Asymmetric envelope expansion of supernova 1987A
Citations Over TimeTop 10% of 1989 papers
Abstract
Analytic models are presented for the spherically symmetric expansion of a supernova envelope like that thought to be present in SN 1987A. The expanded density profile has an outer steep power-law region with p is proportional to r^-9.6^ and an inner, relatively flat region. For SN 1987A, the bend in the profile occurs at a velocity of ~ 4000 km s^-1^. A self-similar solution shows that the outer density profile of an exploded massive star is generally of a power-law form, with index 8.7 or steeper. Polarization observations show that the envelope expansion of SN 1987A is asymmetric, so we model an asymmetric initial density distribution or energy deposition on the assumption of radial flow. A linearized perturbation treatment of a blast wave shows that the nonradial flow has little effect while the shock front is in the stellar envelope. Numerical calculations indicate that there is some tendency for the flow to become more spherical during the expansion phase. Overall, our calculations show that the final degree of asymmetry is comparable in magnitude to the initial asymmetry. For an initial envelope structure like that appropriate to SN 1987A, an asymmetry in the core energy deposition does not damp out in propagating through a spherical envelope; this is not true for an initial envelope with a flat density distribution. Both our analytic and numerical models show that the sense of density asymmetry changes across the bend in the density profile. We believe that this accounts for the different polarization angles observed in the Hα and Ca II lines by Cropper et al. in 1987 June. Our models do not clearly demonstrate a cause for the asymmetry. If the progenitor envelope was rotationally flattened, angular momentum transfer from a binary companion during the stellar evolution is probably necessary. An asymmetric initial explosion is another possibility.