Detailed analysis of a sample of field metal-poor N-rich dwarfs

Abstract

Aims: The aim of this work is to compare the detailed chemical composition of the field N-rich dwarf stars to the second-generation stars of globular clusters (GCs) in order to investigate the hypothesis that they originated in GCs.
Methods: We measured the abundance of 23 elements (from Li to Eu) in a sample of six metal-poor N-rich stars (three of them pointed out for the first time), and we compared their chemical composition to (i) the chemical composition observed in a sample of classic metal-poor stars and (ii) the abundances observed in the second-generation stars of GCs.
Results: In metal-poor N-rich stars, C and O are slightly deficient, but the scatter of [(C+N+O)/Fe] is very small, a strong indication that the N enrichment is the result of pollution by CNO-processed material. The N-rich stars of our sample, similarly to the second-generation stars in the GCs, show an excess of Na and sometimes of Al, as expected if the material from which these stars were formed, has been polluted by the ejecta of massive asymptotic giant branch (AGB) stars. For the first time, we have been able to establish an anti-correlation Na-O in field stars such as the one observed in NGC 6752. The N-rich star HD 74000 has a rather low [Eu/Ba] ratio for its metallicity. Such an anomaly is also observed in several second-generation stars of M 15.
Conclusions: This analysis supports the hypothesis that the N-rich stars observed today in the field were born as second-generation stars in GCs.

Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere (archives of programmes 090.B-0504(A) PI Chaname; 095.D-0504(A) PI Melendez; 076.B-0166(A) PI Pasquini; 067.D-0086(A) PI Gehren; 071.B-0529(A) PI Silva; 065.L-0507(A) PI Primas), and collected at the W. M. Keck Observatory, archive programme G401H, PI Melendez. One star was also observed at ESO with the spectrograph ESPRESSO, programme 107.22RU.001 PI Spite, and two stars were observed at the Observatoire de Haute-Provence (archives of programme 10A.PNPS.HALB, PI Halbwachs) and at the Narval spectrograph of the Observatoire du Pic du Midi, programme L172N04, PI Spite.