Nitrogen enrichment in atmospheres of A- and F-type supergiants

Abstract

Using new accurate fundamental parameters of 30 Galactic A and F supergiants, namely their effective temperatures T eff and surface gravities log g, we implemented a non-LTE analysis of the nitrogen abundance in their atmospheres. It is shown that the non-LTE corrections to the N abundances increase with T eff . The nitrogen overabundance as a general feature of this type of stars is confirmed. A majority of the stars have a nitrogen excess [N/Fe] between 0.2 and 0.9 dex with the maximum position of the star's distribution on [N/Fe] between 0.4 and 0.7 dex. The N excesses are discussed in light of predictions for B-type main sequence (MS) stars with rotationally induced mixing and for their next evolutionary phase, i.e. A-and F-type supergiants that have experienced the first dredge-up. Rotationally induced mixing in the MS progenitors of the supergiants may be a significant cause of the nitrogen excesses. When comparing our results with predictions of the theory developed for stars with the mixing, we find that the bulk of the supergiants (28 of 30) show the N enrichment that can be expected (i) either after the MS phase for stars with the initial rotational velocities v 0 = 200-400 km s −1 (ii) or after the first dredge-up for stars with v 0 = 50-400 km s −1 . The latter possibility is preferred on account of the longer lifetime for stars on red-blue loops following the first dredge-up. Two supergiants without a discernible N enrichment, namely HR 825 and HR 7876, may be post-MS objects with the relatively low initial rotational velocity of about 100 km s −1 . The suggested range for v 0 is approximately consistent with inferences from the observed projected rotational velocities of B-type MS stars, progenitors of A and F supergiants.

Spectral modelling of theαVirginis (Spica) binary system

“…These values are in good agreement with those found by Aufdenberg et al (2008) for the primary star, (24750 ± 500, 3.71 ± 0.06). For the secondary, our result is in agreement with Aufdenberg et al's log g pole = 4.16 ± 0.05, and Lyubimkov et al (1995) T pole eff = 20800 ± 1500 K.…”

THE DISTRIBUTION OF THE ELEMENTS IN THE GALACTIC DISK. III. A RECONSIDERATION OF CEPHEIDS FROMl= 30° TO 250°

“…The [C/Fe] and [N/Fe] ratios are +0.1 and +0.3, respectively, while the C/O ratio is 0.55. The [N/Fe] ratio is a bit high but could be overestimated by up to 0.2-0.3 dex due to non-LTE effects (Lyubimkov et al 2011). The [C/Fe] and C/O ratios are significantly larger than the typical ratios of −0.31 and 0.25, respectively, found in Cepheids.…”

“…Non-LTE effects in CNO have been considered by Fabbian et al (2006) for carbon, by Lyubimkov et al (2011) for nitrogen, and by Takeda & Takada-Hidai (1998) In the temperature range 5800-6400 K, Lyubimkov et al (2011) give the non-LTE correction for N i as + 0.2 to + 0.3 dex in the sense that LTE abundances are larger. Our gf values for N i 744.2 and 746.8 nm are about 0.2 dex higher than assumed by Lyubimkov et al This means our LTE values would be at −0.2 dex relative to theirs, thus when the non-LTE correction is added our nitrogen abundances should be on the same scale as theirs.…”

The VLT-FLAMES survey of massive stars: Nitrogen abundances for Be-type stars in the Magellanic Clouds

“…Lyubimkov et al (2011) observed such nitrogen enhancements in A-and F-type supergiants which could have obtained their nitrogen enrichment from rotational mixing during a B-type MS lifetime. To examine this further it would have been useful to investigate the relationship between observed nitrogen enrichment and fundamental parameters such as the mass and main sequence lifetime of our objects.…”