The dark star is close to Earth : a better story than Star Wars !



An international team of astrophysicists has recently discovered a brown dwarf or a very low-mass star quite close to us, about 13 light-years away.



In the spring of 2000, two astronomers from the Observatoire de Grenoble noticed a very red and unusually shiny object on images provided by DENIS. This object could either be a very low-mass star, located not too far away, or a very distant giant star. Working in collaboration with a Spanish astrophysicist now at the University of Hawaii, they obtained observation time on the Keck I telescope on Mauna Kea, Hawai; they were thus able to perform a spectrum of the star and determine its nature. The high-resolution spectrum, performed in the night of May 30, 2000, clearly showed that the object is not a giant red star but a very low-mass star or a brown dwarf located near to us. In addition, the absence of lithium in the star's surface or atmosphere means that its mass is somewhere between 60 and 90 times that of Jupiter. The lack of lithium shows the mass is more than 60 times that of Jupiter, but this does not rule out the possibility that it is a brown dwarf (the difference between brown dwarves and stars is 75 times Jupiter's mass).


A star that is near moves much faster than distant stars. Thanks to MAMA, astronomers from the Paris Observatory analyzed old snapshots of the sky taken in 1975 and 1986. The star can be seen on the photographs and its movement over the past two decades shows it is very large. Astrophysicists have estimated the star's distance from us at about 13 light-years. This distance is not yet established for certain, since it is based on a comparison between its luminosity with that of stars of the same spectral type. Measurements show that it is probably situated between the 12th and 40th closest stars.


How is it that a star so close to us has remained unnoticed to date? Even though the star is very near, its luminosity is nonetheless rather faint (for instance, it is 10,000 time too faint to be seen by the naked eye). The faint luminosity is due to the fact that the star is much colder and smaller than the Sun. This is why these objects managed so far to escape the attention of astronomers, especially in the southern hemisphere which has been much less systematically explored than the northern hemisphere. Thanks to the DENIS survey, however, they now can easily be identified and it will very soon be possible to evaluate the number of very low-mass stars and brown dwarves in the vicinity of the Sun.


Upon recommendation of the International Astronomical Union, this star was named DENIS-P J104814.7-395606.1. It will now be easier for astronomers to study a very low-mass object that is quite near to us.


Communicated by Yves - November 2000

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