X-ray Emission from Nearby M-dwarfs: the Super-saturation Phenomenon

A rotation rate and X-ray luminosity analysis is presented for rapidly rotating single and binary M-dwarf systems. X-ray luminosities for the majority of both single & binary M-dwarf systems with periods below ≃ 5−6 days (equatorial velocities, Veq∼>6 km s) are consistent with the current rotation-activity paradigm, and appear to saturate at about 10 of the stellar bolometric luminosity. The single M-dwarf data show tentative evidence for the super-saturation phenomenon observed in some ultra-fast rotating (∼>100 km s ) G & K-dwarfs in the IC 2391, IC 2602 and Alpha Persei clusters. The IC 2391 M star VXR60b is the least X-ray active and most rapidly rotating of the short period (Prot∼<2 days) stars considered herein, with a period of 0.212 days and an X-ray activity level about 1.5 sigma below the mean X-ray emission level for most of the single M-dwarf sample. For this star, and possibly one other, we cautiously believe that we have identified the first evidence of super-saturation in M-dwarfs. If we are wrong, we demonstrate that only M-dwarfs rotating close to their break up velocities are likely to exhibit the super-saturation effect at X-ray wavelengths. The M-dwarf X-ray data also show that there is no evidence for any difference in the X-ray behaviour between the single and binary systems, because for the single stars, the mean log Lx/Lbol = −3.21 ± 0.04 (0.2∼<Prot∼<10.1 days), whereas for the binary stars, the mean log Lx/Lbol = −3.19± 0.10 (0.8∼<Prot∼<10.4 days). Furthermore, we show that extremely X-ray active M-dwarfs exhibit a blue excess of about 0.1 magnitudes in U−B compared to less active field M-dwarfs. Such an excess level is comparable to that observed for extremely chromospherically active M-dwarfs. Moreover, as is the case for M-dwarf Ca II H & K activity levels, there is an exclusion zone of X-ray activity between the extremely active M-dwarfs and the less active ones.