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Astron. Astrophys. 346, 811-818 (1999)

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4. Discussion

The AR Lac light curve observed by the Beppo SAX MECS shows a very narrow primary eclipse and no signature of a secondary eclipse. However, at the times of the three observed secondary eclipses, the system apparently was in a low-level flaring state which could have filled-in any small secondary eclipse dip. During the primary eclipse the reduction in the MECS count rate was [FORMULA]25%, which is comparable to the [FORMULA]32% decrease observed by the Einstein IPC (Walter et al. 1983), but less than the [FORMULA]45%, [FORMULA]40%, and [FORMULA]50% decreases observed by the EXOSAT LEIT (White et al. 1990), ROSAT PSPC (Ottmann et al. 1993), and ASCA GIS&SIS (White et al. 1994), respectively. Such a comparison is justified, because even though the MECS is not very sensitive at the low energies ([FORMULA]1.7 keV), no strong energy dependence of the overall orbital modulation was found by the above quoted ROSAT PSPC and ASCA observations, with the depth of the primary eclipse above 2 keV similar to that at low energies. The important aspect of our Beppo-SAX observations is the short duration of the primary eclipse that seems to begin after the 2nd contact and to end before the 4th contact. Such a short eclipse, indicates a well localized and confined G-star corona. Similar but somewhat longer primary eclipses were observed by the Einstein IPC (Walter et al. 1983) and ROSAT PSPC (Ottmann et al. 1993), while the primary ecplises observed by EXOSAT and ASCA (White et al. 1990, White et al. 1994) were much larger with gradual ingress and egress extending well before and after the 1st and 4th contacts, respectively. Moreover, we recall that no primary eclipse at all was observed by the EXOSAT ME (1-6 keV) detector (White et al. 1990), and that this was interpreted as due to the high temperature component of the X-ray plasma being extended on a scale comparable to the binary separation. The scenario that can be drawn from the comparison of the results of all of the available X-ray observations of AR Lac is that the spatial structure of the corona of this system varies with time.

Our analysis of the AR Lac X-ray spectra confirms the existence of a bimodal temperature structure, with the cool component at about 0.8 keV and the hot at about 2.0 keV, consistent with the results of White et al. (1994) and Kaastra et al. (1996), based on the analysis of ASCA data, and of Walter (1996) who analyzed EUVE data. The MECS light curve gives essential information on the high temperature component, that seems to be very well confined and localized close to the G star at the time of our Beppo-SAX observations. No observations were made with the LECS detector during primary eclipses, so we cannot infer anything about the localization of the low temperature component.

Our data confirm that the metal abundances of the AR Lac corona are different from those of the solar photosphere. We obtain a global metal abundance, Z=0.65 solar, that is a bit less than twice the mean value claimed by White et al. (1994) and by Kaastra et al. (1996), who constrained the individual element abundances using ASCA data, and by Walter (1996), who estimated a [FORMULA] ratio equal to 0.39 solar from EUVE spectra. However, we found that a coronal plasma with a global metal abundance of 0.41 could adequately reproduce also the Beppo-SAX spectra if one let the neutral hydrogen column density be about 10 times higher than expected by Linsky et al. (1998). Conversely, the data in the region below 0.5 keV could not be fitted with the low global metal abundance that best fit the data at high energy, unless one assumes a much larger H I interstellar absorption than is implied by HST observations along nearby lines of sight. It is worth noting that the ASCA instruments are not sensitive under 0.5 keV, therefore the ASCA data cannot lead to a similar result. In at least two other cases - VY Ari (Favata et al. 1997b) and HD 9770 (Tagliaferri et al. 1997) - the Beppo-SAX LECS data could be fitted best by assuming a higher than expected H I column density. We now pose the question if the extra-absorption required by the AR Lac Beppo-SAX spectrum below 0.5 keV can be due to circumstellar H I or to H I in the stellar corona. From H[FORMULA] observations of AR Lac obtained in 1997, Frasca et al. (1999) found evidence of circumstellar material before and during the primary eclipse. Moreover, recent data on Sun, e.g. TRACE data (Hurlburt et al. 1998), showed the presence of bound-free absorption due to H I in the solar coronae. If the solar-stellar analogy is valid also in this context, not to take into account the H I bound-free absorption in the stellar corona, could be reflected in a large value of the H I interstellar column density, as it results from our best-fit of the AR Lac X-ray spectrum.

Our analysis of the AR Lac Beppo-SAX data emphasize the dependence of derived abundances on the precise spectral quantity being fit and the mutual interdependence of NH, Z and the two component emission measure ratios. This result lead us to question the credibility of the abundance values that are found when analyzing stellar coronae. Drake (1998) suggested as alternative explanation for the apparently low metal abundances observed in some stellar coronae, that the He abundance in these coronae is enhanced (e.g. doubled), causing lower line-to-continuum ratios. The He might be selectively enhanced by preferential mechanisms for retention of He nuclei in the corona or by diffusion processes. Conversely, we performed a spectral analysis using a two component MEKAL plasma model with all element abundances fixed equal to those of the solar photosphere, except for He which was left as a free parameter in the fitting procedure, and found that an acceptable solution ([FORMULA]=1.07, 351 d.o.f.) can be found for a He abundance of 2.6[FORMULA]0.5 times the solar photospheric value, and a temperature structure that is almost the same as in the case of a metal deficient plasma (Tc[FORMULA]0.78 keV, Th[FORMULA]2.05 keV and EMc/EMh[FORMULA]0.59). If the He abundance is really enhanced or if the stellar coronae are really metal poor is a topic that will be probably best addressed only by the more sensitive and with better spectral resolution future X-ray missions.

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© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999