Abundances and physical parameters for stars in the open clusters NGC 5822 and IC 4756

G. Pace, J. Danziger, G. Carraro, J. Meléndez, P. François, F. Matteucci, N. C. Santos

Abstract
Context. Classical chemical analyses may be affected by systematic errors that would cause observed abundance differences between dwarfs and giants. For some elements, however, the abundance difference could be real.
Aims. We address the issue by observing 2 solar-type dwarfs in NGC 5822 and 3 in IC 4756, and comparing their composition with that of 3 giants in either of the aforementioned clusters.We determine iron abundance and stellar parameters of the dwarf stars, and the abundances of calcium, sodium, nickel, titanium, aluminium, chromium, silicon, and oxygen for both the giants and dwarfs. For the dwarfs, we also estimate the rotation velocities, and and lithium abundances. We improve the cluster parameter estimates (distance, age, and reddening) by comparing existing photometry with new isochrones.
Methods. We acquired UVES high-resolution, of high signal-to-noise ratio (S/N) spectra. The width of the cross correlation profiles was used to measure rotation velocities. For abundance determinations, the standard equivalent width analysis was performed differentially with respect to the Sun. For lithium and oxygen, we derived abundances by comparing synthetic spectra with observed line features.
Results. We find an iron abundance for dwarf stars equal to solar to within the margins of error for IC 4756, and slightly above for NGC 5822 ([Fe/H]= 0.01 and 0.05 dex respectively). The 3 stars in NGC 4756 have lithium abundances between Log N(Li)≈ 2.6 and 2.8 dex, the two stars in NGC 5822 have Log N(Li)≈ 2.8 and 2.5 respectively.
Conclusions. For sodium, silicon, and titanium, we show that abundances of giants are significantly higher than those of the dwarfs of the same cluster (about 0.15, 0.15, and 0.35 dex). Other elements may also undergo enhancement, but all within 0.1 dex. Indications of much stronger enhancements can be found using literature data. But artifacts of the analysis may be partly responsible for this.

Keywords
open clusters and associations: individual: NGC 5822 - open clusters and associations: individual: IC 4756 - stars: abundances - stars: rotation

Notes
Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, during the observing runs 073.D-0655 and 079.C-0131.
Table 2 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A28

Astronomy and Astrophysics
Volume 515, Page A28_1
June 2010

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