The Atacama Pathfinder Experiment (APEX) 12-m sub-millimetre telescope lives up to the ambitions of the scientists by 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 molecule, and the detection of light emitted at 0.2 mm from CO molecules, as well as light coming from a charged molecule composed of two forms of Hydrogen.
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 - also in the Orion star-forming region - of light emitted by 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.
Light coming from a charged molecule composed of Hydrogen and Deuterium (H2D+) was detected in several cold clouds in the Southern Sky. The H2D+ ion is interesting because it traces gas so cold (a few degrees above the absolute zero!) that only a few molecular species have not frozen out onto the surfaces of dust grains.
These are not the only significant discoveries made. Other highlights include the first observations of atomic carbon in the so-called "Pillars of Creation" in the Eagle Nebula (also known as Messier 16), a sub-millimetre study of a massive hot core, of a high-mass star forming region, as well as of a high velocity outflow coming from a young stellar object. Studies of molecular regions in the dwarf galaxy NGC 6822 and in the starburst galaxy NGC 253 were also done, proving that APEX can also contribute to the exploration of extragalactic objects.
Apart from the astronomical studies, a series of contributions deal with the technical aspects of APEX, such as the telescope itself, its software, its receivers and spectrometers. The latter were developed at the Max-Planck-Institut für Radioastronomie in Bonn, Germany and at the Swedish Chalmers University, while the 0.2 mm receiver was developed at the University of Cologne (Germany).
The APEX telescope, designed to work at sub-millimetre wavelengths, in the 0.2 to 1.5 mm range, passed successfully its Science Verification phase in July 2005, and since then is performing regular science observations. It is located on the 5100 m high Chajnantor plateau in the Atacama Desert (Chile), probably the driest place on Earth. It is a collaborative effort between the Max-Planck-Institut für Radioastronomie, ESO and the Onsala Space Observatory (Sweden).
With its precise antenna and large collecting area, APEX provides, at this exceptional location, unprecedented access to a whole new domain in astronomical observations. Indeed, millimetre and sub-millimetre astronomy opens exciting new possibilities in the study of the first galaxies to have formed in the Universe and of the formation processes of stars and planets. It also allows astronomers to study the chemistry and physical conditions of molecular clouds, that are dense regions of gas and dust in which new stars are forming.
APEX is the pathfinder to the ALMA project. In fact, it is a modified ALMA prototype antenna and is located at the future site of the ALMA observatory. ALMA will consist of a giant array of 12-m antennas separated by baselines of up to 14 km and is expected to gradually start operation by the end of the decade.
The Astronomy & Astrophysics special issue (volume 454 no.2 - August I, 2006) on APEX first results includes 26 articles. They are freely available in PDF format from the publisher web site.
These results are partly based on APEX science verification data that are available from the ESO archive at http://www.eso.org/science/apexsv/.
More information on APEX is available at http://www.apex-telescope.org/.
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