Tracing the Propagation of Shocks in the Equatorial Ring of SN 1987A Over Decades with the Hubble Space Telescope

TL;DR

This study uses Hubble Space Telescope data from 1994 to 2022 to analyze shock interactions in SN 1987A's equatorial ring, revealing hotspot evolution, velocities, and structure, providing insights into shock dynamics and circumstellar medium properties.

Contribution

It presents a detailed, multi-decade analysis of hotspot propagation and shock interaction in SN 1987A's equatorial ring, introducing a model of dense substructures affecting shock behavior.

Findings

26 hotspots identified with evolving velocities

Deceleration phase observed around day 8000

Hotspot structures suggest dense substructures with specific filling factors

Abstract

The nearby SN 1987A offers a unique opportunity to investigate the complex shock interaction between the ejecta and circumstellar medium. We track the evolution of the optical hotspots within the Equatorial Ring (ER) by analyzing 33 Hubble Space Telescope imaging observations between 1994 and 2022. By fitting the ER with an elliptical model, we determine its inclination to be $ 42.85 \pm 0.50^{\circ}$ with its major axis oriented $ -6.24 \pm 0.31^{\circ}$ from the west. We identify 26 distinct hotspots across the ER, with additional ones emerging over time, particularly on the west side. The hotspots initially show high velocities ranging from $390$ to $1660 \ \rm km \ s^{-1}$, followed by a deceleration phase around day $\sim 8000$. Subsequent velocities vary from $40$ to $660 \ \rm km \ s^{-1}$. The light curves of the hotspots reach maxima between 7000 and 9000 days, suggesting a connection with the deceleration. Many spots are spatially resolved and show elongation perpendicular to the direction of motion, indicative of a short cooling time. To explain these results, we propose that each hotspot comprises dense substructures embedded in less dense gas. The initial velocities are then phase velocities, the break occurs when the blast wave leaves the ER, while the late velocities reflect the propagation of radiative shocks in the dense substructures. We estimate that the dense substructures have a volumetric filling factor of $\sim0.3 \left( n_{\mathrm{e}}/10^{6}\ \mathrm{cm^{-3}} \right)^{-2} \%$ and a total mass of $\sim0.24 \left(n_{\mathrm{e}}/10^{6}\ \mathrm{cm^{-3}} \right)^{-1}\times10^{-2}\ \mathrm{M_{\odot}}$.