5 research outputs found
HST/ACS colour-magnitude diagrams of M31 globular clusters
With the aim of increasing the sample of M31 clusters for which a colour
magnitude diagram is available, we searched the HST archive for ACS images
containing objects included in the Revised Bologna Catalogue of M31 globular
clusters. Sixty-three such objects were found. We used the ACS images to
confirm or revise their classification and we obtained useful CMDs for 11 old
globular clusters and 6 luminous young clusters. We obtained simultaneous
estimates of the distance, reddening, and metallicity of old clusters by
comparing their observed field-decontaminated CMDs with a grid of template
clusters of the Milky Way. We estimated the age of the young clusters by
fitting with theoretical isochrones. For the old clusters, we found
metallicities in the range -0.4<=[Fe/H]<=-1.9, that generally agree with
existing spectroscopic extimates. At least four of them display a clear blue
HB, indicating ages >10 Gyr. All six candidate young clusters are found to have
ages <1Gyr. With the present work the total number of M31 GCs with reliable
optical CMD increases from 35 to 44 for the old clusters, and from 7 to 11 for
the young ones. The old clusters show similar characteristics to those of the
MW. We discuss the case of the cluster B407, with a metallicity [Fe/H] ~-0.6
and located at a large projected distance from the centre of M31 and from the
galaxy major axis. Metal-rich globulars at large galactocentric distances are
rare both in M31 and in the MW. B407, in addition, has a velocity in stark
contrast with the rotation pattern shared by the bulk of M31 clusters of
similar metallicity. This, along with other empirical evidence, supports the
hypothesis that the cluster is physically associated with a substructure in the
M31 halo that has been interpreted as the relic of a merging event.Comment: 19 pages, 14 figures, 6 tables. Accepted for publication on Astronomy
and Astrophysic
Family ties: abundances in Terzan 7, a Sgr dSph globular cluster
We study the chemical composition of 5 giant stars in the globular cluster
Terzan 7 associated with the Sagittarius dwarf spheroidal galaxy (Sgr dSph),
based on high resolution UVES-VLT spectra. We confirm the metallicity found by
previous high resolution investigations: [Fe/H] ~ -0.6. We also show that this
cluster displays the same low alpha-element to iron ratio found in Sgr dSph
field stars of similar metallicity, as well as the same low Ni/Fe ratio. These
chemical signatures are characteristic of the Sgr dSph system, and appear to be
shared both by the globular cluster Pal 12, which was most likely stripped from
Sgr by tidal interaction, and by Pal 5, which may also have belonged in the
past to the Sgr dSph system. Intriguingly even globular cluster Ruprecht 106,
although not associated to Sgr, displays similar characteristics.Comment: 9 pages, A&A accepte
Stellar substructures in the solar neighbourhood. I. Kinematic group 3 in the Geneva-Copenhagen survey
Context. Galactic archeology is a powerful tool for investigating the formation and evolution of the Milky Way. We use this technique to study kinematic groups of F- and G-stars in the solar neighbourhood. From correlations between orbital parameters, three new coherent groups of stars were recently identified and suggested to correspond to remnants of disrupted satellites.
Aims. We determine detailed elemental abundances in stars belonging to one of these groups and compare their chemical composition with Galactic disc stars. The aim is to look for possible chemical signatures that might give information about the history of this kinematic group of stars.
Methods. High-resolution spectra were obtained with the FIES spectrograph at the Nordic Optical Telescope, La Palma, and analysed with a differential model atmosphere method. Comparison stars were observed and analysed with the same method.
Results. The average value of [Fe/H] for the 20 stars investigated in this study is â0.69 ± 0.05 dex. Elemental abundances of oxygen and α-elements are overabundant in comparison with Galactic thin-disc dwarfs and thin-disc chemical evolution models. This abundance pattern has similar characteristics as the Galactic thick-disc.
Conclusions. The homogeneous chemical composition together with the kinematic properties and ages of stars in the investigated Group 3 of the Geneva-Copenhagen survey provides evidence of their common origin and possible relation to an ancient merging event. The similar chemical composition of stars in the investigated group and the thick-disc stars might suggest that their formation histories are linked