Carbon monoxide is one of the major constituents of stellar and interstellar media. The near infrared emission of CO due to the ground-state vibrational transitions at 4.67 m (1 0) and 2.3 m (2 0) are used as markers to study the evolution and ages of several stars (for example Greene & Lada 1996, Brooke et al. 1996) and the 4.67 m absorption band is used to investigate the composition of interstellar matter, especially the low temperature grains and ice analogues (Elsila et al. 1997). Rotational lines of CO in the microwave region are commonly used in astronomy (for example George et al. 1994). The vacuum ultraviolet (VUV) features of CO have been observed with International Ultraviolet Explorer (IUE) followed by Hubble Space Telescope (HST) using Goddard High-Resolution Spectrograph (GHRS) (for example Lambert et al. 1994). The dark sunspot umbral spectrum also shows strong absorption bands due to several vibrational transitions in the ground-state (X ) of CO (Wallace & Livingston). On the other hand, the spectroscopic features of CO in the near infrared region (NIR) between .9 and 2 m involving the electronically excited states have not yet been explored in astrophysical observations, presumably due to the lack of sensitive detectors that cover the NIR region. Only recently, due to the development of highly sensitive NIR detectors, the NIR region became accessible for astronomers (Wallace & Livingston, Greene et al. 1993, Kelly & Latter 1995). A compilation of electronic transitions in CO and their relevance for astronomy is published by Morton & Noreau (1994). However, in this review the NIR region mentioned above is not taken into account. There are several important spectroscopic characteristics of CO in the region between 0.9 and 1.6 m, which could be useful to understand more about the composition of the interstellar matter.
Spectroscopic data of CO in the gas-phase at shorter wavelengths than 0.86 m is compiled by Krupenie (1966) and Tillford & Simmons (1972). In the NIR region between 0.75 and 3.0 m relatively few studies are available in the literature (Effantin et al. 1982, Amoit & Islami 1986, Momona et al. 1993), of which only the first two articles reported the triplet-triplet NIR emission bands of CO in the gas-phase. We report here several new NIR emission bands of CO isolated in Ar matrices at 20 K and discuss their astrophysical implications.
There exist four triplet electronic states of CO that lie between the ground-state (X ) and the first optically allowed excited singlet state (A ). These triplet states with increasing energy are: a , a , d and e , respectively. The a-state of CO, which lies at 6.01 eV above the ground-state, is a metastable state with a lifetime of several ms, an estimated value of 94 ms in an unperturbed environment, 90 ms in Ne matrices and 7.2 ms in Ar matrices (Fournier et al. 1980). For this reason, the e a, d a and a a transitions that range from UV to NIR region are important for the processes occurring in both stellar and interstellar environment, where the abundance of CO is significant. The a-state of CO is populated subsequent to the A X excitation between 1570 and 1200 Å. Bahrdt et al. (1987) studied in detail the spectroscopic properties of CO in rare-gas matrices except for the NIR emission. These studies will be referred in the following.
© European Southern Observatory (ESO) 1998
Online publication: November 24, 1997