The morphology of the ejecta of SN 1987A at 31 years from 1150 to 10000 Å

Abstract

<p><br></p><p>We present spectroscopy of the ejecta of SN 1987A in 2017 and 2018 from the <em>Hubble Space Telescope</em> and the Very Large Telescope, covering the wavelength range between 1150 and 10 000 Å. 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 Hα, Mg II λλ2796, 2804 and [O I] λλ6302, 6366 (vacuum) emission lines and study their morphology. All three lines have a similar morphology, but Mg II is blueshifted by ∼1000 km s⁻¹ relative to the others and stronger in the north-west. We also study the evolution of the line fluxes, finding a brightening by a factor of ∼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 Lyα fluorescence. The X-ray deposition is calculated based on merger models of SN 1987A. Far-ultraviolet emission lines of H₂ 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 ∼1500 km s⁻¹, is likely associated with the reverse shock.<br></p>