1994A&AS..108..359G

Up to now the spectral classifications of the cool components of composite spectra obtained in the 3800-4800A wavelength region have been very disparate. These disparities are due to the fact that the spectra of the evolved cool component are strongly veiled by that of the hotter dwarf component, which makes a classification very difficult. We propose to study these systems in the near infrared (8380-8780A). In this spectral domain the magnitude difference between the spectra of the components is in general sufficiently large so that one observes practically only the spectrum of the cool component. In this first paper we provide, for a sample of MK standards, the relations between the equivalent width (W_λ ) of certain lines and the spectral classifications. For the cool G, K and M type stars, the lines considered are those of the calcium triplet (Ca II 8498, 8542 and 8662), of iron (Fe I 8621 and 8688), of titanium (Ti I 8426 and 8435) and of the blend λ8468. The use of certain line intensity ratios permits, after eliminating partially the luminosity effects, a first approach to the spectral type. For the hotter stars of types O, B, A and F we study the behavior of the hydrogen lines (P12 and P14), the calcium lines (Ca II 8498 and 8542) as well as those of the oxygen (O I 8446). The latter line presents a very characteristic profile for stars of low rotation and therefore in Am stars, which are frequently found among the composite spectrum binaries. Among the cooler stars of our sample, only 6% present real anomalies with respect to the MK classifications. This result is very encouraging for undertaking the classification of a sample of composite spectra. The spectra were taken at the Observatoire de Haute-Provence (OHP) with the CARELEC spectrograph at the 193 cm telescope, with a dispersion of 33 A/mm.