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Embracing a Rejected Star

Zeta Ophiuchi is a star with a complicated past, having likely been ejected from its birthplace by a powerful stellar explosion. A new look by NASA’s Chandra X-ray Observatory helps tell more of the story of this runaway star.

Located about 440 light-years from Earth, Zeta Ophiuchi is a hot star that is 20 times more massive than the Sun. Previous observations have provided evidence that Zeta Ophiuchi was once in close orbit with another star, before being ejected when this companion was destroyed in a supernova explosion. Infrared data from NASA’s Spitzer Space Telescope, seen in this new composite image, reveal a spectacular shock wave (red and green) that was formed by matter blowing away from the star’s surface and slamming into gas in its path. Data from Chandra show a bubble of X-ray emission (blue) located around the star, produced by gas that has been heated by the shock wave to tens of millions of degrees.

A team of astronomers led by Sam Green from the Dublin Institute for Advanced Studies in Ireland has constructed the first detailed computer models of the shock wave. The team led by Green, and also including Dr Jonathan Mackey and Dr Patrick Kavanagh from DIAS, have begun testing whether the models can explain the data obtained at different wavelengths, including X-ray, optical, infrared and radio observations. All three of the different computer models predict fainter X-ray emission than observed. The bubble of X-ray emission is brightest near the star, whereas two of the three computer models predict the X-ray emission should be brighter near the shock wave.

Still image (above) and animation (below) of simulation Z02 by Green, showing logarithm of the gas density as a function of time.  The star is at the origin, moving supersonically from left to right, and its strong wind sweeps up the interstellar gas and forming a bow shock.  Blue/black is low density and green/yellow is high.  The two panels shows slices through the 3D simulation: the left panel shows the plane y=0, and the right panel shows z=0.

In the future the researchers plan to test more complicated models with additional physics — including the effects of turbulence, and particle acceleration – to see whether the agreement with X-ray data will improve. A paper describing these results has been accepted in the journal Astronomy and Astrophysics and a preprint is available here and official press release here.

NASA’s Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.