Understanding cosmic jet engines
Most theorists believe that a single mechanism is responsible for ejecting jets, potentially related to the accretion of gas onto accreting stars including stellar-mass black holes. The gas is rapidly accelerated and ejected from circumstellar accretion disks in a clumpy knotted cloud. Symbiotic stars (SySs), a type of binary star system, are an excellent case study, a ‘Rosetta stone’ as it were, for decoding the mechanisms of jet formation. They provide a combination of observational data of high spatial resolution and kinematic data from regions as near as possible to the jet source. In addition, SySs are closely related to cataclysmic variable stars (CVs), binary star systems consisting of a white dwarf and a normal star companion. The white dwarf is very dense, with enormous gravitational energy much like a black hole. It accretes material from the companion star and, in the process, forms an accretion disk around itself. Although the short time scales associated with CVs make them the best understood accreting systems, no jet emission has yet been detected from these objects. European researchers seeking information about jet formation from the study of a SyS initiated the ‘Jets in symbiotic stars - the Rosetta stone for jet formation’ (Symbiojets) project. The focus was on available data concerning the R Aquarii (R Aqr) symbiotic variable star in the constellation of Aquarius, the closest star of its type to Earth. Data from the Hubble Space Telescope consisting of images at different times will be used to infer the widths of different jet knots and, subsequently, to determine the radius of the part of the underlying accretion disk from which the jets have been ejected. Scientists will then develop jet formation models with the goal of elucidating mechanisms that may in fact be common to all astrophysical objects associated with these cosmic jet engines that most likely play an integral role in the structure and evolution of the Universe.