Unveiling the 3D structure of nova shells with MUSE: The case of RR Pic
Authors: Celedón L. , Schmidtobreick L. , Tappert C. , Selman F. 
Published: November 2023

Context. Nova eruptions occur in cataclysmic variables when enough material has been accreted onto the surface of the white dwarf primary. As a consequence, the material that has been accumulated until then is expelled into the interstellar medium, forming an expanding nova shell around the system. Understanding the physical process that shapes the morphology of nova shells is essential to fully comprehend how the ejection mechanism operates during nova eruptions. Because of its closeness and age, the nova shell around the classical nova RR Pic (Nova Pic 1925) is an ideal target for studying the evolving morphology of nova shells.
Aims. The use of integral field spectroscopy (IFS) is a technique that has received little attention in the study of nova shells, despite the advantages in using it when studying the morphology and kinematics of nova shells. In this work, we present an IFS study of the RR Pic nova shell, with a particular emphasis on the extraction of the 3D morphology of the shell.
Methods. The nova shell was observed by the Multi-Unit Spectroscopic Explorer (MUSE) instrument placed at the ESO-VLT. By measuring the extension of the nova shell in these new observations, and comparing it against previous measurements, we were able to determine the expansion history of the ejected material. We used this information, together with the distance to the system based on Gaia EDR3 parallaxes, and the systemic velocity of the system reported in the literature to obtain the physical 3D view of the shell.
Results. The MUSE datacube confirms the presence of the nova shell in Hα, Hβ and [OIII], and very faintly in [NII]. A comparison with previous observations suggests that the shell continues in its free-expansion phase but with the different parts of the shell apparently expanding at different rates. The data analysis corroborates the previous vision that the shell is composed of an equatorial ring and polar filaments traced by Hα. At the same time, the new data also reveal that [OIII] is confined in gaps located in the tropical regions of the shell where no Hydrogen is observed. The flux measurements indicate that ~99% of the shell flux is confined to the equatorial ring, while the polar filaments show a flux asymmetry between the NE and SW filaments, with the latter being ~2.5 times brighter. We have estimated the mass of the shell to be ~5 × 10−5 M⊙. From the analysis of the 3D-extracted data, we determine that the ring structure extends ~8000 au from the central binary, and has a position angle of ~155 deg and an inclination of ~74 deg. The analysis of the equatorial ring reveals it is composed of a main ring and several small clouds, extending up to a height of ~4000 au above and below the main plane of the equatorial ring. The radial profile of the whole ring structure is reminiscent of a bow shock.
Conclusions. Our data have proven the capabilities of observing nova shells using IFS, and how the nova shell around RR Pic is an interesting object of study. Further and continuous observations of the shell across the electromagnetic spectrum are required to confirm the results and ideas presented in this work.