SN 1987A’s Circumstellar Envelope. II. Kinematics of the Three Rings and the Diffuse Nebula

Abstract

We present several di†erent measurements of the velocities of structures within the circumstellar envelope of SN 1987A, including the inner, equatorial ring (ER), the outer rings (ORs), and the di†use nebulosity at radii [5 pc, based on CTIO 4 m and Hubble Space Telescope (HST ) data. A comparison of STIS and WFPC2 [N II] j6583 loci for the rings show that the ER is expanding in radius at 10.5^ 0.3 km s~1 (with the northern OR expanding along the line of sight at D26 km s~1, and for the southern OR, D23 km s~1). The best Ðt to CTIO 4 m echelle spectra of the [N II] j6583 line show the ORs expanding at D23 km s~1 along the line of sight. Accounting for inclination, the best Ðt to all data for the expansion in radius of both ORs is 26 km s~1. The ratio of the ER to the OR velocity is nearly equal to the ratio of the ER to the OR radius, so the rings are roughly homologous, all having been created D20,000 yr before the supernova (SN) explosion. This makes the previously reported, large com- positional di†erences between the ER and ORs difficult to understand. Additionally, a grid of long-slit 4 m echelle spectra centered on the SN shows two velocity components over a region roughly coextensive with the outer circumstellar envelope extending D5 pc (20A) from the SN. One component is blueshifted D10 km s~1 relative to the systemic velocity of the SN, while the other is redshifted by a similar amount. These features may represent a bipolar Ñow expanding from the SN, in which the ORs are propelled 10È15 km s~1 faster than that of the surrounding envelope into which they propagate. The kinetic timescale for the entire nebula is Z350,000 yr (and probably more, since material may be accu- mulating in an outer contact discontinuity). The kinematics of these di†erent structures constrain pos- sible models for the evolution of the progenitor and its formation of a mass-loss nebula.