Paper detail

The outer rings of SN1987A

We used low resolution spectroscopy from VLT/FORS1, and high resolution spectra from VLT/UVES, to estimate the physical conditions in the ORs, using nebular analysis for emission lines such as [O II], [O III], [N II], and [S II]. We also measured the velocity at two positions of the ORs to test a geometrical model for the rings. Additionally, we used data from the HST science archives to check the evolution of the ORs of SN 1987A for a period that covers almost 11 years. We measured the flux in four different regions, two for each outer ring. We chose regions away from the two bright foreground stars, and as far as possible from the ER, and we created light curves for the emission lines of [O III], Hα, and [N II]. The profiles of the lightcurves display a declining behaviour, which is consistent with the initial supernova flash powering of the ORs. The electron density of the emitting gas in the ORs, as estimated through nebular analysis from forbidden [O II] and [S II] lines, is <= 3*10^3 cm^-3, has not changed over the last ~ 15 years, and also the [N II] temperature remains fairly constant at ~ 1.2*10^4 K. We find no obvious difference in density and temperature for the two ORs. The highest density, as measured from the decay of Hα, could, however, be ~ 5*10^3 cm^-3, and since the decay is somewhat faster for the southern outer ring than for the northern, the highest density in the ORs may be found in the southern outer ring. For an assumed distance of 50 kpc to the supernova, the distance between the supernova and the closest parts of the ORs could be as short as ~ 1.7*10^18 cm. Interaction between the supernova ejecta and the outer rings could therefore start in less than ~ 20 years. We do not expect the ORs to show the same optical display as the equatorial ring when this happens.