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Astron. Astrophys. 318, 111-133 (1997)

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6. Stellar population

As stated above, the population of active coronae dominates our sample and down to a flux level of 0.03 cts s-1 stars account for 85% of the total number of field sources. Unfortunately, we do not have spectra for all star candidates in the 'full' area. However, in the restricted 'inner' area (38.1 deg2) we have spectroscopic information from the literature or from our own observations for all 42 identified X-ray stars detected above a count rate of 0.02 cts s-1. We list in Table 6 the repartition in spectral type of this complete subsample.


[TABLE]

Table 6. Repartition in spectral types of all X-ray stars (giant and main sequence) with count rate [FORMULA] 0.02 cts s-1 and located in the restricted 'inner' area


It is known that single A stars are very weak X-ray emitters in agreement with their almost absent convective zone and chromospheric activity. Therefore our identification of a small fraction of X-ray sources with A stars, here only based on a very low probability of random coincidence, may only be understood if one assumes the presence of an X-ray active companion of lower mass. Considering the rather short main sequence life time of A stars ([FORMULA] 109 yr) any coeval companion star will probably have a high X-ray activity as a result of its young age. However, ROSAT HRI observations seem to question this explanation and may point toward intrinsic X-ray emission in some late B stars (Berghöfer & Schmitt 1994). At the distance of the A stars the observed X-ray luminosities (see Fig. 14) are well in the range of those observed for classical active coronae. In two cases (RX J2056.7+4940, index 20 and RX J2117.8+5112, index 34) we obtained multicolour CCD images of the field but failed to identify a nearby likely alternative candidate. Assuming a limiting angular distance of [FORMULA] and a mean distance of 250 pc implies an upper limit of [FORMULA] 250 AU on the projected separation of the two stars.

[FIGURE] Fig. 14. Distribution in X-ray luminosities of all identified active coronae having spectral type information either from the literature or from our own optical observations. X-ray luminosities were computed assuming negligible interstellar absorption and a (0.1-2.4 keV) count to flux conversion factor of 1.0 [FORMULA] 10-11 erg cm2 s-1. As expected in an X-ray flux limited sample we preferentially sample the high luminosity tail of the X-ray luminosity function

Only two giant stars HR 8252 (RX J2133.9+4535, index 2) and HR 8072 (RX J2103.4+5021, index 37) are present in our spectroscopically identified sample in agreement with the low spatial density and moderate X-ray luminosities of the evolved late type stars (Maggio et al. 1990).

The number of identified close systems is also small. We find evidences for a short period binary in two cases, RX J2107.3+5202 (index 26, V1061 Cyg; eclipsing F8+G1, P = 2.3467 d, Dworak 1976) and RX J2035.9+4900 (index 53, exhibiting a clear line doubling, Motch et al. 1996a). In a subsequent article we shall discuss in more detail the contribution of close binaries using as additional material red optical spectra covering the Li [FORMULA] 6707 Å line at a slightly higher resolution than that of the blue medium resolution spectra discussed here. In one additional case, RX J2120.9+4636 (index 9) we find conspicuous line doubling in the red medium resolution spectra indicating a close binary.

Our stellar X-ray population displays all the characteristics of an X-ray flux limited sample. We preferentially detect the high X-ray luminosity tail of the distribution function, several stars exhibiting X-ray luminosities in excess of 1030 erg s-1 (see Fig. 14). X-ray luminosities were computed assuming negligible interstellar absorption and their estimated error is of the order of [FORMULA] 40% (Motch et al. 1996a). The mean X-ray luminosity clearly decreases with cooler photospheric effective temperature. This effect could be the result of a dependence of X-ray luminosity with stellar radius (Fleming et al. 1989). We note that the only M star observed above 1030 erg s-1 is the flaring source RX J2104.1+4912 (index 7, see Fig. 13) which would have been detected at about half of this luminosity in the absence of the flare. No particular spectral type dominates the stellar content of our survey area. Although the number density of dwarf G/K stars is only 10 to 16% that of dwarf M stars our distribution peaks at the G/K types. These number ratios indicate that the detection volume for G and K stars are about 14 and 8 times that of M stars and that accordingly, the mean X-ray luminosities of G and K stars are 5.7 and 4 times that of M stars. These figures are in general in good agreement with the mean Einstein X-ray luminosity ratios for disk stars (Micela et al. 1988, Barbera et al. 1993).

Obviously, our survey maximum sampling distance also depends on the spectral type and we detect F stars up to 500 pc and M stars only up to [FORMULA] 100 pc (see Fig. 15). Among the 24 open clusters recorded in SIMBAD in a [FORMULA] [FORMULA] [FORMULA] region centered at l = [FORMULA], b = [FORMULA], only M39 at (l = [FORMULA], b = [FORMULA]) is close enough (d = 300 [FORMULA] 30 pc; Mohan & Sagar 1985) to possibly introduce some inhomogeneity in our X-ray stellar sample. The relatively young age of the cluster (3 [FORMULA] 1 108 yr, Mohan & Sagar 1985) indeed suggests that some active coronae could be detected. However, we do not observe any strong clustering of identified active coronae (see Fig. 16) nor unidentified sources in that particular direction (see Fig. 22). Among the 4 X-ray stars detected within [FORMULA] from the center of the open cluster, only RX J2130.7+4919 (index 110) identified with Star 1930 in the Platais (1994) catalogue could be member of M39. Therefore, we believe that as far as active coronae are concerned our sample area is indeed representative of average mid galactic plane conditions.

[FIGURE] Fig. 15. Distribution in distance of all identified active coronae having spectral type information either from the literature or from our own optical observations
[FIGURE] Fig. 16. Position of the identified active coronae (maximum likelihood [FORMULA] 8) overlayed on the CO map from Dame et al. (1987) where we also show the area investigated in X-rays. Among the 24 open clusters present in a [FORMULA] [FORMULA] [FORMULA] region centered at l = [FORMULA], b = [FORMULA], only M39 (d [FORMULA] 300 pc) at l = [FORMULA], b = [FORMULA] is close enough to be possibly detected at our sensitivity. The absence of concentration of sources at this position indicates that the presence of the cluster does not significantly bias the stellar statistics

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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