Stellar explosions represent the final stage of the life of stars. These can be defined as a function of the energy emitted during the outburst: we have Classical Novae, with a total energy emitted of the order of about 1e45 erg, and Supernovae whose energy emitted varies between 1e51 and 1e53 erg.
Classical Novae (CNe) are thermonuclear explosions that occur in binary-star systems in which a white dwarf (WD) accretes matter from a main sequence star or an evolved companion. The resulting expansion of the hot envelope is responsible for the initial brightness of the nova, which is 5-7 orders of magnitude above quiescence brightness and it causes the accreted mass to be ejected at high velocities (500-2000 km/s as observed in the optical range). In principle, all novae are recurrent, but the time interval between two consecutive eruptions is in general greater than human observational baseline, so for the majority of them we have only observed one outburst.
Supernovae (SNe) are the definite event that mark the end of the life of a massive star. Their ejecta is very massive (more than a solar mass for core-collapse SNe) and they are the main contributor of heavy elements in the Galaxy. Their ejecta travel at velocities up to 30,000 km/s, generating an expanding shock wave into the surrounding interstellar medium, and in turn, sweeping up an expanding shell of gas and dust, which is observed as a supernova remnant. There are two main flavours of SNe, depending on the presence or not of hydrogen in their ejecta: type I SNe are characterised by the absence of hydrogen, with type Ia likely associated with the explosion of an accreting white dwarf that reaches the Chandrasekhar mass (indeed, Classical Novae are considered as progenitor of type Ia SNe); type II SNe show hydrogen in their ejecta and they result from the collapse of the core of a massive star.
After hundreds of years, the ejected material will result in a structure that reflects the main physical properties of the explosion mechanism. Their sizes depends on the distance to us, but in the Galaxy there are few dozens of nova and SN remnants with bright extended remnants easily accessible from Calar Alto. For many of these objects, also infrared, radio and X-rays observations are available, but no IFU hyper-spectral data cube observations have been obtained and published until now. A CASS IFU survey of these remnants, whose extension is larger (more than tens arcmin-square) than current IFU operating at ground-based telescopes, represents one of our science programs.
With the IFU data of nova and SN remnants we will obtain information on the 3D structure of the remnants: the imaging IFU capabilities providing the typical spatial two-dimensions while the spectroscopic one allowing the computation of the third dimension thanks to the ‘frozen’ velocities of the ejected material at these phases once the distance is known. Compared to previous space- and ground-based archival images, the spatial resolution provided by IFU instruments will give important informations on the evolution of the ejected shells, which is also fundamental for a refined determination of the geometry and the distance of the nova.
Our new 3D data will give us key information on the following:
- total mass ejected using observed calibrated fluxes of hydrogen lines, and a distribution of the mass-geometry of the ejecta as a function of their speed (the t2 parameter, in case of novae),
- distribution of the gas densities and temperatures from emission line diagnostics,
- the existence of gradient abundances across the remnant using forbidden lines such as [OIII], [NII], and emission line ratios, and how these gradient are distributed within the observed structures (e.g. if there is an enhancement across equatorial rings, polar caps and/or single knots),
- the role of possible shock excitation originating in the interaction of the ejecta with material expelled in a previous outburst, which can be identified by the contemporaneous presence of high gas temperatures and [OIII] line strengths are larger than expected,
- the inclination of the orbital plane and the role of the accretion disk around the central remnant (WD or NS) in illuminating the nebula.
Finally, we will develop a code for the 3D reconstruction of the explosion from the final results of the analysis making use of an open source suite for the modelling of 3D structures. The gas density distribution and the total ejected mass inferred from the IFU data analysis will help in refining the exact value of a different elements’ mass ejected in an outburst, and give a more accurate estimate for the respective yields of the galactic nova and SN population. As an example, the lithium yield obtained from a single nova outburst can be corrected for the observed gas distribution refining the role of novae in explaining the overabundance of lithium observed in young star populations, although direct detection of lithium has been also reported in spectroscopic observations of nova remnants. The same approach can be adopted for CNO isotopes, although we still lack a direct signature of these elements in novae.
With this wide collection of diagnostic tools we will constrain the range of physical parameters for a large sample of stellar outbursts, providing the first accurate catalog to date of gradient abundances, geometrical distributions and mass ejected inferred from IFU observations. More details on the science program and target selection will follow.

Four of our most representative IFU targets: NGC 6888 WR nebula (top left), Cassiopea A SNR (top right), Crab nebula SNR (bottom right), GK Persei nova (bottom left).