Interstellar lines

Fig. 3.11. Curves of growth for interstellar lines towards f Oph, observed with the Copernicus satellite. This method of plotting shows the dependence on b along the flat portion and its disappearance as the square-root portion is approached (provided all lines have the same value of yX2). After Spitzer and Jenkins (1975). Copyright by Annual Reviews, Inc.

One of the important quantitative results that chemists are interested in and which can be derived from interstellar molecular lines is the number of molecules in a given area or volume of space. Under the condition of low optical depth (Winnewisser et al. 1979) the number of molecules contained in a column within an area F (e.g. 1 cm2) along the line of sight to the observer can be derived directly from the observed intensity of an interstellar line. This parameter, called the column density N, is related to the actual concentration n by the equation... 

Figure 10.41. Spin-rotation and hyperfine splitting of the N = 0 and 1 rotational levels of CN in its X 2E+ ground state, and the observed transitions. The asterisks denote the transitions responsible for the interstellar lines shown in figure 10.40.

Because of the high precision with which the frequencies of the interstellar lines can be measured (better than 1 part in 10s) there remains usually little doubt about the positive identification of the molecular species, despite the fact that only a few transitions out of the whole rotational spectrum of any one given molecule have been observed to date in the radio frequency range. Confirmation is obtained from observations of other rotational transitions, or from the detection of possible fine-structure components, or from observations of corresponding transitions of isotopically substituted species. However, some uncertainty still remains in the identification of formic acid, HCOOH, whose 1 io-ln transition is located in between two 18OH resonances. An independent search for the l0i — 0Oo transition for formic acid was negative (Snyder and Buhl, 1972). Similarly the identification of H2S and H20 still rests on only one observed interstellar radio transition and awaits further confirmation by the detection of other transitions. 

With the exception of CN, all molecular identifications of interstellar lines are based on direct laboratory measurements. The location of the CN radio transitions can be predicted from laboratory ultraviolet data (Poletto and Rigutti, 1965) and is found to be in reasonable agreement with the positions given by the interstellar observations. However, the hyperfine structure (electric quadra-pole and nuclear spin-electronic spin interaction) cannot be uniquely assigned therefore an element of caution has to be maintained until more observational evidence — either from interstellar or laboratory measurements — confirms the assignment. (See Note Added in Proofs- 33). 

The location of an observed molecular radio transition in its energy level scheme and its measured interstellar intensity contain important information concerning the physical state of the molecular cloud in which the transition is observed. It will therefore be an important task for future interstellar molecular research to observe and measure as many transitions of any one molecule in any one particular cloud. Doppler shifts, i.e. the difference in frequency between the rest frequency (known from laboratory measurements) and the observed interstellar line frequencies, provide information on the large scale motion of the molecular clouds while the linewidths indicate the turbulence within the clouds. 

Although it is presently not possible to distinguish clearly between the different formation mechanisms, it is felt that more observational data will furnish the material necessary for that. There are two areas of immediate help which play a very important role in our understanding of interstellar chemistry Isotopic abundance determinations, notably deuterium to hydrogen ratios, and the identification of the unknown interstellar lines by astronomical, laboratory or theoretical means. 

Turner, B. E. (1974). U93.174 - a new interstellar line with quadrupole hyperfine sphtting. Astrophysical Journal, 193, L83. 

Calculations made for the next terms of the C H series have predicted the rotational constants of the higher members, up to n = 10 [45]. These values have been used very recently to identify C7H, CgH, and C9H in laboratory experiments [47,48,49] the measured rotational constants match exactly those computed in 1991, within the error bar given in the theoretical paper. These coupled theoretical/experimental results immediately led to the assignment of some interstellar lines to C7H and CgH [50,51]. However, no evidence was found for C9H in the published astronomical spectral surveys hopefully this detection can be achieved using significantly higher sensitivity and integration times [48]. 

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