MOLECULAR SPECTROSCOPY WITH ISO Molecules in the Interstellar and Circumstellar Medium J. CERNICHARO CSIC, IEM, Dpto. Fisica Molecular, Serrano 123, E-28006 Madrid, Spain 1. Introduction In spite of the important astrophysical output that could be obtained from infrared observations, the wavelength range 2-200 /im has remained poorly studied due to the absorption of radiation produced by the Earth's atmosphere. The spectrometers on board the Infrared Space Observatory (ISO) provide a unique opportunity to study important molecular species through transitions which are inaccessible from the ground or airborne platforms. The ISO spectrometers offer the possibility to detect, with a reasonably good spectral resolution, the ro-vibrational transitions of important molecules. The SWS spectrometer cover the region 2-45 /jm and it is well adapted to the study of ro-vibrational transitions (stretching and bending modes) while the LWS spectrometer provides a powerful tool to observed the pure rotational transitions of light species like H2O. In this review we present the main results on gas phase molecules obtained with the ISO-SWS and ISO-LWS spectrometers and also the low resolution CVF-ISOCAM spectrometer. 2. Molecular Hydrogen Molecular hydrogen, H2, is the most abundant molecule in the interstellar and circumstellar medium. Due to its lack of permanent dipole moment this molecule can be only observed in the near and medium infrared through its ro-vibrational and quadrupolar rotational transitions. The abundance of H2 in molecular clouds has been established through comparisons of the emission of other molecules, like CO, and the visual absorption using the relation N(H2)/Av derived in the dif- fuse interstellar medium. The ISO-SWS spectrometer allows to observe the quadrupolar lines of H2 in the molecular clouds of the Milky Way and in external galaxies, to estimate directly the mass of H2, and to compare with the mass derived from CO data. In young stellar objects, like LKHa224, LKHa225, Wesselius et al. (1996) have observed the S(l) up to S(7) lines of H2. The rotational levels of H2 seem to be populated with a thermal distribution at 500 K for LKHa224 and 800 K for LKHa225. The amount of warm molecular hydrogen detected in these objects is 0.01 and 0.04 M Q respectively. The S(l) up to S(9) lines of H2 have been observed by Timmermann et al (1996) in the warm PDR of S140. The line intensity ratios are characteristic of fluorescent emission. Valentijn et al. (1996) have observed the S(0), S(l) and S(2) lines of H 2 in the galaxy NGC6946. They have derived a mass of 5 10 6 M 0 and a temperature of 170 K for the emitting gas. The observed warm H2 accounts for only 5-10% of the total molecular gas in the emitting region. Several H2 rotational lines have also detected in Arp220 by Sturm et al. (1996). They also conclude that the warm gas contains only 10% of the total molecular gas in Arp220. Similar results have been obtained by Kunze et al. (1996) in NGC4038/39 and by Rigopoulou et al. (1996) in the starburst galaxy NGC3256. Finally, molecular hydrogen emission has been detected in the Helix Nebula by Cox et al. (in preparation) using the CVF-ISOCAM spectrometer. All the emission in the 5-17 pm band is dominated by the H2 lines. There is not trace of emission from PAHs characteristic of less evolved PN as NGC7027. 1145 J. Andersen (ed.), Highlights of Astronomy, Volume 11B, 1145-1147. © 1998 IAU. Printed in the Netherlands. terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S153929960001981X Downloaded from https://www.cambridge.org/core. IP address: 198.252.40.105, on 24 Apr 2020 at 05:10:53, subject to the Cambridge Core