The outer rings of SN 1987A^⋆

¹ Stockholm Observatory, AlbaNova Science CenterDepartment of Astronomy, 106 91 Stockholm, Sweden
e-mail: anestis@astro.su.se
² Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 12 August 2010
Accepted: 2 November 2010

Abstract

Aims. We investigate the physical properties and structure of the outer rings of SN 1987A to understand their formation and evolution.

Methods. We used low resolution spectroscopy from VLT/FORS1 and high resolution spectra from VLT/UVES to estimate the physical conditions in the outer rings, 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 outer rings to test a geometrical model for the rings. Additionally, we used data from the HST science archives to check the evolution of the outer rings of SN 1987A for a period that covers almost 11 years.

Results. We measured the flux in four different regions, two for each outer ring. We chose regions away from the two bright neighbouring stars and as far as possible from the inner ring and created light curves for the emission lines of [O III], Hα, and [N II]. The light curves display a declining behaviour, which is consistent with the initial supernova-flash powering of the outer rings. The electron density of the emitting gas in the outer rings, as estimated by nebular analysis from the [O II] and [S II] lines, is  ≲ 3 × 10³ cm^-3, has not changed over the last  ~15 years, and the [N II] temperature remains also fairly constant at  ~1.2 × 10⁴ K. We find no obvious difference in density and temperature for the two outer rings. The highest density, as estimated from the decay of Hα, could be  ~5 × 10³ cm^-3 however, and because the decay is somewhat faster in the southern outer ring than it is in the northern, the highest density in the outer rings 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 outer rings could be as short as  ~1.7 × 10¹⁸ cm. Interaction between the supernova ejecta and the outer rings could therefore start in less than  ~20 years. We do not expect the outer rings to show the same optical display as the equatorial ring when this happens. Instead soft X-rays should provide a better way of observing the ejecta - outer rings interaction.