Nov. 9/10: The Battle to Observe Betelgeuse

“SOFIA Flight 86 must rank as the most exciting observing nights that I have ever done!” said Graham M. Harper, an astrophysicist from the School of Physics, Trinity College, Dublin, Ireland, after his all-night expedition on Nov. 9/10. That's an impressive statement from a researcher who focuses on stellar activity in cool stars and routinely uses a variety of ground-based and space telescopes to capture energy ranging from the ultraviolet to centimeter radio wavelengths.

Harper traveled 8,260 kilometers (5,130 miles) from Ireland to Palmdale, Calif., to fly all night as a guest investigator on board SOFIA to make infrared observations of the outflow around the massive red supergiant star Betelgeuse. “The flight started off on time and they let me sit in the cockpit for take-off until we started observing,” Harper said. “When we reached 35,000 feet they opened the telescope cavity door. From my seat in the cockpit you could not tell that anything had changed – very smooth indeed.”

Betelgeuse (also known as Alpha Orionis) is the second brightest star in the constellation Orion, the eighth brightest star in the northern night sky, and one of the largest stars ever measured. For his observations on board SOFIA, Harper used the German Receiver for Astronomy at Terahertz Frequencies, or GREAT, an infrared heterodyne spectrometer developed by a team led by Rolf Güsten from the Max-Planck-Institute for Radio Astronomy in Bonn, Germany. Harper’s investigation called for an examination of the 163- and 217-micron cooling lines (J-12-11 at 217 microns and J=16-15 at 163 microns) of carbon monoxide molecules in the gas flowing away from the red supergiant star. Gas expelled from the star will be recycled over the next several thousand millennia to form future generations of stars and planets. SOFIA and GREAT were chosen for this research as the spectral line Harper was observing cannot be detected by any ground-based telescope because it is at a wavelength completely blocked by water vapor in Earth’s atmosphere.

The SOFIA/GREAT observations were part of a multi-wavelength study of carbon monoxide gas around Betelgeuse involving an international team using the Hubble Space Telescope's STIS (Space Telescope Imaging Spectrograph) ultraviolet spectrometer, the Combined Array for Research in Millimeter-wave Astronomy interferometer (CARMA, Big Pine, Calif.), and the Gemini Observatory's Phoenix near-infrared echelle spectrometer (Cerro Pachon, Chile).

Teamwork in Action

During Flight 86, the first half of the science mission went well. Good data were collected on Jupiter and the “Pillars of Creation” (an area of dust and gas within the Eagle Nebula, made famous by a popular Hubble Space Telescope image), and things were progressing well. Then, as the observatory aircraft made a turn to begin a different observing leg, the instrument and telescope control computers stopped communicating. Nothing.

The on-board avionics technicians rebooted the mission control system because, quoting from a post-flight report, “the platform integrated subsystem computers would not accept data from the telescope assembly imager processing subsystem” a fancy way of saying the telescope was pointing blind, without input from its set of optical-wavelength tracker cameras. A big problem; and one that required the attention of every member of the on-board scientific and engineering teams.

After the computers were rebooted several times, the observatory changed course to capture data on Betelgeuse – guest investigator Harper’s intended target. “Betelgeuse was acquired and the very first science data collection worked well,” said Harper. “But just as we got started the computers again stopped talking to each other. After an hour and three unsuccessful reboots we had no luck and had no imagers to guide subsequent observations. Things looked very dim.

“Then the telescope operators realized that they were still capturing tracking data from Betelgeuse, but could not see it with the usual software. Using a separate stand-alone laptop they were able to follow the star,” Harper continued. “The instrument scientists fooled the software into thinking they were running a simulation and were able to actually collect science observations of the star! This was a perfect demonstration of humans using their skill and any resources available to overcome the problem – something that software alone could not do.”

Mission personnel realized this work-around was only applicable for Betelgeuse because it is so bright, so they worked to continue the observations of the red giant star. Failure of the platform integrated subsystem computers had also knocked out the science flight planner’s ability to reroute the flight in real time. “The science flight planner had no aircraft position information, thus we had no way to replan the flight from the main deck,” said Randy Grashuis, SOFIA Mission Director. “We worked with the navigator and the pilots to stay on Betelgeuse for as long as possible and continue to collect data until the target reached the telescope’s elevation limit and it could no longer be seen. The team effort allowed us to take Betelgeuse data for an additional 30 minutes.”

Although observations were done far outside SOFIA’s standard operating procedures on the Nov. 9/10 flight, teamwork between the science staff, the telescope and instrument teams, and the avionics and flight crews enabled the collection of data that is inaccessible to ground-based observatories. After this flight landed, technicians addressed the computer communication issues.

Upon first glance, it appears guest investigator Harper was successful in detecting the carbon monoxide lines he was looking for, and shortly after he arrives home in Dublin he’ll receive his data from the SOFIA flight. Harper will oversee the data reduction, which will be accomplished by his graduate student Sarah Kennelly. In addition to the SOFIA data, Harper will always have one amazing tale to tell of the night he observed Betelgeuse from 43,000 feet.

Nicholas A. Veronico
SOFIA Science Center

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