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 and binary M-dwarf systems with periods below similar or equal to5-6 d (equatorial velocities, V(eq)greater than or similar to6 km s(-1)) are consistent with the current rotation-activity paradigm, and appear to saturate at about 10(-3) of the stellar bolometric luminosity.

The single M-dwarf data show tentative evidence for the super-saturation phenomenon observed in some ultra-fast rotating (greater than or similar to 100 km s(-1)) G- and 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 (P(rot)less than or similar to2 d) stars considered herein, with a period of 0.212 d and an X-ray activity level of 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 LxLbol=-3.21 +/-0.04 (0.2 less than or similar toP(rot)less than or similar to 10.1 d), whereas for the binary stars, the mean log LxLbol=-3.19 +/-0.10 (0.8 less than or similar toP(rot)less than or similar to 10.4 d).

Furthermore, we show that extremely X-ray active M-dwarfs exhibit a blue excess of about 0.1 magnitudes in U-B compared with 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 and K activity levels, there is an exclusion zone of X-ray activity between the extremely active M-dwarfs and the less active ones.