First stars VI – Abundances of C, N, O, Li, and mixing in extremely metal-poor giants. Galactic evolution of the light elements

Abstract

Abstract.We have investigated the poorly-understood origin of nitrogen in the early Galaxy by determining N abundances from the NH band at 336 nm in 35 extremely metal-poor halo giants, with carbon and oxygen abundances from Cayrel et al. (2004Cayrel et al. ( , A&A, 416, 1117, using high-quality ESO VLT/UVES spectra (30 of our 35 stars are in the range −4.1 < [Fe/H] < −2.7 and 22 stars have [Fe/H] < −3.0). N abundances derived both from the NH band and from the CN band at 389 nm for 10 stars correlate well, but show a systematic difference of 0.4 dex, which we attribute to uncertainties in the physical parameters of the NH band (line positions, gf values, dissociation energy, etc.). Because any dredge-up of CNO processed material to the surface may complicate the interpretation of CNO abundances in giants, we have also measured the surface abundance of lithium in our stars as a diagnostic of such mixing. Our sample shows a clear dichotomy between two groups of stars. The first group shows evidence of C to N conversion through CN cycling and strong Li dilution, a signature of mixing; these stars are generally more evolved and located on the upper Red Giant Branch (RGB) or Horizontal Branch (HB). The second group has [N/Fe] < 0.5, shows no evidence for C to N conversion, and Li is only moderately diluted; these stars belong to the lower RGB and we conclude that their C and N abundances are very close to those of the gas from which they formed in the early Galaxy, they are called "unmixed stars". The enhancements, they were formed before any significant enrichment of the Galactic gas by SNe Ia, and their composition should reflect the yields of the first SNe II. However, if massive AGB stars or AGB supernovae evolved more rapidly than SNe Ia and contaminated the ISM, our stars would also reflect the yields of these AGB stars. At present it cannot be decided whether primary N is produced primarily in SNe II or in massive AGB stars, or in both Key words. Galaxy: abundances -Galaxy: halo -Galaxy: evolution -stars: abundances -stars: evolutionstars: supernovae: general Based on observations obtained with the ESO VLT under ESO programme ID 165.N-0276(A). This work has made use of the SIMBAD database.

The ESO UVES advanced data products quasar sample – III. Evidence of bimodality in the [N/α] distribution

“…These yields of primary N production are still uncertain, since they are computed assuming a very high rotation velocity for the metal-poor stars. The discovery of metal-poor halo stars with high (N/O) ratios seems to confirm the primary N production in massive stars (Israelian et al 2004;Spite et al 2005) with a yield that depends on stellar mass and metallicity (Chiappini et al 2006). The theoretical prescriptions regarding the primary N production are uncertain and model dependent, therefore the relative amount of primary N produced in massive stars (Chiappini et al 2005;Maeder et al 2009) and IMS is not well established.…”

“…In H ii regions of spiral and dwarf-irregular galaxies c 2013 RAS (van Zee et al 1998;van Zee & Haynes 2006), metal-poor emission-line galaxies (Nava et al 2006;Izotov et al 2006;Pérez-Montero & Contini 2009), and H ii regions in blue compact dwarf (BCD) galaxies (Izotov & Thuan 1999, 2004, (N/O) ratios show a primary plateau at low oxygen abundances and a secondary behaviour for [O/H] > −1. Nitrogen abundances were also measured in low-metallicity galactic halo stars (Spite et al 2005) and H ii regions in Large Magellanic Cloud (Bekki & Tsujimoto 2010). In particular, it is important to verify if there is any contribution from massive stars at low metallicities.…”

Abundances of carbon-enhanced metal-poor stars as constraints on their formation

“…Two stars, HE 0400−2030 and HE 2319−5228 (discussed in more detail in Sect. 6.3), are found to have mixed CNO-cycled material to their surfaces, converting some C into N (Spite et al 2005(Spite et al , 2006), while C is normal or enhanced in the rest of the stars. We note that since most of our stars have relatively high gravities (and are subgiants), we do not expect that mixing would alter the surface composition of, e.g., C, and in most cases the C corrections 4 from Placco et al (2014b) are negligible (of the order of 0.0 −0.03 dex).…”

“…Carbon isotopic ratios, 12 C / 13 C Convection, either in AGB stars or in spinstars, drives the CNO cycle and transports 13 C and N, created at the expense of 12 C, to a star's surface. This is detectable in the isotopic abundance ratios of C and N, where low 12 C/ 13 C and [C/N] ratios indicate strong internal mixing with CN-cycled material (Spite et al 2005). For 13 of our programme CEMP stars we find ratios that cover 3 < 12 C/ 13 C < 50, where the higher end of this range is in good agreement with Bisterzo et al (2011Bisterzo et al ( , 2012, who find that their AGB models without strong mixing do not result in the low, observed values (4 < 12 C/ 13 C < 10).…”

Evolution and Nucleosynthesis of Massive Stars

“…This can be seen comparing chemical evolution models for the Galactic halo with the abundances observed at the surface of normal iron-poor halo stars [8]. These authors compare their predictions for the evolution of N/O and C/O upon the adoption of different sets of stellar yields with the surface ratios determined for the metal-poor stars observed by [27] and [45] (see Fig. 2).…”