2002A&A...394..539H

Moderate resolution (≃400) 2.38-45.2µm infrared spectra of stars without dust features were obtained with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). The observations are part of a larger program with the objective to extend and refine the current infrared classification schemes. In particular, our data provide the basis for a more detailed classification of the 1.N-1.NO sources (ordinary and oxygen rich naked stars) as defined by Kraemer et al. (2002ApJS..140..389K) in a comprehensive classification of the ISO-SWS spectra. For our analysis, the continuum was determined by fitting Engelke's function (Engelke, 1992AJ....104.1248E) to the SWS data. The stellar angular diameters derived from these estimates of the continuum are in good agreement with values obtained by other methods. Analysis of the equivalent widths of the CO fundamental and first overtone molecular bands, the SiO fundamental and first overtone, as well as the H₂O bending mode band as a function of MK class, reveals that there is sufficient information in the SWS spectra to distinguish between hot (B, A, F) and cool stars. Furthermore, it is possible to determine the spectral type for the G, K and M giants, and subtype ranges in a sequence of K and M giants. The equivalent widths of the CO and SiO bands are found to be well correlated in K and M stars, such that the equivalent widths of the CO fundamental, the SiO first overtone and the SiO fundamental can be reasonably well extrapolated from the depth of the CO first overtone. We have identified two stars, HR 365 and V Nor, whose mid-infrared spectrum does not correspond to their respective optical classification. HR 365 may have a late M companion, which dominates the observed infrared spectrum while V Nor is a late type giant that was included because its spectrum was classified as featureless under the IRAS LRS scheme. According to Kraemer et al. (2002ApJS..140..389K), V Nor has a thin dust shell, which distorts the analysis of its mid-infrared absorption bands.