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Register a new userThe vertical Na-O relation in the bulge globular cluster NGC 6553
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In this article, we present a detailed chemical analysis of seven red giant members of NGC 6553 using high-resolution spectroscopy from VLT FLAMES. We obtained the stellar parameters (Teff, Log(g), vt, [Fe/H]) of these stars from the spectra, and we measured the chemical abundance for 20 elements, including light elements, iron-peak elements, alpha-elements and neutron-capture elements. The metallicities in our sample stars are consistent with a homogeneous distribution. We found a mean of [Fe/H]=-0.14+/-0.07 dex, in agreement with other studies. Using the alpha-elements Mg, Si, Ca and Ti we obtain the mean of [alpha/Fe]=0.11+/-0.05. We found a vertical relation between Na and O, characterized by a significant spread in Na and an almost non-existent spread in O. In fact, Na and Al are the only two light elements with a large intrinsic spread, which demonstrates the presence of Multiple Populations (MPs). An intrinsic spread in Mg is not detected in this study. The alpha, iron-peak and neutron capture elements show good agreement with the trend of the bulge field stars, indicating similar origin and evolution, in concordance with our previous studies for two other bulge GCs (NGC 6440 and NGC 6528).
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Globular Clusters are among the oldest objects in the Galaxy, thus their researchers are key to understanding the processes of evolution and formation that the galaxy has experienced in early stages. Spectroscopic studies allow us to carry out detailed analyzes on the chemical composition of Globular Clusters. The aim of our research is to perform a detailed analysis of chemical abundances to a sample of stars of the Bulge Globular Cluster NGC 6553, in order to determine chemical patterns that allow us to appreciate the phenomenon of Multiple Population in one of the most metal-rich Globular Clusters in the Galaxy. This analysis is being carried out with data obtained by FLAMES/GIRAFFE spectrograph, VVV Survey and DR2 of Gaia Mission. We analyzed 20 Red Horizontal Branch Stars, being the first extensive spectroscopic abundance analysis for this cluster and measured 8 chemical elements (O, Na, Mg, Si, Ca, Ti, Cr and Ni), deriving a mean iron content of $[Fe/H] = -0.10pm0.01$ and a mean of $[alpha/Fe] = 0.21pm0.02$, considering Mg, Si, Ca and Ti (errors on the mean). We found a significant spread in the content of Na but a small or negligible in O. We did not find an intrinsic variation in the content of $alpha$ and iron-peak elements, showing a good agreement with the trend of the Bulge field stars, suggesting a similar origin and evolution.
We used optical images acquired with the UVIS channel of the Wide Field Camera 3 on board of the Hubble Space Telescope to construct the first high-resolution extinction map in the direction of NGC 6440, a globular cluster located in the bulge of our Galaxy. The map has a spatial resolution of 0.5 over a rectangular region of about 160 X 240 around the cluster center, with the long side in the North-West/South-East direction. We found that the absorption clouds show patchy and filamentary sub-structures with extinction variations as large as $delta {rm E}(B-V)sim0.5$ mag. We also performed a first-order proper motion analysis to distinguish cluster members from field interlopers. After the field decontamination and the differential reddening correction, the cluster sequences in the color-magnitude diagram appear much better defined, providing the best optical color-magnitude diagram so far available for this cluster.
As part of a large HST project to study the formation and evolution of rich star clusters in the Large Magellanic Cloud, we present results for the Galactic bulge metal-rich globular cluster NGC6553. HST observations using WFPC2, NICMOS and STIS were obtained for this cluster. The primarily reason for studying NGC6553 is to allow the possibility to transform our NICMOS and STIS magnitudes directly into stellar masses. This is particularly important for determining the low mass end of the IMF. NGC 6553 was chosen because its metallicity, [Fe/H]$=-0.22pm 0.05$, is representative of the metallicities
We report the discovery of a large timescale candidate microlensing event of a bulge stellar source based on near-infrared observations with the VISTA Variables in the Via Lactea Survey (VVV). The new microlensing event is projected only 3.5 arcmin away from the center of the globular cluster NGC 6553. The source appears to be a bulge giant star with magnitude Ks = 13.52, based on the position in the color-magnitude diagram. The foreground lens may be located in the globular cluster, which has well-known parameters such as distance and proper motions. If the lens is a cluster member, we can directly estimate its mass simply following Paczynski et al. (1996) which is a modified version of the more general case due to Refsdal. In that case, the lens would be a massive stellar remnant, with M = 1.5-3.5 Msun. If the blending fraction of the microlensing event appears to be small, and this lens would represent a good isolated black hole (BH) candidate, that would be the oldest BH known. Alternative explanations (with a larger blending fraction) also point to a massive stellar remnant if the lens is located in the Galactic disk and does not belong to the globular cluster.
Globular clusters associated with the Galactic bulge are important tracers of stellar populations in the inner Galaxy. High resolution analysis of stars in these clusters allows us to characterize them in terms of kinematics, metallicity, and individual abundances, and to compare these fingerprints with those characterizing field populations. We present iron and element ratios for seven red giant stars in the globular cluster NGC~6723, based on high resolution spectroscopy. High resolution spectra ($Rsim48~000$) of seven K giants belonging to NGC 6723 were obtained with the FEROS spectrograph at the MPG/ESO 2.2m telescope. Photospheric parameters were derived from $sim130$ FeI and FeII transitions. Abundance ratios were obtained from line-to-line spectrum synthesis calculations on clean selected features. An intermediate metallicity of [Fe/H]$=-0.98pm0.08$ dex and a heliocentric radial velocity of $v_{hel}=-96.6pm1.3~km s^{-1}$ were found for NGC 6723. Alpha-element abundances present enhancements of $[O/Fe]=0.29pm0.18$ dex, $[Mg/Fe]=0.23pm0.10$ dex, $[Si/Fe]=0.36pm0.05$ dex, and $[Ca/Fe]=0.30pm0.07$ dex. Similar overabundance is found for the iron-peak Ti with $[Ti/Fe]=0.24pm0.09$ dex. Odd-Z elements Na and Al present abundances of $[Na/Fe]=0.00pm0.21$ dex and $[Al/Fe]=0.31pm0.21$ dex, respectively. Finally, the s-element Ba is also enhanced by $[Ba/Fe]=0.22pm0.21$ dex. The enhancement levels of NGC 6723 are comparable to those of other metal-intermediate bulge globular clusters. In turn, these enhancement levels are compatible with the abundance profiles displayed by bulge field stars at that metallicity. This hints at a possible similar chemical evolution with globular clusters and the metal-poor of the bulge going through an early prompt chemical enrichment.
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