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Volume 288 - Accretion Processes in Cosmic Sources (APCS2016) - ACCRETION ONTO WHITE DWARFS, NEUTRON STARS & BLACK HOLES
Flows and Shocks: Some Recent Developments in Symbiotic Star and Nova Research
J. Sokoloski,* S. Lawrence, A.P.S. Crotts, K. Mukai
*corresponding author
Full text: pdf
Pre-published on: 2018 March 05
Published on: 2018 May 16
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.
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