High resolution observations of individual stars of NGC 6553, located at = 18h05m11s, = , were obtained at the 3.6m telescope of the European Southern Observatory - ESO. The Cassegrain Échelle Spectrograph (CASPEC) was used with a thinned and anti-reflection coated Tektronix CCD of 512x512 pixels, of pixel size 27x27µm (ESO # 32). The 31.6 gr/mm grating was used with the short camera and the wavelength region covered is 500-750 nm, at a resolution of R 20.000. The reductions were carried out with the routines for échelle spectra contained in the Midas package. This procedure automatically finds the center of the orders, tracks them across the CCD chip and extracts them by summing over a selected input width of pixels. The package allows wavelength calibration by identifying two pairs of lines on the thorium lamp image.
A typical spectrum is shown in Fig. 1 for order 93 of the giant member III-3 and the log of observations is reported in Table 1, where radial velocities derived from several unblended atomic lines in the spectra are indicated. A mean = 6.4 km/s or heliocentric = 6.1 km/s is found, compatible with the value of -5 km/s given in Zinn & West (1984) or -12 km/s given in Zinn (1985), and in very good agreement with the value of 8.4 km/s measured by Rutledge et al. (1997a). A S/N 50 was measured at order 88 for both stars.
Table 1. Log observations
In Fig. 2 is shown the V vs. (V-I) Colour-Magnitude Diagram of NGC 6553 using our data obtained with the Hubble Space Telescope (Ortolani et al. 1995) where the sample stars are identified. The identification names given in Hartwick (1975) are adopted along the text.
© European Southern Observatory (ESO) 1999
Online publication: December 4, 1998