SOFIA: Molecules and Star Formation in the Milky Way

The Stratospheric Observatory for Infrared Astronomy (SOFIA) has completed its first series of science flights using the German Receiver for Astronomy at Terahertz Frequencies (GREAT). The scientific results are now being published in a special issue of the European scientific journal Astronomy & Astrophysics (Volume 542, 10 May) along with reports on GREAT's advanced technologies. They demonstrate the instrument's versatility, include first detections of interstellar molecules and important spectral lines in space, and address the different stages of star formation. SOFIA is operated by NASA and the German Aerospace Center (Deutsches Zentrum für Luft und- Raumfahrt; DLR).

The first series of astronomical observations with GREAT were successfully completed in November last year. Now, six months later, the scientific results have been published in a special issue of the prestigious European scientific journal Astronomy & Astrophysics. An international group of scientists report on the first astronomical results as well as the technologies employed on board SOFIA in 22 articles. GREAT was developed by a consortium of German research institutes led by Rolf Güsten (Max Planck Institute for Radio Astronomy, MPIfR).

An airborne observatory

SOFIA, a joint project between NASA and DLR, carries a 2.7-metre telescope in a modified Boeing 747SP aircraft, and is the world's largest airborne infrared observatory. SOFIA flies at altitudes of up to 13,700 metres to enable detection of radiation at far-infrared wavelengths, which would otherwise be impossible due to absorption by water vapour in the atmosphere. The SOFIA observatory and the GREAT instrument provide a window to the far-infrared skies for high-resolution spectroscopy, and GREAT's technology allows reaching higher frequencies and sensitivities than ever before.

"The high resolving power of the GREAT spectrometer allows us to analyse and study interstellar gas and the stellar life cycle, from a protostar's early embryonic phase while still embedded in its parental cloud, to an evolved star's death, when the stellar envelope is ejected back into space," says Rolf Güsten from the Max Planck Institute for Radio Astronomy, Principal Investigator of the GREAT project. "This stunning collection of first scientific results is the reward for many years of development work, and underlines the huge scientific potential of airborne far-infrared spectroscopy."

GREAT provides new scientific insights

Many of the contributed papers study the star formation process in its earliest phases - from when the protostellar molecular cloud contracts and condenses to the moment in which the embryonic star vigorously interacts with its surrounding parental molecular cloud, tearing it apart and ionising it. The high spectral resolution capabilities of GREAT enabled the calculation of the velocity field of gas in the parental molecular clouds, which is tracked by the important ionised carbon spectral line in several star forming regions.

GREAT detected the velocity signature of the motion of the infalling gas (collapse) in three protostellar envelopes, directly examining the dynamics of forming stars. Two molecular species were detected for the first time ever in space: OD, an isotopic substitute of hydroxyl (OH), in which with the hydrogen atom is substituted by the heavier deuterium, and the mercapto radical SH. Crossing a technological barrier, observations of the ground-state transition of OH at 2.5 terahertz (120 microns wavelength) allow the exploration of new astrochemical territories. 

The remnant envelope of an evolved star, ionised by its hot stellar core, was also investigated,  as was the violent shock interaction of a supernova remnant with its surrounding interstellar medium. Furthermore, researchers also studied the circumnuclear accretion disc, which is ultimately feeding the black hole in the centre of the Milky Way, and star formation in the circumnuclear region of the nearby galaxy IC342.

Exceptional results

"The rich harvest of scientific results from this first observing campaign with SOFIA and the GREAT instrument gives a first glimpse of the tremendous scientific potential of this observatory," states Alois Himmes, DLR's SOFIA programme manager. SOFIA will take advantage of rapid instrument innovations, allowing instruments like GREAT to be continuously adapted to the latest technologies, and thus promising exciting astronomical observations in the coming years.