ROSAT EUV LUMINOSITY FUNCTIONS OF NEARBY LATE-TYPE STARS

The ROSATEUV all-sky survey was conducted with the UK Wide Field Camer a (WFC) over the six-month period 1990 July to 1991 January, in two 'c olours' using broadband filters to define wavebands centred at about 1 00 and 140 angstrom. We present EUV luminosity functions (LFs) for vol ume-limited samples of stars selected from the Third Catalogue of Near by Stars (CNS3), using distance limits of 25 and 16 pc for the F-K and M stars respectively. LFs have been computed for both single stars an d spectroscopic binaries of each spectral type in both of the WFC surv ey passbands. The LFs may be adequately described by power laws and ar e sufficiently flat that the stellar number-flux relations ('log N - l og S') will be heavily influenced by the higher luminosity objects. Sp ectroscopic binaries have higher mean EUV luminosity and flatter slope than single stars and, indeed, the high-luminosity tail apparent in t he G-star LF can be almost entirely attributed to spectroscopic binari es, the majority of which are RS CVn systems. A comparison with earlie r Einstein Observatory (soft X-ray) results shows that there is broad agreement for the mean luminosities of F, G and K stars. Recently publ ished Einstein results for M stars are also consistent with the EUV LF s, though earlier Einstein studies indicated much higher mean luminosi ties. This provides an explanation as to why the EUV log N - log S, co ntrary to pre-launch predictions, is not dominated by M stars. Compari son of log N - log S distributions predicted from our EUV LFs with tho se of late-type stars in the WFC Bright Source Catalogue (BSC), sampli ng EUV-bright stars over a larger distance range than our CNS3 study, shows an excess of K stars in BSC relative to the CNS3 sample. Most of these objects have recently been identified as active stars, and may well constitute a 'yellow-star' excess similar to that found in Einste in X-ray surveys.