next previous
Up: Carbon and nitrogen abundances


4 Results, discussion and conclusions

An inspection of Fig. 2 and the corresponding measurements given in Table 1 shows for the stars $\epsilon$ Eri, $\alpha$ Cen A and B, YY Gem, $\pi^1$ UMa, and AD Leo that the measured values of  $R_{\rm NC}$ are consistent (to within the measurement errors) with the theoretical expectations from a cosmic abundance plasma in collisional equilibrium. However, the giant stars Capella, HR 1099, UX Ari, $\beta $ Cet, and Algol lie - in increasing strength - significantly above the theoretical expectations. For the latter no emission from C  VI (or any other carbon ion) has been measured so that only a lower limit ( $R_{\rm NC,Algol} \ge 23.3$) can be derived, and the value measured for $\beta $ Cet ( $R_{\rm NC,\beta Cet}
= 8.48 \pm 2.4$) deviates by more than one order of magnitude from theoretical expectations. Thus all giants in our sample show significantly enhanced $R_{\rm NC}$-ratios, while all dwarf stars have $R_{\rm NC}$-ratios consistent with cosmic abundances. We propose to attribute these anomalously large $R_{\rm NC}$-values to the nitrogen enrichment expected from mass transfer in Algol and nitrogen dredge-up in $\beta $ Cet and suggest that also in the cases of HR 1099 and UX Ari CNO-cycle burning with ensuing nitrogen dredge-up has begun.

If we accept a value of $R_{\rm NC}= 0.57$ as the maximally possible one for a cosmic abundance plasma, the measured $R_{\rm NC}$-values can be directly converted to lower limits for the deviations of the N/C abundance with respect to cosmic abundances. We note that these values are independent of the actually prevailing differential emission measure distribution. We specifically find for Algol $\log \Delta {\rm N/C}> 1.61$, for $\beta $ Cet $\log \Delta {\rm N/C}> 1.17$, while for UX Ari we find $\log \Delta {\rm N/C} > 0.55$ and for HR 1099 $\log \Delta {\rm N/C}> 0.16$.

How reliable are those numbers? In Table 1 we quote the statistical measurement errors. In order to obtain flux ratios, one has to multiply with the ratio of the effective LETGS areas at 24.74 Å and 33.74 Å, i.e., only the relative error enters, which is expected to be <5%. The theoretical  $R_{\rm NC}$ ratio depends on the collisional excitation rates and the ionization equilibrium. These errors are difficult to specify; the former are believed to have accuracies of $\approx$20%. As far as the latter are concerned, different equilibrium calculations may result in a small shift in temperature scale. We therefore conclude that the deviations of the observed  $R_{\rm NC}$ ratio for HR 1099, UX Ari, and specifically of $\beta $ Cet and Algol cannot be attributed to errors in the model, but have a physical cause.

How unique is our proposed interpretation? Can we be sure that we actually do see the late-type star in the Algol system? While a mathematical proof for the absence of X-ray emission from Algol A cannot be given, such an assumption appears extremely unlikely. First, none of the single A-type stars in the solar neighborhood are X-ray sources (cf. Schmitt 1997). Second, in the case of the eclipsing binary $\alpha$ CrB, consisting of a primary of spectral type A0V and a secondary of type G2V, a total X-ray eclipse is observed, demonstrating that the early-type star can contribute a minute fraction of the total X-ray flux at best (Schmitt & Kürster 1993). And third, a long duration flare observed on Algol with BeppoSAX (Schmitt & Favata 1999) shows a total eclipse of the flare plasma at (optical) secondary minimum, when Algol A is seen in front of Algol B, thus demonstrating that at least the flare was associated with Algol B. We therefore conclude that it is extremely unlikely that Algol A significantly contributes to the total X-ray flux from this system.

In summary, we have shown that the LETGS spectra of Algol, $\beta $ Cet, and the RS CVn binaries Capella, HR 1099, and UX Ari can only be understood by an enhanced nitrogen abundance. It is natural to attribute this enhancement to the CNO-cycle operating in the cores of these stars. X-ray spectroscopy can therefore be used to test the predictions of stellar evolution theory.

Acknowledgements

J.-U.N. acknowledges financial support from Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) under 50OR98010. Fruitful discussions with Prof. H. Kähler on stellar evolution are acknowledged.


next previous
Up: Carbon and nitrogen abundances

Copyright ESO 2002