Ultra-cool Diminutive Star Weighs In

The power of the some of the world's biggest telescopes has been brought to bear to directly measure the mass, for the first time, of one of the smallest stars ever seen in the universe. Barely the size of the planet Jupiter, the dwarf star weighs in at just 8.5 percent of the mass of our Sun.

This is the first ever mass measurement of an L-type dwarf star belonging to a new stellar class of very low mass objects in space discovered just a few years ago. The observation is a major step toward our understanding of the types of objects that occupy the gap between Sun-like stars and planets.

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 Telescopes (Hawaii), and the Hubble Space Telescope. The mass was measurable because the so-called L-Dwarf is in orbit around an even smaller object, a brown dwarf that is 6.6 percent of the Sun's mass, and too puny to shine by nuclear fusion. The L-Dwarf is precariously close to the theoretical minimal fusion limit, which is 8 percent of our Sun's mass.

The precise mass measurements were made by an international team of astronomers led by Hervé Bouy from the Max Planck Institute for Extraterrestrische Physik in Garching, Germany and the Observatoire de Grenoble in France; Eduardo Martin (Instituto de Astrofisica de Canarias, Spain); and Wolfgang Brandner (Max Planck Institute for Astronomie, Germany).

In addition to using the latest evolutionary models from the group of Ecole Normale Superieure de Lyon, detailed observations of each component of the binary system were required to be able to compute their masses. Because both objects are very close to each other, telescopes capable of 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 objects 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. Once the orbit was known, the astronomers were able to compute the total mass of the system using Kepler's laws first formulated four centuries ago.

Once the total mass of the system was known, 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. Finally, the mass of each component could be determined.

Only the world's very largest telescopes have a resolution comparable to Hubble's to resolve the binary pair. This is possible after using adaptive optics to cancel out atmospheric blur.

Both components of the binary system belong to the L spectral class that includes the lowest mass stars and the highest mass brown dwarfs in our solar neighborhood. This stellar class was discovered in 1997 by Eduardo Martín, Gibor Basri, Xavier Delfosse and Thierry Forveille, and was added to the spectral classification that had remained unchanged for half a century. The L class is characterized by the formation of dust grains in the object's atmosphere, which dramatically changes the visible-light spectrum.

Theoretically predicted for a long time, these sub-stellar objects called "brown dwarfs" were only discovered in 1995. Indirect techniques were conceived to identify brown dwarf candidates. However, the mass measurement is the only direct way to identify a star as a brown dwarf. There is no technique to directly measure the mass of any star in the universe, unless the star belongs to a binary system. Binary brown dwarfs are especially challenging because they are often faint and lie very close to each other. Large telescopes are therefore required to perform such studies.

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