The N/O Ratio in Early B-Type Main Sequence Stars as an Indicator of Their Evolution

Abstract

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 T eff > 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 V 0 < 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 mainsequence B-stars do actually have low initial rotation velocities V 0 . The few early main-sequence Bstars with higher [N/O] = 0.4-0.8 correspond to models with rotation velocities V 0 = 200-300 km/s. This conclusion is consistent with earlier data for stars with the same masses in a later stage of evolution: the AFG-supergiant and bright giant stage.

Oxygen abundance and the N/C versus N/O relation for AFG supergiants and bright giants

“…As an example of good agreement with these results, one may indicate an analysis of the N/O ratio for 46 early B-type MS stars (Lyubimkov 2016(Lyubimkov , 2018. It was found that most of the stars near the MS termination (82 %) show the relatively low values [N/O] < = 0.3; from the viewpoint of the theory (Georgy et al 2013), these stars had the low initial rotational velocities V 0 from 0 to 130 km/s.…”

Analysis of the Non-LTE Lithium Abundance for a Large Sample of F-, G-, and K-Giants and Supergiants

“…Data from a recent analysis of the N/O ratio in the atmospheres of 46 early MS B-stars [13] can also serve as confirmation of this effect. N/O can be regarded as one of the indicators of stellar evolution.…”

“…Strictly speaking, a comparison with models of rotating stars implies that they have V 0 < 100km/s. Only when V 0 = 200-300 km/s are elevated values of [N/O] = 0.4 -0.8 observed at the end of the MS, but there are few stars of this kind [13].…”

“…N/O can be regarded as one of the indicators of stellar evolution. It was shown [13] that for most of these stars this ratio did not vary during evolution in the MS (i.e.,   0…”