Interstellar spectra

Microwave spectra (giving pure rotational spectra) are especially usefiil for the detection of interstellar molecular ions (in some cases the microwave spectrum has first been observed in interstellar spectra ). 

In this article the infrared spectroscopic evidence for interstellar PAHs will be reviewed. The spectroscopic properties of PAHs studied in salt pellets rather than amorphous carbons will be primarily used since a wealth of very detailed information is available (thanks to the sustained, dedicated effort of Cyvin and his coleagues over many years) and molecule-sized emitters can account for many details of the interstellar spectra. Infrared spectra of amorphous carbon particles and carbonaceous films, synthesized to study the connections with interstellar carbonaceous material, are just now becoming available. The work of Bussoletti and coworkers ([33] and references therein) and Sakata and colleagues ([34] and references therein) is particularly noteworthy in this regard. 

The spectroscopic assignments are based on a rough similarity between the interstellar spectra and laboratory spectra of PAHs and related materials. As the... 

This molecular soccer ball was discovered in 1985, as a result of studies into the structures formed by carbon atoms in space, which were in turn prompted by unexplained features in interstellar spectra. 

If the pure rotational spectrum of a molecule is known, in principle it can be identified in space. However, interstellar spectra can be quite complex, exhibiting emission from hundreds of lines, as shown in Figure 4. Therefore, it... 

The H2 bands observed in the interstellar spectra are actually features of excitation from the electronic ground state for which the affect of vibrations can be seen in the fine structure. The primary excitation is electronic rather than vibrational, and so a different set of selection rules will apply. The origin of vibrational fine structure can be understood in terms of the Franck-Condon approach to electronic excitations discussed in Appendix 9. 

Typical abundances of PAHs are of the order of 10 relative to hydrogen, which makes these species the most abundant polyatomic molecules present in space. Vibrational spectroscopy is eminently suited for detecting the presence of classes of molecules, but it is much more difficult to identify specific molecules within a collection of species. However, while the gross characteristics of these stellar and interstellar spectra are very similar, when examined in detail, they vary from source to source. Analysis of these variations may well provide us with a tool to identify specific molecules within the circumstellar and interstellar PAH family and such efforts, supported by extensive laboratory studies, are now underway. 

A. S. Webster. It may or may not be, but the spectrum is peculiar. It doesn t look like a normal interstellar spectrum. 

The hydrogen atom and its spectrum are of enormous importance in astrophysics because of the large abundance of hydrogen atoms both in stars, including the sun, and in the interstellar medium. 

Auger electron spectrum, 320 dipole moment, 116 interstellar, 120 SO2... 

HC5N (cyanodiacetylene) interstellar, 120 rotational spectrum, 110 Na2S203 (sodium thiosulphate)... 

F1= F1 F10 (s-lranx-crotonaldehyde) infrared spectmm, 159ff Raman spectrum, 159ff 14 9X (cyanotetraacetylene) interstellar, 120... 

Solvent properties and dipoles, 313 Sorbitol, 423 Sprensen pH scale, 190 Space, interstellar, 448 Spectrograph mass, 242 simple, 247 Spectroscopy, 187 infrared, 249 microwave, 249 X-ray, 248 Spectrum... 

The rotational spectrum has been calculated accuratly by ab-initio methods [2], and has been measured in the laboratory with high precision [3,4], so that the radio detection of C3H2can be done without ambiguity, encouraging its search in different environments as dense dark clouds [5], diffuse interstellar medium [6] or Hll regions [7]. 

There are many situations in which scientists need to know how alike a number of samples are. A quality control technician working on the synthesis of a biochemical will want to ensure that each batch of product is of comparable purity. An astronomer with access to a large database of radiofrequency spectra, taken from observation of different parts of the interstellar medium, might need to arrange the spectra into groups to determine whether there is any correlation between the characteristics of the spectrum and the direction of observation. 

Now today, we have found about a hundred different masers in space and some lasers. The difference between a maser and a laser is of course only in the wavelength. But there are some astronomical systems where infrared is getting amplified. Now as has been pointed out, amplification in interstellar space doesn t involve resonances, but it does involve stimulated emission. You know, somebody could have seen these interstellar masers in the radio regions of the spectrum many years ago. Anybody who used the radio technology of 1936, and looked up into the sky, could have detected this water frequency. They didn t bother to look, but it was there all the time. So now we know, lasers have been there for billions of years. Masers have been there billions of years. So that s another way we might have discovered them, but we didn t. Now I emphasize this to indicate that we need to search, we mustn t be too confined by what we think is going to work, we ve got to explore. 

One of the oldest problems in molecular astronomy concerns identification of the molecules responsible for diffuse interstellar bands (DIBs). Since their first observation in 1922 some 127 bands have been detected all over the electromagnetic spectrum, shown schematically in Figure 3.19, but the origins of the transitions, the so-called carriers of DIBs, have not been determined. 

The interstellar extinction has a great effect on distance determination for stars. The B/V index derived in Chapter 2 will be distorted by the presence of interstellar dust, with an amount of radiation in the blue part of the spectrum removed. The difference between the observed colour index and the colour index on which it should have based its temperature is called the colour excess. We defined m to be the measured apparent magnitude, which must now be corrected by an amount Av and added to the distance modulus equation ... 

Fig. 12.9. Composite spectrum of Lyman-break galaxies showing a combination of interstellar and stellar absorption lines, P Cygni features and nebular emission lines, dominated by Lyman-a. After Shapley et al. (2003).

When a Rydberg atom reduces its principal quantum number by one unit, when emitting a photon, the light is in the microwave region of the electromagnetic spectrum. With this radiation isolated Rydberg atoms can be observed in interstellar space, where interatomic collisions are rare. Atoms with n up to 350 have been observed by radio astronomical methods. 

Disregarding for a moment the bright and dark bands that decorate the spectrum of a heavenly body at specific wavelengths, the overall hue of that spectrum can tell us the surface temperature of the object. A blue star is thus hotter than a yellow one, and a yellow star is hotter than a red one. The Sun is hotter at the surface than the red star Antares, which in turn stands as a torrid desert before the brown dwarfs or interstellar clouds. The stars go red with cold. 

It is very useful to complement the compositional analysis of stars by a like analysis of the interstellar medium. This can be done by making use of absorption lines which the latter removes from the UV spectrum of hot, bright stars (Fig. 8.8). Measured abundances only concern gases lying between the source star and the observer. Matter contained in dust grains escapes detection. 

---