Search for O VI Emission from the Shocked Circurmstellar Gas of SN 1987A

The Far Ultraviolet Spectroscopic Explorer (FUSE) was used to search for broad O VI emission from the shock interaction zones produced by the collision of high-velocity supernova ejecta with the dense inner circumstellar ring of SN 1987A. Since the shock interaction with the inner ring began in 1997, broad (FWHM = 300 km/sec) emission from optical coronal lines (e.g. [Fe X], [Fe XI], and [Fe XIV]) has emerged and increased exponentially in strength. O VI 1032-1038 Angstrom emission is expected to track the coronal lines. O VI is also expected to be the primary cooling transition for the million-degree shocked gas. An accurate measurement of the O VI line strength would significantly improve current models of the shock interaction. FUSE observations of SN 1987A in 2000 and 2001 did not detect broad O VI due to spectral contamination fiom two earlytype stars within a few arc seconds of the SN. However, O VI emission was detected with narrow line widths (FWHM less than 35 km/sec) and a heliocentric radial velocity of +280 km/sec. This places the emitting gas at rest relative to the supernova and is interpreted as emission from unshocked circumstellar gas. A new FUSE observation of SN 1987A obtained in May 2007 used a narrow slit (1.25 x 20 arcsec) to significantly reduce the spectral contamination from the two early-type stars. Yet the 2007 spectrum does not reveal any significant O VI emission. The implications of these results are discussed.