A multinational team of researchers using ESO’s Very Large Telescope has discovered that almost three quarters of high-mass stars have close companions, far more than previously thought.
Surprisingly most of these pairs are also experiencing disruptive interactions, such as mass transfer from one star to the other, and about one third are even expected to ultimately merge to form a single star.
The Universe is a diverse place, and many stars are quite unlike the Sun. An international team has used the VLT to study what are known as O-type stars, which have very high temperature, mass and brightness. These stars have short and violent lives and play a key role in the evolution of galaxies. They are also linked to extreme phenomena such as ‘vampire stars,’ where a smaller companion star sucks matter off the surface of its larger neighbor, and gamma-ray bursts.
“These stars are absolute behemoths,” said Dr Hugues Sana of the University of Amsterdam, Netherlands, who led a study published in the July 27 issue of the journal Science. “They have 15 or more times the mass of our Sun and can be up to a million times brighter. These stars are so hot that they shine with a brilliant blue-white light and have surface temperatures over 30 000 degrees Celsius.”
The astronomers studied a sample of 71 O-type single stars and stars in pairs in six nearby young star clusters in the Milky Way.
By analyzing the light coming from these targets in greater detail than before, the team discovered that 75% of all O-type stars exist inside binary systems, a higher proportion than previously thought, and the first precise determination of this number. More importantly, though, they found that the proportion of these pairs that are close enough to interact is far higher than anyone had thought, which has profound implications for our understanding of galaxy evolution.
“The life of a star is greatly affected if it exists alongside another star,” said co-author Dr Selma de Mink of the Space Telescope Science Institute. “If two stars orbit very close to each other they may eventually merge. But even if they don’t, one star will often pull matter off the surface of its neighbor.”
Mergers between stars, which the team estimates will be the ultimate fate of around 20–30% of O-type stars, are violent events. But even the comparatively gentle scenario of vampire stars, which accounts for a further 40–50% of cases, has profound effects on how these stars evolve.
Until now, astronomers mostly considered that closely-orbiting massive binary stars were the exception, something that was only needed to explain exotic phenomena such as X-ray binaries, double pulsars and black hole binaries. The new study shows that to properly interpret the Universe, this simplification cannot be made: these heavyweight double stars are not just common, their lives are fundamentally different from those of single stars.
Bibliographic information: Sana H et al. 2012. Binary Interaction Dominates the Evolution of Massive Stars. Science vol. 337, no. 6093, pp. 444-446; doi: 10.1126/science.1223344