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

Abstract

At fast rotation rates the coronal activity of G- and K-type stars has been\nobserved to "saturate" and then decline again at even faster rotation rates --\na phenomenon dubbed "super-saturation". In this paper we investigate coronal\nactivity in fast-rotating M-dwarfs using deep XMM-Newton observations of 97\nlow-mass stars of known rotation period in the young open cluster NGC 2547, and\ncombine these with published X-ray surveys of low-mass field and cluster stars\nof known rotation period. Like G- and K-dwarfs, we find that M-dwarfs exhibit\nincreasing coronal activity with decreasing Rossby number N_R, the ratio of\nperiod to convective turnover time, and that activity saturates at L_x/L_bol ~\n10^-3 for log N_R < -0.8. However, super-saturation is not convincingly\ndisplayed by M-dwarfs, despite the presence of many objects in our sample with\nlog N_R < -1.8, where super-saturation is observed to occur in higher mass\nstars. Instead, it appears that a short rotation period is the primary\npredictor of super-saturation; P <=0.3d for K-dwarfs and perhaps P <=0.2d for\nM-dwarfs. These observations favour the "centrifugal stripping" model for\nsuper-saturation, where coronal structures are forced open or become\nradiatively unstable as the Keplerian co-rotation radius moves inside the X-ray\nemitting coronal volume.\n