2. Observations and data analysis
2.1. Program stars
This X-ray study is based on the known sample of stars within a distance 13 pc around the Sun as listed in the Third Catalog of Nearby Stars; I use the updated version made available to me by Jahreiss (1991; personal communication). From this catalog those dwarf stars were selected with distances less than or equal to 13 pc and with B-V colors in the range , thus corresponding to a spectral type of A, F and G. Since the selection was made on B-V color, there are also four K0 type stars in the sample, i.e., Gl 309, Gl 454, Gl 635 B, and Gl 764. A volume-limited study of stars with colors in excess of has been performed by Schmitt et al. (1995), while a similar study of giant stars within a distance range of 25 pc has been presented by Hünsch et al. (1996). All stars classified as white dwarfs, subdwarfs or giants have been excluded from the sample in an attempt to restrict the study to main-sequence stars only; I did, however, retain some stars classified as luminosity class IV or VI. My full sample consists out of a total of 74 stars. A number of objects are in binary systems which cannot be resolved or for which no high angular resolution X-ray data are available at present; a detailed account of these cases and how they were treated is presented in Sect. 3.2 In Table 1 I list my sample stars by their Gliese (Gl) and HD numbers, give spectral types (as given in the most recent version of this catalog), absolute and apparent magnitudes, distances (in pc), B-V colors, and U, V, W space motions (in km/sec). Obviously, the composition of my sample by spectral type represents the space densities of stars of different masses. Thus, the sample contains only five A-type stars: Sirius (= Gl 244A) of type A0V, Leo (= Gl 448) of type A3V, Vega (= Gl 721) of type A0V, Altair (=Gl 768) of type A7 IV-V, and Fomalhaut (= Gl 881) of type A3V. In this paper I will not discuss the X-ray properties of Sirius A, which is accompanied by the famous X-ray bright white dwarf Sirius B. The two stars cannot resolved in PSPC data. A ROSAT HRI pointing on the Sirius system was carried out in March 1991, at a time when the separation between the two stars was only 3 arc sec. Inspection of the image (after despeckling, cf., Schmitt et al. 1994) shows an elongation of the image along the known position angle, however, the separation is too close to derive a count rate reliable and precise enough to assess whether this rate is consistent with that expected from the known ROSAT HRI UV leak (cf., Schmitt et al. 1993). Sirius A is therefore left out from all further considerations, and my useful sample thus contains only 73 stars.
Table 1. Solar-like stars
Table 1. (continued)
2.2. X-ray observations and data analysis
The primary data base is that provided by the ROSAT all-sky survey. A detailed description of the ROSAT satellite and its hardware is given by Trümper (1983) and Pfeffermann et al. (1986). Here it suffices to note that during the all-sky survey the ROSAT satellite was scanning the sky along great circles perpendicular to the plane of the ecliptic. By following the apparent solar motion the whole sky was covered. During a given scan, an X-ray source stayed in the 2 degree field of view of the ROSAT Position Sensitive Proportional Counter (PSPC) for up to 30 seconds; the overall survey observations thus consist of a series of individual co-added "snapshots" yielding a total useful exposure time of typically 400 - 600 seconds . For the analysis of the survey observations the same procedures as used by Schmitt et al. (1995) were employed. Program stars not detected in the all-sky survey were observed in the ROSAT pointing program. The sources were placed on-axis with typical exposure times between 1 and 2 ksec, but in some cases deeper exposures were obtained (cf., information provided in Table 2).
Table 2. X-ray data for solar-like stars
Table 2. (continued)
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998