A binary star in Sauron’s ‘pupil’

Arp 87: Merging Galaxies from Hubble Image Credit: NASA, ESA, Hubble Space Telescope; Processing: Douglas Gardner

Our Sun, with an approximate age of 4,500 million years, is in the middle of its life. Within another 5,000 million years its size will increase until it almost reaches the orbit of Mars, expelling the outer layers of its atmosphere and forming what is known as a planetary nebula, which will end up dissipated in the stellar medium. Almost all the stars in the universe will end their lives like this.

Today we know of more than 3,000 planetary nebulae in our galaxy. 80% of them present complex morphologies, very difficult to explain with the basic model of planetary nebula formation, which would give rise to rather spherical nebulae.

The fascinating morphologies observed seem to indicate asymmetries during the star’s mass loss process, something that could be the result of binary interactions, that is, of the central star with another companion star.

However, this binarity hypothesis does not yet have clear observational evidence since, to date, only about 60 central binary stars of planetary nebulae have been detected. Therefore, the detection of more double stars is essential for the in-depth study of this phase of the life of stars like the Sun.

Now, a study led by Alba Aller Egea, a researcher at the Center for Astrobiology (CAB, CSIC-INTA) and published in the journal Astronomy & Astrophysics, has used data from the TESS space telescope (Transiting Exoplanet Survey Satellite, exoplanet search satellite in transit) to detect binary stars in the nucleus of several planetary nebulae.

“Of the eight central stars of planetary nebulae analyzed, seven show clear signs of variability, compatible in most cases with the presence of a companion star,” says Aller. “And the eighth, which does not seem to have variability at least in the 27 days in which the data have been taken, it is the only apparently spherical planetary nebula in the sample, something that would corroborate the binarity theory ”.

In the center of the eye of God or Sauron
Of special interest are the results obtained for the Helix Nebula or NGC 7293, also popularly known as the Eye of God or the Eye of Sauron (from the Lord of the Rings). This nebula is one of the most studied and also one of the closest to Earth, located about 650 light years away.

As Aller explains: “Thanks to the TESS data, we detected, for the first time, a clear variability in the light that reaches us from its central star, what we call the light curve. This variability could be consistent with the presence of a low-mass companion star or substellar object (such as a brown dwarf or exoplanet), which could be reflecting light from the much hotter primary star. The period of this signal is 2.8 days, which would be, if that companion star is confirmed, the time in which both would orbit around each other ”.

“The TESS data have also allowed us to analyze other intrinsic characteristics of the stars themselves, such as the frequencies of their pulsations or their rotation, something that we have been able to see in two of the stars in the sample (NGC 246 and RWT 152)”, indicates Sebastià Barceló Forteza, also a CAB researcher and co-author of the study.

“Although this satellite was born with the main objective of detecting planets the size of the Earth, its capabilities provide a unique opportunity to investigate in many other fields of astrophysics”.

“By observing the entire sky – he adds – TESS gives us access to countless scientific cases to be exploited. And by doing so with a 2-minute cadence it allows us to see phenomena with a very fast and precise variability, since it allows us to detect variations in starlight of up to 100 parts per million (100 windows off in a skyscraper with 1 million windows on). “

“In this case, we have used the data from this satellite for a different field than the main purpose of the telescope, which will allow us to better understand the death of stars like our Sun,” concludes Lillo-Box.