Clumps and rings of ejecta in SNR 0540-69.3 as seen in 3D
  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 VLT/MUSE to
study H\beta, [O III] \lambda \lambda 4959, 5007, H\alpha, [S II] \lambda
\lambda 6717, 6731, [Ar III] \lambda 7136 and [S III] \lambda 9069. This is
complemented by 2D spectra from VLT/X-shooter, which also cover [O II] \lambda
\lambda 3726, 3729 and [Fe II] \lambda 12567. We identify three main emission
components: (i) Clumpy rings in the inner nebula (<1000 km/s) with similar
morphologies in all lines; (ii) Faint extended [O III] emission dominated by an
irregular ring-like structure with radius ~1600 km/s and inclination ~40 \dg,
but with maximal velocities reaching ~3000 km/s; and (iii) A blob of H\alpha
and H\beta located southeast of the pulsar at velocities ~1500-3500 km/s. We
analyze the geometry using a clump-finding algorithm and use the clumps in the
[O III] ring to estimate an age of 1146 \pm 116 years. 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.
