1 Introduction

"Upgren's Unclassified Stars: A New Type of G-Giant Stars?'' - was the title of a paper by Sturch & Helfer (1971) in which UBVRI photometry was presented for 17 stars for which Upgren (1962) could not obtain luminosities. Upgren (1962) had conducted objective-prism observations with dispersion of 580 Å/mm^-1 for late-type stars near the north galactic pole. For the G stars, luminosity criteria were the two CN bands at $\lambda 3820{-}80$ Å and $\lambda 4130{-}4215$ Å. It appeared that for some G stars, luminosity determination from these features was inaccurate. Sturch & Helfer, however, also met with difficulties: the position of stars observed in the U-B,R-I diagram matched neither the Hyades nor nearby field dwarfs, nor field giants with r<100 pc, nor M $\thinspace 67$ or the giant branches of a variety of globular clusters. The authors concluded that these unclassified stars probably belong to the field equivalents of the red horizontal branch (RHB) stars of metal-rich globular clusters. This paper marked the beginning of a serious effort to study red horizontal branch stars in the Galactic field (see Tautvaisiene 1996a for a review).

64 G stars from Upgren's list were investigated by Rose (1985) using a quantitative three-dimensional spectral classification system employing 2.5 Å resolution spectra in the blue. A number of Upgren's unclassified stars were found to be dwarfs. Quite a large group of Upgren's G stars were, however, shown to be evolved, based on the strength of their Sr II $\lambda 4077$ Å line. They were also distinguished from post-main-sequence stars evolving through the same region of the HR diagram because of the unique appearance of their CN $\lambda 3883$ and 4216 Å bands. It was concluded that a class of red horizontal-branch stars, similar to those in the "metal-rich'' globular cluster M 71, has been identified in the Galactic disk. Moreover, it was noticed that these stars have metallicities and kinematics which are common for the "thick disk'' of the Galaxy revealed by Gilmore & Reid (1983). Detailed measurements of kinematic parameters of the stars by Stetson & Aikman (1987) have confirmed that they belong to the thick disk of the Galaxy.

Norris (1987) reported DDO observations for ten of Upgren's red giants which Rose (1985) identified as RHB stars and presented arguments that these stars could equally well be the core-helium-burning "clump'' stars similar to those seen in the old, metal deficient open cluster NGC 2243. Consequently they could be as young as about 5-7 Gyr rather than about 14 Gyr as would follow from their identity to the population of metal-rich galactic disc globular clusters.

Photometric observations and three-dimensional classification in the Vilnius photometric system were carried out for 13 of the Roses's RHB stars by Tautvaisiene (1996b). The results were photometric spectral types, metallicities, effective temperatures, surface gravities, absolute magnitudes and ages. The stars form a group with mean $\rm [Fe/H]=-0.6\pm0.1$ which is between -0.7 as evaluated by Rose (1985) and -0.5 as determined by Norris (1987) from the cyanogen excess parameter $\delta_{\rm CN}$. An age of about 10-12 Gyr was ascribed to the group from comparison with model isochrones. This age is intermediate between the ages of the disk globular clusters and the oldest open clusters.

The aim of the present study is to perform a high resolution spectroscopic analysis of 13 red horizontal branch stars which were identified in Upgren's list by Rose (1985). We expect that C/N and ¹²C/¹³C abundance ratios, and possibly also the abundances of sodium, aluminium and s-process elements, will provide information on the extent of mixing processes in these evolved stars. Abundances of other chemical elements will be useful for the interpretation of the chemical evolution of the thick disk of the Galaxy.