Stars with spectral types later than F5 show magnetic surface activity,
chromospheres and coronae like the Sun. Stars with earlier spectral types
appear to have a weaker outer atmosphere, supposedly due to the shallowness or
absence of outer convection zones in these stars
(Schrijver 1993, and references therein). As part of a study of the onset of stellar activity along
the main sequence, we have used ROSAT all-sky survey data to determine X-ray
fluxes of a sample of 173 main-sequence stars, with spectral types between A8
and G2. In order to derive detailed quantitative information on the location of
these stars in the Hertzsprung-Russell diagram, we have also made Walraven
five-colour photometric observations.
The sample studied here was selected from the Bright Star Catalogue (BSC;
Hoffleit & Jaschek 1982, lists almost all stars with mag)
according to the following criteria:
The selected sample is listed in Table 1, excluding five stars for which no (photometric and X-ray) data are available (HR 591, HR 5542, HR 6593, HR 8245, HR 8735). Four stars have not been observed in the Walraven photometric system, and for 11 stars we have no ROSAT data available. These 15 stars with incomplete data are indicated as such in Table 1.
This paper reports the results of the ROSAT and Walraven observations. Spectroscopic observations and determinations of the rotational velocities for the sample stars have been described in Groot et al. (1996).