It is shown that, in the case of early B-type main-sequence stars, of the three ratios N/C, C/O, and N/O which are regarded as indicators of stellar evolution, the ratio N/O is more reliable since it seems to be insensitive to overionization of the NII and OII ions. On the other hand, the N/C and C/O ratios, which include carbon, may contain systematic errors for stars with Teff > 18500 K because of neglected overionization of CII ions. The ratio N/O is studied in the atmospheres of 46 early main-sequence B stars. These values of N/O are examined as functions of the effective temperature, age, rotation speed, and mass of the stars. Most early B-stars in the main sequence are found to have [N/O] ≈ 0, which indicates that N/O varies little during the main sequence stage, and this result is independent of the basic parameters listed above. There are two explanations for the large number of stars with [N/O] ≈ 0 : it is predicted theoretically that for an initial rotation velocity V0 < 100 km/s, N/O varies little toward the end of the main sequence stage ([N/O] < 0.2) and observations show that most early main-sequence B-stars do actually have low initial rotation velocities V0. The few early main-sequence B-stars with higher [N/O] = 0.4-0.8 correspond to models with rotation velocities V0 = 200-300 km/s. This conclusion is consistent with earlier data for stars with the same masses in a later stage of evolution: the AFGsupergiant and bright giant stage.
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Translated from Astrofizika, Vol. 59, No. 4, pp. 519-534 (November 2016).
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Lyubimkov, L.S. The N/O Ratio in Early B-Type Main Sequence Stars as an Indicator of Their Evolution. Astrophysics 59, 461–474 (2016). https://doi.org/10.1007/s10511-016-9449-2
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DOI: https://doi.org/10.1007/s10511-016-9449-2