Journal Article

Investigating coronal saturation and supersaturation in fast-rotating M-dwarf stars

R. D. Jeffries, R. J. Jackson, K. R. Briggs, P. A. Evans and J. P. Pye

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 411, issue 3, pages 2099-2112
Published in print March 2011 | ISSN: 0035-8711
Published online February 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17848.x
Investigating coronal saturation and supersaturation in fast-rotating M-dwarf stars

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At fast rotation rates, the coronal activity of G- and K-type stars has been observed to ‘saturate’ and then decline again at even faster rotation rates – a phenomenon dubbed ‘supersaturation’. In this paper, we investigate coronal activity in fast-rotating M-dwarfs using deep XMM–Newton observations of 97 low-mass stars of known rotation period in the young open cluster NGC 2547 and combine these with published X-ray surveys of low-mass field and cluster stars of known rotation period. Like G- and K-dwarfs, we find that M-dwarfs exhibit increasing coronal activity with decreasing Rossby number NR, the ratio of period to convective turnover time, and that activity saturates at LX/Lbol≃ 10−3 for log NR < −0.8. However, supersaturation is not convincingly displayed by M-dwarfs, despite the presence of many objects in our sample with log NR < −1.8, where supersaturation is observed to occur in higher mass stars. Instead, it appears that a short rotation period is the primary predictor of supersaturation; P ≤ 0.3 d for K-dwarfs and perhaps P ≤ 0.2 d for M-dwarfs. These observations favour the ‘centrifugal stripping’ model for supersaturation, where coronal structures are forced open or become radiatively unstable as the Keplerian corotation radius moves inside the X-ray-emitting coronal volume.

Keywords: stars: activity; stars: coronae; stars: rotation; open clusters and associations: individual: NGC 2547; X-rays: stars

Journal Article.  12318 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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