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Astron. Astrophys. 340, 476-482 (1998)

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2. Observations

Blue-visual spectra of the program stars were obtained by U. Heber using CASPEC, a Cassegrain échelle spectrograph, on the ESO 3.6m telescope at La Silla, Chile, on 1984 April 4, 1985 April 5 and 8, and 1987 April 18. The data reduction has been described by Heber et al. (1986) and by Jeffery & Heber (1992), and yields a single wavelength calibrated spectrum corrected for the blaze function in each échelle order.

In general, observations of stellar spectra are affected by photon noise, wavelength calibration errors, instrumental calibration errors, cosmic ray events and non-stellar spectral features. These may introduce many problems when fitting a large section of synthetic spectrum ([FORMULA]Å) in order to analyse many lines simultaneously.

To ensure that the wavelength registration was correct, a synthetic spectrum of approximately the correct effective temperature, gravity and composition was constructed. The velocity shift between the observed and synthetic spectra was measured by cross-correlation, and the wavelengths in the observed spectrum were then shifted to the laboratory rest frame. This global velocity correction did not correct for local calibration errors, in individual échelle orders for example, which became apparent during the subsequent analysis. However, integrated over the entire spectrum, their nett contribution is stochastic rather than systematic.

The reconstruction of the stellar spectrum from several overlapping orders of an échellogram in which the blaze correction is difficult to calibrate is another major source of error. The problem is too identify the continuum correctly without compromising the wings of broad absorprion lines, especially those of He i. The original normalization of the CASPEC spectrum was carried out by attempting to correct each échelle order for the convolution of the order blaze function and the local stellar continuum. Whilst adequate over small sections of spectrum, the overall result is not satisfactory in the current sample. A procedure has been applied which re-normalizes the spectra in an objective fashion using regions of spectrum dominated by continuum. Further details are given in an appendix.

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998