Results of a volume-limited X-ray survey of stars of luminosity classes IV and V in the spectral range F7-G9 observed with the Einstein Observatory are presented. Using survival analysis techniques, the stellar X-ray luminosity function in the 0.15-4.0 keV energy band for both single and multiple sources. It is shown that the difference in X-ray luminosity between these two classes of sources is consistent with the superposition of individual components in multiple-component systems, whose X-ray properties are similar to those of the single-component sources. The X-ray emission of the stars in our sample is well correlated with their chromospheric CA II H-K line emission and with their projected equatorial rotational velocity. Comparison of the X-ray luminosity function constructed for the sample of the dG stars of the local population with the corresponding functions derived elsewhere for the Hyades, the Pleiades, and the Orion Ic open cluster confirms that the level of X-ray emission decreases with stellar age.
Table 1 lists the sample of 173 stars observed for this study. They are selected from the Bright Star Catalogue (Cat. <V/50>), with the following selection criteria: - Spectral type between A8 and G2; no spectral peculiarities noted; not double in spectral type classification (e.g., HR 32 with spectral type F2V+F6V is excluded); - Luminosity class V; - Right ascension between 0h and 2h, or between 14h and 24 h, declination south of +10 degrees (defining the region on the sky visible during the appointed observation times); - Binaries for which both components occurred in the BSC are excluded, if the separation is less than 10". Not listed are five stars for which no (Walraven photometric and ROSAT X-ray) data are available. These are HR 591, HR 5542, HR 6593, HR 8245 and HR 8735. Table 2 lists the Walraven photometric (VBLUW) data for all but four stars from Table 1. Also listed in Table 2 are the effective temperature, surface gravity and the reddening, as derived from comparison with theoretical colours. Table 5 lists the ROSAT All Sky Survey data for all but 11 stars from Table 1. For a description of the Walraven photometric system, see e.g. <GCPD/11>
Within the NaCo-ISPY exoplanet imaging program, we aim at detecting and characterizing the population of low-mass companions at wide separations (>~10AU). We observed R CrA twice with the NaCo instrument at the Very Large Telescope (VLT) in the L' filter with a one year time baseline in between. We used artificial negative signals to determine the position and brightness of the companion and the related uncertainties. The companion is detected at a separation of 196.8+/-4.5/196.6+/-5.9mas (18.7+/-1.3/18.7+/-1.4AU) and position angle of 134.7+/-0.5{deg}/133.7+/-0.7{deg} in the first/second epoch observation. We measure a contrast of 7.29+/-0.18/6.70+/-0.15mag with respect to the primary. A study of the stellar proper motion rejects the hypothesis that the signal is a background object. Depending on the assumed age, extinction, and brightness of the primary, the stellar companion has a mass between 0.10+/-0.02M_{sun}_ and 1.03^+0.20^_-0.18_M_{sun}_ range, if no contribution from circumsecondary material is taken into account. The presence of this companion needs to be taken into account when analyzing the complex circumstellar environment of R CrA.
