Theory predicts that the temperature of the X-ray-emitting gas (∼10 6 K) detected from planetary nebulae (PNe) is a consequence of mixing or thermal conduction when in contact with the ionized outer rim (∼10 4 K). Gas at intermediate temperatures (∼10 5 K) can be used to study the physics of the production of X-ray-emitting gas, via C , N and O ions. Here we model the stellar atmosphere of the CSPN of NGC 1501 to demonstrate that even this hot H-deficient [WO4]-type star cannot produce these emission lines by photoionization. We use the detection of the C lines to assess the physical properties of the mixing region in this PNe in comparison with its X-ray-emitting gas, rendering NGC 1501 only the second PNe with such characterization. We extend our predictions to the hottest [WO1] and cooler [WC5] spectral types and demonstrate that most energetic photons are absorbed in the dense winds of [WR] CSPN and highly ionized species can be used to study the physics behind the production of hot bubbles in PNe. We found that the UV observations of NGC 2452, NGC 6751 and NGC 6905 are consistent with the presence mixing layers and hot bubbles, providing excellent candidates for future X-ray observations.

We present GTC MEGARA high-dispersion integral field spectroscopic observations of the nova remnant QU Vul, which provide a comprehensive 3D view of this nova shell. The tomographic analysis of the H𝛼 emission reveals a complex physical structure characterized by an inhomogeneous and clumpy distribution of the material within this shell. The overall structure can be described as a prolate ellipsoid with an axial ratio of 1.4±0.2, a major axis inclination with the line of sight of 12 • ± 6 • , and polar and equatorial expansion velocities ≈560 km s −1 and 400±60 km s −1 , respectively. The comparison of the expansion velocity on the plane of the sky with the angular expansion implies a distance of 1.43±0.23 kpc. The ionized mass is found to be ≈ 2 × 10 −4 M , noting that the information on the 3D distribution of material within the nova shell has allowed us to reduce the uncertainty on its filling factor. The nova shell is still in its free expansion phase, which can be expected as the ejecta mass is much larger than the swept-up circumstellar medium mass. The 3D distribution and radial velocity of material within the nova shell provide an interpretation of the so-called "castellated" line profiles observed in early optical spectra of nova shells, which can be attributed to knots and clumps moving radially along different directions.