Astronomers discover ultra-near stars in the M3-1 planetary nebula

Image obtained with the Hubble Space Telescope from the M3-1 planetary nebula, whose central star is actually a binary system with one of the shortest known orbital periods. Credit: David Jones / Daniel López – IAC

An international team of astronomers has discovered two stars in a binary pair that orbit one around each other in just over three hours, in the M3-1 planetary nebula. Remarkably, the stars could lead to an explosion of nova, a totally unexpected event based on our current understanding of the evolution of binary stars. The team, led by David Jones of the Instituto Astrofisica de Canarias and the Universidad de La Laguna, reports its findings in Monthly Notices of the Royal Astronomical Society: Letters.

Planetary nebulae are the glowing shells of gas and dust formed from outer layers of stars, like our own Sun, which they reject during the later stages of their evolution. In many cases, interaction with a nearby companion star plays an important role in the ejection of this material and in the formation of the elaborate structures observed in the resulting planetary nebulae.

The M3-1 planetary nebula is located in the Canis Major constellation at a distance of about 14,000 light-years. M3-1 was a serious candidate to host a binary central star, as its structure with jets and filaments is very visible typical of these interactions between binary stars.

A two-star scale animation of the M3-1 planetary nebula orbiting one around the other. Their combined brightness varies depending on whether the two are visible or if one or the other is eclipsed by his companion. Credit: David Jones – IAC.

Using telescopes from the Southern European Observatory (ESO) in Chile, Jones' team examined M3-1 over a period of several years. In doing so, they discovered and studied the binary stars located in the center of the nebula.

"We knew that M3-1 was to host a binary star, so we undertook to acquire the necessary observations to prove it and relate the properties of the nebula to the evolution of the star or stars who have trained, "says Brent Miszalski, researcher at Southern African Large Telescope and co-author of the study.

Since the two stars are so close to each other that they can not be solved from the ground, the presence of the second star is deduced from the variation in their observed combined luminosity – the most obviously by periodic eclipses of one star by the other which produce marked drops. in the brightness.

"When we started the observations, it was immediately clear that the system was a binary system," says Henri Boffin, a researcher at the Southern European Observatory (European Southern Observatory) in Germany. "We found that the brightness of the seemingly unique star in the center of the nebula was changing rapidly, and we knew it had to be due to the presence of a companion star."

The team discovered that the central star of the M3-1 planetary nebula possessed one of the shortest binary central stars of the orbital period known to date, at just over three o'clock . ESO's observations also show that the two stars – most likely a white dwarf with a main sequence companion of low mass – are almost touching.

As a result, the pair is likely to experience a so-called nova eruption, a result of material transfer from one star to the other. When this reaches a critical mass, a violent thermonuclear explosion occurs and the system temporarily increases the brightness by nearly one million times.

"After the various observation campaigns in Chile, we had enough data to understand the properties of the two stars: their mass, their temperature and their radius," explains Paulina Sowicka, PhD student at the Nicolaus Copernicus Astronomical Center in Poland. "It's a real surprise that the two stars are so close and so big that they almost touch each other. An explosion of nova could take place in a few thousand years from now.

The theory suggests that binary stars should be well separated after the formation of a planetary nebula. It should take a long time before they start interacting again and events like novae become possible.

In 2007, astronomers observed another nova explosion, known as Nova Vul 2007, inside another planetary nebula.

Jones comments: "The 2007 edition was particularly difficult to explain. At the moment when the two stars are close enough for a nova, the material of the planetary nebula should have expanded and dissipated to the point of no longer being visible. "

Jones adds, "In the central stars of M3-1, we found another candidate for a similar eruption in New Zealand in the relatively near future."

The team now hopes to deepen their research on the nebula and others, helping to illuminate the physical processes and origins of novae and supernovae, the most dramatic and violent phenomena in the universe.

The new work appears in: "The binary star of the short orbital period at the heart of the M 3-1 planetary nebula", D. Jones, H.M.J. Boffin, P. Sowicka, et al., Monthly Notices of the Royal Astronomical Society: Letters (2018), 482 (1) (DOI: 10.1093 / mnrasl / sly142).