Asymmetry in the Wind from the Precursor to SN 1987A

Abstract

This work has been accepted for publication by Nature (ALLEN et al. [1]). The initial UV flash from the surface of SN 1987A ionized the circumstellar gas to produce narrow emission lines which were seen first in the UV (IUE) (WAMSTEKER et al. [2]) and later in the optical spectra (WAMPLER [3]). Since May 1988 we have recorded a narrow component to the 1083 nm He I line. Measurements were obtained on six occasions between days 434 and 737 during which time the narrow component exhibited little evidence for variation, remaining at an intensity of 15 ± 2 x 10^-13 erg/s/cm² (see Fig. 1.) On day 737 we explored the spatial distribution of the narrow component (Refs [1],[4]). We set the grating so that one of the detectors in the spectrometer (FIGS) received radiation from the narrow component while several straddled the broad component. By repeated raster scanning of the telescope we thus created simultaneous maps of both the broad and narrow components. While the broad component, due to the supernova itself, originated in an unresolved point source, approximately half the total intensity of the narrow component arose in a compact source, of diameter less than ~ 1.5 arcsec, displaced by 0.8±0.3 arcsec at PA 200±15 deg (see Fig 2). Geometrical and other considerations demonstrate that the source lay behind the supernova, at a distance of about one light year.