Abundâncias químicas em estrelas Gigantes vermelhas dos Aglomerados abertos NGC 2420, NGC 6819 e NGC 6705 a partir dos espectros APOGEE

Abstract

Chemical Abundances in red giants of the open clusters NGC 2420, NGC 6819 and NGC 6705 from APOGEE spectra Studying chemical abundances of stars belonging to open clusters helps in understanding the formation and evolution of the galactic disc, since these are tracers of the chemical structure of the disc. We study the abundances of the heavy elements cerium and neodymium in a sample of red giant stars belonging to the open clusters NGC 2420, NGC 6705 and NGC 6819, as well as a control sample composed by field stars previously studied in the literature. In addition, we study in detail the chemical patterns of the young open cluster (age ~300 Million years) NGC 6705. This study is based on a detailed manual spectroscopic analysis of infra-red spectra between 1.5 and 1.7 \mum obtained by SDSS IV - APOGEE. Determination of the abundances was made in ETL via spectral synthesis and using the MOOG code and Kurucz atmospheric models. The results obtained for the [Ce/Fe] and [Nd/Fe] behaviors as a function of metalicity indicate a slight increase in [Ce, Nd /Fe] abundance as the metalicity decreases in the range of -1.5 to ~ +0.15 dex. The mean abundance of <[Ce/Fe]> obtained for the cluster NGC 2420 is +0.18±0.07; for NGC 6705, +0.09±0.03; for NGC 6819, -0.06±0.01 and, for neodimium, <[Nd/Fe]> are: 0.19±0.08, 0.18±0.10 and 0.06±0.01, respectively. When comparing Nd abundances obtained for the three clusters with the gradient of [Nd/Fe] with a galactocentric distance it is found that [Nd/Fe] ratio for the cluster NGC 6705, located in the innermost part of the disc, does not seem to follow exactly the positive gradient of [Nd/Fe] vs. Rg obtained in the literature. With the discovery of field stars that are enriched in ⍺-process elements although they are young, and some controversial results in the literature in regard to NGC 6705 also being enriched in ⍺-elements, the study of the abundance pattern of the open cluster NGC 6705 is very relevant. In this work, we determinated the abundances of C, N, Fe and ⍺-elements (O, Si, Mg, Ca and Ti) of four giant stars belonging to the red clump of the open cluster NGC 6705. The mean abundances obtained are: <A(C)>=8.58±0.11, <A(O)>=8.83±0.24, <A(N)>=8.58±0.11, <A(Fe)>=7.58±0.06, <A(Si)>=7.69±0.11, <A(Mg)>=7.60±0.11, <A(Ca)>=6.44±0.14 and <A(Ti)>=5.10±0.15, and the mean metalicity obtained of the cluster was <[⍺/Fe]> = 0.01 ± 0.04, these results are consistent with the expectation that the disc young open cluster NGC 6705 has a solar ratio of ⍺-elements to iron, not confirming the previous results of the literature for this cluster.