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

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