Infrared spectral classification of normal stars^*

¹ Astrophysics Missions Division, Research and Scientific Support Department of ESA, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands e-mail: aheras@rssd.esa.int
² Space Research Organization of The Netherlands, PO Box 800, 9700 AV Groningen, The Netherlands
³ Air Force Research Laboratory/Space Vehicles Directorate, AFRL/VSB, Hanscom AFB, USA
⁴ Space Science Department, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, UK
⁵ LESIA, Observatoire de Paris, 92190 Meudon, France
⁶ Laboratorio de Astrofísica Espacial y Física Fundamental, INTA, PO Box 50727, 28080 Madrid, Spain
⁷ Science Operations and Data Systems Division, Research and Scientific Support Department of ESA, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
⁸ Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
⁹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Corresponding author: A. M. Heras, aheras@rssd.esa.int

Received: 7 May 2002
Accepted: 1 August 2002

Abstract

Moderate resolution (≈400) 2.38–45.2 μm 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. ([CITE]) in a comprehensive classification of the ISO-SWS spectra. For our analysis, the continuum was determined by fitting Engelke's function (Engelke [CITE]) 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. ([CITE]), V Nor has a thin dust shell, which distorts the analysis of its mid-infrared absorption bands.