4 Conclusions

We have presented an X-ray selected sample of solar-type stars observed by ROSAT in its all-sky survey. We have obtained the spectral classification from $BV(RI)_{\rm C}$ observations, as well as the distances and X-ray luminosities. Most of the sample objects lie within a radius of 200 pc from the Sun. Apart from a few exceptions, they are all active stars. The X-ray luminosities are of the order of $10^{29} - 10^{30}\ {\rm erg} \ {\rm s}^{-1}$,with a few objects having lower or higher luminosities. The hardness ratios are centered around a mean value of 0.13, quite typical for active stars.

A comparison with the Einstein EMSS sample shows that both samples have similar characteristics, and that they can therefore be considered as coming from the same population, representative of the solar-type X-ray population in the solar neighbourhood. The only difference between the samples is the presence of a high-luminosity tail ($10^{32}\ {\rm erg} \ {\rm s}^{-1}$) in the EMSS sample which is absent in our sample. Uncertainties in the determination of the stellar absolute magnitude could possibly be responsible for this difference, however, from the comparison with known RS CVn systems, we argue that very active A-K coronal sources are indeed rare.

The EXOSAT sample shows an apparently strong difference in the distribution in spectral types, but the low number of sources does not allow any statistical conclusions.

Our results allow us to determine at least some selection criteria for solar-type stellar sources in the RASS. The best candidates are sources with apparent visual magnitude brighter than 15mag and situated at less than 20$^{\prime\prime}$ from the X-ray position. The sources should have a hardness ratio between -0.4 and 0.7, and a ratio of X-ray to visual flux included in the interval of $\log (f_{\rm X}/f_V)$from -5 to -1.8.

Acknowledgements

This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. The ROSAT project has been supported by the Bundesministerium für Forschung und Technologie (BMFT) and the Max-Planck-Gesellschaft (MPG). GC thank the European Southern Observatory for financial support, under the "Senior Visitor Program'', that made possible his permanence at ESO-Santiago

  

Table 3: Summary of X-ray and observational data as well as results for the studied sample

 

Table 3: continued

 

Table 3: continued