Astronomical Spectroscopy
Astronomical spectroscopy has attracted perhaps the largest body of scientists and engineers working in the Submillimeter/Terahertz spectral region. That the line between scientists and the engineers is often blurred in this field is perhaps one of the chief reasons it has been so successful. The challenging technical problems of astronomy have not only driven and motivated the development of technology, but also have provided generally accepted and verifiable benchmarks.
The genesis of this astronomical activity can be traced back to the early days of microwave spectroscopy when techniques developed for millimeter spectroscopy were adapted in 1955 to detect radiation from the sun in the atmospheric window around 94 GHz [1].
At a basic level astronomical spectroscopy and atmospheric spectroscopy are very similar. Both detect the emission from molecules and, to first order, both use similar technology to do so: heterodyne receivers being extended from lower frequencies and optical interferometric techniques being extended from shorter wave-lengths.
It is instructive to start by considering the differences in the science (as well as in the resulting technology) for atmospheric and astronomical applications. In general the interstellar medium is colder, with temperatures typically not too many times that of the microwave background (2.7 K), but with hotter (100K - 1000K) regions as protostellar cores are approached. Additionally, the astronomical collision times are much longer in all circumstances. This long collision time, combined with fluxes of energetic particles, produces molecular systems which can be far from equilibrium in rotational state populations, partial pressures of gases (which for almost all species would approach zero under conditions dictated by vapor pressure), and abundances of ions, free radicals, and other reactive species. A useful measure of this non-equilibrium is that the lifetime of gaseous species in the interstellar medium is ~ 105 - 106 years before they freeze out on dust grains.
From the point of view of laboratory astrophysics, this leads to a rather different set of problems than those motivated by atmospheric science. Table V.B.1-1 shows a list of the molecular species that have been detected in the interstellar medium. A comparison of it with the species found in
Figure: Ozone Production and Destruction.
Simplified diagram which shows the ozone prod-
uction and destruction cycles in the upper atmo-
sphere.
Table V.B.1-1 lists a number of major millimeter, submillimeter, and far infrared telescope facilities. These include ground, aircraft, and space based telescopes. Expecially noteworthy for the future of the field are the large new systems in the active development stage: Atacama Large Millimeter Array (ALMA) at Llano de Chajnantor in Chile, Herschel (FIRST), and SOFIA, a general purpose observing platform in a converted 747. By the historical standards of the spectral region, these represent an enormous investment and are a testimony both to the importance of the science that drives these projects and the technological advances that have made them possible.
| Location | Size | Name | Website |
| Ground Based Telescopes | |||
| Mauna Kea | 15m | James Clerk Maxwell (JCMT) | JCMT |
| Mauna Kea | 10m | Caltech Submillimeter Observatory (CSO) | CSO |
| Mauna Kea | 8 x 6m | Submillimeter Array (SMA) | SMA |
| Boston | 1.2m | The CFA | CFA |
| Goernergrat (Switzerland) | 3m | KOSMA | KOSMA |
| Llano de Chajnantor (Chile) | 64 x 12m | Atacama Large Millimeter Array (ALMA) | ALMA |
| Mt. Graham | 10m | Arizona Radio Observatory | ARO |
| South Pole | 1.7m | Center for Astrophysical Research in Antarctica (CARA) | CARA |
| Airborne Telescopes | |||
| Stratospheric Observatory of Infrared Astronomy (SOFIA) | SOFIA | ||
| Space Based Telescopes | |||
| Submillimeter-Wave Astronomy Sattelite (SWAS) | SWAS | ||
| Herschel | Herschel |
An important first order effect in this comparison is simply one of atomic abundance. The atmospheric species are derivative of the major atmospheric components (N2, O2, and H2O) and man-made injections into the atmosphere. The interstellar species are driven more by cosmic abundances (H, C, O, N, . . .). Additionally, the interstellar list has many prominent ions and free radicals whose lifetimes under terrestrial conditions are very short. While spectral line frequencies are independent of the molecular environment after correction to rest velocity, their shapes and widths are not. Moreover, the inelastic rotational energy transfer rates which are closely related to pressure broadening (which is absent in the interstellar medium but of great significance in the recovery of atmospheric parameters from remote sensing data) are similarly necessary for the recovery of astronomical information from non-equilibrium interstellar medium.
Thus much of the emphasis in laboratory astrophysics has been on the development of laboratory environments both for the production of reactive species and ones in which low temperature collisional studies can be carried out. These have been considered in more detail above in Section IV.
References
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- Detection of NH3 molecules in the interstellar medium by their microwave emission Phys. Rev. Lett. 21, 1701-1705 (1968). Google Scholar
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- How the Laser Happened: Adventures of a Scientist (Oxford, 1999). Google Scholar
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- Spectroscopy of the Earth's Atmosphere and Interstellar Medium () 423-517 (Academic Press, 1992).