Spectral lines interstellar

Although oxygen was found to be the only oxidant for conversion of coproporphyrinogen III to protoporphyrin IX, anaerobic systems must obviously exist for the biosynthesis of the latter molecule (43). Porphine itself has not been found in nature but spectral lines identical to those of bis-pyridylmagnesiumtetrabcnzoporphine have been detected in interstellar space (53). 

A major step forward in our knowledge of interstellar matter was made in 1951 with the discovery of the first interstellar spectral line in the radio range, the famous X21 cm hyperfine structure line of atomic hydrogen. Most of our present knowledge of the distribution and physical state of interstellar matter in our Galaxy is based on observations of this line. At first it looked as... 

However, the detection of radio spectral lines in the frequency range of about 22 to 23 GHz from the polyatomic molecules NH3 and H20 (in 1968 and 1969) by Cheung et al. and the discovery of the organic molecule H2CO (in 1969) by Snyder et al. marked the beginning of a long series of discoveries. From these and all subsequent discoveries it became evident that dense and cool condensations of the interstellar matter are particularly rich in molecules. 

So far, unsuccessful searches for Hj in BN, GL 2591, LkH 101, NGC 2024/IRS, W33IR, NGC 2264 and AFGL 2591 have been reported. Black et alP observed spectral lines of CO simultaneously with their search for Hj. The abundance of CO thus obtained together with the upper limit of the Hj column density set a limit on the rate of the cosmic ray ionization C through eqn. (7). van Dishoeck and Black have proposed on chemical grounds that the abundance of Hj may be equally high in diffuse interstellar clouds. ... 

P13.30 The question of whether to use CN or CH within the interstellar cloud of constellation Ophiuchus for the determination of the temperature of the cosmic background radiation depends upon which one has a rotational spectrum that best spans blackbody radiation of 2.726 K. Given flo(CH) = 14.90 cm-1, the rotational constant that is needed for the comparative analysis may be calculated from the 226.9 GHz spectral line of the Orion Nebula. Assuming that the line is for the l2Cl4N isotopic species and J + 1 <— 7=1, which gives a reasonable estimate of the CN bond length (117.4 pm), the CN rotational constant is calculated as follows. 

Here D is the centrifugal distortion constant. Since we can measure the frequencies of spectral lines to one part in 10, small corrections such as centrifugal distortion are absolutely necessary to identify any interstellar species. 

Particle velocity effects. Particle velocity can cause Doppler broadening of spectral lines. The effect is extremely small for interstellar clouds at 10 K but is appreciable for clouds near high temperature stars. Outflows of gas from pulsing stars exhibit a red Doppler shift when moving away at high speed and a blue shift when moving toward us. 

The intensities of spectral lines depend not only on the population density of the molecules in the absorbing or emitting level but also on the transition probabilities of the corresponding molecular transitions. If these probabilities are known, the population density can be obtained from measurements of line intensities. This is very important, for example, in astrophysics, where spectral lines represent the main source of information from the extraterrestrial world. Intensity measurements of absorption and emission lines allow the concentration of the elements in stellar atmospheres or in interstellar space to be determined. Comparing the intensities of different lines of the same element (e.g., on the transitions Ei Ek and Ee -> Ek from different upper levels Ei, Ee to the same lower level Ek) furthermore enables us to derive the temperature of the radiation source from the relative population densities A/, Ne in the levels Ei and Ee at thermal equilibrium according to (2.18). All these experiments, however, demand a knowledge of the corresponding transition probabilities. 

Among the molecules detected so far in the interstellar medium, some species have been more widely used than others as physical and chemical probes. Some of the more commonly used species are described below. This choice is somewhat subjective and care must be taken when analysing molecular spectral lines, to compare with detailed physical and chemical models before drawing definitive conclusions. 

Specific instruments have been developed to enable the detection of the faint and usually extended emission of interstellar nebulae, as well as the narrow spectral lines, that can appear either as emission or absorption features. The key parameters for such studies are the spatial and spectral resolution of the instmments, their wavelength coverage and sensitivity. Depending on the wavelength domain, specific detectors and instrument layout must be used that take advantage of the most recent advances in light collecting and detection. 

Similarly, the proper interpretation of radio astronomical spectral lines from dense interstellar clouds requires collisional information involving ions at low temperatures. By combining the considerations reflected in Figs. 27 and 29, it is possible to obtain the pressure broadening of molecular ions at very low temperatures. This has been demonstrated for the ion HCO" and the collision partner H2. Likewise, extension of direct time-resolved measurements, as discussed in Section IV.D, has been recently carried out incorporating the collisional cooling technique discussed here. 

Spectral lines of N2H were first detected in 1974. A triplet of lines centered near 93174 MHz in the spectrum of a dense interstellar cloud [1] was identified as the hyperfine structure of the J = 1 - O transition [2, 3]. Further observations on N2H and some of its isotopomers in various interstellar sources were reported see for example [4 to 8]. It belongs to the most abundant interstellar ions see for example [9, 10]. 

Diffuse interstellar bands (DIBs) Unassigned spectral features that are seen along many lines of sight, especially towards reddened stars. 

Reddening of the continuum by interstellar dust (which leads to excess redness, known as colour excess, relative to the spectral type from H or other line features). 

The lithium resonance doublet line X 6707 is fairly easy to observe in cool stars of spectral types F and later, and it has also been detected in diffuse interstellar clouds. There is thus an abundance of data, although in the ISM the estimation of an abundance is complicated by ionization and depletion on to dust grains. The youngest stars (e.g. T Tauri stars that are still in the gravitational contraction phase before reaching the main sequence) have a Li/H ratio that is about the same as the Solar System ratio derived from meteorites, Li/H = 2 x 10-9, which is thus taken as the Population I standard. 

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