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Astron. Astrophys. 318, 215-230 (1997)

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3. Results

3.1. Basic data

The basic results of our X-ray observations are presented in Table 2, where I provide the star name in column 1, the observing mode ("SU" for survey data, "PO" for pointing data, and "BO" for pointing data obtained with the boron filter) in column 2, the exposure time (in sec) in column 3. In column 4 I give an upper limit flag "UL" for those stars not detected, which remains empty for the detections. For those cases the entry in column 5 provides a 95 % confidence upper limit to the PSPC count rate in the broad band while column 7 remains empty. For the detections I give the PSPC broad band count rates and their "1 [FORMULA] " errors in columns 5 and 6 of Table 2. In column 7 of Table 1 the existence likelihood (ML) of the detections is listed; it provides a measure of how much the assumption "source plus background" improves the statistical description of the data over the assumption "background only" (cf., Cruddace et al. 1988). Candidate positions, where sources with an existence likelihood [FORMULA] have been found, I consider as upper limits. Col. 8 in Table 2 denotes the hardness ratio as determined from the PSPC data. Denoting by H and S the counts recorded in the soft and hard PSPC pulse height channels respectively, the hardness ratio HR is defined through

[EQUATION]

as soft band I use the PSPC pulse height channels 11 - 41, as hard band those between 52 and 201, respectively. In those cases where a significant detection was obtained only in the soft band, a hardness ratio value HR = -1 was adopted. In col. 9 of Table 1 I provide the logarithmic X-ray luminosities in the 0.1 - 2.4 keV pass band, calculated with the distances in Table 1 and a conversion factor (from count rate to energy flux) computed from the hardness ratio HR through [FORMULA] i.e., using the same procedure as Schmitt et al. (1995): in those cases where only soft counts (i.e., HR=-1) were recorded, a value of CEF = 6  [FORMULA] was used. In column 10 I indicate the fraction f of the total count rate assigned to individual binary components; for single stars, [FORMULA] by definition. In the absence of any further information the total flux was divided equally among all system components. In those cases where HRI pointings were available I used the flux division as measured with the HRI. In the following section I discuss how the all individual cases were treated.

3.2. Notes on individual stars

In the following sections some notes on individual stars are provided. Specifically I discuss how the known binary or multiple systems were treated; in some cases ambiguities in the attribution of the X-ray emission could be resolved with new ROSAT HRI images. I also give a very brief discussion of the PSPC observation of Vega.

Gl 34 A/B (= [FORMULA] Cas): Consists of a G0V and M0V star, separated by [FORMULA] 12 arc sec. In Fig. 1a I display a ROSAT HRI image of [FORMULA] Cas, clearly showing two sources. The northern source is Gl 34 B, the flux is distributed in the ratio 1: 2.25 between the A and B components, i.e., the M-star is the X-ray brighter one, consistent with Einstein Observatory HRI observations (cf., Harris & Johnson 1985). Gl 53 A/B (= µ Cas): Close pair, unresolvable for the ROSAT PSPC and HRI. [FORMULA] ; µ Cas could not be detected in the RASS data.

[FIGURE] Fig. 1a. ROSAT HRI images of resolved binary systems containing nearby solar-like stars in visual double stars. All the images show the actual recorded photon events in the highest possible angular resolution (which oversamples the point response by a factor of 10) in relative coordinates. This is Gl 34 A/B; for details see text.

HD 12545: Long-Period RS CVn star (cf., Bopp et al. 1993.

Gl 107 A/B (= [FORMULA] Per): Wide pair, consisting of F7V and M1V star at 22 arc sec separation; since no ROSAT HRI observations are available, X-ray emission is equally attributed to both components.

Gl 216 A/B (= [FORMULA] Lep): Wide pair, consisting of F6V and K2V star. Einstein Observatory HRI observations yielded a flux ratio of 1:6.6 (cf., Schmitt et al. 1985).

Gl 222 A/B (= [FORMULA] Ori): Wide pair consisting of G0V star and dM star. Following Haisch et al. (1994), we attribute all the X-ray emission to the G-type star.

Gl 280 A/B (= [FORMULA] CMi) = Procyon: Consists of F5 IV-V star and cool white dwarf; the X-ray emission comes exclusively from the non-degenerate star (cf., Schmitt et al. 1985).

Gl 356 A/B: (= 11 LMi) Faint, late-type companion at 4 arc sec distance (cf., Poveda et al. 1994). All the X-ray emission is attributed to primary.

Gl 423 A/B (= [FORMULA] UMa): Close pair, unresolvable for the ROSAT PSPC and HRI. Consists of two G0Ve stars, both of which are spectroscopic binaries. The X-ray emission is equally distributed among components.

Gl 442 A/B (= HR 4523): Very faint companion (LHS 313) 25 arc sec away (cf., Poveda et al. 1994). All the X-ray emission is attributed to primary.

Gl 451 A/B: Close pair, unresolvable for the ROSAT PSPC and HRI. Faint companion, all the X-ray emission is attributed to primary.

Gl 482 A/B: Close pair, unresolvable for the ROSAT PSPC and HRI. Consists of two almost identical F0V stars, thus X-ray emission is equally distributed among components.

Gl 534.1A (= HR 5236): Poveda et al. (1994) list faint companion 33 arc sec away. Gl 534.1A was not detected in the RASS data.

Gl 549 A/B (= [FORMULA] Boo): Wide pair, consisting of G2V and late-type star more than 1 arc min away. All the X-ray emission is attributed to primary since RASS position agrees very well with primary position.

Gl 559 A/B (= [FORMULA] Cen): Wide pair, consisting of G2V and K0V star. The X-ray emission of the [FORMULA] Cen system was first resolved with the Einstein Observatory HRI (cf., Golub et al. 1981); a ROSAT HRI observation displayed in Fig. 1b confirms that the K component is brighter.

[FIGURE] Fig. 1b. ROSAT HRI image of [FORMULA] Cen A/B; for details see text.

Gl 566 A/B (= [FORMULA] Boo): Wide pair, consisting of G8Ve and K4Ve star with a separation of 7 arc sec at position angle [FORMULA]. In Fig. 1c I display a ROSAT HRI image of the [FORMULA] Boo system; the observations were despeckled in a manner very similar to that described and applied by Schmitt et al. (1994) in their analysis of the Castor system. The ROSAT HRI image appears to be elongated along the (optical) position angle, however, it is also clear, that most of the emission of the [FORMULA] Boo system comes from only one component. If one interprets the emission in the vicinity of the secondary as real, the primary is also the dominant X-ray source; if only one source contributes, one cannot discriminate between the two sources. In the following I attribute all of the X-ray emission to the optical primary.

[FIGURE] Fig. 1c. ROSAT HRI image of [FORMULA] Boo; no emission from secondary is detected, for details see text.

Gl 575 A/B (= 44 Boo): W UMa system.

Gl 601 A/B (= [FORMULA] Tra): Poveda et al. (1994) list faint companion 157 arc sec away. All the X-ray emission is attributed to primary. RASS position agrees very well with primary position.

Gl 611 A/B (= HD 144579): Poveda et al. (1994) give rather faint companion at a distance of 70 arc sec. All the X-ray emission is attributed to primary and RASS position agrees very well with primary position.

Gl 620.1A (= HR 6094): Poveda et al. (1994) list a white dwarf more than 5 arc min away. All the X-ray emission is attributed to primary.

Gl 635 A/B (= [FORMULA] Her): Close pair, unresolvable for the ROSAT PSPC and HRI. Consists of F and G star, the X-ray emission is equally distributed among components.

Gl 666A (= HR 6416): Wide pair at 10 arc sec distance, consisting of G8V and M0V star at an angular distance of 10 arc sec. PSPC pointing data is available, but only a few counts were detected. No ROSAT HRI data available, the X-ray emission is attributed to primary component.

Gl 695 A/B (= µ Her): Triple, consisting of a wide pair and a close pair, unresolvable for the ROSAT PSPC and HRI. The two components of the close pair are M dwarfs, the other component of the wide pair is a G5IV star at a separation of 34 arc sec. The ROSAT HRI image (cf., Fig. 1d) shows emission from both the M stars (western source) as well as the G star equally distributed among the two components.

[FIGURE] Fig. 1d. ROSAT HRI image of µ Her A/B; for details see text.

Gl 713 A/B (= [FORMULA] Dra): Close pair consisting of F and G star, unresolvable for the ROSAT PSPC and HRI. The X-ray emission is equally distributed among components.

Gl 721 (= [FORMULA] Lyr = Vega): An extremely weak source is found at the position of Vega; it is present only in the very softest PSPC channels, and hence completely consistent with UV contamination, which is predicted at a level of 2.2  [FORMULA] cts/sec. I interpret this "detection" as upper limit.

Gl 853 A/B (= HR 8501): Poveda et al. (1994) list somewhat fainter companion 3.4 arc sec away, unresolvable for the ROSAT PSPC and HRI. X-ray emission is equally distributed among components.

Gl 914 A/B (= 85 Peg): Triple system, consisting of a close pair, unresolvable for the ROSAT PSPC and HRI, and a distant K7 component more than 1 arc min away. The X-ray emission is equally distributed among the components of close pair. The RASS position is 55 arc sec off the nominal primary position, therefore the majority for the X-ray flux might come from the late-type companion; for the moment I assign half the X-ray flux to the primary.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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