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Astron. Astrophys. 346, L1-L4 (1999)

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2. Observational data

We selected our targets from the photographic photometry of Buonanno et al. (1986) to cover the range 14.5[FORMULA]15.5. 19 stars were observed with the ESO 1.52m telescope (61.E-0145, July 22-25, 1998) and the Boller & Chivens spectrograph using CCD # 39 and grating # 33 (65 Å/mm). This combination covered the 3300 Å - 5300 Å region at a spectral resolution of 2.6 Å. The data reduction will be described in Moehler et al. (1999a). Prompted by the suggestion of Grundahl et al. (1999) that radiative levitation of heavy metals may enrich the atmospheres of BHB stars, we searched for metal absorption lines in these spectra. Indeed we found FeII absorption lines in almost all spectra (for examples see Fig. 1).

[FIGURE] Fig. 1. The iron and magnesium lines as seen in the spectra of some of the stars in NGC 6752. The solid line marks a model spectrum for which we used a solar metallicity model stratification but adjusted all metals to [M/H] = -1.5 for the spectrum synthesis except iron. The iron abundance was then adjusted to the noted values to reproduce the marked FeII lines. The dashed line marks a model spectrum for which we used the cluster metal abundance for all metals. Effective temperature and surface gravity for both models are those plotted in Fig. 2, upper and central panel, respectively. The temperatures of the stars range from 12,000 K (B3348) to 16,000 K (B1152). Obviously iron is strongly enriched whereas magnesium is consistent with the mean cluster abundance.

13 stars were observed as backup targets at the NTT during observing runs dedicated to other programs (60.E-0145, 61.E-0361). The observations and their reduction are described in Moehler et al. (1999b). Those spectra have a spectral resolution of 5 Å covering 3350 to 5250 Å. No metal lines could be detected due to this rather low spectral resolution.

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© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999