A. M. Heras 1, R. F. Shipman 2, S. D. Price 2, M. Cohen 3, T. de Graauw 4, H.J. Walker 5, M. Jourdain de Muizon 6, M.F. Kessler 1, & T. Prusti 1
1 ISO Science Operations Centre, Astrophysics Division of ESA, P. O. Box 50727, E-28080 Villafranca, Spain 2 Phillips Laboratory/Geophysics Directorate, 29 Randolph Rd., PL/GPOB, Hanscom AFB, MA 01731-3010, USA
3 Radio Astronomy Laboratory, 601 Campbell Hall, University of California, Berkeley, California 94720
4 Space Research Organization of The Netherlands, Postbus 800, NL-9700 AV, Groningen, The Netherlands
5 CLRC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK
6 DESPA, Observatoire de Paris, F-92190 Meudon, France
7 ISO Science Operations Centre, Astrophysics Division of ESA, P. O. Box 50727, E-28080 Villafranca, Spain
We present an overview of the spectroscopic characteristics of ordinary stars observed with SWS in the low-resolution AOT1 mode, which covers the full wavelength range of the instrument, 2.38-45.2 microns. The stars have been selected according to the following criteria: (1) their IRAS LRS spectrum is classified as 1n, that is, it is feature less; (2) for those stars not observed by IRAS, there is no indication in the ISO spectrum of silicate features. The resulting sample contains stars that fill a grid with MK classifications between B and M, and luminosity classes dwarf to supergiant. We have identified the spectral features (lines and molecular bands) that are of interest for classification purposes. In particular, we have determined the presence in the spectra and calculated the intensities of H lines, the CO fundamental and CO first overtone (around 4.6 and 2.5 microns, respectively), the SiO fundamental and SiO first overtone (around 8 and 4 microns, respectively), the OH band (3-3.5 microns) and the water bands at 2.7 and 5.5-6.5 microns. In this way we have been able to make a bidimensional analysis, relating the relative contribution of these species in each spectrum both to the MK and to the luminosity class. As a result we discuss the choice of parameters to be used in an infrared classification of stars (i.e. H line ratios, intensity of certain molecular bands). The study of the continua has been carried out by applying the analytical approximation to the 2-60 microns infrared continua proposed by Engelke (ApJ 104, 1248, 1992). Through fits to the observations this method produces estimates of the effective temperature and angular diameter of cool stars. We discuss the results and compare with the values obtained by other means when possible. This work is part of an extensive ISO dedicated time program STARTYP1/2 and ZZSTARTY, which aims to provide a comprehensive spectral classification scheme by using SWS and PHOT-S spectra of bright representative infrared sources.