CARA Science: Overview

Cosmic Microwave Background Radiation

The Cosmic Microwave Background (CMB) has a wealth of information about the origin and evolution of the Universe in its signal. Its spectrum is that of a blackbody at 2.7 K, confirmation of the prediction made by the standard Big Bang model. Its angular power spectrum contains information on the structure that existed at decoupling. Anisotropy in the matter distribution at this time left imprinted upon the CMB a small anisotropy (deltaT/T ~10^-5) in the angular distribution of microwave power. The amplitude and spatial distribution of the anisotropy are directly related to the conditions in the early Universe which gave rise to the onset of structure formation. All current models of structure formation predict specific power spectra and morphologies for the CMB anisotropy. Within the context of a given theory, the cosmological parameters such as the Hubble constant H0, and the mass density of the Universe omega = rho/rhoc, can be estimated from observations of the anisotropy in the CMB. Determining the angular power spectrum of the fluctuations, their distribution function, and imaging the CMB anisotropy at intermediate angular scales (20 arcseconds to 1 degree) is the scientific goal of the CARA CMB program.

Measuring the angular power spectrum of the CMB fluctuations, and producing images of the primary and secondary anisotropy of the CMB, are key science goals of several experiments either just completed (Python), ready for deployment (Viper), or under construction (DASI) by members of CARA. Each experiment addresses a unique range of angular scales and frequency coverage. Together, they span a range of angular scales from 3 degrees to 2 arcminutes, and frequencies from 26 - 400 GHz.

CARA CMB project goals include:

CARA CMB instruments include:

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The earliest stars formed from a pristine cosmic gas with essentially no elements heavier than helium. After galaxies formed, subsequent stars formed heavier elements in their cores which were returned to the interstellar medium through winds and supernova explosions, in a continual process of enrichment which constantly increases the heavy element abundance in galaxies. Because these heavy elements play an essential role, the formation of stars and planets, and ultimately, of life is intimately tied to the cycling of material back and forth between stellar interiors and the interstellar medium. The physics of the interstellar medium and its interaction with the stars that it spawns is a central problem in modern astrophysics.

Perhaps the most important element heavier than helium is carbon. Current observing techniques permit observation of almost all manifestations of carbon atoms in the interstellar medium: carbon in dust grains (graphite), molecular carbon (CO), neutral atomic carbon (C I), and ionized carbon (C II). This is not the case for any other common atom. Consequently, observations of carbon allow us to study all phases of the interstellar medium. Carbon in these forms can be seen throughout the Galaxy and in other galaxies.

Although carbon in dust and molecular forms is routinely observed from the ground, atomic carbon observations are quite difficult, because atmospheric water vapor is nearly opaque to their submillimeter spectral lines except at the very driest sites. Neutral carbon is particular important because of its central roles in (1) the formation of giant molecular clouds, which in turn form new stars, and (2) the interaction of radiation from newly formed stars with its ambient parental material. CARA's AST/RO telescope is the first to observe neutral carbon routinely and almost continuously.

Star formation occurs inside molecular clouds. The process of star formation destroys dense interstellar clouds. Material in the clouds is converted in part into stars, and clouds are disrupted and destroyed by the energy released from the newborn stars. Star formation is inefficient in converting dense gas into stars, so the rate at which material is incorporated into star-forming clouds must exceed the star formation rate. Little is known about the formation of dense interstellar clouds. Do spiral density waves drive cloud formation? Is material added to clouds primarily by condensation, or by collision and dissipation? How do the cloud formation processes affect cloud chemistry? Are magnetic field effects important? Are metallicity effects important? Do the mechanisms which form a cloud affect its subsequent star formation?

Cloud formation occurs at the boundaries of dense clouds. These regions are best studied in the ground-state fine-structure lines of atomic carbon at submillimeter wavelengths. Carbon has the lowest ionization energy (11.26 eV) of all common elements, so it is ionized by the UV radiation field in the intercloud medium. In dense clouds, chemistry drives almost all the carbon into CO and dust grains. Models of photodissociation regions (PDRs) indicate that atomic carbon [C I] emission arises primarily at cloud boundaries, the interface between molecular material and the intercloud medium.

CARA submillimeter goals include:

CARA submillimeter instruments include:

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Infrared Experiments

Once stars form in the interstellar medium, they begin to destroy their parental clouds via photodissociation, photoionization, and stellar winds and outflows. It is difficult to study very young stars and their interaction with the surrounding medium because they are deeply embedded in visibly opaque molecular clouds. Fortunately, young stars and their surroundings also emit strongly at infrared wavelengths: (1) in thermal continuum radiation from dust grains in protostellar accretion disks, (2) in fluorescent emission from molecular hydrogen and PAH dust grains excited by the ultraviolet component of the starlight, (3) in radiation from molecular hydrogen shocked by high-velocity outflows, and (4) in infrared recombination lines from ionized hydrogen.

The South Pole is uniquely suited to large-scale infrared surveys of star-forming regions. The low thermal background from the very cold telescope provides an enormous advantage in sensitivity. Although the seeing at the South Pole is not as good as at the best ground-based sites, it is adequate for wide-field imaging with small telescopes, and because astronomical sources never set, very long integrations and deep imaging are practical and efficient.

In the next two years, CARA scientists will perform extensive large-scale infrared and submillimeter surveys of star-forming regions in the Milky Way and Magellanic Clouds. Such studies are crucial for understanding the frequency and location of star formation in molecular clouds, the global morphology, energy balance, and chemistry of the interstellar medium, and the differences between star formation in high and low-metallicity systems.

The SPIREX telescope will be used to perform a number of wide-field infrared surveys, including the following:

CARA's infrared instruments include: Return to top.

Site Testing

Over the past few years, careful measurements at the South Pole have shown that conditions there are exceptionally good for certain kinds of astronomy. The advantages of Antarctic sites over those at temperate locations stem principally from the extreme cold and dryness, combined with the high altitude and the stability of the atmosphere.

These studies are carried out at all wavelengths, and so considerably more information is available on a separate page.

CARA's site testing instruments include:

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