Science: Baby red dwarf screams across the radio bands


By KEN CROSWELL in BERKELEY A young red dwarf star in the southern constellation of Dorado is broadcasting radio waves more strongly than any other known red dwarf. It may give astronomers insight into the early evolution of these stars, which make up 70 per cent of all stars in the Galaxy. Red dwarfs are much fainter and cooler than the Sun. When they are young, most spin fast and this generates strong magnetic fields which create huge flares that can outshine the rest of the star. Also, as electrons whirl around the magnetic field lines, they radiate intense radio waves. The record-breaking red dwarf is called Rositter 137B. Jeremy Lim of the California Institute of Technology in Pasadena discovered its intense radio emission while using a new array of radio telescopes near Narrabri, Australia, to observe another star. Rositter 137B was close by. Lim detected two flares during the time he observed the star. He also found that the star’s steady, or ‘quiescent’, radio emission is stronger than that of any known red dwarf. For instance, it is 100 times as strong as that coming from UV Ceti, the first red dwarf found to produce flares. In fact, the quiescent radio emission is even stronger than the strongest flare ever detected on UV Ceti. Lim thinks that Rositter 137B owes its intense radio emission and strong magnetic field to its rapid rotation. He estimates that the star spins on its axis in less than 9 hours, making it the fastest rotating red dwarf field star (one that is not part of a cluster of stars). Before this discovery, the fastest spinning red dwarf field star was Gliese 890, which rotated every 10.3 hours in the constellation of Aquarius. Lim thinks that Rositter 137B must be young because red dwarfs spin more slowly as they age. Consequently, his discovery suggests that many red dwarfs pass through a stage of intense radio emission when they are young. This emission then decays to levels seen in stars such as UV Ceti, whose quiescent radio emission is still far stronger than the Sun’s. Rositter 137B is so young that it may be an escaped star from the Pleiades, a cluster of stars only 70 million years old, or 1.5 per cent the age of the Sun. Gliese 890 may also have escaped from the Pleiades (Astronomy, January 1991, p 28). The Pleiades are 410 light years from Earth, whereas Rositter 137B and Gliese 890 are at distances of 80 and 65 light years,
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