Abstract
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 "super-saturation". 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 N_R, the ratio of period to convective turnover time, and that activity saturates at L_x/L_bol ~ 10^-3 for log N_R < -0.8. However, super-saturation is not convincingly displayed by M-dwarfs, despite the presence of many objects in our sample with log N_R < -1.8, where super-saturation is observed to occur in higher mass stars. Instead, it appears that a short rotation period is the primary predictor of super-saturation; P <=0.3d for K-dwarfs and perhaps P <=0.2d for M-dwarfs. These observations favour the "centrifugal stripping" model for super-saturation, where coronal structures are forced open or become radiatively unstable as the Keplerian co-rotation radius moves inside the X-ray emitting coronal volume.
Highlights
X-ray emission from the hot coronae of photospherically cool stars arises from magnetically confined and heated plasma with temperatures in excess of 106 K
A summary of the findings of Sections 3 and 4 would be that X-ray activity rises with decreasing Rossby number, reaches a saturation plateau at a critical Rossby number that is approximately independent of the stellar mass and becomes supersaturated at a smaller Rossby number in a way that is dependent on the stellar mass – either in the sense that supersaturation does not occur at all in low-mass M-dwarfs or if it does, it occurs at much lower Rossby numbers than for K-dwarfs
The evidence for supersaturation in M-dwarfs with M < 0.55 M is weak, limited to three objects with short periods and very small Rossby numbers, which have 3σ upper limits to their coronal activity of LX/Lbol ≤ 10−3.3–10−3.4. These data were combined with X-ray measurements of fastrotating low-mass stars in the field and from other young clusters, and the results were considered in terms of stellar rotation period and in terms of the Rossby number, which is thought to be diagnostic of magnetic dynamo efficiency
Summary
X-ray emission from the hot coronae of photospherically cool stars arises from magnetically confined and heated plasma with temperatures in excess of 106 K (see the review by Gudel 2004). X-ray observations of cool stars with convective envelopes have long since shown that X-ray activity increases with the rotation rate and has led to the paradigm that a magnetic dynamo process produces and maintains the required magnetic fields, in analogy to processes observed to occur in the Sun The influence of a regenerative dynamo is supported by observations of coronal activity in stars at a range of masses. These demonstrate, in accordance with expectations from dynamo models, that rotation is not the only important parameter.
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