2. The observations
High resolution (R=100.000) spectra were obtained with the 1.4m ESO/CAT telescope and the CES spectrograph, configured with the long camera and CCD detector number 38, on November 19 and 21, 1996. With this setting, each resolution element is 0.07 Å and covered by 3 pixels. The bandwidth is about 100 Å centered at 9390 and 9604 Å.
Six early type stars were observed in each wavelength range. Because of heavy atmospheric lines in these spectra, near equality of airmass of the reddened and unreddened stars took priority over an exact spectral type match. The three reddened stars are HD 80077 (B2Iape, E = 1.41), HD 63804 (A0Iap, E = 1.17) and HD 43384 (B3Ia, E = 0.57). Reddening values were derived from Hipparchos Input Catalogue data. In addition, we observed the reference stars HD 34085 (B8Ia, E = 0.00, only used for the 9400 Å window), HD 47105 (A0IV, E = 0.03), HD 81188 (B2IV, E = 0.05) and HD 52089 (B2II, E = 0.02).
Figure 1 shows part of the spectrum of the unreddened star HD 81188, after the telluric lines have been removed using HD 47105 as a reference: it is clear that the stellar lines of HD 47105 show up as 'emission' features, while some residual telluric lines are present due to different airmasses. Note that the 9632 Å line in HD 47105 is broad, showing two components. These are the MgII lines, at 9631.888 and 9632.435 Å (NIST Atomic Spectroscopic Database), recognized by Foing & Ehrenfreund (1994) as overlaying the DIB at 9632 Å.
Luckily enough, HD 47105 is either a very slow rotator or pole-on, so its stellar lines are rather narrow and very recognizeable as such. We made use of this fact to improve the removal of telluric contamination by adopting the following procedure: we first removed the telluric lines from the spectrum of HD 47105 using HD 81188 as a reference, then we fitted the stellar lines of HD 47105 with splines and finally divided them away from the original spectrum of HD 47105, obtaining a "pure" telluric spectrum. This pure telluric spectrum was used to remove the tellluric lines from other reference stars. That, in turn, allowed us to recognize and remove the (much broader) stellar lines before using these spectra as telluric reference for our program stars.
The 9400 window is even more affected by telluric water lines. We were unable, in this case, to accurately separate, and thus remove, the stellar lines from the spectra of our reference stars. Hence, the resulting spectra from this window show some 'emission' features due to stellar lines in the spectra of the telluric reference stars.
© European Southern Observatory (ESO) 1997
Online publication: April 8, 1998