2021ApJ...922..265L

The distribution of ejecta in young supernova remnants offers a powerful observational probe of their explosions and progenitors. Here we present a 3D reconstruction of the ejecta in SNR 0540-69.3, which is an O-rich remnant with a pulsar wind nebula located in the LMC. We use observations from the Very Large Telescope (VLT)/MUSE to study Hβ, [O III] λλ4959, 5007, Hα, [S II] λλ6717, 6731, [Ar III] λ7136, and [S III] λ9069. This is complemented by 2D spectra from VLT/X-shooter, which also cover [O II] λλ3726, 3729, and [Fe II] λ12567. We identify three main emission components: (i) clumpy rings in the inner nebula (<=1000 km s^–1) with similar morphologies in all lines; (ii) faint extended [O III] emission dominated by an irregular ring-like structure with radius ∼1600 km s^–1 and inclination ∼40°, but with maximal velocities reaching ∼3000 km s^–1; and (iii) a blob of Hα and Hβ located southeast of the pulsar at velocities ∼1500-3500 km s^–1. We analyze the geometry using a clump-finding algorithm and use the clumps in the [O III] ring to estimate an age of 1146 ± 116 yr. The observations favor an interpretation of the [O III] ring as ejecta, while the origin of the H-blob is more uncertain. An alternative explanation is that it is the blown-off envelope of a binary companion. From the detection of Balmer lines in the innermost ejecta we confirm that SNR 0540 was a Type II supernova and that hydrogen was mixed down to low velocities in the explosion.