The cooler companion spirals into the atmosphere of its larger companion and helps eject the shared atmosphere which we now see as the nebula
THE SOUTHERN African Large Telescope (SALT), situated in the heart of the Northern Cape at the South African Astronomical Observatory (SAAO) in Sutherland, has discovered a binary star system in the Hourglass Nebula, one of the most famous nebulae captured by the Hubble Space Telescope.
The Hourglass Nebula consists of two hourglass-shaped lobes of gas and what appears to be an eye staring right back at us. Shells of gas form the eye surrounding the hot central star that illuminates the nebula like a neon sign.
Astronomers have long suspected the peculiar nebula was formed by two interacting stars in a binary system but until now, no one could prove it. The SALT discovery of two stars orbiting each other every 18.15 days in the Hourglass Nebula, firmly settles the matter and gives new insight into how a wide variety of close binary stars and hourglass-shaped nebulae may form.
An international team of astronomers, led by SALT astronomer Dr Brent Miszalski at the SAAO, used SALT to peer into the “sparkle” of the eye of the Hourglass Nebula – its central star.
“A total of 26 SALT measurements were taken that detected the small movements of the central star towards or away from us caused by the gravity of a second companion star. This Doppler or “wobble” method, that can also be used to find planets around other stars, revealed a hidden companion orbiting the central star every 18.15 days,” Miszalski said.
Co-author of the study, Rajeev Manick, formerly a Masters student at SAAO and the University of Cape Town and now completing his PhD at the Katholieke Universiteit Leuven in Belgium, analysed the SALT measurements and found that the companion must be a small, cool star about five times lighter than the Sun.
Another surprise came with the binary – the relatively wide separation between the two stars is remarkable.
Co-author Professor Joanna Mikoajewska of the Nicolaus Copernicus Astronomical Center in Warsaw, Poland, a major partner in SALT, stated that previous authors suggested that a nova explosion could explain many aspects of the Hourglass Nebula, “but curiously we found the stars were too far apart for this to have ever been possible”.
Instead of a nova explosion, the orbital period indicates the Hourglass Nebula formed through an interaction that many close binary stars experience – a so-called common-envelope stage.
In this scenario the cooler companion spirals into the atmosphere of its larger companion and helps eject the shared atmosphere which we now see as the nebula.
The Hourglass Nebula is one of very few such examples to show an orbital period above 10 days, making it helpful to improve our understanding of this brief phase that many types of binary stars experience during their lifetime.
While astronomers still do not quite understand how hourglass-shaped nebulae form, the discovery of a binary in the Hourglass Nebula considerably strengthens the long suspected, but difficult to prove, connection between binary stars and hourglass-shaped nebulae.
A famous example is the nebular remnant of Supernova 1987A that is often compared against the Hourglass Nebula because of its very similar shape. It is thought to have resulted from the merger of two massive stars before the supernova event.
This process shares similarities with that which formed the Hourglass Nebula, hinting at some shared physics resulting in two of the most unusual nebulae in the sky.