayleigh scattering off from small particles (f\lambda \propto \lambda-4).

In addition to details of the observations and reductions, we will present comparisons between the far-UV and optical data for this echo location. The supernova itself was extremely well-observed throughout its optical maximum (~83~d after core collapse), and the echo has also been well-sampled. Combining the known reflecting geometry and SN spectrum, the optical reflecting properties of the dust can be strongly constrained. Applying these dust properties to the IUE spectrum, we can make a reasonable extraction of the far-UV spectrum of the SN at shock breakout. We will also compare our IUE results with data from the Ultraviolet Imaging Telescope.

The author(s) of this abstract have provided an email address for comments about the abstract: lawrence@astro.columbia.edu

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