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.
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.
Astronomy
Aeronomy
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:
Comments or questions may be sent to: adair@cfa.harvard.edu (Adair P. Lane)