No close partner for young, massive stars in the Omega Nebula
Astronomers Maria Ramirez-Tannus, Lex Kaper and Alex de Koter of the University of Amsterdam and Hugues Sana of the University of Leuven have discovered that, contrary to expectations, none of the massive stars in the star-forming region M17 in the Omega Nebula is part of a close binary star. They lead their lives alone or with a distant partner star. These findings will be published soon in Astronomy & Astrophysics Letters.
The Omega Nebula is an open cluster in the Sagittarius constellation. The constellation is located more than 5,000 light years away and is home to several dozen young, hot stars. The astronomers discovered that these massive stars show surprisingly little variation in their radial velocity, the speed at which they move toward us or away from us. If these stars were binary stars, their radial velocity would vary by several dozen kilometres a second, because they orbit each other within their own orbits. The variation in star-forming region M17 is just five kilometres a second.
The researchers base their conclusion on data from the X-shooter spectrograph of the ESO’s Very Large Telescope in northern Chile. Most stars are not alone. Recent research shows that 70% of massive stars with a mass of 10 to 100 times the mass of the sun, which end their lives as a neutron star or black hole, have one or more close companions.
A statistical analysis shows that only roughly 10% of the massive stars in M17 are close binary stars. At the same time, M17 has many wide binary stars, compared with older star-forming regions which accommodate close as well as wide binary stars.
This is the first time that a young star-forming region such as this one has been studied for the presence of binary stars. The reason for this is that such regions are hidden from view by the gas and dust from which new stars are born. It is therefore a challenge to obtain high-quality spectra of such stars from which the radial velocity can be determined.
Lead author Sana: ‘If M17 does not have any close binary stars, it must mean that systems of this kind do not arise until later in the evolutionary process. Maybe they first form wide binary stars, which do not migrate to each other until later on.’ Co-author Ramirez-Tannus is enthusiastic about the results. ‘We have now observed ten of them and will be studying many more in order to understand how wide binary stars change into close binary stars.’
The answer to the question of whether massive stars are usually binary stars is important for a greater understanding of the star-forming process. It is also an indication of the number of neutron binary stars and binary black holes created, either of which may ultimately produce a gravitational wave.