SOFIA: first science highlights and future science potential

Abstract

SOFIA, the Stratospheric Observatory for Infrared Astronomy, is a joint project between NASA and the German Aerospace Agency (DLR) to develop and operate a 2.5 m airborne telescope in a highly modified Boeing 747SP aircraft that can fly as high as 45 000 feet (13.7 km). This is above 99.8 % of the precipitable water vapor which blocks much of the midand far-infrared radiation from reaching ground-based telescopes. In this review, we briefly discuss the characteristics of the Observatory and present a number of early science highlights obtained with the FORCAST camera in 5-40 micron spectral region and with the GREAT heterodyne spectrometer in the 130-240 micron spectral region. The FORCAST images in Orion show the discovery of a new high-mass protostar (IRc4), while GREAT observations at 1 km s −1 velocity resolution detected velocity-resolved, redshifted ammonia spectra at 1.81 THz in absorption against several strong farinfrared dust continuum sources, clear evidence of substantial protostellar infall onto massive (non-ionizing) protostars. These powerful new data allow us to determine how massive stars form in our Galaxy. Another highlight is the stunning image taken by FORCAST that reveals the transient circumnuclear 1.5 pc radius (dust) ring around our Galactic center, heated by hundreds of massive stars in the young nuclear star cluster. The GREAT heterodyne spectrometer also observed the circumnuclear ring in highly excited CO rotational lines, indicative of emission from warm dense molecular gas with broad velocity structure, perhaps due to local shock heating. GREAT also made superb mapping observations of the [C II] fine structure cooling line at 158 microns, for example in M17-SW molecular cloud-star cluster interface, observations which disprove the simple canonical photodissociation models. The much better baseline stability of the GREAT receivers (compared to Herschel HIFI) allows efficient on-the-fly mapping of extended [C II] emission in our galaxy and also in other nearby spiral galaxies. Of particular note is the GREAT discovery of two new molecules outside the solar system: OD (the deuterated OH hydroxyl radical) as well as mercapto radical SH, both in absorption near 1.4 THz, a frequency gap where Herschel was blind. A special highlight was the 2011 June 23 UT stellar occultation by Pluto using the HIPO high speed photometer and the FDC fast diagnostic camera. This difficult but successful observation, which was both space-critical (within 100 km) and time-critical (within 1 min), proved that SOFIA can be in the right place at the right time, when important transient events occur.

The Sofia Observatory at the Start of Routine Science Operations: Mission Capabilities and Performance

“…3.6 at λ = 5. In just this first year of science flights, SOFIA has furnished ample proof of its worth and future promise to infrared astronomers (Zinnecker 2013). During that time, the facility has provided data to support several interstellar molecular line studies, including the first observations of the neutral forms of the mercapto (SH) and deuterated hydroxyl (OD) radicals (Neufeld et al 2012;Parise et al 2012).…”

Exoplanet Research with the Stratospheric Observatory for Infrared Astronomy (SOFIA)

“…Shear layer seeing (air flow over the cavity) is negligible at the longer MIR and FIR wavelengths, but can be up to 5 arcsec in the optical and less in the near-infrared. Pointing of the SOFIA telescope is accurate to about 0.5 arcsec and the tracking is good, with about 0.5 arcsec over a timespan of half an hour to an hour (Zinnecker 2013). SOFIAs first generation science instruments consist of high speed photometers, imaging cameras, and spectrographs capable of resolving both broad features due to dust and large molecules, and molecular and atomic gas lines at km/s resolution.…”

“…For flights that take off and land from the same field, some fraction of the targets must be located to the north since the telescope can only view from the portside of the aircraft. A single target can in principle be viewed for an entire flight by flying one-way flights between widely separated airfields (Zinnecker 2013). However, SOFIA is usually limited to start and land in Palmdale, CA.…”

Application of Superconducting Hot-Electron Bolometer Mixers for Terahertz-Band Astronomy

“…SOFIA employs a photometer, near-, mid-, and far-infrared cameras, infrared spectrometers, and the GREAT heterodyne spectrometer, covering wavelengths from 0.3 µm to 1.6 mm [43], [44]. GREAT uses waveguide HEB mixer detectors cooled at 4 K with cryogenic amplifiers in a liquid-heliumcooled cryostat, giving a hold time of >15 h [45].…”