Recent observational and theoretical studies of the classical nova outburst1This review is part of a Special Issue on the 10th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas.

Abstract

Many facets of astrophysics are captured in classical novae (CNe) eruptions, making these systems unique laboratories to investigate several poorly understood processes, many in real time. CNe may also be related to the super-soft sources, the likely progenitors of SN Ia explosions. Spectra of CNe ejecta, from X-ray to infrared (IR) wavelengths, are remarkable for the changing elemental and ionic content as the ejecta temporally evolves — often exhibiting a variety of low-energy permitted lines of CNO and Fe II, high ionization lines, e.g., [Fe VIII] λ 6078 Å, and IR “coronal” lines (few 100 eV transitions) of metals such as Ne, Si, S, and Ar. At higher energies, X-ray and UV emission in CNe comes from the nuclear burning of residual accreted material on the white dwarf surface after the initial outburst; this emission directly probes processes powering the post-outburst evolution of the white dwarf and the ejecta. Some nova systems are observed to form dust, making CNe one of the known in situ stellar sources of dust grains. However, from a theoretical standpoint the interpretation of the emission line spectra, the derivation of metal abundances and estimates of ejecta mass are vexed by limited and (or) uncertain atomic and molecular data that is becoming increasingly acute, as spectral resolution and wavelength coverage expands into heretofore new observational phase space. Here we highlight IR observations of select CNe, including V2467 Cygni and V2361 Cygni, studied with the NASA Spitzer telescope and contemporaneously with ground-based optical spectroscopy as well as Swift, Chandra, and XMM-Newton spectrophotometry. We discuss new paradigms derived from photoionization models, recent issues associated with ejecta abundances derived from hydrodynamical codes simulating the nova outburst, and future challenges.