First direct measurement of the mass of an ultra-cool brown dwarf star

An international team of astronomers, led by Hervé Bouy from the Max Planck Institute, Garching, Germany and the Observatoire de Grenoble, France, have for the first time measured the mass of an ultra-cool brown dwarf star. The team performed the measurements using four of the most powerful telescopes available. This is the first-ever mass measurement of an L-type star belonging to the new stellar class of very low-mass stars, discovered a few years ago. With a mass of 6.6% of the solar mass, this celestial object is a “failed” star, lying between stars and planets in the evolutionary scheme.

Making use of four of the most famous telescopes worldwide, an international team of astronomers made the first-ever direct measurement of the mass of a so-called L-type star. The star, named 2MASSW J0746425+2000321, is a binary star that was observed for four years with the ESO Very Large Telescope (Chile), the Keck and Gemini Telescopes (Hawaii), and the Hubble Space Telescope.

Precise observations of each component of the binary system were required to be able to compute their masses. As both stars are very close to each other, telescopes providing high-resolution images were needed. Additionally, observations had to be performed over a long period of time (four years) to follow the motion of both stars around each other. Very accurate measurements of the relative position of the individual components were made, so that the full orbit of the binary system could be reconstructed, as illustrated in the following picture. Once the orbit was known, the astronomers were able to compute the total mass of the system using Kepler’s laws. In addition, very precise measurements of the brightness of each star were needed to be able to compute the individual mass of each component of the system. The astronomers calculated the mass ratio of the system from these brightness measurements, using the theoretical models by G. Chabrier and collaborators (Centre de Recherche Astronomique de Lyon, France). Finally, the mass of each component could be determined.

Both stars of the binary system belong to the L stellar class that includes the lowest mass stars. This stellar class was discovered in 1997 and was added to the stellar classification that had remained unchanged for half a century. The L class is characterized by the formation of dust grains in their atmospheres, which strongly changes the shape of the spectrum. For the first time, Hervé Bouy and his team have directly measured the mass of a star from this new class of ultra-cool stars.

The more massive component of the system weighs 8.5% of the solar mass, and is likely to be a very low-mass star. Weighing 6.6% of the solar mass, the secondary star is clearly not a star, but a so-called “sub-stellar” object, a failed star that occupies an intermediate position between the lightest stars and the heaviest planets.

Theoretically foreseen for a long time, these sub-stellar objects called “brown dwarfs” were only discovered in 1995. Indirect techniques were conceived of to identify brown dwarf candidates; however, mass measurement is the only direct way to identify a star as a brown dwarf. Indeed, following stellar evolutionary models, the mass IS the criterion to determine whether a given object is a “true” star or a brown dwarf. A “true” star is heavy enough to, at some point, stabilize its temperature through fusion in its interior. For example, for 5 billion years our Sun has been burning hydrogen – it is thanks to this hydrogen fusion that the Sun shines – and it will go on burning hydrogen for 5 billion years more. A brown dwarf will never have such a stable life. Its brightness originates in the energy that remains from its birth; as this energy decreases, the brown dwarf becomes cooler and fainter. Direct mass measurements such as the one made by Bouy and his team, are a key to a better understanding of the physics of these fascinating objects.

Such mass measurements, however, are much more challenging than one could imagine. There are no means to measure the mass of a star in the Universe, except if the star belongs to a binary system. Additionally, binary brown dwarfs are often faint and close to each other: large telescopes are therefore required to perform such studies. These requirements make this research topic particularly challenging; the mass measurement performed by Hervé Bouy and his colleagues is thus a major step toward our understanding of these sub-stellar objects that occupy the gap between stars and planets.

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