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Abstract
We present ultraviolet and optical spectra of the first bright spot (P.A. = 29°) on SN 1987A's equatorial circumstellar ring taken with the Space Telescope Imaging Spectrograph. We interpret this spot as the emission produced by radiative shocks that occur where the supernova blast wave strikes an inward protrusion of the ring. The observed line widths and intensity ratios indicate the presence of radiative shocks with velocities ranging from 100 to 250 km s-1 entering dense (104 cm-3) gas. These observations, and future observations of the development of the spectra and line profiles, provide a unique opportunity to study the hydrodynamics of radiative shocks.
Highlights
After the discovery of the circumstellar ring around SN 1987A, several authors pointed out that the supernova blast wave should strike the ring some 10–20 yr after the supernova explosion (Luo & McCray 1991; Luo, McCray, & Slavin 1994; Chevalier & Dwarkadas 1995; Borkowski, Blondin, & McCray 1997b)
A shock interpretation for these spots was suggested by Space Telescope Imaging Spectrograph (STIS) observations of the ring taken in 1997 (Sonneborn et al 1998)
The gas in the photoionization zone will have the same velocity as the shock front, and the optical lines would be expected to have velocity profiles representative of the hydrodynamics of the shock. The presence of both redshifted and blueshifted emission can be explained by shocks traveling both into and out of our line of sight. It will not be an easy task to develop a quantitative model for the spectrum and line profiles of the hot spot
Summary
After the discovery of the circumstellar ring around SN 1987A, several authors pointed out that the supernova blast wave should strike the ring some 10–20 yr after the supernova explosion (Luo & McCray 1991; Luo, McCray, & Slavin 1994; Chevalier & Dwarkadas 1995; Borkowski, Blondin, & McCray 1997b). The estimated time of first contact was uncertain, mainly because it depended on the unknown density of circumstellar gas between the supernova and the ring. In 1997 an unresolved brightening spot was detected at position angle ∼29Њ (spot 1) on the near (north) side of the ring (Pun et al 1997; Garnavich, Kirshner, & Challis 1997). Lawrence et al (2000) trace the appearance of this spot back to 1995 This spot continues to brighten, and within the last year several new spots have appeared (Lawrence & Crotts 2000; Garnavich, Kirshner, & Challis 2000; Lawrence et al 2000).
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