Astronomers and their work: Pavel Kroupa (left) and Ingo Thies in front of the formula describing the mass distribution of pairs of brown dwarfs. Image: Argelander Institute for Astronomy
Read aloud Since their discovery several years ago, the so-called "Brown Dwarfs" are giving up astronomers' puzzles. How do these strange objects emerge, whose mass is too big for a planet, but too small for a star? Now, the astronomers Ingo Thies and Pavel Kroupa of the University of Bonn give the answer: brown dwarfs are not just too small stars that have not got enough hydrogen for nuclear fusion, but they form an independent class of celestial bodies. Two peculiarities of the brown dwarfs have already indicated that they arise under different conditions than ordinary stars. Thus, the distance between two partners of a normal binary star system is very variable. Some are circling at a closer distance than the Sun and Earth, a distance astronomers call the Astronomical Unity. Others are separated by several thousand astronomical units. This is different with brown dwarfs: "The track radii of pairs of brown dwarfs are cut off above about 15 astronomical units, " says Ingo Thies. "Couples with longer distances are the exception."

In addition, there are hardly any pairs of ordinary stars and brown dwarfs. If both celestial bodies were created by the same mechanism, there would have to be many such unequal pairs. "According to the classical model, both brown dwarfs and stars should be formed from interstellar gas clouds, which aggregate due to their mass attraction, " explains Pavel Kroupa of the Argelander Institute. "Therefore, there should not be these differences."

To prove that stars and brown dwarfs are as different as stars and planets, Thies and Kroupa analyzed the mass distribution of many newborn celestial bodies in three interstellar clouds, the birthplaces of new stars. "Our study reveals a jump in the mass distribution that makes the division of the stellar population obvious, " says Ingo Thies. The two researchers conclude that brown dwarfs are born in a different way than ordinary stars.

Possible birth scenarios were developed by a European team of astronomers back in 2001. Thus, brown dwarfs could begin their lives as part of a system of three-star embryos. But as the weakest member of such a trio, they were eventually catapulted out of the community, so that their further growth was stopped. According to another idea, brown dwarfs could form around a nascent star in the outer regions of the gas disk. But by a passing third star they are finally separated from this. Since almost all stars are born in star clusters, such encounters are not rare, say the Bonn researchers. display

Unlike ordinary stars, brown dwarfs can not be born alone, but only in the vicinity of stars. These are, so to speak, misfired stars whose development process was interrupted at an early stage.

Ingo Thies and Pavel Kroupa (University of Bonn): Monthly Notices of the Royal Astronomical Society, online pre-publication Ute Kehse

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