The 3-D structure of SN 1987A's inner ejecta

Abstract

Context. Observing the inner ejecta of a supernova is possible only in a handful of nearby supernova remnants. The core-collapse explosion mechanism has been extensively explored in recent models and predict large asymmetries. SN 1987A is the first modern stellar explosion that has been continuously observed from its beginning to the supernova remnant phase. Twenty years after the explosion, we are now able to observe the three-dimensional spatially resolved inner ejecta of this supernova. Aims. Detailed mapping of newly synthesised material and its radioactive decay daughter products sheds light on the explosion mechanism. This may reveal the geometry of the explosion and its connection to the equatorial ring and the outer rings around SN 1987A. Methods. We have used integral field spectroscopy to image the supernova ejecta and the equatorial ring in the emission lines of [Si I] + [Fe II] (λ1.64 μm) and He I (λ2.058 μm). The spectral information can be mapped into a radial velocity image revealing the expansion of the ejecta both as projected onto the sky and perpendicular to the sky plane. Results. The inner ejecta are spatially resolved in a North-South direction and are clearly asymmetric. Like the ring emission, the northern parts of the ejecta are blueshifted, while the material projected to the South of the supernova centre is moving away from us. We argue that the bulk of the ejecta is situated in the same plane as defined by the equatorial ring and does not form a bipolar structure as has been suggested. The exact shape of the ejecta is modelled and we find that an elongated triaxial ellipsoid fits the observations best. The velocity measured in the [Si I] + [Fe II] line corresponds to ∼3000 km s −1 and is the same as the width of the IR [Fe II] line profiles during the first years. From our spectral analyses of the ejecta spectrum we find that most of the He I, [Si I] and [Fe I-II] emission originates in the core material which has undergone explosive nucleosynthesis. The He I emission may be the result of α-rich freeze-out if the positron energy is deposited locally. Conclusions. Our observations clearly indicate a non-symmetric explosion mechanism for SN 1987A. The elongation and velocity asymmetries point towards a large-scale spatial non-spherical distribution as predicted in recent explosion models. The orientation of the ejecta in the plane of the equatorial ring argues against a jet-induced explosion through the poles due to stellar rotation.

Matter Mixing in Aspherical Core-collapse Supernovae: Three-dimensional Simulations with Single-star and Binary Merger Progenitor Models for SN 1987A

“…As mentioned in Section 1, the 3D morphology of the inner ejecta of SN 1987A is globally elliptical/elongated (Kjaer et al 2010;Larsson et al 2013Larsson et al , 2016. The distributions of the observed [Fe II] lines in the Doppler velocity are biased toward the redshifted side.…”

“…In this paper, we assume the form of the initial radial velocities as v r ∝ r f (θ). Since observations of SN 1987A have shown that the inner ejecta is globally elliptical (Kjaer et al 2010;Larsson et al 2013Larsson et al , 2016, bipolar-like explosions may be justified. Then, the following four cases for the shape of f (θ) (0 ≤ θ ≤ π) are considered.…”

“…Recent observations of emission lines of [Si I] + [Fe II] and He I (1.644 µm and 2.058 µm, respectively) by HST and VLT (Kjaer et al 2010;Larsson et al 2013Larsson et al , 2016 have revealed that the three-dimensional (3D) morphology of the inner ejecta of SN 1987A is globally elliptical/elongated (the ratio of the major to minor axes of the inner ejecta is 1.8 ± 0.17 (Kjaer et al 2010). It has been known that in the nebula around SN 1987A there is a triple ring structure consisting of an inner equatorial ring (ER) and two outer rings (ORs) and the configurations with respect to the Earth have been deduced (e.g., Tziamtzis et al 2011;Sugerman et al 2005a).…”

The Matter Beyond the Ring: The Recent Evolution of SN 1987A Observed by the Hubble Space Telescope

“…The distances between the SN and the centers of the NOR and SOR were set to 1.3 and 1.0 × 10 18 cm, respectively, consistent with the range of distances to the ORs reported by Tziamtzis et al (2011). We also apply a small rotation of the system around the north-south axis in order to match the 7 • tilt of the major axis of the ER (obtained from the fits in Section 6, see also Kjaer et al 2010) and the similar observed tilts of the ORs. Finally, we shift the centers of the NOR and SOR to the west by 5.5 and 4.0 × 10 16 cm, respectively, which results in a good agreement between the projected model and the images.…”

“…The fact that molecules and dust are observed in the inner ejecta also shows that these regions are not significantly affected by the X-rays (Matsuura et al 2015;Fransson et al 2016;Abellán et al 2017;Larsson et al 2019;Cigan et al 2019). The 3D emissivities of a number of atomic and molecular emission lines from the ejecta have been determined using a combination of imaging and spectra, revealing a highly asymmetric distribution (Kjaer et al 2010;Larsson et al 2013Larsson et al , 2016Abellán et al 2017;Larsson et al 2019). Several different early observations also provided strong evidence for an asym- metric explosion, including polarimetry (Schwarz 1987;Jeffery 1991), the "mystery spots" (Nisenson et al 1987;Meikle et al 1987;Nisenson & Papaliolios 1999) and the "Bochum event" (Hanuschik & Thimm 1990).…”