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Astron. Astrophys. 350, 598-602 (1999)

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5. Discussion

While inspecting Table 4 one feature draws attention: all of the investigated stars possess an apparent carbon deficiency. Nitrogen shows either an under- or over- abundance with respect to the solar value. This hints that 1) the program stars represent the different initial abundances of carbon and nitrogen and 2) some of them show probably the signs of CN-processed material at the surface. Let us discuss the implications further.

Incomplete CNO cycling leads to transformation of some part of the carbon nuclei into nitrogen nuclei without a significant alteration in the number of oxygen nuclei. Upon being brought to the surface and mixed with the photospheric gas (provided a mixing mechanism is present) the CN-processed material alters the initial atmospheric chemical composition - carbon becomes deficient, while nitrogen becomes proportionately overabundant. To express the resulting nitrogen abundance one can use the following formula:

[EQUATION]

where

[EQUATION]

In Fig. 2 we show several dependencies representing [FORMULA] as a function of the [FORMULA] value for different sets of the initial carbon and nitrogen abundances scaled by a factor of 0.2 dex starting from the solar composition (i.e., 8.55/7.97). Of course, the initial C and N abundances cannot with absolute confidence be proportionally scaled to the solar abundances of these elements, but the figure does indicate the overall behavior of the final nitrogen abundance.

[FIGURE] Fig. 2. Resulting nitrogen abundance as a function of carbon content. Solid lines are dependencies obtained by decreasing the carbon abundance from initial value with a step of [FORMULA]=0.1 [FORMULA] (indicated near dashed lines). The initial values of carbon and nitrogen abundances for different cases are indicated near the solid lines. Stars of Ori OB1 association (crosses) have been added to our sample

As one can see from Fig. 2, three stars of our sample likely possess CN-processed material in their atmospheres, while the rest have carbon and nitrogen abundances which are close to the presumed initial values. It is interesting to note that from the whole sample only two stars: HD 45546 and HD 74280 have an initial CN-abundance which could be considered as solar-like. (Note that the result for HD 74280 is not quite reliable as only one carbon and three nitrogen lines were used).

Carbon appears to be deficient in main-sequence B stars. This circumstance indicates that the rather high initial abundance inherent to Sun is likely not representative of main-sequence B stars.

As a particular example, in Fig. 2 we added to our sample of stars those belonging to Ori OB1 association which were analysed by Kilian (1992). Ori OB1 stars are compactly clustered on the figure (as should be expected for the stars that were simultaneously born from rather homogeneous gas) and their position also clearly indicates the common carbon deficiency.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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