We are in Palmdale, California, Building 703. Mission Briefing. In two hours we will take off to another ten-hour SOFIA observing flight. SOFIA is short for Stratospheric Observatory for Infrared Astronomy and is a heavily modified Boeing 747 SP. The telescope is hidden behind a giant bulkhead. The mission briefing starts with the role call: everyone who expects to fly is called by the mission director, some 20 or 30 people. Most wear brown fire-proof flight suits. On board are pilots, engineers, scientists and guest observers, whose data are going to be taken. Usually, the observations are fully operated by the instrument team, leaving little to do for the guest observers but to watch and marvel. They are obliged to present their science goals at the mission briefing to an always interested audience. Flight plans are handed out to everyone and explained with delay options. Then the weather forecast with water vapor expectations are presented. Since water vapor absorbs infrared radiation, this is of crucial importance for the data quality. Last but not least we are warned about trip hazards from chords and not to use microwaves, a special requirement for GREAT, the German Receiver for Astronomy at Terahertz Frequencies, the instrument we are using. Of course, cell phones must be switched off.

Figure 1: 3 RGB color Ionized carbon map of SOFIA taken with the upGREAT receiver.  Red is an integrated image from 7 to 9 km/s, green from 9 to 11 km/s and blue is 11 to 13 km/s. The inset plot shows the average spectrum of the entire map with the RGB velocity ranges overplotted, each map pixel is made of individual spectrum allowing astronomers to disentangle the kinematics of the region.

An impressive 2 million spectra of the [CII] fine-structure line from the Orion Nebula, the most nearby massive star-forming region. Now the fun begins. Such a large 3D map requires new tools for data reduction that have to be developed as we proceed. It takes time and patience. Spikes from a cell phone have to be removed, beam efficiencies corrected, baseline problems solved. The reduced map still contains too much information to be analyzed by one person alone. Generations of PhD students may work on it. We focus on the low-hanging fruit. By using position-velocity diagrams we are able to vizualize the expansion of the Extended Orion Nebula, the bubble blown by the central stars. The result is of surprising beauty. The wind of the central Trapezium stars is powerful enough to drive the large-scale expansion of the interstellar medium.

The scientific result is only the tip of the iceberg. The unrelenting dedication of everybody who is involved in the project makes flying with SOFIA an unforgettable experience. May she live long.

Cornelia Pabst and Ronan Higgins

Figure 2: Cornelia Pabst (lead author, University of Leiden) giving an on board interview

Figure 3: Denise Riquelme (Max Planck institute for Radio Astronomy) and Ronan Higgins (Co-author, University of Cologne) inspecting the data during flight

Links:

• DLR promotional video taken during Orion observation flight partly in german with english subtitles)
• University of Cologne press release
• University of Leiden press release