Discovery of Radio Emission from the Tight M8 Binary LP 349−25

[[abstract]]We present radio observations of eight ultracool dwarfs with a narrow spectral type range (M8-M9.5) using the Very Large Array at 8.5 GHz. Only the tight M8 binary LP 349 - 25 was detected. LP 349 - 25 is the tenth ultracool dwarf system detected in radio wavelengths and its trigonometric parallax pi = 67.6 mas, recently measured by Gatewood and coworkers, makes it the furthest ultracool system detected by the Very Large Array to date, and the most radio luminous outside of obvious flaring activity or variability. With a separation of only 1.8 AU, masses of the components of LP 349 - 25 can be measured precisely without any theoretical assumptions, allowing us to clarify their fully convective status and hence the kind of magnetic dynamo in these components, which may play an important role in explaining our detection of radio emissions from these objects. This also makes LP 349 - 25 an excellent target for further studies with better constraints on the correlations between X-ray, and radio emission and stellar parameters such as mass, age, temperature, and luminosity in ultracool dwarfs.[[fileno]]2010504010056[[department]]天文