An inspection of Fig. 2 and the corresponding measurements given in
Table 1 shows for the stars Eri,
Cen A and B,
YY Gem,
UMa, and AD Leo that the measured values of
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,
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 (
)
can be derived, and the value measured for
Cet (
)
deviates by more than one order of magnitude from
theoretical expectations. Thus all giants in our sample show
significantly enhanced
-ratios, while all dwarf stars have
-ratios consistent with cosmic abundances.
We propose to attribute these anomalously large
-values
to the nitrogen enrichment expected from mass transfer in Algol and
nitrogen dredge-up in
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
as the maximally possible one for
a cosmic abundance plasma, the measured
-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
,
for
Cet
,
while for UX Ari we find
and
for HR 1099
.
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
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
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
ratio for HR 1099, UX Ari, and specifically of
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 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, 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.
Copyright ESO 2002