Light Chemical Elements in Stars: Mysteries and Unsolved Problems

Abstract

them were initially synthesized in the Big Bang. The primordial abundances of these elements calculated using the Standard Model of the Big Bang (SMBB) are presented in this review. The good agreement between the SMBB and observations of the primordial abundances of the isotopes of hydrogen and helium, D, 3 He, and 4 He, is noted, but there is a difference of ~0.5 dex for lithium (the isotope 7 Li) between the SMBB and observations of old stars in the galactic halo that has not yet been explained. The abundances of light elements in stellar atmospheres depends on the initial rotation velocity, so the typical rotation velocities of young Main Sequence (MS) stars are examined. Since the data on the adundances of light elements in stars are very extensive, the main emphasis in this review is on several unsolved problems. The helium abundance He/H in early B-type MS stars shows an increment with age; in particular, for the most massive B stars with masses M 12 19 M  , He/H increases by more than a factor of two toward the end of the MS. Theoretical models of stars with rotation cannot explain such a large increase in He/H. For early B-and late O-types MS stars that are components of close binary systems, He/H undergoes a sharp jump in the middle of the MS stage that is a mystery for the theory. The anomalous abundance of helium (and lithium) in the atmospheres of chemically peculiar stars (types He-s, He-w, HgMn, Ap, and Am) is explained in terms of the diffusion of atoms in surface layers of the stars, but this hypothesis cannot yet explain all the features of the chemical composition of these stars. The abundances of lithium, beryllium, and boron in FGK-dwarfs manifest a trend with decreasing effective temperature T eff as well as a dip at T eff ~ 6600 K in the Hyades and other old clusters. These two effects are among the unsolved problems. In the case of lithium, there is special interest in FGK-giants and supergiants that are rich in lithium (they have log ε(Li) > 2 ). Most of them cannot be explained in terms of the standard theory of stellar evolution, so nonstandard hypotheses are invoked: the recent synthesis of lithium in a star and the engulfment by a star of a giant planet with mass equal to that of Jupiter or greater. An analysis of the abundances of carbon, nitrogen, and oxygen in early B-and late O-type MS stars indicates that the C II, N II, and O II ions are overionized in their atmospheres. For early B-type MS stars, good agreement is found between observations of the N/O ratio and model calculations for rotating stars. A quantitative explanation of the well-known "nitrogen-oxygen" anticorrelation in FGK-giants and supergiants is found. It reflects the dependence of the anomalies in N and C on the initial rotation velocity V 0 . The stellar rotation models which yield successful explanations for C, N. and O cannot, however, explain the observed increased helium enrichment in early B-type MS stars.

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.…”

“…Nevertheless, it should be stated that such accordance between observations and the theory exists not always. As noted in Lyubimkov (2018) review, there is a significant discrepancy between observations and the theory for helium, the main product of CNO-cycle. The same computations of Georgy et al (2013), which seem to be successful in the case of C, N and O, cannot explain the observed helium enrichment in the B-type MS stars.…”

“…We note once again that the stars of both types have masses M = 4-19 M ⊙ . The problem of observed rotational velocities for the MS stars of various spectral types is discussed in Lyubimkov (2018) review on light elements in stars.…”

Three Nearby K-Giants with Planets: Accurate Determination of Basic Parameters, Including an Analysis of Metallicity Based on Fe I Lines

“…The first of these two problems concerning giants and supergiants that are rich and super-rich in lithium has been discussed in some detail in a review [2]; some details on light elements are also given in Ref. 3. As noted there [2,3], most lithium-rich stars and all stars that are super-rich in lithium cannot be explained in terms of the standard theory of stellar evolution.…”

Stark Broadening of Neutral Boron Lines

“…The standard Big Bang nucleosynthesis (BBN) theory is not effective to explain the generation of 6 Li, 9 Be, 10 B, 11 B [27-29], what is reflected in the low abundance of these simple species. Recently, according to [30], there are modern data indicating that first chemical elements up to oxygen are formed in Big Bang nucleosynthesis. Nevertheless, the abundance questions are among unsolved problems.…”