* Astronomy

The Atacama Pathfinder Experiment (APEX) 12-m sub-millimetre telescope lives up to its ambitions of providing access to the "Cold Universe" with unprecedented sensitivity and image quality. As a demonstration, no less than 26 articles based on early science with APEX are published this week in the research journal Astronomy & Astrophysics. Among the many new findings, most in the field of star formation and astrochemistry, are the discovery of a new interstellar ion and the detection of CO radiation at 0.2 mm and of H2D+ radiation.

Using both APEX and the IRAM 30-metre telescope the first astronomical detection of a charged molecule composed of carbon and fluorine - the 'CF+ ion' - was made. Prior to this discovery, only one fluorine-containing molecular species had been found in space so far, the HF molecule ('hydrogen fluoride'), consisting of one atom of hydrogen and one of fluorine. The newly discovered molecule, produced through a reaction between carbon and the HF molecule, was found in a region adjoining the Orion Nebula, one of the nearest and most active stellar nurseries in the Milky Way. This detection provides support to the astronomers' understanding of interstellar fluorine chemistry, suggesting that hydrogen fluoride is ubiquitous in interstellar gas clouds.

Another premiere is the detection - again in the Orion region - of radiation from carbon monoxide (CO) at a wavelength of 0.2 mm. These short wavelengths are very difficult to investigate, both because the water vapour in the atmosphere attenuates the signal even more severely than elsewhere in the submillimeter range, but also because they are at the limit of the telescope’s operating range. The detection of CO at these wavelengths, the very shortest accessible from Earth in any of the submillimeter "windows", proves the superb efficiency of APEX.

Source

Title: First observations with CONDOR, a 1.5 THz heterodyne receiver
Authors: M. C. Wiedner, G. Wieching, F. Bielau, M. Emprechtinger, K. Rettenbacher, N. H. Volgenau, U. U. Graf, C. E. Honingh, K. Jacobs, B. Vowinkel, K. M. Menten, K. M., L. Nyman, R. Güsten, S. Philipp, D. Rabanus, J. Stutzki, F. Wyrowski

The THz atmospheric windows centered at roughly 1.3 and 1.5~THz, contain numerous spectral lines of astronomical importance, including three high-J CO lines, the N+ line at 205 microns, and the ground transition of para-H2D+. The CO lines are tracers of hot (several 100K), dense gas; N+ is a cooling line of diffuse, ionised gas; the H2D+ line is a non-depleting tracer of cold (~20K), dense gas. As the THz lines benefit the study of diverse phenomena (from high-mass star-forming regions to the WIM to cold prestellar cores), we have built the CO N+ Deuterium Observations Receiver (CONDOR) to further explore the THz windows by ground-based observations. CONDOR was designed to be used at the Atacama Pathfinder EXperiment (APEX) and Stratospheric Observatory For Infrared Astronomy (SOFIA). CONDOR was installed at the APEX telescope and test observations were made to characterise the instrument. The combination of CONDOR on APEX successfully detected THz radiation from astronomical sources. CONDOR operated with typical Trec=1600K and spectral Allan variance times of 30s. CONDOR's first light observations of CO 13-12 emission from the hot core Orion FIR4 (= OMC1 South) revealed a narrow line with T(MB) = 210K and delta(V)=5.4km/s. A search for N+ emission from the ionisation front of the Orion Bar resulted in a non-detection. The successful deployment of CONDOR at APEX demonstrates the potential for making observations at THz frequencies from ground-based facilities.

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In November 2005, CONDOR, the CO N+ Deuterium Observation Receiver, opened its eye to the universe for the first time. CONDOR was installed at APEX (Atacama Pathfinder EXperiment) in the Chilean Andes and detected hot gas in the vicinity of young massive stars from radiation at the extremely high radio frequency of 1.5 terahertz (THz), i.e. 1.5 million million Hertz.
The CONDOR detections are the first THz-frequency observations acquired with a large telescope (12-m diameter). The observations reveal several surprises, and the expectation that THz astronomy would yield valuable scientific results has been met. The success of CONDOR is a combined effort of researchers from the First Physical Institute of the University of Cologne and the Max Planck Institute for Radio Astronomy.

"CONDOR has fully met our expectations. We prepared the receiver well, we had an excellent team at the site, but we also were lucky that the weather was so good" - Dr. Martina Wiedner, CONDOR project leader.


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The first CONDOR (CO N+ Deuterium Observation Receiver) observations were performed on the APEX (Atacama Pathfinder EXperiment) telescope. Currently, APEX is the only telescope with a mirror larger than 5 m that can observe terahertz frequencies.
Credit Arnaud Belloche

Because of the difficulty in detecting electromagnetic waves at such high frequencies (a thousand times higher than those of a cellular phone and a million times higher than "short wave" radio), state-of-the-art receivers have to be used. A special type of device called a Hot Electron Bolometer, developed at the University of Cologne by Dr. Karl Jacobs and his colleagues, was essential for CONDOR’s success. This device converts the THz frequency radiation into frequencies around 1 GHz, which are much easier to manipulate. To achieve high sensitivity, the receiver is cooled to a temperature of -269°C, only 4°C above absolute zero.

The CONDOR observations require that the amount of water vapour in the Earth atmosphere is exceptionally small, because water vapour readily absorbs THz radiation. Located in the Atacama Desert of Chile at an elevation of 5100 m, the site of the APEX telescope is extraordinarily dry. APEX has a 12 m primary mirror that resembles a perfect paraboloid to within 15 microns (7 times thinner than a human hair). The telescope is currently equipped with receivers between 300 and 900 GHz. CONDOR, which requires different technology, is the first APEX receiver operational above 1 THz.


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Star formation in the Orion Nebula. The plot on the top shows the first CONDOR detection of highly excited carbon monoxide (CO J = 13 → 12) from the massive star formation region FIR4 in Orion. The line is a clear indicator of hot gas. In the background is an optical image from the Hubble Space Telescope, which shows a larger region of stars and glowing dust. FIR 4 is not visible at optical frequencies because it is hidden behind a thick layer of dust and gas.
Credit: ESA/NASA/CONDOR

"The CONDOR observations are done at the highest frequencies that APEX expects to ever reach. At even higher frequencies, the Earth’s atmosphere becomes opaque until one reaches the infrared wavelengths" - Dr. Rolf Güsten, APEX project manager.

The CONDOR observations on APEX open up the nearly unknown THz universe for exploration.

"If one could only see blue things, one would never know about trees and grass. Similarly, one discovers new things in the universe by looking at it in different frequencies. The spectral signatures of hot gas (high rotational transitions of the carbon monoxide (CO) molecule) are seen at THz frequencies. Since hot gas is an essential component of massive star formation, regions that give birth to massive stars can be observed at these frequencies" - Dr. Martina Wiedner.

CONDOR detected emission from CO at 1.5 THz during its first observations. The line width is surprisingly narrow, suggesting that the gas is heated by ultraviolet (UV) radiation from stars, rather than collisions within gases as originally expected. Astronomers on the CONDOR project team are eager to make further observations.

The CONDOR project is carried out at the First Physical Institute of the University of Cologne (I. Physikalisches Institut of the Universität zu Köln) in an Independent Junior Research Group of the Collaborative Research Centre 494 (Nachwuchsgruppe im Sonderforschungsbereich (SFB) 494). About 100 scientists from Germany are working in this Collaborative Research Centre, entitled "The Evolution of Interstellar Matter: Terahertz Spectroscopy in Space and in the Laboratory." The APEX telescope was built jointly by the Max Planck Institute for Radio Astronomy in Bonn, Germany, the Onsala Space Observatory, Sweden, and the European Southern Observatory.

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