Cool stars with spectral types F to M are abundantly found in the solar neighborhood and in nearby open clusters. A comparison of these populations allows the study of solar-like stars with a range of masses, ages, and rotation rates, parameters that seem to be crucial for magnetic activity. In this fashion, one also hopes to improve our understanding of solar activity. While, obviously, the Sun can only be observed, but not changed in its physical parameters, the study of the stars enables us to study how X-ray emission - and activity in general - depend on fundamental stellar parameters. This is the concept of the solar-stellar connection.
To study the X-ray properties of solar-like stars in an unbiased manner and to compare the Sun to the stars in a fair way, one, therefore, has to consider volume-limited samples. The RASS was a spatially complete but flux-limited all-sky survey. Flux-limited surveys tend to find the intrinsically brightest objects of any given population since these are sampled over the largest spatial volumes, while low-luminosity objects are sampled only over small spatial volumes and are therefore quite sparse in the RASS data.
Volume-limited samples with very large detection rates were constructed for F, G, K, and M dwarfs by combining RASS data with ROSAT pointed observations of stars not detected in the RASS. All ROSAT observations of nearby cool stars are summarized in the NEXXUS data base . The NEXXUS catalog shows that stellar X-ray emission is detected down to an absolute magnitude of MV = 20 mag, i.e., down to the very bottom of the main sequence.
What about the completeness of the X-ray detections in the NEXXUS database? Within a volume of 12 pc all F/G stars have been detected. Out of 51 K stars within 12 pc only two stars have not been detected (detection rate of 96%), while out of 65 M stars within 6 pc 6 stars have not been detected (detection rate of 91%), and most of the nondetected M stars are brown dwarfs or very low-mass stars (see discussion in Sect. 10.3). The obvious conclusion is that the detection rate in these volume-limited samples of the nearest stars is very large and in fact, there is no reason to expect that the remaining few undetected stars will not be detected with more sensitive X-ray observations. Therefore, the formation of X-ray emitting coronae appears to be universal for solar-like main-sequence stars, and intrinsically X-ray dark solar-like stars do not exist (at least within the immediate solar environment). The observed mean X-ray surface flux distribution of the NEXXUS sample displays a rather well-defined lower envelope of Fxiiim « 104 erg cm-2 s-1 (Fig. 10.7).
This lower limit to the X-ray surface flux actually compares well with the observed X-ray surface flux from solar coronal holes and it is suggestive to interpret the stars observed at their minimum flux levels as stars surrounded by coronal holes without any active regions.
absolute magnitude vs. X-ray surface flux
Fig. 10.7 X-ray surface flux vs. absolute V band magnitude for a volume-limited sample of nearby cool stars; + , F/G stars (d < 14 pc); * , K stars (d < 12 pc); x , M stars (d < 6 pc)
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