Flows and Shocks: Some Recent Developments in Symbiotic Star and Nova Research

How — and how efficiently — do white dwarfs (WDs) accrete and expel material? The answers to these questions have bearing on binary stellar evolution and the production of type Ia supernovae, the physics of accretion disks and jets, and our understanding of stellar eruptions as we enter the golden age of time-domain astrophysics. Optical observations have contributed crucial informa- tion about accreting WDs and nova eruptions for more than half a century. In the past decade, however, observations at the more extreme ends of the electromagnetic spectrum have driven a series of breakthroughs. X-ray and UV observations of WDs that accrete from red giants hint at the existence of a heretofore hidden population of these objects. Whereas a WD that accretes from a red giant and maintains quasi-steady shell burning on its surface is very likely to show the distinctive ‘symbiotic phenomenon’ in its optical spectrum, a similar WD without shell burning might only reveal the interaction with its companion in the UV and/or X-rays. Furthermore, these non-burning symbiotic stars may be as numerous as burning symbiotics, and afford a particularly good view of WDs that drive powerful jets. Regarding nova eruptions, the Fermi satellite showed that they are transient GeV γ-ray sources and therefore capable of particle acceleration. For novae in cataclysmic variables, this implicates internal shocks. Other signatures of shocks include ther- mal X-rays and non-thermal radio emission, and a substantial fraction of optical emission may be shock-powered in the early phase of novae. Radio (V959 Mon) and HST (V959 Mon and T Pyx) images of nova shells within a few years of their respective eruptions suggest that internal shocks commonly arise as a fast flow or wind collides with a slower flow that is concentrated in an equatorial ring. The flow structure within the ejecta and the properties of its internal shocks are also providing new constraints on the ejection mechanism for nova remnants, the origin of features in optical light curves, dust formation, and particle acceleration in dense environments. In both symbiotic stars and nova eruptions, emission from shocks enhances the degree to which multiwavelength observations probe the inflows and outflows from accreting WDs.