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

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11. Summary and conclusions

The purpose of this paper is to determine stellar parameters (e.g. [FORMULA],  [FORMULA] & [FORMULA]) and chemical abundances that will allow us to isolate a local sample of BHB stars by their physical properties. All of our sample of thirty one candidate stars appear to belong to the halo, but BD +32 2188 (a post-AGB star), BD +00 0145 (a possible cool sdB star) and HD 16456 (the RR Lyrae star CS Eri) are not BHB stars. HD 202759, although classified as an RR Lyrae star (AW Mic), has such a low V-amplitude ([FORMULA] 0.1 mag) and high [FORMULA]  (7 500 K) that it has been included with the BHB stars. Our spectra of HD 14829, HD 78913, HD 106304 and HD 213468 were not of sufficient quality for a complete abundance analysis although we were able to estimate [Fe/H] from their Mg II  ([FORMULA]4481) lines.

Of the twenty eight stars which we classify as BHB stars, the most doubtful is HD 139961 because it has the largest [FORMULA] and also an unusually low orbital eccentricity (0.22) 17. It is also NSV 7204 in the New Catalogue of Suspected Variable stars, Kukarkin et al. (1982). Corben et al. (1972) found a range of 0.08 magnitudes in V over six observations. The 85 observations of this star in the ESA Hipparcos catalogue, however, show a range of only 0.05 magnitudes; this corresponds to an rms deviation of only 0.01 magnitudes. Its colour, moreover, does not put it near the edge of the instability strip, so that its variablity seems questionable. The existence of stars such as HD 139961 shows how difficult the classification of BHB stars can be and how necessary it is to use all available criteria . When large numbers of stars are to be surveyed, simpler methods may have to suffice but one must then expect to get more misclassifications. Thus, Wilhelm et al. (1999) classify BHB stars with broad band UBV colours, Balmer-line widths and the Ca II (K-line) equivalent widths. Among the 18 stars in common with our sample, they classify the broad-lined A-star HD 203563 as an FHB star and their [Fe/H] average 0.32[FORMULA]0.08 more metal-poor than ours with individual stars differing from our [Fe/H] by as much as 0.8 and 0.9 dex.

Projected rotational velocities ([FORMULA]) were determined for each star by calibrating the FWHM of the Mg II  ([FORMULA]4481) line against the [FORMULA] of seven of the stars in our sample that had previously been determined from echelle spectra by Peterson et al. (1983). No obvious trend of [FORMULA] was found with either [FORMULA] or abundance. A simple analysis of the [FORMULA] (following Chandrasekhar & Munch 1950) shows that the deprojected distributions of these rotational velocities are similar to those found in globular clusters. Both have a [FORMULA] of [FORMULA]17 km s[FORMULA] that is intermediate between that of the high rotational velocity clusters (M13 and M92) and the low rotational clusters (M3 and NGC 288).

BD +00 0145, HD 14829, HD 78913, HD 106304 and HD 213468 should be reobserved since we did not obtain spectra of sufficient quality for a complete analysis. Improved equivalent widths and [FORMULA] could be obtained for all our BHB stars by using a higher resolution and a larger waveband (e.g. by using an echelle spectrograph) so that more lines would be available. Improved abundances, however, require a better understanding of the physical conditions in the stellar atmospheres and more accurate gf values as well as more certain determinations of the interstellar extinctions. In this latter connection, more reliable determinations of the extinction would be possible if ([FORMULA]) colours were available for our entire sample. It is possible that HD 130095 is a binary. Its reported velocity variations should be checked so that (if these are real) a period can be derived.

As we noted earlier, many of our BHB stars were selected from the early type stars that were found in surveys for high proper motion; our sample may therefore be expected to have a kinematic bias. This bias (inter alia) will be examined in a following paper, where we shall compare the galactic orbits of these BHB stars with those of other nearby halo stars.

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© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000