Radio detection of late-type (G-K) dwarf stars in the Pleiades

Abstract

We report deep 3.6 cm radio observations of a small sample of the most rapidly rotating G-K dwarf stars in the Pleiades. Of the four ultrafast rotators (UFRs) observed, three were detected. The G8 dwarf H II 1136, the fastest rotating G dwarf known in the Pleiades, displayed a flare that rose to a peak flux density of ∼1 mJy (peak radio luminosity of ∼2 × 10 16 ergs Hz -1 s -1) in less than ∼1 hr, stayed at approximately this level for ∼2 hr, and then decayed apparently exponentially with an e-folding time of ∼1.4 hr. Following the flare, H II 1136 was detected in what may be its quasi steady state with a flux density of 0.16 ± 0.02 mJy. The K2 dwarf H II 1883, the fastest rotating K dwarf in the Pleiades, was detected on two separate occasions, both times as an apparently steady source. The stellar flux density, however, appeared to change from 0.10 ± 0.02 mJy to 0.05 ± 0.01 mJy in the two observations separated by about 3 months. By contrast, the KO dwarf H II 625 displayed slowly varying emission with an average flux density of 0.16 ± 0.02 mJy. For all the stars detected, the average radio luminosity of their quasi-steady (perhaps quiescent) emission is 1-3 × 10 15 ergs Hz -1 s -1. For the single undetected star, the K2 dwarf H II 3163, we placed an upper limit (5 σ) of 0.12 mJy on its flux density or 2 × 10 15 ergs Hz -1 s -1 on its luminosity. Our results represent the first detection of late-type dwarf stars in the Pleiades and indeed of any apparently single stars in an open cluster. It demonstrates that solar-type stars recently descended to the zero-age main sequence can be copious radio emitters. Both in their quasi-steady and flaring states, the radio luminosities of the Pleiades stars are similar to those of equally rapidly rotating but relatively nearby late-type dwarf stars belonging to the Local Association, which provides further support for the idea that such stars are physical counterparts of UFRs in the Pleiades. When averaged over the stellar surface, the surface radio luminosity of the Pleiades stars is comparable to that of the most active T Tauri stars, a trend recently noted for the surface soft X-ray emission of these two classes of stars. This may suggest that the magnetic dynamo of rapidly rotating late-type stars operates at a saturated level as these stars descend to the main sequence.