2. Observational data
Spectra for the stars RLWT-13, RLWT-41, S285-6, Bo-1a and NGC-1983a were obtained during the nights of 24-28 Dec 1996 at the 2.5 m Isaac Newton Telescope (INT) on La Palma in the Canary Islands. The Intermediate Dispersion Spectrograph (IDS) was employed with the 500 mm camera and the H1800V grating. With a pixel size of 24 µm for the TEK3 1124 1124 CCD, a resolution of 0.55 at the central wavelength of was achieved, as measured from the full width at half maximum of the arc lines. Bias frames and flat fields (tungsten lamp) were recorded at the beginning of each night, and wavelength calibration was undertaken using spectra of a CuAr arc lamp obtained at regular intervals throughout the night.
Spectra covering the red C II lines for the stars S208-6, S283-2 and S289-2 and S289-4, together with additional data for S285-6 were recorded at the William Herschel Telescope on La Palma over two observing runs in December 1991 and November 1993, details of which are summarised in Smartt et al. (1996a). In both cases the ISIS spectrograph was employed with the R1200R grating on the red arm giving a resolution of 0.8 . To ensure consistency, these data were re-reduced together with the INT data at the Northern Ireland STARLINK node at Queen's University Belfast using standard FIGARO techniques (Shortridge et al. 1996) as outlined in Smartt et al. (1996a).
Where observed, the equivalent widths of the C II features were determined by fitting Gaussian profiles after normalising to a pseudo-continuum fitted to the red wing of the H line. The errors in the equivalent widths were estimated from the rms of the fit to the pseudo-continuum and that of the Gaussian fit to the profiles and are in the range . Fig. 1 shows the spectrum of Bo-1a which is the sum of four 1200 s exposures giving a continuum signal to noise of approximately 100, typical of those obtained. The adopted continuum and the Gaussian fits to the two lines are shown.
Table 1. Equivalent widths for the red C II lines recorded towards the nine target stars
© European Southern Observatory (ESO) 1998
Online publication: March 23, 1998