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AST/RO
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AST/RO Science Goals

Astronomy

The 1.7 meter diameter AST/RO telescope was conceived as a submillimeter-wave survey instrument and a prototype telescope for automated winter operations at the South Pole. It was designed to carry out large-scale mapping of the southern Galactic plane, the Galactic Center, nearby galaxies, and selected star-formation regions using a wide variety of submillimeter-wave line and continuum detection systems.

The immediate scientific goals of the instrument are heterodyne spectroscopy of atomic and molecular clouds in the Milky Way and nearby galaxies. Current targets are the 492 GHz fine-structure line of neutral atomic carbon (CI) and the J=4-3 rotational line of carbon monoxide (CO) at 460 GHz. The 1.7 meter aperture yields a beamsize of about 2 arcminutes at these frequencies, large enough to permit large-scale mapping programs, yet small enough to map distant clouds in the Galaxy and to just resolve hundreds of external galaxies. It is not small enough, however, to study distant galaxies or to study protostellar regions in any detail.

Carbon is the fourth most abundant element in the universe and an important component in the chemistry and cooling of the interstellar medium of the Milky Way and other galaxies. Current observing techniques now permit observation of all phases of carbon in the interstellar medium: neutral atomic carbon (CI), ionized carbon (CII), molecular carbon (CO), and carbon in dust grains (graphite). This is not the case for any other common element. Consequently, observations of carbon reveal the complete range of physical conditions found in the interstellar medium.

The 3P electronic ground state of neutral atomic carbon is split by spin-orbit coupling into three levels, which have energies of 0 K, 23.6 K, and 62.4 K, respectively. The resulting 3P1 to 3P0 magnetic dipole transition is at a frequency of 492.1607 GHz (wavelength 610 microns), and the 3P2 to 3P1 transition is at a frequency of 809.345 GHz (370 microns). These fine-structure transitions are easily excited by collisions at the cool gas temperatures (T < 50 K) characteristic of the denser phases of the interstellar medium. The low-lying rotational transitions of CO (up to J = 4-3) are likewise easily excited in cold gas, while the 158 micron [CII] ground-state line and higher rotational transitions of CO are easily observed only from warmer gas.

Observations of neutral carbon are important to understand the interaction of starlight with the interstellar medium. When an O or B star forms, it emits copious amounts of far-ultraviolet (FUV, 6 to 13.6 eV) radiation. These FUV photons impinge on the surfaces of neutral clouds and are sufficiently energetic to eject electrons from dust grains. Through collisions with atoms and molecules, the photoelectrons heat the gas which in turn results in excitation of neutral and ionized carbon. Regions where FUV radiation dominates the heating are called photodissociation regions (PDRs) .

Over the last ten years, chemical and thermal models of PDRs have been developed (e.g., Tielens and Hollenbach 1985; Hollenbach et al. 1991) which predict the intensity of emission lines from these regions as a function of cloud density and incident FUV flux. These models show that neutral carbon is the by-product of the equilibrium of two competing processes taking place in the envelope of a molecular cloud: the photodissociation of CO, which creates C0, and the photoionization of C, which destroys it to create C+. The [CI] fine-structure lines at 492 and 810 GHz are thus tracers of the transition between C+ (the photodissociation region) and CO (the dense molecular cores). In PDRs with densities in the range 102 to 105 cm-3), the models predict "the ubiquitous presence of [CI] emission" (Hollenbach et al. 1991, ApJ, 377, 192).

A primary goal of AST/RO is to improve our understanding of photodissociation regions, molecular clouds, and star formation by studying the distribution and properties of atomic carbon throughout the interstellar medium.

Aeronomy

A submillimeter and far-infrared Fourier Transform Spectrometer (FTS) is under construction by Richard Chamberlin of the Caltech Submillimeter Observatory (Richard was AST/RO's first winterover scientist in 1995). The FTS will be installed at the South Pole in late 2000 and will measure atmospheric emissivity at wavelengths between 60 and 1000 microns, providing a definitive test of atmospheric opacity models.

Also planned are observations of molecules in Earth's stratosphere. AST/RO's receivers and spectrometers are much more sensitive and have much higher spectral resolution than the instruments usually used for remote sensing of stratospheric molecules. The observed spectra can be inverted to obtain ozone and CO abundances as a function of altitude. Given the bandwidth and sensitivity of the AST/RO system, we estimate that we will be able to determine ozone abundance for altitudes between 15 km and 60 km, with an altitude resolution of about 2 km and an accuracy of about 5%, with two hours of observing time. This will provide a uniform, well-sampled database for studies of the time evolution of ozone, measuring its disappearance in late winter and early spring. Unlike most other methods of measuring stratospheric molecules, this technique can be used at night as well as in daylight and samples a range of altitudes at the same time.

Site Characterization

One of the very first goals of the AST/RO project was to characterize the properties of the South Pole as a site for millimeter and submillimeter-wave astronomy. Both the transparency and the stability of the atmosphere are excellent. Results are described in detail in:

  • Chamberlin, R. A., and Bally, J. 1994, "225 GHz Opacity of the South Pole Sky Derived From Continual Radiometric Measurements of the Sky Brightness Temperature", Applied Optics, 33(6), 1095.

  • Chamberlin, R. A., and Bally, J. 1995, "The Observed Relationship Between the South Pole 225 GHz Atmospheric Opacity and the Water Vapor Column Density", Int. J. of IR and MM Waves, 16(5), 907.

  • Chamberlin, R. A., Lane, A. P., and Stark, A. A. 1997, "The 492 GHz Atmospheric Opacity at the Geographic South Pole", Astrophysical Journal, 476, 428-433.

  • Lane, A. P. 1998, "Submillimeter Transmission at South Pole", in Astrophysics From Antarctica, A. S. P. Conf. Ser., eds. G. Novak and R. H. Landsberg, (San Francisco, CA: Astr. Soc. of the Pacific), 141, 289-295.

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