Stars and planets are born in molecular clouds that form and finally get disrupted again in the interstellar medium, the matter that exists in the space between the star systems in a galaxy. Astronomers still don't fully understand how this life cycle works in our Milky Way. The Stratospheric Terahertz Observatory (STO2), a NASA balloon-borne mission, led by the University of Arizona and with significant contributions from SRON, traveled in December 2016 to the edge of space above Antarctica to provide a missing piece of the puzzle.
At an altitude of 39 kilometers above Antarctica the sky is crystal clear. There is hardly any water vapor which can block the far-infrared radiation (also called terahertz radiation) from space. The edge of space is therefore a perfect environment for astronomical observations in the terahertz range. NASA uses super pressure balloons to lift observatories to that altitude. STO2 is one of them.
Once high above the Antarctic, STO2 circled along with the polar vortex for a period of three weeks. During that time STO2 picked up as much radiation as possible at the frequencies of 1.4 and 1.9 THz to find ionized nitrogen (NII) and ionized carbon respectively (CII) in a part of our Milky Way. These substances indicate the process of star formation from dust and gas.
This project is co-supported by NWO, NASA and SRON. The teams of Prof. Alexander Tielens (Leiden University) and Prof. Floris van der Tak (SRON/RUG) will contribute to the data analysis and the science.
STO2 was an exploratory mission under the leadership of the University of Arizona for astronomy in these terahertz frequencies. As leading experts in the field of terahertz receivers, SRON and the Delft University of Technology (TU Delft) were asked to deliver the STO2-receivers for the three different channels (4.7, 1.9, and 1.4 terahertz) and the 4.7 terahertz local oscillator unit. The receivers are based on superconducting Hot Electron Bolometers, which were fabricated at TU Delft. The 4.7 THz local oscillator was built in collaboration with a group at Massachusetts Institute of Technology (MIT) USA (for the quantum cascade laser) and with two groups in Nizhny Novgorod, Russia (for the frequency stabilization technology).
During the flight STO2 picked up as much radiation as possible at the frequencies of 1.4 and 1.9 THz to find ionized nitrogen (NII) and ionized carbon respectively (CII) in a part of our Milky Way. These substances indicate the process of star formation from dust and gas. The 4.7 THz detector that would measure neutral atomic oxygen (OI) also worked fine. However, something went wrong in the system for the local oscillator that had to generate the required reference signal of 4.7 THz. An electrical component needed for the communication between this local oscillator and ground control became overheated by the sun.
OI reveals that a star is actually being born. This is an observation that astronomers are keen to obtain, especially if that observation is being done for the first time beyond the earth's atmosphere, as would have been possible with STO2. However, STO2 delivered large quantity of data for the other two frequencies. After an initial hiccup in the orientation mechanism of the telescope, the collection of that data proceeded really well. Once the rough data have been processed to reveal spectral lines for CII and NII STO2 will have drastically expanded the area mapped so far for these substances.
The detector of STO2 is based on a superconducting Hot Electron Bolometer Mixer (HEBM), using nanotechnology developed at TU Delft. This detector is the most sensitive heterodyne detector now available in the terahertz domain. A heterodyne receiver can convert a high-frequency spectral line signal from space into a spectral line at a microwave frequency without losing any information. Similar to FM radio, this mixing of frequencies makes the reception clearer and the signal from space can be amplified better. It can deliver an unparalleled high spectral resolution.
A quantum cascade laser at 4.7 terahertz, actually a tiny semiconductor chip developed through a collaboration between TU Delft, SRON and MIT, will be used as a so-called local oscillator, providing a reference frequency for the incoming signals from space.