Discovery of remarkably intense radio emission from a young and rapidly rotating dMe star

Abstract

I report the serendipitous discovery of remarkably intense radio emission from the dM4e star Rositter 137B. This star is rotating very rapidly (P rot ≤ 9 hr) and is believed to have an age of ∼5-8 × 10 7 yr and therefore still contracting onto the main sequence. The remarkably intense radio emission of Rst 137B suggests that (some) older solar-neighborhood dMe flare stars, which are believed to be near or at the zero-age main sequence (ZAMS; age of 10 8-10 9 yr), may have evolved through a stage of even higher radio activity than their presently already elevated levels. This in turn may indicate that the magnetic dynamo of M dwarf stars decays in strength well before these stars reach the ZAMS; on the other hand, if as is suggested by Linsky & Saar the magnetic dynamo of many near-ZAMS solar-neighborhood dMe flare stars are still operating in a regime of saturation, then younger and/or more rapidly rotating M dwarf stars may be able to more efficiently convert magnetic energy to radio energy. Rst 137B appears to display quite steady quiescent emission. On one occasion its quiescent emission was observed to be circularly polarized in the opposite sense to that of coherent flares, providing perhaps the first unambiguous evidence that the quiescent emission of dMe flare stars can attain quite high degrees of circular polarization. If, as current evidence suggests, both the quiescent and flaring emission of flare stars originate predominantly from magnetic fields with the same polarity, this observation also could provide the first tentative identification of the magnetoionic mode of coherent stellar microwave flares. The two coherent flares observed on Rst 137B were inferred to be polarized in the sense of the o-mode, not the x-mode as predicted by the popular interpretation of electron-cyclotron maser theory. This theory, however, does predict that under stellar conditions less favorable for the growth of electromagnetic maser waves than has generally been assumed, o-mode can dominate over x-mode emission.