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Astron. Astrophys. 364, 102-136 (2000)

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1. Introduction

The field blue horizontal branch (BHB) stars have often been used to trace the galactic halo. Recent surveys of distant BHB stars include those of Pier (1983), Sommer-Larsen & Christensen (1986), Flynn & Sommer-Larsen (1988), Sommer-Larsen et al. (1989), Preston et al. (1991), Arnold & Gilmore (1992), Kinman et al. (1994, hereafter KSK), Beers et al. (1996) and Sluis & Arnold (1998).

The nearby BHB stars have been discovered sporadically over the past sixty years - the majority by Strömgren 4-colour photometry. Pre-eminent among the discoverers have been A. G. Davis Philip (Philip 1994) and Stetson (1991). The only attempt, however, to obtain a complete sample of the nearby BHB stars (and hence a local space density) appears to be that by Green & Morrison (1993). Following Philip et al. (1990), they showed that a BHB star must not only have the appropriate Strömgren [FORMULA] and c1 indices, but must also show little or no rotational broadening in high-resolution spectra. This criterion must now be somewhat modified since Peterson et al. (1995) found BHB stars with [FORMULA] as large as 40 km s[FORMULA] in the globular cluster M13. Philip et al. also considered that a BHB star must have an appropriate location in the C(19-V)0 vs. (b-y)0 diagram 1. In the solar neighbourhood, disk stars greatly outnumber halo stars and there is a relatively high probability of finding disk objects whose Strömgren indices are close to those of BHB stars. To emphasize this, we give, in the Appendix A, a non-exhaustive list of stars whose colours resemble those of BHB stars but which most probably do not belong to this category. The use of high-resolution spectra is mandatory for the selection of BHB stars in the solar neighbourhood since both accurate abundances and [FORMULA] are needed as criteria.

High resolution studies of nearby RR Lyrae stars have been made by Clementini et al. (1995) and by Lambert et al. (1996). Both the RR Lyrae and BHB stars may be expected to have similar galactic kinematics. There are, however, disk RR Lyrae stars in the nearby field, but there are (as far as we know) no corresponding nearby field BHB stars that have disk kinematics 2.

While it is known that the field BHB stars generally show the low metal abundances that characterize halo stars, early determinations of these abundances show a rather wide scatter (see Table A27 in KSK). The first reliable determination was probably that based on the co-added photographic spectra of HD 161817 by Adelman et al. (1987). The metallic lines in the visible spectra of BHB stars are relatively weak and early photographic spectra did not have adequate signal-to-noise to measure these lines with sufficient accuracy. Also, until relatively recently, it was not known with certainty whether or not the evolution from the tip of the giant branch to the blue end of the horizontal branch (with significant mass-loss) would change the composition in the stellar atmospheres and whether diffusion effects would be present. Glaspey et al. (1989) observed two HB stars in the globular cluster NGC 6752. The hotter (16 000 K) showed low rotation, a strong overabundance of iron and a helium deficiency. The cooler (10 000 K) showed a higher rotation and no abundance anomalies compared to the red giants in the cluster. An example of a hot (16 430 K) field HB star is Feige 86 which was analyzed by Bonifacio et al. (1995). They found overabundances of the heavy elements and other pecularities which might be attributed to diffusion. Lambert et al. (1992) used an echelle spectrograph with a CCD detector to obtain moderately high-resolution spectra ([FORMULA]/[FORMULA] 18 000) of two BHB stars (with [FORMULA] [FORMULA]10 000 K) in the globular clusters M4 and NGC 6397. They found that their metallicities agreed well with those found previously for the red giants in these clusters. Caloi (1999) proposed that the gap observed in the HB sequence in many globular clusters at a ([FORMULA]) of about zero is a surface phenomenon and that stars with [FORMULA] [FORMULA]10 000 K will show peculiar chemical compositions. Grundahl et al. (1999) have noted that a jump in both Strömgren u and [FORMULA] occurs for stars hotter than [FORMULA] = 11 500 K in the EHB of globular clusters and suggest that this marks the onset of radiative levitation. This would explain the results of Glaspey et al. (1989) and the more recent discoveries of strong overabundances of Fe in these hotter stars in the globular clusters NGC 6752 (Moehler et al. 1999) and M13 (Behr et al. 1999). Behr et al. (2000) find that the HB stars in M13 that are cooler than [FORMULA] = 11 000 K have high rotation ([FORMULA] [FORMULA] 40 km s[FORMULA]) while the hotter stars have a low rotation as might be expected if radiative levitation is operating.

All stars in our sample are cooler than 11 000 K because hotter stars cannot easily be identified as BHB stars by their Strömgren indices. We should therefore expect them to have chemical abundances that are similar to those of other halo field stars such as halo RR Lyrae stars and halo red giants. We should not expect abundance anomalies to be present, and none have been reported, in the cooler field BHB stars that have previously been observed. In addition to HD 161817 (Adelman & Hill 1987), abundances have been derived from CCD spectra for ten other nearby field BHB stars (Adelman & Philip 1990, 1992, 1994, 1996a, 1996c). We consider that the abundances of BHB stars based on photographic spectra by Klochkova & Panchuk (1990) are less accurate because of the poor agreement of their equivalent widths with those obtained from CCD spectra.

The aim of the present study is to provide data for a reliable sample of the BHB stars in the solar neighbourhood. This includes the colour distribution, the reddenings, the stellar parameters, the projected stellar rotations ([FORMULA]) and the abundance ratios. These data can be compared with data for BHB in globular clusters and in other parts of the galaxy. The galactic orbits of about half the stars in our present sample have recently been calculated and analyzed by Altmann & de Boer (2000). It is intended, in a future paper, to derive the galactic orbits not only for this BHB sample but also for other local samples of halo stars so that these may be compared. These samples can help us to determine a better overall definition of the local halo and determine to what extent it may be distinguished from the disk populations 3.

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Online publication: December 15, 2000