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

Lyubimkov, L. S.

doi:10.1007/s10511-013-9301-x

Cited by 10 publications

(13 citation statements)

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“…This is confirmed by UV observations of two early B-type stars, b CMa (B1 II-III) and e CMa (B2 II), with the EUVE satellite. The observed flux was greater than the theoretical value by two orders of magnitude for λ  504Å and by several times for λ 912Å .It has been reported[39] that there appears to be superionization of C II, N II, and O II ions in the atmospheres of early B-and late O-type stars that is not taken into account in conventional calculations. It becomes significant at temperatures T eff > 18500 K in the case of C II lines and T eff > 26000 K for N II and O II lines.…”

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confidence: 59%

Light Chemical Elements in Stars: Mysteries and Unsolved Problems

Lyubimkov

2018

Astrophysics

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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.

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confidence: 59%

Light Chemical Elements in Stars: Mysteries and Unsolved Problems

Lyubimkov

2018

Astrophysics

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“…Nieva & Przybilla (2012) estimated "cosmic" standard abundances (rather the present-day abundances in the solar neighborhood) using bright, slowlyrotating, early-type B stars. Due to their short main sequence lives, B stars contain abundance information that is both contemparaneous and local, and their purely radiative atmospheres are comparatively simple to model, although UV photoionization rates must be correctly modeled (Lyubimkov 2013). Nieva & Przybilla (2012) report metallicities that are very close to solar, depsite 4.5 Gyr of intervening GCE, and a mean C/O of 0.37, lower than the solar value of 0.55.…”

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confidence: 99%

What Are Little Worlds Made Of? Stellar Abundances and the Building Blocks of Planets

Gaidos

2015

ApJ

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If the photospheres of solar-type stars represent the composition of circumstellar disks from which any planets formed, spectroscopic determinations of stellar elemental abundances offer information on the composition of those planets, including smaller, rocky planets. In particular, the C/O ratio is proposed to be a key determinant of the composition of solids that condense from disk gas and are incorporated into planets. Also, planets may leave chemical signatures on the photospheres of their host stars by sequestering heavy elements, or by being accreted by the stars. The presence, absence, and composition of planets could be revealed by small differences in the relative abundances between stars. I critically examine these scenarios and show that (i) a model of Galactic chemical evolution predicts that the C/O ratio is expected to be close to the solar value and vary little between dwarf stars in the solar neighborhood; (ii) spectroscopic surveys of M dwarf stars limit the occurrence of stars with C/O 1 to < 10 −3 ; and (iii) planetesimal chemistry will be controlled by the composition of oxygen-rich dust inherited from the molecular cloud and processed in a dust-rich environment, not a gas with the stellar composition. A second generation of more reduced planetesimals could be produced by re-equilibration of material with dust-depleted gas. Finally, I discuss how minor differences in relative abundances between stars that correlate with condensation temperature can be explained by dust-gas segregation, perhaps in circumstellar disks, rather than planet formation.

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“…It should be noted that the solar C, N and O abundances are known presently with high accuracy. For instance, if one takes Caffau et al (2011) Apart from these very close solar data, we used the above-mentioned CNO-abundances determined for the unevolved early B-type MS stars (Lyubimkov 2013). These data can be actually considered as the initial abundances for AFG supergiants and bright giants, which are descendants of the B-type MS stars.…”

Section: The N/c Vs N/o Relationmentioning

confidence: 99%

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

Lyubimkov

Коротин

Lambert

2019

Monthly Notices of the Royal Astronomical Society

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Non-LTE analysis (LTE is local thermodynamic equilibrium) of the oxygen abundances for 51 Galactic A-, F-and G-type supergiants and bright giants is performed. In contrast with carbon and nitrogen, oxygen does not show any significant systematic anomalies in their abundances log ε(O). There is no marked difference from the initial oxygen abundance within errors of the log ε(O) determination across the T eff interval from 4500 to 8500 K and the log g interval from 1.2 to 2.9 dex. This result agrees well with theoretical predictions for stellar models with rotation. With our new data for oxygen and our earlier non-LTE determinations of the N and C abundances for stars from the same sample, we constructed the [N/C] vs [N/O] relation for 17 stars. This relation is known to be a sensitive indicator of stellar evolution. A pronounced correlation between [N/C] vs [N/O] is found; the observed [N/C] increase from 0 to 1.6 dex is accompanied by the [N/O] increase from 0 to 0.9 dex. When comparing the observed [N/C] vs [N/O] relation with the theoretical one, we show that this relation reflects a strong dependence of the evolutionary changes in CNO abundances on the initial rotation velocities of stars. Given that the initial rotational velocities of these stars are expected to satisfy V 0 <150 km/s, it is found that they are mostly the post first dredge-up (post-FDU) objects. It is important that just such initial velocities V 0 are typical for about 80 % of stars in question (i.e. for stars with masses 4-19 M ⊙ ). A constancy of the total C+N+O abundance during stellar evolution is confirmed. The mean value log ε(C+N+O)=8.97±0.08 found for AFG supergiants and bright giants seems to be very close to the initial value 8.92 (the Sun) or 8.94 (the unevolved B-type MS stars).

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

Cited by 10 publications

References 29 publications

Light Chemical Elements in Stars: Mysteries and Unsolved Problems

Light Chemical Elements in Stars: Mysteries and Unsolved Problems

What Are Little Worlds Made Of? Stellar Abundances and the Building Blocks of Planets

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

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