Binary Stars Orbiting Each Other INSIDE a Planetary Nebula

Planetary nebulae are a fascinating astronomical phenomena, even if the name is a bit misleading. Rather than being associated with planets, these glowing shells of gas and dust are formed when stars enter the final phases of their lifespan and throw off their outer layers. In many cases, this process and the subsequent structure of the nebula is the result of the star interacting with a nearby companion star.

Recently, while examining the planetary nebula M3-1, an international team of astronomers noted something rather interesting. After observing the nebula’s central star, which is actually a binary system, they noticed that the pair had an incredibly short orbital period – i.e. the stars orbit each other once every 3 hours and 5 minutes. Based on this behavior, the pair are likely to merge and trigger a nova explosion.

The team, led by David Jones of the Instituto Astrofisica de Canarias and the Universidad de La Laguna, reported their findings in Monthly Notices of the Royal Astronomical Society: Letters. The team included other members of the (IAC) as well as the European Southern Observatory (ESO), the Nicolaus Copernicus Astronomical Center (CAMK), the South African Astronomical Observatory (SAAO), and the Observatorio Astronómico Nacional (OAN-IGN).

For the sake of their study, the team relied on the ESO’s New Technology Telescope (ESO-NTT), located at the La Silla Observatory in Chile, to examine M3-1 over a period of several years. This planetary nebula is located in the constellation Canis Major, roughly 14,000 light-years from Earth. In the process, the team discovered and studied the binary stars in the center of the nebula.

As Brent Miszalski – a researcher at the Southern African Large Telescope and co-author of the study – indicated in a recent Royal Astronomical Society press release, this discovery confirmed what many astronomers already suspected. “We knew M3-1 had to host a binary star,” he said, “so we set about acquiring the observations required to prove this and to relate the properties of the nebula with the evolution of the star or stars that formed it.”

For some time, M3-1 has been considered a firm candidate for a binary central star based on its structure (which features prominent jets and filaments that are indicative of binary interactions). However, because the stars are so close together, they cannot be resolved separately from the ground. As a result, scientists have inferred the presence of a second star from the variation of their combined brightness.

The most obvious cause of these variations would be how the stars periodically eclipse one another, which would produce a marked drop in brightness. As Henri Boffin – a ESO researcher in Garching, Germany – explained:

“When we began the observations, it was immediately clear that the system was a binary. We saw that the apparently single star at the center of the nebula was rapidly changing in brightness, and we knew that this must be due to the presence of a companion star.”

However, the team was surprised to find that the pair had one of the shortest orbital periods (3 hours and 5 minutes) of any binary stars discovered inside a nebula to date. They further concluded that the stars are so close that they are practically touching. As a result, the two are likely to undergo a nova eruption in the future, where material is transferred from one star to the other, creating a critical mass that triggers a violent thermonuclear explosion.

As Paulina Sowicka, a PhD student at the Nicolas Copernicus Astronomical Center in Poland, indicated:

“After the various observing campaigns in Chile, we had enough data to begin to understand the properties of the two stars – their masses, temperatures and radii. It was a real surprise that the two stars were so close together and so large that they were almost touching one another. A nova explosion could take place in just a few thousand years from now.”

When the two stars merge and trigger a nova explosion, the system will increase in luminosity by up to a million times, which will brighten the surrounding nebula considerably and create an incredible light show. What’s more, the detection of this binary pair is also inconsistent with conventional thinking about how binary stars evolve within a planetary nebula.

Previously, astronomers had been working under the assumption that binary stars are well separated after the formation of a planetary nebula. Basically, it was thought that it would not be until the nebula’s gases expanded and dissipated (to the point that it was no longer visible) that a binary pair could start interacting again, leading to a merger and a nova explosion.

But with this latest observation, that theory may come to be challenged. This study is also bolstered by a similar nova explosion (known as Nova Vul 2007) that was observed inside a planetary nebula in 2007. As Jone explained:

“The 2007 event was particularly difficult to explain. By the time the two stars are close enough for a nova, the material in the planetary nebula should have expanded and dissipated so much that it’s no longer visible. In the central stars of M3-1, we’ve found another candidate for a similar nova eruption in the relatively near future.”

Looking ahead, the team hopes to carry out further studies of M3-1 and other nebulas like it. These observations could give astronomers greater insight into the physical processes and origins of some of the most powerful phenomena in the Universe. These include cataclysmic variables (where one star siphons material from another) novae, and perhaps even supernovae.

Further Reading: RAS, MNRAS