Using the HiPERCAM imager on the William Herschel Telescope (WHT), astronomers have been able to observe in exquisite detail the eclipse of the central star of the planetary nebula Outters 5 by its binary companion.

Planetary nebulae are the glowing shells of gas and dust expelled by sun-like stars at the end of their lives as they evolve towards becoming white dwarfs. It is thought that many planetary nebulae get their spectacular, axisymmetric shapes from the interactions of their progenitor stars with close binary companions. However, the process is poorly understood.

One of the major problems facing astronomers, in their quest to understand how binary stellar evolution can lead to aspherical planetary nebulae, is that the central stars of planetary nebulae are generally faint and their binary companions, where known, are on very short orbits. This means that large telescopes and very efficient instrumentation is required in order to be able to resolve the properties of the stars at different points in their orbit.

Preliminary observations of the central star of Outters 5, with the Wide Field Camera (WFC) mounted on the Isaac Newton Telescope (INT), already showed that the central star must be a binary due to the very deep eclipses occurring roughly every 8 hours.

David Jones (Instituto de Astrofísica de Canarias) explains: "However, with the INT we weren't able to detect the star during eclipse, it was just too faint. We needed the much larger WHT to be able to measure the eclipses, during which the stars become 90% fainter."

It was for just this reason that the central stars of Outters 5 were considered the perfect test for the HiPERCAM instrument - an extremely sensitive imager capable of taking simultaneous images in five different filters at once - during its first light on the WHT. These data allowed the team to measure the precise depth and duration of one eclipse, placing strong constraints on the sizes and temperatures of the central stars.

Using the ISIS spectrograph on the WHT, the astronomers then measured the changes in radial velocity of the stars throughout the orbit - an indirect way of measuring the masses of the two stars. Collecting all these measurements together enabled them to fully characterise the properties of both stars, revealing important information about the history of the system.

The mass of the nebular progenitor was found to be roughly half that of the Sun, strongly indicative that the nebula was formed relatively early in the star's life while it was still on the red giant branch rather than the asymptotic giant branch, as is typically expected. This is almost certainly due interactions with the companion cutting the nebular progenitor's evolution short.

On the other hand, the companion was found to be a red dwarf star that is both hotter and larger than typical for stars of the same mass. Again, this is almost certainly due to interactions which led to the formation of the nebula, during which the red dwarf star accreted material from the nebular progenitor, expanding and heating up in the process.

Very few binary central stars of planetary nebulae have been characterised with such precision. More studies of this kind are essential to truly understand the processes at work during the formation of planetary nebulae and, ultimately, what fate beholds our own Sun? A spectacular, aspherical planetary nebula, like Outters 5, or a faint round (somewhat boring) planetary nebula, like Abell 39?