An international team of astronomers, including Alex de Koter (UvA and KU Leuven) and Rens Waters (UvA and SRON), has dethroned twelve red giants that seemed to be losing a record amount of mass. The stars in question appear to have a hidden partner that distorts the measurements. The researchers published their findings on 25 February in the journal Nature Astronomy.
Red giants are giant, old stars near the end of their life cycle, that emit gasses and dust particles through a stellar wind. Some red giants appeared to lose a lot of mass in this manner. But new observations correct that. The stellar wind of these giants is not more intense than normal, but is influenced by a partner that has remained under the radar until now.
Astronomers had previously discovered twelve record holder mass losing red giants, which emit the equivalent of a hundred earth masses per year in just one hundred to two thousand years. This was difficult to explain, says astronomer and principal researcher Leen Decin (KU Leuven): ‘If you look at the mass of such a star in their next phase of life, the intense stellar wind does not last long enough to explain the observed mass loss. It is statistically unlikely that we would have discovered twelve of these red giants, if you know that it is a phase of barely a few hundred or thousands of years in their billion year life cycle. That's like finding a needle in a haystack twelve times.’
Thanks to new observations made with the ALMA telescope in Chile, it has become clear what is going on with two of these red giants. The stellar wind of these stars forms a spiral. That is an indication that the red giant is not alone, but is part of a binary star. The red giant forms the main star and a second star revolves around it. Both stars influence each other and their environment by means of gravity, in two distinct ways. On the one hand the star wind is drawn in the direction of the second star. On the other hand the red giant itself also wobbles a bit. These movements shape the stellar wind into a spiral.
The discovery of these partner stars has made the pieces of the puzzle fall into place. The red giants that astronomers thought are losing massive amounts of mass, are not really record holders in weight loss. It just seems like they are losing so much mass. Between the two stars, there is an area where the stellar wind is much more concentrated because of the gravity of the partner star. When the astronomers included this in their calculations, the red giants did not lose the equivalent of a hundred earths per year, but only of ten earths. That is similar to ordinary red giants.
The researchers have now scrutinized and dethroned two of the twelve record stars. They expect to be able to dethrone the ten remaining record holders fairly quickly, because they are very similar to the first two. Decin: ‘Until now, we thought that many stars lived alone, but we probably need to adjust that image. Star with a partner are probably more common than we think.’
'Reduction of the maximum mass-loss rate of OH/IR stars due to unnoticed binary interaction'. Door: L. Decin, W. Homan, T. Danilovich, A. de Koter, D. Engels, L.B.F.M. Waters, S. Muller, C. Gielen, D.A. García-Hernández, R.J. Stancliffe, M. Van de Sande, G. Molenberghs, F. Kerschbaum, A.A. Zijlstra & I. El Mellah. In: Nature Astronomy, 25 februari 2019. Link: https://www.nature.com/articles/s41550-019-0703-5