27 Apr. 2011

 

The Tycho supernova had a companion

 

A group of Chinese astronomers led by Fangjun Lu (Institute of High Energy Physics, Chinese Academy of Sciences in Beijing) have shown that the progenitor of the Galactic supernova that exploded in 1572, and observed in particular by Tycho Brahe (from whom it takes its name) had a companion star, and that it wasn't a white dwarf but probably a solar-type star.
Understanding this supernova is of particular importance because it was a type Ia explosion, the type used as a "standard candle" in the distance measurements used to determine the rate of expansion of the Universe, and the role played by dark energy. Although it is thought that these explosions are due either to the fusion of two white dwarfs or the transfer of mass from a normal star onto a white dwarf, it is actually very difficult to distinguish between these two scenarios observationally.
Lu and co-workers studied features in the left-hand image of the Tycho supernova remnant (above), which is about 20 light years across, to draw their conclusions. This includes the X-ray emission in blue (from the Chandra space telescope) caused by the shock wave which forms as the ejected material collides with the gas and dust in the surrounding medium.
The evidence for the presence of a companion star is the blue arc at the lower left of the image. This is interpreted as material that was stripped from the companion star which was located very near the white dwarf that exploded. The explosion tore away material from the companion and accelerated it in the 'wind' of the explosion. The point of impact between this wind and the stripped material creates a shock, which we see because of its X-ray emission.
So if this is what happened, where is the companion star now? Previous studies with optical telescopes have found a star within the remnant that is moving much faster than all the others (greater proper motion). Using the dynamical properties of this star and the properties of the arc, Lu's team has estimated that the companion had an orbital period of around 5 days and an orbital radius of less than 1/10 of an astronomical unit. The illustration to the right (not to scale) illustrates the scenario.
Finding the material stripped from a companion star shows that one of the popular scenarios for type Ia supernovae is correct. This will reassure those trying to use these supernovae to study the geometry of the Universe and the nature of dark energy.

 

by Michele Ferrara & Marcel Clemens

credit: NASA/CXC/Chinese Academy of Sciences/F. Lu et al, M. Weiss