Interacting binaries and their transients

 
A large fraction of stars is not isolated, but in binaries or multiple systems. While many of those might evolve as isolated stars, a number of them is expected to interact during their lifetime. These binary systems can share their photosphere, exchange mass, produce a variety of transient phenomena which are either destructive for the system or not (accretion instabilities, nova outbursts, common envelope formation, few types of supernovae). Blue and red stragglers, rapidly rotating yellow giants, and stars with anomalous abundances are believed to have experienced some type of interaction in their past life. Cataclysmic variables, low and high mass X-ray binaries, and accreting millisecond pulsars, are interacting at present.

This is the realm of time domain astronomy. Observations with slow (week to months) or high (hours to seconds or millisecond) cadence are needed to characterize transients or the stars orbiting around their common center of mass.
This is also the realm of multi wavelength spectroscopic observations at the highest possible spectral resolution. Multi wavelength because most of the systems emit across the whole electromagnetic spectrum (from the gamma and X-ray to the infrared), and, therefore, observing across large wavelength ranges allows characterizing the various energy sources and their effects. High spectral resolution because it allows separating the different emission components and determining the dynamics of the observed structures.

We are currently after non-degenerate star mergers (i.e. the common envelope on the making), the aftermath of nova outbursts (what happens to the accreting star after it explosively ejected matter into the circumstellar space?), evidences that rapidly rotating cool giants might result from stellar mergers, and the physical characterization of the filaments in novae ejecta, including their possible interaction with the interstellar material. All these require new high quality series of multi-band spectra at R≥10000.
We also dig into data archives to assess mass accretion rate, spectral energy distribution and the state of the donor stars in cataclysmic variables. 

 
 
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