Spatially Resolved STIS Spectroscopy of SN 1987A: Evidence for Shock Interaction with Circumstellar Gas

Visual and ultraviolet spatially resolved (~0.″1) spectra of SN 1987A obtained on days 3715 and 3743 with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope show that the high-velocity supernova debris is colliding with circumstellar gas. Very broad Lyα emission with velocities extending to ~±20,000 km s-1 originates inside the inner circumstellar ring and appears to fill most of the surface area within 0.″67 ± 0.″03 (0.14 pc at a distance of 50 kpc) of the ring's center. The observed Lyα flux from the shocked ejecta is (1.85±0.53) × 10−13 and (1.25±0.51) × 10−12 ergs cm-2 s-1 after correcting for extinction. A spatially unresolved blueshifted emission feature was discovered in Hα (and other lines) on the inner ring at position angle 31°±8°. The Hα emission extends to -250 km s-1 with no corresponding redshifted emission. This highly localized interaction appears to be the initial contact of the supernova blast wave with an inward protrusion of the inner ring. The broad Lyα emission and the "hot spot" are separate interaction phenomena associated with the reverse and forward shocks, respectively. We also find that the size of the inner ring in forbidden lines of oxygen has a dependence on ionization potential, in agreement with photoionization models of the ring.