Carbon, nitrogen and oxygen abundances in atmospheres of the 5–11 M⊙ B-type main-sequence stars

Abstract

Fundamental parameters and the carbon, nitrogen and oxygen abundances are determined for 22 B-type stars with distances d 600 pc and slow rotation (v sin i 66 km s −1 ). The stars are selected according to their effective temperatures T eff and surface gravities log g, namely: T eff is between 15300 and 24100 K and log g is mostly greater than 3.75; therefore, stars with medium masses of 5-11 M ⊙ are selected. Theory predicts for the stars with such parameters that the C, N and O abundances in their atmospheres should correspond to their initial values. Non-LTE analysis of C II, N II and O II lines is implemented. The following mean C, N and O abundances are obtained: log ǫ(C) = 8.31±0.13, log ǫ(N) = 7.80±0.12 and log ǫ(O) = 8.73±0.13. These values are in very good agreement with recent data on the C, N and O abundances for nearby B stars from other authors; it is important that different techniques are applied by us and other authors. When excluding for the stars HR 1810 and HR 2938, which can be mixed, we obtain the following mean abundances for the remaining 20 stars: log ǫ(C) = 8.33±0.11, log ǫ(N) = 7.78±0.09 and log ǫ(O) = 8.72±0.12; these values are in excellent agreement with a present-day Cosmic Abundance Standard (CAS) of Nieva & Przybilla.The derived mean N and O abundances in unevolved B stars are very close to the solar photospheric abundances, as well as to the protosolar ones. However, the mean C abundance is somewhat lower than the solar one; this small but stable carbon deficiency is confirmed by other authors. One may suggest two possibilities to explain the observed C deficiency. First, current non-LTE computations of C II lines are still partially inadequate. In this case the C deficiency is invalid, so one may conclude that the Sun and the local unevolved B stars have the same metallicity. This would mean that during the Sun's life (i.e., for the past 4.5 · 10 9 yr) the metallicity of the solar neighbourhood has not markedly changed; so, an intensive enrichment of the solar neighbourhood by metals occurred before the Sun's birth. Second, the C deficiency in the local B stars is valid; it is supposed that the Sun can migrate during its life from inner parts of the Galactic disk where it has born, so its observed chemical composition can differ from the composition of young stars in its present neighbourhood.

Possible Overionization of C II, N II, and O II Ions in the Atmospheres of Early B- and Late O-Type Stars

“…In addition the accuracy of the determination of the C, N and O abundances was increased, also for the B-type MS stars. The most recent work on these stars [6,7] shows that the average abundances of nitrogen and oxygen in B-type MS stars are now in good agreement with modern estimates for the sun. However, there is still a small deficiency for C. For example, an average carbon abundance of ( ) 04 0 33 8 C log .…”

“…As noted above, of the group of elements C, N, and O, a deficit of carbon is of greatest interest. with T eff , specifically, the relatively cool stars with effective temperatures T eff < 18100 K have a higher abundance of C on the average than the stars with T eff > 18500 K. The abundance of C for stars with T eff = 15300-18100 K is essentially the same as the solar abundance, while it is systematically lower for stars with T eff = 18500-24100 K. This can be seen in Table 2, where the average abundances of C for the relatively cool and hot B-stars [7] are compared. For the stars with T eff < 18100 K the average abundance [7] dex.…”

“…On the other hand, the amounts of many metals in nearby young stars agree well with the solar abundances; this does not support the alternative hypothesis. For example, agreement of this sort has been found between nearby B-type MS stars and the sun for the following elements: N and O [6,7], Mg [9], and Mg, Si, and Fe [10]. In the case of class A, F, and G supergiants, which are the direct descendents of B-type MS stars, agreement with the sun has been obtained for such indicators of metallicity as Fe [11] and Cr and Ti [12].…”

“…In fact, their age does not generally exceed 100 million years, or considerably less than the sun's age of 4.5 billion years. The distances to these stars [7] do not exceed 600 pc. Is it possible that the initial abundances of metals in such young and nearby stars could differ from the metal content of the sun?…”

“…In addition, the accuracy of the data is somewhat lower here. Nevertheless, as For further discussion it is necessary to recall briefly the method for determining the parameters T eff and log g in the earlier papers [7,24] that have been used to construct Figs. 1-3.…”