The Shape of SN 1987A's Circumstellar Nebula (and The Distance to the LMC)
Session 39 -- SN1993J, SN1978K and SN1987A
Display presentation, Thursday, January 13, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [39.09] The Shape of SN 1987A's Circumstellar Nebula (and The Distance to the LMC)

A.P.S. Crotts (Columbia Univ.), W.E. Kunkel (Carnegie Observatories), S.R. Heathcote (CTIO), E.L. Blanton (Columbia Univ.)

The circumstellar continuum nebula around SN 1987A, discovered in 1989, disappeared in early 1992 after evolving during the intervening period. We interpret this continuum nebulosity as the light echo from the SN maximum light pulse propagating through reflecting material. In this case the nebula appears consistent with a squat, double-lobed structure with a rotational axis inclined about $45^{\circ}$ to the line of sight (and oriented nearly north-south). A least-squares fit, in spherical coordinates, yields $R(\phi) = [1 - 0.29\phi^{2}](2.47~ly)$, for $-0.45\pi < \phi < 0.45\pi$, where $\phi = 0$ along the rotational axis. The equatorial pinch corresponds closely to the position inferred for the small elliptical ring centered on SN 1987A, assuming that it is expanding, and assuming that it is roughly circular. We investigate, by examining the morphology of the diffuse wind beyond this double-lobed nebula, how this non-spherical geometry was created. Presumably this nebula is created at the interface of the inner, blue-supergiant (BSG) wind, and the earlier, denser and slower red-giant (RG) wind. The distribution of this RG wind suggests that the anisotropy of this interface is due both to anisotropic RG wind density and velocity fields.

Panagia $et$ $al.$ 1991 suggested that the angular diameter of the inner ring compared to the light-propagation time from its front to back can be used to establish the distance to the SN, therefore the Large Magellanic Cloud (LMC). This idea was further refined by Dwek \& Felten 1992 and Gould 1993. A major uncertainty in this determination is the ring's angle of inclination. The double-lobed nebula we have mapped suggests this is close to $45^{\circ}$, indicating a distance to the LMC of about 52 kpc. (We will give a more detailed determination of $D_{_{LMC}}$ in our poster!)