The morphology of the ejecta of SN 1987A at 31 years from 1150 to 10000 {\AA}

TL;DR

This study presents multi-epoch ultraviolet to near-infrared spectroscopy of SN 1987A's ejecta at 31 years, revealing detailed morphology, line flux evolution, and insights into excitation mechanisms and ejecta interactions.

Contribution

First comprehensive UV-to-NIR spectral mapping of SN 1987A's ejecta at this epoch, analyzing morphology, flux evolution, and excitation sources with new velocity maps and emission line studies.

Findings

Mg II is blueshifted by ~1000 km/s and brighter in the northwest.

Mg II flux increased ninefold since 1999, unlike other lines.

Ultraviolet Mg II lines are mainly excited by X-ray heating, while infrared lines are driven by Lyα fluorescence.

Abstract

We present spectroscopy of the ejecta of SN 1987A in 2017 and 2018 from the Hubble Space Telescope and the Very Large Telescope, covering the wavelength range between $1150$ and $10000$ {\AA}. At 31 years, this is the first epoch with coverage over the ultraviolet-to-near-infrared range since 1995. We create velocity maps of the ejecta in the H$\alpha$, Mg II $\lambda\lambda2796,2804$ and [O I] $\lambda\lambda6302,6366$ (vacuum) emission lines and study their morphology. All three lines have a similar morphology, but Mg II is blueshifted by $\sim$1000 km s$^{-1}$ relative to the others and stronger in the northwest. We also study the evolution of the line fluxes, finding a brightening by a factor of $\sim$9 since 1999 in Mg II, while the other line fluxes are similar in 1999 and 2018. We discuss implications for the power sources of emission lines at late times: thermal excitation due to heating by the X-rays from the ejecta-ring interaction is found to dominate the ultraviolet Mg II lines, while the infrared Mg II doublet is powered mainly by Ly$\alpha$ fluorescence. The X-ray deposition is calculated based on merger models of SN 1987A. Far-ultraviolet emission lines of H$_2$ are not detected. Finally, we examine the combined spectrum of recently-discovered hotspots outside the equatorial ring. Their unresolved Balmer emission lines close to zero velocity are consistent with the interaction of fast ejecta and a clumpy, slowly moving outflow. A clump of emission in this spectrum, south of the equatorial ring at $\sim$1500 km s$^{-1}$, is likely associated with the reverse shock.