Radio Detection of Late { Type ( G { K )
نویسنده
چکیده
We report deep 3.6 cm radio observations of a small sample of the most rapidly{ rotating G{K dwarf stars in the Pleiades open cluster. Of the 4 ultra{fast rotators (UFRs) observed, 3 were detected. The G8 dwarf HII 1136, the fastest rotating G dwarf known in the Pleiades cluster, displayed a are that rose to a peak ux density of 1 mJy (peak radio luminosity of 210 16 erg Hz ?1 s ?1) in less than 1 hr, stayed at approximately this level for 2 hrs, and then decayed apparently exponentially with an e{folding time of 1.4 hrs. Following the are, HII 1136 was detected in what may be its nonimpulsive state with a ux density of 0:16 0:02 mJy. The K2 dwarf HII 1883, the fastest rotating K dwarf in the Pleiades cluster, was detected on two separate occasions. On the rst occasion the star displayed two weak ares superposed on apparently steady emission with a ux density of 0:100:02 mJy, whereas three months later the star was detected only as a steady source with a factor of 2 lower ux density of 0:050:01 mJy. By contrast, the K0 dwarf HII 625 displayed slowly{varying emission with an average ux 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 erg Hz ?1 s ?1. For the single undetected star, the K2 dwarf HII 3163, we placed an upper limit (5) of 0.12 mJy on its ux density, or 2 10 15 erg Hz ?1 s ?1 on its luminosity. Our results represent the rst detection of late{type dwarf stars in the Pleiades cluster, 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 copius radio emitters. Both in apparent quiescence and in ares, the radio luminosity of the Pleiades cluster stars is similar to that of equally rapidly rotating but relatively nearby 1 postal address { 2 { late{type dwarf stars belonging to the Local Association, providing further support for the idea that such stars are physical counterparts of UFRs in the Pleiades cluster. When averaged over the stellar surface, the surface radio luminosity of the Pleiades cluster stars is comparable to that of the most active T Tauri stars, a trend recently noted for the surface soft X{ray emission …
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