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

Abstract

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).

Light Elements in the Universe

“…The evolution of nitrogen, and in particular of the N/O abundance ratio, has been studied in our Galaxy with observations of samples of stars and H II regions (e.g., Christlieb et al, 2004;Israelian et al, 2004;Carigi et al, 2005;Esteban et al, 2005;Spite et al, 2005;Lyubimkov et al, 2013;Lyubimkov et al, 2019) to put constraints on both Galactic evolution and nucleosynthesis processes. Chiappini et al (2005) interpreted the origin and evolution of nitrogen in our Galaxy comparing the observed abundances in the Milky Way Galaxy with their two-infall model (originally developed in Chiappini et al, 1997), similarly to Gavilán et al (2006), who introduced a primary component in the production of N from intermediatemass stars to explain to observed abundances.…”

Fundamental parameters and non‐local thermodynamic equilibrium abundances of key light elements for nine bright and nearby K giants

“…How do the derived abundances of light elements correspond to this evolutionary status? In Table 11 we present the mean values of the C and O abundances and the C/O ratio; apart from nine program K giants, we give there for comparison data for 17 AFG supergiants and bright giants, the evolved stars with masses M = 4–19 M ⊙ (Lyubimkov et al 2019b). The non‐LTE C and O abundances for these stars were determined using the same technique as for our K giants.…”

“…Earlier the non-LTE determination of the C and O abundances was implemented for AFG-type supergiants and bright giants by Lyubimkov et al (2015Lyubimkov et al ( , 2019b. Now we perform a similar analysis for cooler and less massive stars, our program K giants.…”

“…Our technique of the non-LTE computations of C I and O I lines were considered in detail in the works of Lyubimkov et al (2015Lyubimkov et al ( , 2019b. Using the updated code SYN-THV of Tsymbal (1996), we calculated synthetic spectra for selected wavelength regions; all lines in these regions from database VALD3 (Ryabchikova et al 2015) were taken into account.…”

“…How do the derived abundances of light elements correspond to this evolutionary status? In (Lyubimkov et al 2019b). The non-LTE C and O abundances for these stars were determined using the same technique as for our K giants.…”