4. Astrophysical implications
The NIR emission of CO presented here may have important astrophysical implications. The strongest emission band at 9506 cm-1 (1.052 m) occurs in the highly transparent window between the telluric absorption reagions due to water at 0.95 and 1.12 m (Sommerville 1995) and thus should be relatively easy to detect. In fact the NIR region, especially between 0.9 and 1.5 m, is still largely unexplored by astronomers (Foing & Ehrenfreund 1995) and our spectroscopic data could be of use for future endeavours. It is an advantage that in the NIR region the continuum extinction is significantly smaller than in the visible region. A further advantage comes from the influence of the environment on the energies of the triplet states. The triplet-triplet transitions of CO occur among the higher excited states involving vibrationally excited electronic states. As mentioned in the previous section, the energy levels of these vibronic states are highly sensitive to the environment in which CO exists. Consequently some of the bottlenecks may disappear and new ones may appear.
For example, in Ne matrices, where the energy levels are least perturbed compared to the gas-phase (Fig. 3), the a -state is stabilised with respect to the a-state when compared to the corresponding energy levels in Ar matrices. As a consequence, the a (5) is shifted to lower energy than the a(8)(Fig. 3) in Ne matrices (Bahrdt & Schwentner 1988). This results in appearance of a bottleneck at a (5) in Ne matrices and a bottleneck at a (6) in Ar matrices. The positions of these NIR emission bands are also sensitive to the environment of CO. For example, the gas-phase energy of the a (2) a(0) transition at 9278 cm-1 (Effantin et al. 1982) is shifted to lower energy by 228 cm-1 compared to the energy in Ar matrices (amounting to a 0.0258 m shift).
Kelly & Latter (1995) have studied the NIR emission spectra ( = 0.9 - 1.3 m) of several evolved stars. These authors reported some unidentified lines in addition to several identified ones. We find that three out of four unidentified emission lines from HM Sge lie close to the NIR emission bands of CO measured in Ar matrices in the present study. These are: 0.9495 (0.9445), 1.0447 (1.0520) and 1.2893 (1.2870) m, the values in parentheses being from the present study. Though the differences can be attributed to matrix shifts, further investigations need to be carried out to positively identify these lines.
The interpretation and assignment of the diffuse interstellar absorption bands (DIBs) is still a puzzle in spite of the spectacular progress made in the recent years (see Herbig 1995 for a review). A variety of organic molecules and ions have already been proposed to be the carriers of some of these DIBs (Foing & Ehrenfreund 1997, Ehrenfreund et al. 1997 and references therein). Multiphotonic processes in molecular hydrogen are also proposed to cause the appearance of some of the DIBs (Sorokin & Glownia 1996) and positively identified recently by Ubachs et al. (1997). CO is one of the most abundant molecules in space and its triplet-triplet transitions involving the a state range between 0.3 m and 3 m (present work, Krupenie 1966, Effantin et al. 1982, Amoit & Islami 1986, Bahrdt & Schwentner 1988). The a state of CO, which is a metastable state, is populated after the A X excitation at wavelengths between 1570 and 1200 Å. In this region several DIBs and the A X absorption bands of CO have been measured (Cardelli 1995). Keeping in mind lifetimes of the a state of CO (see Introduction), a significant population of this state can be achieved under the conditions existing in the interstellar clouds. Consequently, the triplet-triplet transitions may also result in some of the DIBs. We would like to propose, for example, that the infrared DIB observed by Joblin et al. (1990) at 1.3175 m may be due to the a (2) a(1) transition in CO. The corresponding emission occurs at 1.3221 m in the gas-phase (Effantin et al. 1982) and at 1.287 m in Ar matrices. Similarly the DIB at 0.9577 m whose carrier has been proposed to be C60 (Foing & Ehrenfreund 1994), lies close to the a (3) a(0) transition in CO, which occurs at 0.9575 m in the gas-phase (Effantin et al. 1982). Further work is warranted to positively identify whether or not CO can be the carrier of these DIBs.
As discussed in the previous section, the relative energy levels of the triplet states are strongly influenced by the environment of CO. For this reason, the gas-phase energies may be of use only if free CO is in consideration. The low temperature grains and ice analogues containing CO strongly influence its spectroscopic properties. Controlled laboratory experiments using synchrotron radiation as the source of excitation and low temperature ices having the composition similar to the interstellar ice particles are in progress. These studies and the astrophysical observations that complement each other should render precise information on the nature and composition of the interstellar matter.
© European Southern Observatory (ESO) 1998
Online publication: November 24, 1997