ASTROPHYSICS: Big Flares from a Teeny Star

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Science  03 May 2002:
Vol. 296, Issue 5569, pp. 807b
DOI: 10.1126/science.296.5569.807b

The distinction between planets and stars has become more difficult over the past 7 years with the discovery of extrasolar massive giant gas planets, low-mass brown dwarfs, and other low-mass objects that seem to lie somewhere in between. One criterion used to separate the species is the deuterium-burning mass limit, that is, the minimum mass an object needs to sustain deuterium fusion for at least a brief time and thus be considered a star.

Enter the enigmatic S Ori 55, a low-mass object, isolated and apparently free-floating in the sigma Orionis cluster. New spectra from the Keck I telescope obtained by Zapatero Osorio et al. indicate that this object is intensely flaring, giving off bursts of hydrogen alpha emission in a highly variable manner. Evolutionary models suggest that S Ori 55 is young (about 3 million years old) and has a low mass of about 0.012 solar masses, which puts it right at the deuterium-burning mass limit. If the flares are from a stellar chromosphere, due to magnetospheric mass infall, then this low-mass star is somehow able to maintain substantial magnetic activity while being at the limit of deuterium fusion. Alternatively, the flares could be due to mass accretion by S Ori 55 of surrounding disks as the star grows.—LR

Astrophys. J. 569, L99 (2002).

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