Stars are divided into two groups by mass. Low-mass stars have a mass of a few times the mass of our Sun or less, while high-mass stars have a mass above this limit. Theory and observation show that the interior structure of stars are very different above and below this mass dividing line. Below the surface, low mass stars boil, transporting energy from hot inner regions to cooler outer regions through bulk motions of fluid, similar to a pot of water on a hot stove. But stars (unlike pots of water) consist of a plasma of ionized gas, in which positive and negative charges are no longer locked together in neutral atoms. The organized circulation of the plasma's charged particles, coupled with the star's rotation, drive strong, unstable magnetic fields by a "dynamo" process. These fields can intensify greatly and produce sudden eruptions of electromagnetic energy and star stuff, in the form of powerful stellar flares which produce cosmic rays and hard X-ray emission. Hig mass stars, on the other hand, do not show these boiling motions of plasma below their surface, and, in general, the stellar dynamos are inefficient, and magnetic fields are generally weak. These massive stars, in general, do not show the electromagnetic flaring activity seen in lower mass stars. But there are some high mass stars that do seem to show relatively strong magnetic fields, puzzling scientists. But even these "magnetic" massive stars have not been observed to show the violent type of flaring emission seen in lower mass stars like the Sun. Until now, that is. Recent observations of X-ray emission from a young massive star called Rho Ophiuchi A by the XMM-Newton X-ray satellite observatory, have, for the first time, shown evidence of a stellar flare from a high-mass star. Rho Oph A is the bright source at the center of the XMM-Newton X-ray image shown above (the other sources are mostly other members of Rho Ophiuchus association). During an observation of nearly 40 hours, XMM-Newton saw that the X-ray emission flared by more than a factor of 3 over its usual X-ray brightness. Not only did the brightness increase, but the temperature of the emission increased as well, similar to the behavior of flares from low-mass stars. This sudden brightening has been attributed to a flare from a long-lasting, highly-magnetized hot spot on the surface of Rho Oph A. An alternative explanation, that the flare comes from a lower-mass, otherwise hidden companion star orbitally locked to Rho Oph A, is unlikely but can't be ruled out yet.
XMM-Newton: X-ray Flares from Massive Stellar Lighthouse
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