Astron. Astrophys. 358, 233-241 (2000)

1. Introduction

and Reticuli form a striking couple of dwarf stars because they present some contradictory properties. As judged from their kinematical parameters they are very likely old disk population stars (Eggen, 1971). However  Ret presents emission in the CaII H and K lines with a strength that is typical of the young disk population. A previous paper (da Silva & Foy, 1987) put them under the zero age main sequence (ZAMS) corresponding to their metallicity. The similarities between their kinematical characteristics and the little distance that separates them from each other (310" or  pc) hint at a common origin for these stars.

and  Reticuli are classified as G3-5 V and G2 V, respectively, according to the Michigan Catalogue of Spectral Types (Houk & Cowley, 1975). They have common proper motions and radial velocities. Da Silva & Foy (1987), who made a detailed spectroscopic analysis of these stars, found for both of them a high surface gravity (log g = 4.70) typical of metal deficient, unevolved stars. Their gravity was determined based on the obtained by the authors and on the parallax values available at the time. In spite of their high surface gravity, the metal content determined by them for these stars was a bit higher than the solar one ([Fe/H] = +0.10, with the usual notation [A/H] = log(A/H)_* - ). This discrepancy could be explained, according to the authors, if the stars were helium rich, relatively to the Sun (Perrin et al., 1977).

Woolley (1970) has proposed that these two stars belong to the Herculis kinematic group defined by Eggen (1958). Porto de Mello & da Silva (1991) studied the physical existence of this group on the basis of an abundance analysis. For this purpose they used the and  Ret abundances determined by da Silva and Foy. However, there are great discrepancies between different authors who have analysed these stars. The effective temperatures determined for  Ret, for example, range from 5600 K to 6072 K. This variation is 3 times larger than the probable errors cited for the determinations. The determined iron abundances ([Fe/H]) present a variation of 0.52 dex between different authors. Precise determinations of these atmospheric parameters are needed to obtain accurate abundances and to analyse the evolutionary stage of the stars by way of evolutionary diagrams and isochrones, thus determining if and  Ret belong or not to the  Her group.

In the present analysis we seek to obtain the atmospheric parameters and element abundances of and  Ret with as high a precision as possible. Using these data we study the evolutionary stage of these stars to verify the hypothesis of helium superabundance. We finally analyse the physical existence of the  Her kinematic group.

© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000
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