Terahertz Astronomy at the Pole Opens New Vistas
Brent D. Johnson
The South Pole hardly offers a welcoming environment, even during its relatively warm summer months. For terahertz astronomers, however, the pole is an ideal location for performing observations at these frequencies, especially during the perilous austral winter.
The Amundsen-Scott South Pole Station, maintained by the US government, is the most remote outpost on the planet. Perched on a glacial plateau at an altitude of approximately 11,000 feet, the station endures summer temperatures of —40 °C. After mid-February, when the temperature dips below —50 °C, no planes can land on the plateau, and the population at the pole dwindles from 200 to 40. A skeleton crew remains throughout the winter, cut off from the nearest supply base 3362 miles away in New Zealand.
The station in winter is perfect for terahertz astronomy, unlike the more pleasant conditions at the observatories on Mauna Kea in Hawaii, for example. Although Mauna Kea stands at nearly 14,000 feet and has lower air pressure, it has more precipitable water vapor in the air above it that absorbs radiation at terahertz frequencies. The extremely dry air at the pole, in contrast, offers median transparencies of 5 to 11 percent at important wavelengths around 200 µm, and it can be three times better than this on particularly good days.
The extremely dry atmospheric conditions at the Amundsen-Scott South Pole Station are ideal for performing terahertz astronomy. The low moisture content in the winter air permits more far-infrared radiation to pass, revealing cosmic phenomena at the spectral lines of carbon monoxide and singly ionized nitrogen. Photo by Petty Officer 2nd Class John K. Sokolowski, US Navy. Courtesy of US Department of Defense.
Making the long journey to the white continent in November, K. Sigfrid Yngvesson, a professor of electrical engineering at the University of Massachusetts Amherst, deployed a local laser oscillator receiver for the 1.7-m Antarctic Submillimeter Telescope and Remote Observatory. The Trend, or the terahertz receiver with niobium nitride hot electron bolometer device, operates on a heterodyne principle. Mixing the incoming terahertz radiation from the telescope with that from a Coherent SIFIR-50 terahertz laser in a superconducting niobium nitride detector yields a signal that is the difference of the two interfering frequencies. This gigahertz signal then is directly amplified by a cooled electronic amplifier using high-electron-mobility transistor.
The new receiver -- the result of a joint effort of UMass Amherst and Lowell, the University of Arizona, Smithsonian Astrophysical Observatory, Harvard University and Chalmers University of Technology in Sweden -- is at least 10 times more sensitive than the Schottky-barrier diodes used in the Kuiper Airborne Observatory. For the past 20 years, that instrument, mounted on a Lockheed C-141 transport plane, has performed infrared astronomy at altitudes greater than 40,000 feet to escape the absorption effects of moisture in the atmosphere. Although devices like Trend likely will find ultimate application on similar airborne platforms as well as on space-based ones, it is crucial to gain familiarity with the technology on terrestrial sites.
Targets of interest
After the receiver was calibrated using laboratory sources at room temperature and at liquid nitrogen temperature, the telescope was left in the hands of Chris Martin, a solid-state physicist turned astronomer. Martin has been operating the instrument from the South Pole since its installation and attempted to make observations in May. He hopes to collect more astronomical data this month, when the weather is optimal for terahertz work.
The researchers have several targets at terahertz frequencies. The first is carbon monoxide. CO has a higher-order line at approximately 1267 GHz, and observation at such higher excitation states exposes stars that are in the process of formation in the interstellar medium, such as star bursts, or jets of gas that occur when stars are born. The center of the galaxy may be the best place to look for these events.
Another, more interesting target, Yngvesson said, is singly ionized nitrogen. With a line at approximately 1461 GHz, it can be observed only using terahertz astronomy. When the Cosmic Background Explorer investigated the infrared and microwave remnants of the big bang, it detected singly ionized nitrogen with crude, angular resolution. Heterodyne receivers may allow scientists to tease kinematic information from their studies of the skies to determine how stars and clusters affect the ionized interstellar medium.
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